Patent Application
20240404810
The present invention relates to charge detection mass spectrometers (CDMS).
Charge Detection Mass Spectrometry (CDMS) is a technique that allows deconvolution of complex spectra of macromolecules. As molecules increase in size, the number of different charge states they may acquire increases. In the limit, overlapping charge states of molecules having different masses cause a blurred continuum on the mass to charge m/z scale of conventional mass spectrometers (MS). Such mass spectra yield little or no analytically useful information because individual species no longer stand out as distinct peaks. This is particularly problematic in the case of the electrospray of macromolecules as this ionisation technique yields many different charge states as molecular mass increases. In contrast to MS, which determines mass to charge m/z of ions, CDMS determines masses (i.e. not merely mass to charge m/z) by determining both mass to charge m/z and charge z of the ions. In conventional CDMS, individual ions are injected into an ion trap and are made to oscillate backwards and forwards through an inductive charge detection tube. As a particular ion enters the inductive charge detection tube, the particular ion induces a small, measurable voltage, the amplitude of which is proportional to its charge. The measured periodic time of the oscillation yields the mass to charge ratio m/z of the particular ion and the product of these two measurements gives the true mass of the particular ion. Allowing many oscillations within the ion trap and analysing the resulting signal by Fourier Transform (FT) improves the accuracy of both the charge and the mass to charge ratio m/z measurements. The measurement of true mass is in contrast to conventional MS such as orthogonal-acceleration time-of-flight (oa-TOF) MS which determine only mass to charge ratios m/z. Generally, the accuracy of CDMS depends on two limiting factors: electronic noise in the detection electronics giving uncertainty in charge measurements; and energy spread of incoming ions giving variations in oscillation periods.
In 2012, Contino and Jarrold [1] presented a Charge Detection Mass Spectrometer (CDMS, clear from context, also known as CDMS analyser) with a limit of detection of 30 elementary charges for a single ion. This paper gives a comprehensive review of CDMS at that time and is incorporated in its entirety by reference herein. This CDMS comprised an electrospray source coupled to a dual hemispherical deflection analyser (HDA) followed by a cone trap incorporating an image charge detector. Ions were energy selected by the dual HDA prior to entering the trap. The fundamental oscillation frequency of the trapped ions was extracted by a fast Fourier transform (FFT). The oscillation frequency and kinetic energy provided the mass to charge ratios m/z of the trapped ions. The magnitude of the FFT at the fundamental frequency was proportional to the charge. Particularly, this CDMS required use of the dual HDA as an energy filter to limit the spread of ion energies entering the electrostatic cone trap and thereby reduce the variation in oscillation frequency, so as to achieve the limit of detection of 30 elementary charges for a single ion. However, limiting the spread of ion energies entering the electrostatic cone trap reduced the throughput of the CDMS. Lower noise electronics meant that by 2015, Keifer, Shinholt and Jarrold [2] demonstrated improved charge accuracy to better than integer level—which is sufficient for true mass determination.
In 2018, Hogan and Jarrold [3] employed a segmented Electrostatic Linear Ion Trap (ELIT), which had a lower dependence on oscillation period with ion energy than the cone trap of their previous CDMS. This CDMS also required use of the dual HDA energy filter while significant dependence on oscillation frequency due to ion energy spread and radial position remained.
Particularly, for this CDMS, the kinetic energy dependence of the ion oscillation frequency was reduced by an order of magnitude, which should have led to an order of magnitude reduction in the uncertainty of the mass to charge ratio m/z ratio determination. However, only a factor of four improvement was achieved, attributed to the trajectory dependence of the ion oscillation frequency.
It is one aim of the present invention, amongst others, to provide a method, a computer, a computer program, instructions and/or a CDMS which at least partially obviates or mitigates at least some of the disadvantages of the prior art, whether identified herein or elsewhere. For instance, it is an aim of embodiments of the invention to provide a method enabling an improved limit of detection. For instance, it is an aim of embodiments of the invention to provide a method enabling an increased dynamic range. For instance, it is an aim of embodiments of the invention to improve an accuracy and/or a resolution of the mass to charge, charge and/or mass of ion.
A first aspect provides a method of discriminating noise, the method implemented by a computer comprising a processor and a memory, the method comprising:
A second aspect provides a computer, comprising a processor and a memory, configured to implement a method according to the first aspect; a computer program comprising instructions which, when executed by a computer, comprising a processor and a memory, cause the computer to perform a method according to the first aspect; and/or a non-transient computer-readable storage medium comprising instructions which, when executed by a computer, comprising a processor and a memory, cause the computer to perform a method according to the first aspect.
A third aspect provides a charge detection mass spectrometer, CDMS, comprising:
According to the present invention there is provided a method, as set forth in the appended claims. Also provided is a computer, a computer program, instructions and/or a CDMS. Other features of the invention will be apparent from the dependent claims, and the description that follows.
The first aspect provides a method of discriminating noise, the method implemented by a computer comprising a processor and a memory, the method comprising:
In this way, noise in the frequency-domain spectrum is attenuated since the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs is rejected therefrom, referred to herein as harmonic filtering. That is, the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs is discriminated (i.e. differentiated, distinguished) as noise (i.e. a false positive) from other frequency-amplitude pairs of the series of frequency-amplitude pairs having respective harmonic frequency-amplitude pairs amongst the series of frequency-amplitude pairs, corresponding with the charge induced in the inductive charge detector by ion moving in the CDMS (i.e. a positive, such as a true positive). Particularly, the inventor has identified that charge induced in the inductive charge detector due to noise, such as electronic noise, is typically described by a single frequency-amplitude pair (i.e. a fundamental or first harmonic) not having a higher order harmonic. In contrast, the inventor has identified that charge induced in the inductive charge detector due to an ion is typically described by two or more frequency-amplitude pair (i.e. a fundamental or first harmonic and at least one higher order harmonic). Furthermore, the inventor has identified that a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) of the ion may be predetermined according to a design, for example a geometry, of the CDMS and/or the inductive charge detector. In other words, noise components in the time-domain signal are eliminated therefrom by rejection of corresponding noise frequency-amplitude pairs from the frequency-domain spectrum. In this way, a limit of detection of the CDMS is improved and/or a dynamic range of the CDMS is increased, since noise is attenuated. Particularly, by rejecting the particular frequency-amplitude pair as noise from the series of frequency-amplitude pairs, an accuracy and/or a resolution of the mass to charge, charge and/or mass of the ion, calculated using the retained (i.e. not rejected, remaining) frequency-amplitude pairs of the series of frequency-amplitude pairs may be improved, since this noise is eliminated.
Particularly, the inventor has determined that noise, for example electronic noise such as due to applying a trapping potential and/or arising in the inductive charge detector, an amplifier thereof and/or associated electronics, severely limits the limit of detection and/or dynamic range of conventional CDMS. Without appropriate noise reduction, the tiny, induced charges from ions as they move through the inductive charge detector may be swamped (i.e. masked, dominated, hidden) by noise, thus severely restricting the limit of detection and/or the dynamic range of the CDMS.
Generally, an ion or a population of ions is introduced into a typically grounded electrostatic field ion trap of a CDMS and trapped therein, by a potential applied to a set of electrodes of the electrostatic field ion trap. The ion or a population of ions moves along, for example oscillating along or oscillating around, an ion path defined at least in part, by the electrostatic field ion trap via (i.e. through) the inductive charge detector, causing charges (generally, signals) to be induced therein. The respective amplitudes of the induced signals are proportional to the charge of the ion or respective charges of the population of ions and the respective periodic times of movement yield the mass to charge ratio m/z of the ion or the respective mass to charge ratios m/z of the population of ions. In a conventional CDMS, noise, for example electronic noise, may modify the measured amplitudes of the induced charges, vary the measured periodic times of movement and/or prevent measurement of the ion or the population of ions, thereby adversely affecting measurement accuracy and hence severely limiting the limit of detection and/or dynamic range of the conventional CDMS.
In more detail, the inventor has identified at least two different sources of noise, for example electronic noise, that are preferably reduced or eliminated:
Notably, the inventor has recognised that both of these sources of noise result from capacitive pick up from an electric field region, for example between electrostatic electrodes of the CDMS, leaking into the inductive charge detector itself and thereby inducing noise signals that are received by the amplifier. That is, noise signals are induced in the inductive charge detector as a result of changes in the electric fields of the set of electrostatic electrodes. Examples of changes resulting in induced noise signal include the rising edge of switching of the set of power supplies at a trapping initiation event (i.e. the potential applied to the electrostatic field ion trap at the time to), voltage ripple and/or electronic noise from the set of power supplies during acquisition time and the falling edge of switching of the set of power supplies at a trapping termination event.
Additionally and/or alternatively, the inventor has recognised that the method according to the first aspect is agnostic of the source of noise and thus is suitable for discriminating noise arising from any source, including from ion decay when an ion disintegrates during acquisition of the time-varying signal. In this way, attenuation of noise may be handled generally by the method according to the first aspect, without requiring specific handling for noise arising from specific sources. In this way, attenuation of noise is simplified compared with conventional methods, since the method of discriminating noise is generally applicable, independent of source thereof. Additionally and/or alternatively, the method according to the first aspect requires relatively lower compute resource compared with conventional methods, since the method of discriminating noise is generally applicable, independent of source thereof. For example, the method according to the first aspect may be implemented in real time, suitable for data directed acquisition (DDA), for example, as described in more detail below.
The first aspect provides the method of discriminating noise. It should be understood that noise, for example electronic noise, comprises and/or is an unwanted component in the time-domain signal (i.e. a false positive), being an unwanted (also known as an undesired) component. It should be understood that the noise is discriminated (i.e. differentiated, distinguished) from a wanted (also known as desired) component in the time-domain signal, arising from the charge induced in the inductive charge detector by the ion moving in the CDMS (i.e. a true positive). Hence, in one example, the method comprises and/or is a method of discriminating unwanted components, for example noise, in the time-domain signal from wanted components, for example arising from the charge induced in the inductive charge detector by the ion moving in the CDMS, in the time-domain signal. Additionally and/or alternatively, one example, the method comprises and/or is a method of rejecting noise.
More generally, the first aspect provides a method of identifying noise, the method implemented by a computer comprising a processor and a memory, the method comprising: obtaining a time-domain signal representative of a charge induced in an inductive charge detector by an ion moving in a charge-detection mass spectrometer, CDMS;
In this way, the particular frequency-amplitude pair of the series of frequency-amplitude pairs is identified as noise, thereby enabling rejection thereof and/or investigation into a source of noise, for example.
The method is implemented by the computer comprising the processor and the memory. Suitable computers are known. In one example, the method is implemented by the computer off-line, at-line, on-line and/or in real-time (i.e. with respect to acquiring, using the CDMS, the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS). For example, the time-domain signal may be acquired using the CDMS, stored in storage and consecutively (i.e. subsequently) post-processed using the method off-line. For example, the time-domain signal may be acquired using the CDMS, stored in storage and concurrently (i.e. simultaneously) and/or consecutively processed and/or post-processed using the method at-line. For example, the time-domain signal may be acquired using the CDMS and concurrently (i.e. simultaneously) processed using the method on-line. For example, the time-domain signal may be acquired using the CDMS and concurrently (i.e. simultaneously) processed using the method in real-time (i.e. fast enough for control of acquisition using the CDMS of the same sample or a subsequent sample).
The method comprises obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the charge-detection mass spectrometer, CDMS.
It should be understood that the time-domain signal comprises and/or is a time-varying signal (also known as a transient). It should be understood that the induced charge (i.e. the charge induced in the inductive charge detector) arises from presence of the ion proximal thereto. It should be understood that the induced charge has a polarity opposite to that of the ion. It should be understood that the magnitude of the induced charge is proportional to the charge of the ion. It should be understood that a frequency (also known as a frequency component) of the time-domain signal is equal to a frequency of periodic movement, for example oscillation such as continuously around an ion path at constant speed or oscillating around along an ion path or reciprocating along an ion path, from which the mass-to-charge of the ion may be calculated. It should be understood that an amplitude of the time-domain signal is representative of the charge of the ion. It should be understood that noise contributes to frequencies and/or amplitudes of the time-domain signal and hence noise degrades a limit of detection, dynamic range, mass to-charge accuracy and/or resolution, charge accuracy and/or resolution and/or mass accuracy and/or resolution.
Suitable inductive charge detectors for CDMS are known.
In one example, the ion comprises and/or is a single ion and obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the single ion moving in the CDMS. In this way, the method is suitable for mass to-charge, charge and/or mass analysis of a single ion.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the charge induced in the inductive detector by a plurality of ions, including the ion, moving in the CDMS, optionally wherein the plurality of ions have mutually different mass to charge ratios. In other words, the plurality of ions includes the ion. In this way, the method is suitable for mass to-charge, charge and/or mass analysis of a plurality of ions, for example the respective mass to-charge, charge and/or mass analysis of a plurality of ions (i.e. individually). It should be understood that the plurality of ions may include similar or dissimilar ions (i.e. having the same or different mass to-charge, charge and/or mass). It should be understood that the time-domain signal is thus contributed to by the respective charges induced by the plurality of ions and that the respective mass to-charge, charge and/or mass of the respective ions may be determined individually therefrom.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the respective charges induced in the inductive charge detector by a population of ions, including the ion, moving in the CDMS. In this way, the method is suitable for mass to-charge, charge and/or mass analysis of a population of ions, for example the respective mass to-charge, charge and/or mass analysis of a population of ions (i.e. individually). It should be understood that the population of ions may include similar or dissimilar ions (i.e. having the same or different mass to-charge, charge and/or mass). It should be understood that the time-domain signal is thus contributed to by the respective charges induced by the population of ions and that the respective mass to-charge, charge and/or mass of the respective ions may be determined individually therefrom.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS via (i.e. passed and/or through) the inductive charge detector N times, where N is a natural number greater than or equal to 1, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, 200, 500 or more. Generally, ions move around an ion path defined by the CDMS through at least 1 turn (also known as orbit), preferably through at least N turns where N is a natural number greater than or equal to 1, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, 200, 500 or more. Hence, the CDMS may be known as a multi-turn CDMS. Generally, increasing the number of turns through which the ion moves and hence the measurement time reduces uncertainties in masses determined therefrom. However, increasing the number of turns also increases an analysis time while likelihood of loss of a particular ion, such as through collisions such as with residual gas, other ions and/or walls of the CDMS, increases. By improving the vacuum, for example to at most 2×10−9 Torr or better, the likelihood of loss of a particular ion through collisions with residual gas may be reduced, thereby increasing the number of turns. Hence, the number of turns through which the ions move may be balanced accordingly.
Suitable CDMS are known, as described in more detail with respect to the third aspect.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the charge induced in the inductive detector by the ion moving unidirectionally and/or with constant speed in the CDMS. In this way, the method is suitable for orbit electrostatic sector field ion traps, for example, as described in more detail with respect to the third aspect. For example, ions generally orbit electrostatic sector field ion traps unidirectionally and/or with constant speed.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive detector by the ion moving in the CDMS comprises obtaining the time-domain signal representative of the charge induced in the inductive detector by the ion moving with harmonic motion and/or bidirectionally in the CDMS. In this way, the method is suitable for ELITs and/or Orbitraps, for example, as described in more detail with respect to the third aspect. For example, ions generally oscillate bidirectionally in ELITs with harmonic motion axially. For example, ions generally oscillate radially and axially in Orbitraps, with simple harmonic motion axially.
The method comprises transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs. It should be understood that the series of frequency-amplitude pairs describes the frequencies present in the time-domain signal. It should be understood that the series of frequency-amplitude pairs includes one or more frequency-amplitude pairs, depending upon the number of frequencies present in the time-domain signal, for example. It should be understood that the amplitude of a more frequency-amplitude pair may represent the peak area or the peak height of a peak in the frequency-domain spectrum.
In one example, the series of frequency-amplitude pairs comprises S frequency-amplitude pairs, wherein S is a natural number greater than or equal to 1, for example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, 90000, 100000 or more frequency-amplitude pairs. It should be understood that generally, the number S of frequency-amplitude pairs increases with the number of ions and/or noise.
In one example, transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs comprises transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs using a Fourier Transform, FT, such as a Fast Fourier Transform, FFT, and/or a Fourier-related transform such as a two-sided Laplace transform, a Mellin transform, a Laplace transform, a Fourier series transform, a sine and/or cosine transform, a Hartley transform, a short-time Fourier transform (or short-term Fourier transform) (STFT), a rectangular mask short-time Fourier transform, a chirplet transform, a fractional Fourier transform (FRFT), a Hankel transform, a Fourier-Bros-Iagolnitzer transform and/or a linear canonical transform. Other suitable transforms are known. In one preferred example, the FT comprises and/or is a FFT. In this way, transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs may be executed efficiently, requiring relatively lower compute resource.
In one example, transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs comprises transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs using a maximum entropy method. Suitable maximum entropy methods are known.
In one example, transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs comprises transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs using a Fourier Transform, FT and a maximum entropy method, as described above.
In one example, the method comprises pre-processing the time-domain signal before transforming the time-domain signal into the frequency-domain spectrum comprising the series of frequency-amplitude pairs, for example wherein pre-processing the time-domain signal comprises windowing (such as to exclude particular time ranges such as beginning or end and/or to include only particular time ranges of interest such as when the ion is moving and/or to exclude particular time ranges not of interest such as upon ion decay when the ion disintegrates during the acquisition of the time-varying signal), data cleaning (such as find, remove and/or replace bad and/or missing data), smoothing (such as to eliminate high variance data), detrending (such as to remove a trend) scaling or normalizing (such as to change bounds) and/or filtering (such as passband filtering, high-pass filtering and/or low-pass filtering). In this way, artefacts in the time-domain signal may be removed, for example.
In one example, the method comprises examining the time-domain signal for consistency of the frequency-domain spectrum as a function of time, for example by subdividing (for example as consecutive time windows or moving time windows) the time-domain signal into a plurality of time-domain signal segments, transforming the plurality of time-domain signal segments into a corresponding plurality of frequency-domain spectra, mutually comparing the plurality of frequency-domain spectra and discarding one or more of the corresponding time-domain signal segments based on a result of the comparing. In this way, ion decay, for example, may be identified and the one or more of the corresponding time-domain signal segments discarded.
In one example, the method comprises removing one or more frequency-amplitude pairs of the series of frequency-amplitude pairs having a frequency greater than or equal to a threshold frequency, for example a predetermined absolute or relative threshold frequency (such as top 30%, 20% or 10%). In this way, frequency-amplitude pairs having relatively higher frequencies may be removed from the series of frequency-amplitude pairs, for example before the step of rejecting, thereby reducing a number of frequency-amplitude pairs to be potentially rejected (i.e. reducing computing).
In one example, the method comprises removing one or more frequency-amplitude pairs of the series of frequency-amplitude pairs having an amplitude less than or equal to a threshold frequency, for example a predetermined absolute amplitude or relative threshold amplitude (such as bottom 30%, 20% or 10%). In this way, frequency-amplitude pairs having relatively lower amplitudes may be removed from the series of frequency-amplitude pairs, for example before the step of rejecting, thereby reducing a number of frequency-amplitude pairs to be potentially rejected (i.e. reducing computing).
The method comprises rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs. It should be understood that harmonics of the particular frequency-amplitude pair are determined conventionally, wherein the respective harmonic frequency-amplitude pair has a frequency equal to an integral multiple (within a tolerance, as described below) of the frequency of the frequency-amplitude pair (typically the fundamental or first harmonic).
Particularly, the inventor has identified that charge induced in the inductive charge detector due to noise, such as electronic noise, is typically described by a single frequency-amplitude pair (i.e. a fundamental or first harmonic) not having a higher order harmonic. In contrast, the inventor has identified that charge induced in the inductive charge detector due to an ion is typically described by two or more frequency-amplitude pair (i.e. a fundamental or first harmonic and at least one higher order harmonic). Furthermore, the inventor has identified that a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) of the ion may be predetermined according to a design, for example a geometry, of the CDMS and/or the inductive charge detector.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises iterating through the series of frequency-amplitude pairs and rejecting each particular frequency-amplitude pair of the series of frequency-amplitude pairs not having a respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs within a predetermined frequency tolerance range, for example wherein the predetermined frequency tolerance range is absolute (i.e. constant) or relative (i.e. a proportion of the frequency). In this way, frequency-amplitude pairs having respective frequencies proximal a harmonic frequency of the particular frequency-amplitude pair may be identified. In contrast, if a frequency-amplitude pair is not identified proximal a harmonic frequency of the particular frequency-amplitude pair, the particular frequency-amplitude may be rejected.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs within a predetermined amplitude tolerance range, for example wherein the predetermined amplitude tolerance range is absolute (i.e. constant) or relative (i.e. a proportion of the amplitude, for example of the particular frequency-amplitude pair). In this way, frequency-amplitude pairs having respective amplitudes relative to the amplitude of the particular frequency-amplitude pair may be identified. In contrast, if a frequency-amplitude pair is not having an amplitude relative to the amplitude of the particular frequency-amplitude pair, the particular frequency-amplitude may be rejected. Particularly, the inventor has identified that the relative amplitudes of a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) may be characteristic of an ion moving in the CDMS and may be calculated by modelling, for example.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting a particular frequency-amplitude pair of the series of frequency-amplitude pairs not having a predetermined harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum). Particularly, the inventor has identified that a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) of the ion may be predetermined according to a design, for example a geometry, of the CDMS and/or the inductive charge detector. For example, the harmonic signature may include only a fundamental or first harmonic and a second harmonic. For example, the harmonic signature may include only a fundamental or first harmonic, a second harmonic and a third harmonic. For example, the harmonic signature may include only a fundamental or first harmonic and a third harmonic. For example, the harmonic signature may include only a fundamental or first harmonic and odd harmonics. For example, the harmonic signature may include only a fundamental or first harmonic and even harmonics.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting a particular frequency-amplitude pair of the series of frequency-amplitude pairs having a predetermined harmonic frequency-amplitude pair, preferably a predetermined third and/or higher respective harmonic frequency-amplitude pair, amongst the series of frequency-amplitude pairs.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting a particular frequency-amplitude pair of the series of frequency-amplitude pairs not having a predetermined harmonic frequency-amplitude pair signature amongst the series of frequency-amplitude pairs.
In one example, rejecting the particular frequency-amplitude pair of the series of frequency-amplitude pairs not having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises rejecting a, all and/or only particular frequency-amplitude pairs of the series of frequency-amplitude pairs not having respective harmonic frequency-amplitude pairs amongst the series of frequency-amplitude pairs. In this way, all unwanted frequency-amplitude pairs, such as arising from noise, may be eliminated, for example.
In one example, the method comprises retaining a particular frequency-amplitude pair of the series of frequency-amplitude pairs having a respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs. In this way, wanted frequency-amplitude pairs, such as arising from the ion or ions, may be retained, for example.
In one example, the method comprises retaining all particular frequency-amplitude pairs of the series of frequency-amplitude pairs having respective harmonic frequency-amplitude pairs amongst the series of frequency-amplitude pairs. In this way, all wanted frequency-amplitude pairs, such as arising from the ion or ions, may be retained, for example.
Additionally and/or alternatively, the first aspect provides a method of discriminating noise, the method implemented by a computer comprising a processor and a memory, the method comprising:
In one example, retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs within a predetermined frequency tolerance range, for example wherein the predetermined frequency tolerance range is absolute (i.e. constant) or relative (i.e. a proportion of the frequency). In this way, frequency-amplitude pairs having respective frequencies proximal a harmonic frequency of the particular frequency-amplitude pair may be identified and retained. In contrast, if a frequency-amplitude pair is not identified proximal a harmonic frequency of the particular frequency-amplitude pair, the particular frequency-amplitude may be rejected.
In one example, retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs within a predetermined amplitude tolerance range, for example wherein the predetermined amplitude tolerance range is absolute (i.e. constant) or relative (i.e. a proportion of the amplitude, for example of the particular frequency-amplitude pair). In this way, frequency-amplitude pairs having respective amplitudes relative to the amplitude of the particular frequency-amplitude pair may be identified. In contrast, if a frequency-amplitude pair is not having an amplitude relative to the amplitude of the particular frequency-amplitude pair, the particular frequency-amplitude may be rejected. Particularly, the inventor has identified that the relative amplitudes of a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) may be characteristic of an ion moving in the CDMS and may be calculated by modelling, for example. In one example, retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having a respective predetermined harmonic frequency-amplitude pair, preferably only a predetermined second harmonic frequency-amplitude pair, amongst the series of frequency-amplitude pairs.
In one example, retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having a predetermined harmonic frequency-amplitude pair signature amongst the series of frequency-amplitude pairs.
In one example, retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having the respective harmonic frequency-amplitude pair amongst the series of frequency-amplitude pairs comprises retaining the particular frequency-amplitude pair of the series of frequency-amplitude pairs having a predetermined harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum). Particularly, the inventor has identified that a harmonic signature (i.e. the particular harmonic frequency-amplitude pairs included in the frequency-domain spectrum) of the ion may be predetermined according to a design, for example a geometry, of the CDMS and/or the inductive charge detector. For example, the harmonic signature may include only a fundamental or first harmonic and a second harmonic. For example, the harmonic signature may include only a fundamental or first harmonic, a second harmonic and a third harmonic. For example, the harmonic signature may include only a fundamental or first harmonic and a third harmonic. For example, the harmonic signature may include only a fundamental or first harmonic and odd harmonics. For example, the harmonic signature may include only a fundamental or first harmonic and even harmonics.
In one example, the method comprises calculating a mass-to-charge ratio of the ion using the frequency of the retained frequency-amplitude pair and optionally, using the frequency of the respective harmonic frequency-amplitude pair.
In one example, the method comprises calculating a charge of the ion using the amplitude of the retained frequency-amplitude pair and optionally, using the amplitude of the respective harmonic frequency-amplitude pair.
In one example, the method comprises calculating a mass of the ion using the calculated mass-to-charge ratio of the ion and the calculated charge of the ion.
In one example, obtaining the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS comprises acquiring, using the CDMS, the time-domain signal representative of the charge induced in the inductive charge detector by the ion moving in the CDMS.
In one example, the method comprises controlling the CDMS, for example in real-time, using the calculated mass to charge ratio of the ion and/or the charge of the ion.
The second aspect provides a computer, comprising a processor and a memory, configured to implement a method according to the first aspect; a computer program comprising instructions which, when executed by a computer, comprising a processor and a memory, cause the computer to perform a method according to the first aspect; and/or a non-transient computer-readable storage medium comprising instructions which, when executed by a computer, comprising a processor and a memory, cause the computer to perform a method according to the first aspect.
The third aspect provides a charge detection mass spectrometer, CDMS, comprising:
In one example, the CDMS is generally as described in WO 2022/049388 A1 and/or WO 2022/214815 A1, the subject matter of which is incorporated herein by reference.
The CDMS comprises the electrostatic field ion trap, comprising the set of electrostatic electrodes including a first electrostatic electrode and a second electrostatic electrode. As described below, the electrostatic field ion trap may comprise and/or be an electrostatic sector field ion trap and/or an electrostatic linear field ion trap, for example. It should be understood that the set of electrostatic electrodes includes the first electrostatic electrode and the second electrostatic electrode (i.e. a plurality of electrostatic electrodes). In one example, the set of electrostatic electrodes includes E electrostatic electrodes, where E is a natural number greater than or equal to 2, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, 200, 500 or more. In one example, the first electrostatic electrode comprises two or more electrostatic electrodes, for example inner and outer electrodes of an electrostatic sector electrode or a stack of electrodes of a reflecting or mirror electrode, such as a cone trap or segmented electrode, as described below. The second electrostatic electrode may be as described with respect to the first electrostatic electrode mutatis mutandis.
In one example, the electrostatic field ion trap comprises and/or is an electrostatic sector field ion trap, for example wherein the first electrostatic electrode comprises and/or is a first electrostatic sector electrode and wherein the second electrostatic electrode comprises and/or is a second electrostatic sector electrode. In one example, the first electrostatic sector electrode is an inner electrode and the second electrostatic sector electrode is an outer electrode, or vice versa, of a cylindrical, toroidal or spherical electrostatic sector. It should be understood that the electrostatic sector field ion trap is a periodic structure and defines, at least in part, a closed ion path (also known as an orbit), such that ions may move (also known as oscillate) around the closed ion path repeatedly, for example an integral or a non-integral number of turns. Generally, ions move around the ion path through at least 1 turn, preferably through at least N turns where N is a natural number greater than or equal to 1, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, 200, 500 or more. Hence, the electrostatic sector field ion trap may be known as a multi-turn (also known as multi-pass) electrostatic sector field ion trap and in turn, the CDMS may be known as a multi-turn CDMS. Generally, increasing the number of turns through which the ions move and hence the measurement time reduces uncertainties in masses determined therefore. However, increasing the number of turns also increases an analysis time while likelihood of loss of a particular ion, such as through collisions such as with residual gas, other ions and/or walls of the CDMS, increases. By improving the vacuum, for example to at most 2×10−9 Torr or better, the likelihood of loss of a particular ion through collisions with residual gas may be reduced, thereby increasing the number of turns. Hence, the number of turns through which the ions move may be balanced accordingly. The fundamental frequency f (and/or a harmonic thereof) of an ion moving around the ion path is dependent on the mass to charge ratio m/z and is measured as described below, using the inductive charge detector. If the electrostatic sector field ion trap is isochronous to first order, the uncertainty in mass is reduced, compared with the ELIT of reference [3], for example, and as described below in more detail.
It should be understood that the electrostatic sector field ion trap is configured to define, at least in part, the ion path via the inductive charge detector. That is, in use, the electrostatic sector field ion trap defines, at least in part, the ion path via the inductive charge detector. By at least in part it should be understood that the ion path may be entirely defined by the electrostatic sector field ion trap or alternatively, may be partly defined by the electrostatic sector field ion trap and partly defined by one or more ion optical elements, for example lenses and/or magnets. In one example, the electrostatic sector field ion trap is configured to define (i.e. entirely) the ion path via the inductive charge detector. It should be understood that the ion path is the ion optical axis for a hypothetical perfect ion, in which the ion optical axis is an arcuate, closed line defining a plane. In contrast, the energy spread together with the angular and spatial deviations of a population of ions means that the respective ion trajectories deviate from the ion optical axis, in and out of the plane, such that the ion path sweeps volumetrically around the ion optical axis. It should be understood that a cross-section, for example a shape and/or dimension thereof, of the ion path, orthogonal thereto, may vary around the ion path. Particularly, as described in more detail below, the ions may be brought into point focus and parallel focus, for example alternately, around the ion path, while spherical electrostatic sectors may, for example, give rise to at least partly spherical ion paths. Additionally and/or alternatively, the ion path may be described and/or defined as an ion beam.
It should be understood that ions move around the ion path with a constant sense, which for convenience may be termed unidirectional, in that the orbital direction is constant, notwithstanding that the instantaneous directions of motion of the ions change constantly. In contrast, ions move in ELITs, such as the ELITs of references [2] and [3], with alternating, opposite senses, forwards and backwards, which for convenience may be termed bidirectional or reciprocating, thereby resulting in mutual interaction between ions moving in opposite directions, that precludes introducing more than a single ion therein. In more detail, typically thousands of ions are measured to generate a mass spectrum, depending on sample heterogeneity. According to the continuous (or random) trapping mode of reference [3], the probability that the ELIT contains zero ions, one ion, or more than one ion is given by a Poisson distribution such that the maximum number of single ion ELIT trapping events that can be realized is just 37% (i.e. a duty cycle of 37%). For 100 ms long trapping periods, the optimum fraction of single ion trapping events equates to a maximum of around 13,300 single ion events per hour for the ELIT such that a spectrum of a homogeneous sample may be acquired in under half an hour under optimum conditions (i.e. when the signal is stable and the number of single ion trapping events is close to the maximum that may be realized). In contrast, the unidirectional, closed ion path defined, at least in part, by the electrostatic sector field ion trap may be used to simultaneously determine the respective masses of a plurality of ions, for example as described below in detail, thereby reducing the acquisition time compared with the ELIT. Particularly, as discussed previously, a space charge capacity of the electrostatic sector field ion trap is increased compared with reflecting based ion traps, in which ions must slow to a low speed as they turn around in the mirror sections, since the unidirectional ion path reduces or eliminates mutual interactions between ions of the plurality thereof. Furthermore, for a given ion path length, the effective mean cross-sectional area of the ion path and hence the volume thereof is greater for the electrostatic sector field ion trap than for an ELIT, for example, since the ion path for the electrostatic sector field ion trap generally permits ions to fan out, for example arcuately, transversely to the ion optical axis. Hence, the electrostatic sector field ion trap may be filled with relatively more ions, for example one or more orders of magnitude more compared with the ELIT, while the kinetic energies of the ions are relatively constant. Furthermore, the bidirectional or reciprocating ion path of the ELIT of references [2] and [3], for example, results in overlapping signals induced in the charged tubes thereof if more than one ion is trapped, thereby preventing mass determination. In contrast, the unidirectional ion path means that a likelihood of overlapping signals induced in the inductive charge detector by two or more ions is reduced. It should be understood that generally, partially overlapping induced signals may be separated in the frequency-domain but completely overlapping induced signals, for example due to phase coherent ions, overtaking ions or ions moving in opposite directions, may preclude mass determination. Hence, ion packets or clouds are preferably avoided, to avoid phase coherent ions, while the unidirectional path eliminates moving in opposite directions. Particularly, as described below in more detail, the plurality of ions may be introduced so as to be mutually spatially and/or temporally separated, thereby reducing or eliminating likelihood of overlapping signals induced in the inductive charge detector. Thus, by increasing the number of ions in the electrostatic sector field ion trap to even just 10 and since the duty cycle is not limited to just 37%, as for the ELIT, the same mass spectrum may be instead acquired in less than a minute.
Generally, the ion-optical description of transfer of ions between the entrance and exit of an analyser may be expressed as a transfer matrix or via ray tracing. A curvilinear coordinate system (x,y,z) may be defined having its origin on the optical axis and the z therealong. For ions of equal masses, such a transfer matrix may be expressed, where time is of interest, as:
For applications of interest, the ions originate from low intensity sources, having relatively low energy (i.e. low-energy ions), but have relatively large energy spreads δK.
Ions having the same mass to charge ratios m/z but different energies will move through the analyser in the same time if (δT∨δK)=0. Such an analyser is energy isochronous. In one example, |(δT∨δK)|≤0.1; preferably |(δT∨δK)|≤0.05; more preferably |(δT∨δK)|≤0.01. That is, the analyser may be quasi-energy isochronous, thereby still allowing a relatively large number of turns while relaxing tolerancing of the geometry. Ranges for (δT∨X), (δT∨A), (X∨δK) and/or (A∨δK) may be defined similarly.
It should be understood that focal plane is the position where ions sent from the optic axis with an angular distribution are brought to a focal point after passing through the analyser. In one dimension x, this is mathematically expressed in aberration theory notation as (x∨a)=0. An analyser behaves stigmatically (i.e. is stigmatic) if (x|a)=(y|b)=0 at the focal plane. More generally, ions having the same mass to charge ratios m/z but different energies and different angles of inclination at the entrance will move through the exit independently of energy and entrance angle of inclination if (X∨A)=(A∨X)=(A∨δK)=0. Such an analyser is stigmatic and achromatic focusing and the trajectories are mirror symmetric. In one example, |(X∨A)|,|(A∨X)| and/or |(A∨δK)|≤0.1, preferably |(X∨A)|,|(A∨X)| and/or |(A∨δK)|≤0.05, more preferably |(X∨A)|,|(A∨X)| and/or |(A∨SK)|≤0.01. That is, the analyser may be quasi-stigmatic and/or quasi-achromatic, thereby still allowing a relatively large number of turns while relaxing tolerancing of the geometry.
Generally, an achromatic system is one where the transfer matrix elements for the transverse coordinates do not depend on momentum. Generally, an isochronous system is one where the transit time of a trajectory through the system does not depend on the initial coordinates. It is well known that a first-order achromatic system is also isochronous, except for pure momentum dependence. The converse is also true. This result is entended to higher orders. Conditions may be found so that for a system whose chromatic terms all vanish up to a certain order the transit time will be independent of the transverse coordinates up the same order. Under the same conditions, the converse will also be true.
However, the spatial focusing requirement of (X∨A)=(A∨X)=0 requires identical ion trajectories for the particular ion for every turn. For the applications of interest, spatial focusing requirement may be relaxed by postulating only that the particular ion moves stably in phase space, thus requiring:
In this way, the particular ion may move on different trajectories during different turns. This relaxation also may increase design freedom and/or tolerate constructional errors, for example, while alternatively and/or additionally accommodate spatial and/or angular deviations arising from ion injection, for example.
Conversely, perfect spatial and temporal focusing eliminates ion beam divergence and mass resolution degradation as the number of turns increases, by returning the particular ion to the same position and at the same angle of inclination upon every turn. TOF MS analyser geometries having such perfect spatial and temporal focusing have been proposed (MULTUM, MULTUM II and planar figure of eight) and some constructed, as described in more detail below with reference to [15], which is incorporated herein in entirety by reference.
In one example, the electrostatic sector field ion trap comprises a set of electrostatic sectors, including a first electrostatic sector and a second electrostatic sector. It should be understood that the first electrostatic sector and the second electrostatic sector are mutually spaced apart, for example by a field-free region (also known as a drift space), traversed by the ion path. It should be understood that the inductive charge detector is disposed in a field-free region. While the inductive charge detector could be disposed in an electric field, direct capacitive coupling of noise from the power supply limits detection. In one example, the electrostatic sector field ion trap comprises a set of electrostatic sectors, including a first electrostatic sector and a second electrostatic sector, and a set of electric quadrupole lenses, including Q quadrupole lenses, wherein Q is a natural number greater than or equal to 1, for example wherein Q is four or six times the number of electrostatic sectors. Generally, a quadrupole lens focuses in one coordinate direction and defocuses in a mutually orthogonal coordinate direction. Hence, a single quadrupole lens cannot be used to focus an ion beam to a point or to produce a two-dimensional image, for example. However, two-dimensional focusing may be accomplished with combinations of quadrupole lenses, such as two quadrupole lenses (doublets) and three quadrupole lenses (triplets). For example, two quadrupole lens doublets may be arranged corresponding with the entrance and the exit of an electrostatic sector, respectively. In one example, the electrostatic sector field ion trap does not comprise a set of electric quadrupole lenses and/or RF electric lenses, thereby reducing a complexity. It should be understood that an electrostatic sector comprises two corresponding electrodes mutually spaced apart radially, having corresponding radii of curvature in two mutually orthogonal dimensions, to which corresponding and opposed electrical DC potentials are applied to thereby provide a toroidal electric field defining the ion optical axis therethrough, wherein the electrical potential on the ion optical axis (i.e. the central trajectory) is preferably the same as that in the field free region, for example ground. It should be understood that the electrostatic sector field ion trap comprises a set of power supplies, for example DC power supplies, electrically coupled thereto.
Additionally and/or alternatively, the electrostatic sector field ion trap may be defined by a set of cells (also known as unit or elements), including a first cell and a second cell, wherein the first cell comprises a set of drift spaces, a set of electrostatic sectors including a first electrostatic sector and optionally, a set of quadrupole lenses. It should be understood that the second cell may be as described with respect to the first cell. Symmetrical geometries of cells are more readily understood but the principles extend to asymmetrical geometries cells. An electrostatic sector field ion trap defined by four cells may be considered to be a doubly symmetric geometry of two cells, such as the MULTUM and the MULTUM II of reference [15]. While the planar figure of eight geometry of reference [15] prima facie appears to be defined by two cells, perfect focussing is achieved after two turns and hence this planar figure of eight geometry is defined also by four cells.
In one example, the first electrostatic sector comprises and/or is a cylindrical, a toroidal or a spherical electrostatic sector. A cylindrical electrostatic sector provides the simplest geometry, having effectively a single radius of curvature in one dimension only (the second radius of curvature in a mutually orthogonal dimension being infinite) but does not confine ions in that orthogonal dimension e.g. y direction and thus generally requires confining electric fields in said y direction (curvilinear coordinates). Cylindrical electrostatic sectors are generally used together with electric quadrupole lenses. MULTUM and the planar figure of eight geometry of reference [15] comprise respectively four and two cylindrical electrostatic sectors, each together with eight electric quadrupole lenses. A toroidal electrostatic sector has two different radii of curvature in the two mutually orthogonal dimensions, the ratio of which must be defined for enablement, and may confine ions in both dimensions such that electric quadrupole lenses may not be required. Since toroidal electrostatic sectors have two different radii of curvature, construction thereof is relatively more complex. MULTUM II of reference [15] and the rhomboid geometry of reference [16] each comprise four toroidal electrostatic sectors and do not require electric quadrupole lenses. A spherical electrostatic sector is a special case of a toroidal electrostatic sector, having two radii of curvature that are the same, and may confine ions in both dimensions such that electric quadrupole lenses may not be required. The figure of eight geometry of reference [4] comprises two spherical electrostatic sectors and does not require electric quadrupole lenses. Hence, by basing a cell on a spherical electrostatic sector, the number of ion optical components may be reduced compared with a cell based on a cylindrical electrostatic sector or a toroidal sector.
In one example, the first electrostatic sector has a deflection angle ψ0 greater than 45.0°, preferably at least 60.0°, for example in a range from greater than 60.0° to 270.0°, preferably in a range from 90.0° to 240.0°. In one example, the second electrostatic sector is as described with respect to the first electrostatic sector, for example having the same or a different deflection angle ψ0. In one example, each electrostatic sector of the set of electrostatic sectors has the same deflection angle ψ0. In this way, a complexity is reduced and/or a symmetry increased. In one example, alternate electrostatic sectors of the set of electrostatic sectors have the same respective deflection angles ψ0.
In one example, the set of electrostatic sectors has a total deflection angle ψ0 greater than 360.0°, preferably at least 390.0°, for example in a range from greater than 360.0° to 720.0°, preferably in a range from 390.0° to 660.0°. That is, the ion path includes a crossover.
In one example, the set of electrostatic sectors does not comprise or consist of a ring of eight 45° toroidal electrostatic sectors.
In one example, the first electrostatic sector and the second electrostatic sector are mutually opposed, for example directly, diagonally and/or diametrically wherein the ion path and/or ion optical axis is linear between the exit of the first electrostatic sector and the entrance of the second electrostatic sector. In one example, the ion path and/or ion optical axis between the exit of the first electrostatic sector and the entrance of the second electrostatic sector is in a field-free region. That is, the ion path and/or ion optical axis between the exit of the first electrostatic sector and the entrance of the second electrostatic sector does not include a quadrupole lens, for example.
In one example, the set of electrostatic sectors includes only the first electrostatic sector and the second electrostatic sector, preferably wherein the first electrostatic sector and the second electrostatic sector are spherical electrostatic sectors, of radius r, having a deflection angle ψ0 of nominally 199.2°, for example within a range from 198.2° to 200.2°, preferably in a range from 198.7° to 199.7°, more preferably in a range from 199.0° to 199.4°, for example 199.2°, and wherein the electrostatic sector field ion trap comprises four field-free regions of length grof nominally 5.9r (for example, within 2%, preferably within 1%), thereby providing a three-dimensional figure of eight geometry according to reference [4].
In one example, the electrostatic sector field ion trap comprises a set of field free regions (also known as drift regions), including a first field free region and a second field free region. In one example, a length of the ion path through the set of field free regions is at least 50%, preferably at least 55%, more preferably at least 60%, most preferably at least 65% of the total length of the ion path. In this way, the inductive charger detector may be arranged in the set of field free regions to extend along about 50% of the ion path, thereby increasing a measurement duty cycle.
In one example, the first electrostatic sector comprises a set of shunts, including a first shunt, arranged to delimit a field due to the first electrostatic sector. In this way, fringe fields due to the first electrostatic sector may be controlled and/or the inductive charge detector shielded from the field due thereto. For example, shunts may attenuate noise coupled to the inductive charge detector up to several orders of magnitude. For example, a 100 V power supply electrically coupled to the first electrostatic sector may exhibit <1 mV RMS noise. Using shunts, this noise may be attenuated to about 1 μV RMS, and hence compatible with suitable charge sensitive amplifiers having sensitivities of typically 0.6 μV/charge.
In one example, the electrostatic sector field ion trap (i.e. the behaviour thereof) is isochronous, for example to first order with respect to energy (i.e. energy isochronous), as discussed above, for example after 1 turn and/or after an integral number of turns. In this way, the uncertainty in the mass to charge ratio m/z is reduced, compared with an ELIT such as the ELIT of reference [3]. Example geometries for such an electrostatic sector field ion trap include the figure of eight of reference [4], the MULTUM, MULTUM II and planar figure of eight of reference [15] and the rhomboid of reference [16]. Other geometries are known. In one example, the electrostatic sector field ion trap is isochronous to first order with respect to energy, having a residual to second order, for example a parabolic residual to second order. In this way, the uncertainty in the mass to charge ratio m/z due to a small dispersion in energy ΔE of the ion is further reduced. Second order spatial aberrations result in precession of the ions while orbiting, which may be controlled using constraining electric fields, for example to prevent ion losses arising from precession in electrostatic sector field ion traps based on cylindrical and/or toroidal electrostatic sectors, or may be allowed for electrostatic sector field ion traps based on spherical electrostatic sectors, for example.
In one example, the electrostatic sector field ion trap is configured to define, at least in part, the ion path in two or three mutually-orthogonal dimensions. For example, the electrostatic sector field ion trap may be configured to define, at least in part, the ion path in two mutually-orthogonal dimensions, such as in the x,z dimensions, and may be referred to as a planar electrostatic sector field ion traps, in which the ion optical axis defines the plane. It should be understood that deviations in position, angle of inclination and/or energy cause ions to depart from the ion optical axis such that the ion beam may be represented by a distribution transverse thereto, for example in phase space. Examples planar electrostatic sector field ion traps include the MULTUM, MULTUM II and planar figure of eight of reference [15] and the rhomboid of reference [16]. Construction of such planar electrostatic sector field ion traps may be simplified and may be based on cylindrical, toroidal and/or spherical electrostatic sectors, including quadrupole lenses as required.
In one example, the ion path defined by the electrostatic sector field ion trap includes a crossover or point focus, for example the MULTUM, MULTUM II and planar figure of eight of reference [15] and the figure of eight of reference [4]. In this way, the electrostatic sector field ion trap may be isochronous with respect to energy while a length of the ion path increased for a given perimeter or area of the electrostatic sector field ion trap, compared with a ring, for example.
In one example, the electrostatic field ion trap comprises and/or is an electrostatic linear field ion trap, also known as electrostatic linear field ion trap (ELIT) for example wherein the first electrostatic electrode comprises and/or is a first reflecting or mirror electrode and wherein the second electrostatic electrode comprises and/or is a second reflecting or mirror electrode, for example as described in [2], [3], [17] and [18], the subject matter of which is incorporated herein by reference. ELITs are known, typically comprising a pair of mutually opposed reflecting or mirror electrodes, such as cones traps or segmented electrodes. In 2012, Contino and Jarrold [1] presented a Charge Detection Mass Spectrometer (CDMS, clear from context, also known as CDMS analyser) with a limit of detection of 30 elementary charges for a single ion. This CDMS comprised an electrospray source coupled to a dual hemispherical deflection analyser (HDA) followed by a cone trap incorporating an image charge detector. Ions were energy selected by the dual HDA prior to entering the trap. The fundamental oscillation frequency of the trapped ions was extracted by a fast Fourier transform (FFT). The oscillation frequency and kinetic energy provided the mass to charge ratios m/z of the trapped ions. The magnitude of the FFT at the fundamental frequency was proportional to the charge. Particularly, this CDMS required use of the dual HDA as an energy filter to limit the spread of ion energies entering the electrostatic cone trap and thereby reduce the variation in oscillation frequency, so as to achieve the limit of detection of 30 elementary charges for a single ion. However, limiting the spread of ion energies entering the electrostatic cone trap reduced the throughput of the CDMS. Lower noise electronics meant that by 2015, Keifer, Shinholt and Jarrold [2] demonstrated improved charge accuracy to better than integer level—which is sufficient for true mass determination.
In 2018, Hogan and Jarrold [3] employed a segmented Electrostatic Linear Ion Trap (ELIT), which had a lower dependence on oscillation period with ion energy than the cone trap of their previous CDMS. This CDMS also required use of the dual HDA energy filter while significant dependence on oscillation frequency due to ion energy spread and radial position remained. Particularly, for this CDMS, the kinetic energy dependence of the ion oscillation frequency was reduced by an order of magnitude, which should have led to an order of magnitude reduction in the uncertainty of the mass to charge ratio m/z ratio determination. However, only a factor of four improvement was achieved, attributed to the trajectory dependence of the ion oscillation frequency.
It should be understood that the electrostatic linear field ion trap is a periodic structure and defines, at least in part, a closed ion path, such that ions may move (also known as reciprocate or oscillate) along the closed ion path repeatedly, for example an integral or a non-integral number of turns. Generally, ions move along the ion path through at least 1 pass, preferably through at least N passes, where N is a natural number greater than or equal to 1, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, 200, 500 or more. Hence, the electrostatic linear field ion trap may be known as a multi-pass) electrostatic linear field ion trap and in turn, the CDMS may be known as a multi-pass CDMS. Generally, increasing the number of passes through which the ions move and hence the measurement time reduces uncertainties in masses determined therefore. However, increasing the number of passes also increases an analysis time while likelihood of loss of a particular ion, such as through collisions such as with residual gas, other ions and/or walls of the CDMS, increases. By improving the vacuum, for example to at most 2×10−9 Torr or better, the likelihood of loss of a particular ion through collisions with residual gas may be reduced, thereby increasing the number of passes. Hence, the number of passes through which the ions move may be balanced accordingly. The fundamental frequency f (and/or a harmonic thereof) of an ion moving along the ion path is dependent on the mass to charge ratio m/z and is measured as described below, using the inductive charge detector.
It should be understood that the electrostatic linear field ion trap is configured to define, at least in part, the ion path via the inductive charge detector. That is, in use, the electrostatic linear field ion trap defines, at least in part, the ion path via the inductive charge detector. By at least in part it should be understood that the ion path may be entirely defined by the electrostatic linear field ion trap or alternatively, may be partly defined by the electrostatic linear field ion trap and partly defined by one or more ion optical elements, for example lenses and/or magnets. In one example, the electrostatic linear field ion trap is configured to define (i.e. entirely) the ion path via the inductive charge detector. It should be understood that a cross-section, for example a shape and/or dimension thereof, of the ion path, orthogonal thereto, may vary along the ion path. Additionally and/or alternatively, the ion path may be described and/or defined as an ion beam.
It should be understood that ions move along the ion path of ELITs, such as the ELITs of references [2] and [3], with alternating, opposite senses, forwards and backwards, which for convenience may be termed bidirectional or reciprocating, thereby resulting in mutual interaction between ions moving in opposite directions, that precludes introducing more than a single ion therein.
In one example, the electrostatic field ion trap comprises and/or is a curved trap, (referred to colloquially as a “c-trap”) of the type utilized in Orbitrap mass spectrometers sold by Thermo Fisher Scientific, for example as described in US 2022/0246414 A1, the subject matter of which is incorporated herein by reference. The curved trap is composed of a set of generally parallel rod electrodes that are curved concavely toward the ion exit. Radial confinement of ions within ion store may be achieved by applying oscillatory voltages in a prescribed phase relationship to opposed pairs of the rod electrodes, while axial confinement may be effected by applying static voltages to end lenses positioned axially outwardly of the rod electrodes.
The CDMS comprises the inductive charge detector and the electrostatic field ion trap is configured to define, at least in part, the ion path via (i.e. through) the inductive charge detector. In other words, the inductive charge detector encloses or surrounds, at least in part, the ion path.
Generally, when an ion moves through the inductive charge detector, the ion induces a charge that is detected by a charge sensitive amplifier, which outputs a signal. It should be understood that the inductive charge detector comprises a charge sensitive amplifier and optionally a digitiser, communicatively coupleable or coupled to a computer comprising a processor and a memory. The mass of the ion may be determined using the signal by Fourier analysis for example using a Fourier Transform (FT) or fast Fourier Transform (FFT), least squares, Filter Diagonalization Method (FDM) and/or Maximum Likelihood method or similar. In one example, the signal comprises and/or is a time-domain signal, which may be amplified and/or digitized for analysis. The use of FFTs, for example, enables detection of charges which do not rise above the noise in the time-domain and lowers the LOD to <7 e (elementary charges). The mass to charge ratio m/z is inversely proportional to the square of the fundamental frequency f by the relationship:
In one example, the inductive charge detector comprises a first set of charge detector tubes, including a first charge detector tube. For example, charge detector tubes may be disposed in one or more of the field free regions, for example in all of the field free regions. For example, the first set of charge detector tubes may comprise and/or be a segmented charge detector tube, including a plurality of charge detector tubes. In one example, the inductive charge detector comprises C sets of charge detector tubes, including the first set of charge detector tubes, wherein C is a natural number greater than or equal to 1, for example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, for example wherein C is equal to the number of field free regions. In this way, a duty cycle of inductive charge detection may be increased. In one example, the first set of charge detector tubes comprise and/or is a segmented charge detector tube, for example an axially and/or a radially segmented charge detector tube. By segmenting a charge detector tube axially, such that the segments are in series or tandem, signals may be induced in each of the segments successively by an ion moving therethrough. For example, as described below, increasing the length of a segment beyond about twice the width thereof does not substantially increase the magnitude of the induced signal while a plurality of segments and hence induced signals improves measurement statistics. By segmenting a charge detector tube radially, such that the segments are in parallel, two ions substantially coincident in the z dimension (curvilinear coordinates) but mutually separated in the x and/or y dimensions may induce signals in different radial segments, thereby reducing the likelihood of signal overlap due to the two ions otherwise moving together through the charge detected tube. In this way, an ion capacity of the CDMS may be increased. In one example, an internal cross-section, for example a shape, of the inductive charge detector corresponds with, for example is similar to, a cross-section, for example a shape, of the ion path therethrough. For example, while a charge detector tube having a cylindrical bore is suitable for a generally cylindrical ion path, the first charge detector tube may be adapted to have a tapered bore to correspond with a frustoconical ion path entering and/or exiting an electric quadrupole lens or to provide an annulus or a tapered annulus for an ion path entering and/or exiting a spherical electrostatic sector, for example. In one example, the first charge detector tube comprises an outer electrode and an inner electrode, thereby providing an annulus or a tapered annulus there through for the ion path, optionally comprising one or more supports therebetween, for example being disposed and/or having cross sections adapted to reduce likelihood of ion collisions therewith.
In one example, the first charge detector tube, having a length L and a width W, has a ratio of the length L to the width W in a range from 3:2 to 8:2, preferably in a range from 3:2 to 5:2, for example 2:1 and/or a ratio of the length L to the width W of at least 2:1. Particularly, a magnitude of the induced signal does not substantially increase by further lengthening the first charge detector tube relative to its width.
In one example, a portion of the ion path via the inductive charge detector is in a range from 30% to 70%, preferably in a range from 40% to 60%, for example 50% of the ion path defined by the electrostatic sector field ion trap. In this way, artifacts, such as signal processing artifacts, in a time-domain signal of an ion, as analysed by FT for example, are reduced. Particularly, if the portion of the ion path via the inductive charge detector is about 50% of the ion path defined by the electrostatic sector field ion trap, even order harmonics when analysing the time-domain signal by FT maybe reduced or eliminated. An ideal, 50% duty cycle square wave does not have even order harmonics in its FFT and fewer harmonics results in a fundamental peak having a larger magnitude. Since the magnitude of the fundamental peak is proportional to the charge of the ion, an increase in the magnitude may decrease uncertainty in the charge by increasing the signal-to-noise ratio of the fundamental peak.
In one example, the CDMS comprises a set of electrostatic focus lenses, including a first electrostatic focus lens, arranged to constrain, at least in part, the ion path in a first dimension, for example transverse thereto. In other words, the ion path may be compressed, for example uniaxially. In one example, the set of electrostatic focus lenses is arranged to constrain, at least in part, the ion path in a second dimension, for example transverse thereto, wherein the first dimensional and the second dimension are mutually orthogonal. In other words, the ion path may be compressed, for example biaxially. In this way, construction of the CDMS may be simplified while spatial aberrations, if any, arising due to the set of electrostatic focus lenses are not significant. Additionally and/or alternatively, by constraining the ion path, an internal dimension, for example an internal diameter, of the inductive charge detector may be reduced since a cross-sectional area of the ion path is reduced, thereby improving a rise time thereof. In one example, the first dimension is orthogonal to a direction of the ion path via the inductive charge detector. In one example, the first focus lens comprises and/or is a cylinder lens, an einzel lens and/or a plate lens, for example disposed across a crossover in the ion path. In this way, symmetry about the crossover may be maintained while the geometry simplified. For example, an einzel lens may maintain at least rotational symmetry. In one example, the set of electrostatic focus lenses is arranged to constrain, at least in part, the ion path in a second dimension, for example transverse thereto, wherein the first dimensional and the second dimension are mutually orthogonal. It should be understood that the set of electrostatic focus lenses, including the first focus lens, are arranged to constrain, at least in part, the ion path in the first dimension rather than, for example, an RF field and/or a magnetic field.
In one example, a cross-section of the ion path via the inductive charge detector is arcuate, having a central angle in a range from −3° to +3°, preferably in a range from −2° to +2°, more preferably in a range from −1° to +1°. This way, the ion beam is confined to a relatively narrow arc whereby an internal dimension, for example an internal diameter, of the inductive charge detector may be reduced since a cross-sectional area of the ion path is reduced, thereby improving a rise time thereof.
In one example, the inductive charge detector is configured to operate at ground potential. In this way, a noise level thereof is reduced, thereby enabling detection of very low induced signals.
The CDMS comprises the controller, for example comprising a computer including a processor and a memory, configured to determine the respective masses of an ion population (a single ion or a plurality of ions) moving in the defined ion path using the frequencies and the magnitudes of the respective signals, wherein the respective signals are induced by the ion population in the inductive charge detector. As a particular ion enters the inductive charge detection, the particular ion induces a small, measurable voltage (i.e. an induced signal), the amplitude of which is proportional to its charge. For improved measurement, this induced signal is amplified by the amplifier. The measured periodic time of the oscillation yields the mass to charge ratio m/z of the particular ion and the product of these two measurements gives the true mass of the particular ion. Allowing many oscillations within the ion trap and analysing the resulting signal by Fourier Transform (FT) improves the accuracy of both the charge and the mass to charge ratio m/z measurements.
The CDMS comprises the set of power supplies, including the first power supply, electrically coupled to the electrostatic field ion trap and configured to apply the potential thereto at the time t0. It should be understood that the power supplies of the set thereof are high voltage (HV) power supplies. Suitable power supplies are known.
In one example, the set of power supplies comprises the first power supply and a second power supply, respectively electrically coupled to the first electrostatic electrode and the second electrostatic electrode, for example the inner electrode(s) and the outer electrode(s) of a set of electrostatic sectors, configured to respectively apply the potential and a mutually inverted (i.e. opposed or antiphase) potential thereto at the time to. Typically, the potentials applied to the inner and the outer electrode of an electrostatic sector are mutually opposed but not necessarily equal in magnitude, though similar in magnitude (for example within 95%).
In one example, the CDMS comprises a set of electrostatic focus lenses, including a first focus lens, and the set of power supplies includes a third power supply electrically coupled to the set of electrostatic focus lenses, for example the first focus lens.
The CDMS comprises the amplifier, electrically coupled to the inductive charge detector and configured to amplify the signals received thereby. It should be understood that the amplifier has an input for receiving signals and an output, for outputting amplified signals therefrom. Suitable amplifiers are known.
In one example, the CDMS comprises means for introduction of ions into the ion path. In one example, introduction of the ions is via a field-free region, for example by switched ion displacement in the x direction and/or the y direction (curvilinear coordinates), such as using deflecting electrodes as described in [14]. In one example, introduction of the ions is via the electrostatic sector field ion trap, for example by switching the first electrostatic sector for example only the first electrostatic sector, such as described in [13] or by switching the set of electrostatic sectors, for example two or more or all of the electrostatic sectors, mutatis mutantis. By switching only the first electrostatic sector, thereby operating the remaining electrostatic sectors at their respective operating potentials, ions may be successively introduced into the ion path so as to fill the ion path until the first ion introduced reaches proximally the first electrostatic sector, whereupon the first electrostatic sector is switched back to its respective operating potential. For example, for a figure of 8 geometry, ions may be introduced so as to fill about an upper ¾ of the figure of 8. Conversely, by switching all the electrostatic sectors of the set thereof, control is simplified while the number of power supplies required is reduced. In one example, the electrostatic sector field ion trap, for example the first electrostatic sector, comprises an ion inlet for introduction of ions therethrough into the ion path. In one example, the ion inlet comprises and/or is a passageway through the outer electrode of the first electrostatic sector.
In one example, the CDMS does not comprise an energy filter to limit the spread of ion energies entering the electrostatic sector field ion trap, for example a dual HDA as described previously. In contrast to the cone trap and the ELIT of references [2] and [3], the electrostatic sector field ion trap the CDMS has a relatively broader tolerance to deviations to ion energy spread and/or radial and/or angular position ions, while ions outside of this relatively broader tolerance, for example high-energy or off-axis ions, are unstable and thus rapidly collide with walls of the electrostatic sector field ion trap without adversely affecting mass determination of other ions. In one example, the CDMS has an energy acceptance of greater than 0.40%, preferably at least 0.5%, more preferably at least 1%, most preferably at least 2%, 3% or 4%. In one example, the CDMS has an energy acceptance of at most 20%, preferably at most 15%, more preferably at most 10%.
In one example, the CDMS comprises a lift device configured to increase an ion energy of the ions to be introduced into the ion path. In one example, the lift device is configured to trap the ions to be introduced into the ion path. In this way, the ions may be gated for introduction into the electrostatic sector field ion trap, for example to mutually spatially and/or temporally separate a plurality of ions introduced therein, such that the plurality of ions moves generally mutually spatially and/or temporally separated therearound simultaneously. In one example, the lift device is configured to collimate the ions to be introduced into the ion path, thereby providing a pencil of mutually spatially separated ions. In one example, the lift device is configured to introduce a plurality P of ions (i.e. a population of ions), wherein P is a natural number greater than 1, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100 or more ions, preferably wherein P is at least 4, more preferably wherein P is at least 10, 10, 20, 50, 100 or more ions. It should be understood that P is the mean number of ions introduced, the number of ions having a typically Poisson distribution and wherein the ions are mutually spatially separated randomly, thereby adopting substantially random discrete initial positions along a portion of the ion path i.e. mutually spatially and/or temporally separated, such that the plurality of ions moves generally mutually spatially and/or temporally separated therearound simultaneously. In the high intensity limit, like on a magnetic sector, the ion beam would include many ions per linear distance along the beam. At the other end of the scale, ions are injected individually according to the Poisson distribution (but it is still a beam in its behaviour apart from space charge considerations). In one example, the lift device is configured to introduce the ions into the ion path by pulsing the ions into the ion path. More generally, in one example, the CDMS comprises means for introducing the ions into the ion path, generally as described with respect to the lift device but optionally increasing the ion energy. In one example, the means for introducing the ions into the ion path comprises means for introducing a plurality P (i.e. a population) of ions into the ion path, wherein P is a natural number greater than 1, wherein the plurality of ions respectively adopt substantially random discrete initial positions along a portion of the ion path, such that the plurality of ions moves generally mutually spatially and/or temporally separated therearound simultaneously. That is, the ions, for example the plurality of ions, are not focussed spatially and/or temporally, as typically focussed for introduction into a convention mass to charge analyser, such as quadrupole analyser, a time of flight analyser or an ion trap analyser, such as a three-dimensional quadrupole ion trap, a cylindrical ion trap, a linear quadrupole ion trap or an orbitrap. In other words, no effort is made to focus the ions. Rather, a disordered population of ions is introduced into the ion path, for example as provided by thermalizing the ions in a gas cell, which reduces the energy distribution of the ions without focussing. If these ions are trapped and/or the respective energies of the ions increased (for example, using a lift device) before introduction into the ion path, the ions may be introduced into the ion path by applying a potential gradient that substantially maintains disorder and/or non-correlation, spatially and/or temporally, of the ions upon introduction into the ion path.
In one example, the CDMS comprises an ion source, for example an atmospheric pressure ionisation (API) source such as an electrospray ionisation (ESI) source or a Nano spray ionisation source. Other ion sources are known.
In one example, the CDMS does not comprise a magnetic deflector or sector. That is, the CDMS may comprise only electrostatic electrodes and/or sectors and optionally, electric quadrupole lenses and/or electric lenses as described herein.
In one example, the CDMS comprises one or more devices upstream and/or downstream of the electrostatic sector field ion trap for ion processing, for example activation of ions using externally injected particles such as reagent ions or reagent ions co-trapped with precursor ions, interactions with (e.g. externally injected) electrons, manipulation of the mass-to-charge ratio of ions preferably by electron detachment (e.g. using energetic charged particles, including fast electrons), proton attachment or charge reduction processes, interactions between ions and neutral molecules (e.g. externally injected) in ground or excited state, interactions with photons, excitation of ion motion using auxiliary AC waveforms or duty cycle variations of the RF trapping waveform, ion isolation using AC waveforms or duty cycle control, collisional activation dissociation, ion accumulation and transfer. Processing may involve one or more of the above functions to be performed simultaneously or sequentially.
In one example, the CDMS comprises a vacuum system, for example including a chamber housing the electrostatic sector field ion trap and the inductive charge detector therein, a vacuum pump and a controller. In one example, the chamber is a differentially pumped chamber, for example having a vacuum, of at most 1×10−8 Torr, preferably of at most 5×10−9 Torr, more preferably of at most 2×10−9 Torr or better.
In one example, the CDMS comprises and/or is a stand-alone CDMS. Conversely, in one example, the CDMS is included in a mass spectrometer, for example integrally included (i.e. ab initio) therein or included as an upgrade or retrofit.
In one example, the CDMS comprises a fragmentation device. a fragmentation device. Tandem mass spectrometry is a well-established technique that yields structural information and increased specificity (also known as selectivity) for selected precursor ions. Generally, an ion (i.e. a precursor ion) having a targeted (i.e. selected) mass to charge ratio m/z is isolated and subsequently fragmented to yield information on the ion's structure in full spectral mode. In triple quadrupole (also known as tandem quadrupole) experiments, targeted fragment ions are monitored to increase the specificity of the experiment yielding highly quantitative information on the target molecule. Tandem mass spectrometry experiments are performed using a variety of mass spectrometer combinations including Quadrupoles, Ion Traps, Time-of-Flight, FTICR, Magnetic Sector and Orbitrap® instruments. For all these experiments, knowledge of the mass to charge ratio m/z of the selected precursor ion (and hence mass) is required in order to perform the desired isolation step. In the particular case of very high mass ions, utilising electrospray ionisation knowledge of the mass to charge ratio m/z of the precursor ion alone is insufficient to determine the true mass of the selected ion, since this ionisation technique yields many different charge states as molecular mass increases, as described previously. Hence there is a need to be able to select a large molecular weight species of targeted mass in order to subsequently fragment it to obtain structural and/or quantitative information. As described herein, CDMS is a proven technique to measure the mass of these very large electrospray created ions. Fragmentation studies have been performed using CDMS instruments utilising cone trap geometries described initially by Benner [17], and also the ELIT [18]. In these studies, photofragmentation takes place within the cone trap and ELIT, respectively. However, this approach results in sharing of ion kinetic energy between the produced photofragments (i.e. product ions), thereby causing some of these photofragments to become unstable within the traps (and hence loss of these photofragments) and a degradation of the mass resolution of remaining photofragments due to their deviations from the optimal design energy of the trap itself. Furthermore, this approach does not allow for selection of the precursor ion or ions to be photofragmented, not withstanding that these CDMS are inherently limited to determination of mass of a single ion.
In contrast, the CDMS comprising the fragmentation device enables determination of respective masses of a mixture of ions (i.e. having different masses), isolation of a single chosen species (i.e. a single precursor ion or a plurality of precursor ions having the same mass) for fragmentation internal or external to the electrostatic sector field ion trap device, with the product ions (generally, product ion(s) i.e. or one or more product ions or a plurality of product ions) reintroduced into the ion path, thereby providing true tandem mass spectrometry experiments across the full mass range of the product ions.
In one example, the fragmentation device comprises and/or is an RF ion trap equipped with one or more of the following fragmentation techniques: Collisional Induced Dissociation (CID), Light based photofragmentation (Lamp or laser based, UV, visible or Infra-Red), Electron Capture Dissociation (ECD), Electron Transfer Dissociation (ETD), Electron Induced Dissociation (EID), Surface Induced Dissociation (SID), Resonantly Induced Dissociation. Other fragmentation techniques are known. In one example, the fragmentation device is configured to trap the precursor ion and/or a product ion thereof, for example by comprising one or more regions for trapping the precursor ion and/or a product ion thereof. In this way, the precursor ion may be introduced (directly or indirectly from the ion path) and trapped, for example in the first region and subsequently moved for example by applying a potential gradient to a second region for fragmentation therein, whereupon the product ions are similarly moved back to the first region or to third region, optionally the respective energies there are increased, before being moved to the ion path.
In one example, the fragmentation device is configured to increase (more generally, control) an ion energy of the product ion to be introduced into the ion path, for example by accelerating the product ion by a predetermined potential difference. In this way, the ion energy of the product ion may be controlled to correspond with a design, for example optimum, of the electrostatic sector field trap, thereby overcoming a disadvantage of conventional CDMS including photofragmentation, as described previously.
In one example, the fragmentation device is configured to thermalize the product ion, for example by collisional cooling with a buffer gas, and accelerate the product ion (i.e. increase an ion energy thereof) to a desired kinetic energy into the electrostatic sector field trap for mass determination.
In one example, the fragmentation device comprises and/or is an RF ion trap, for example a multipole linear ion trap such as a quadrupole, hexapole or octopole, for example as described in U.S. Pat. No. 5,847,386, incorporated herein by reference in entirety. In one example, the fragmentation device comprises and/or is a stacked electrode device, for example as described in U.S. Pat. No. 5,206,506, incorporated herein by reference in entirety. In one example, the fragmentation device comprises and/or is a 3D quadrupole ion trap. In one example, the fragmentation device comprises and/or is a linear ion trap, for example as described in U.S. Pat. No. 7,312,442B2, U.S. Pat. No. 7,755,034B2, U.S. Pat. No. 6,995,366B2, EP1706890B1 or WO/2017/134436, incorporated herein by reference in entirety.
In one example, the electrostatic sector field ion trap is configured to define, at least in part, the ion path via the fragmentation device (c.f. via the inductive charge detector such that the fragmentation device and the inductive charge detector are in series along the ion path). That is, the fragmentation device is between, for example, a first electrostatic sector and a second electrostatic sector of a set of electrostatic sectors, such that the ion path traverses through the fragmentation device. In other words, the fragmentation device may be described as being internal or in-line with respect to the electrostatic sector field ion trap, such that the ion path is continuous through the fragmentation device. That is, entering of ions into the fragmentation device and exiting of ions therefrom is directly from and to the ion path, respectively. For example, the respective masses of one or more precursor ions are determined, for example as described with respect to the first aspect, while the fragmentation device is deactivated (i.e. configured in a first state to permit the one or more precursor ions to move therethrough without fragmentation). Subsequently, the fragmentation device is activated (i.e. configured in a second state to fragment the one or more precursor ions), whereupon the one or more precursor ions are fragmented into respective product ions therein upon moving therethrough, since the ion path is via the fragmentation device. Thereafter, the fragmentation device is deactivated and the respective masses of one or more product ions are determined, for example as described with respect to the first aspect. In one example, the fragmentation device is configured to trap a precursor ion, as described below, upon entering the fragmentation device, fragment the precursor ion thereby providing a plurality of product ions therefrom, optionally trap the plurality of product ions and increase respective ion energies of the plurality of product ions, as described below, whereupon the plurality of product ions exit the fragmentation device having the increased respective ion energies for determination of the respective masses of the plurality of product ions. That is, the precursor ion is trapped, fragmented and the ion energies of the resulting product ions increased for determination of the respective masses of the product ions in-line.
In one example, the CDMS comprises means for ejecting ions from the ion path into the fragmentation device. That is, the fragmentation device is outside the electrostatic sector field ion trap, such that the ion path does not traverse through the fragmentation device. In other words, the fragmentation device may be described as being external or off-line with respect to the electrostatic sector field ion trap. That is, entering of ions into the fragmentation device and exiting of ions therefrom is indirectly from and to the ion path, respectively. For example, the respective masses of one or more precursor ions are determined, for example as described with respect to the first aspect. Since the ion path is not via the fragmentation device, the fragmentation device may be deactivated or activated. Subsequently, the precursor ions are ejected from the ion path into the fragmentation device, which is activated (i.e. configured in a second state to fragment the one or more precursor ions), whereupon the one or more precursor ions are fragmented into respective product ions therein. Thereafter, product ions are injected (i.e. introduced) from the fragmentation device into the ion path and the respective masses of one or more product ions are determined, for example as described with respect to the first aspect. In one example, the means for ejecting ions is configured to eject a precursor from the ion path into the fragmentation device, the fragmentation device is configured to trap the ejected precursor ion, as described below, upon entering the fragmentation device, fragment the precursor ion thereby providing a plurality of product ions therefrom, optionally trap the plurality of product ions and increase respective ion energies of the plurality of product ions, as described below, and inject (i.e. introduce) the plurality of product ions having the increased respective ion energies into the ion path for determination of the respective masses of the plurality of product ions. That is, the precursor ion is ejected and the precursor ion trapped, fragmented and ion energies of the resulting product ions increased offline and the product ions injected into the ion path for determination of the respective masses of the product ions. In one example, the means for ejecting ions from the ion path into the fragmentation device comprises and/or is one or more deflection electrodes and/or a deflection/focussing field, for example applied in the orthogonal Y direction, such as described below. In one example, the electrostatic sector field ion trap comprises an ion outlet for exit of ejected ions therethrough from the ion path. The ion outlet may be as described with respect to the ion inlet. In one example, the ion inlet provides the ion outlet i.e. a combined ion inlet/outlet.
In one example, the fragmentation device is configured to introduce the product ion into the ion path by pulsing the product ion into the ion path, for example as described with respect to introducing precursor ions into the electrostatic sector field ion trap. In this way, the product ions enter the electrostatic sector field ion trap as a pencil of spatially-separated ions, thereby improving mass determination thereof.
In one example, the fragmentation device is combined with a lift device configured to increase an ion energy of ions to be introduced into the ion path, as described previously. That is, the fragmentation device and the lift device may be combined as a single device and the functions thereof performed independently or dependently e.g. lift only without fragmentation for precursor ions or fragmentation and lift for product ions.
In one example, the CDMS comprises an ion isolating optical element configured to isolate a precursor ion for fragmentation by the fragmentation device. In this way, a particular precursor ion, having a mass or a mass to charge m/z determined by the CDMS, for example of a plurality of precursor ions such as a pencil of spatially-separated precursor ions, is selected for fragmentation by the fragmentation device, such that structural information may be obtained for that particular precursor ion, thereby overcoming a disadvantage of conventional CDMS including photofragmentation, as described previously. In one example, the ion isolating optical element is configured to isolate a precursor ion species (i.e. a single precursor ion or a plurality of precursor ions having a particular mass or a particular mass to charge m/z) for fragmentation by the fragmentation device. In one example, the ion isolating optical element is configured to isolate the precursor ion by causing loss of ions other than the precursor ion to be isolated, for example by causing these other ions to be unstable in the ion isolating optical element and/or the electrostatic sector field trap.
In one example, the ion isolating optical element comprises and/or is a quadrupole lens, an einzel lens, a deflection plate; and/or is provided by the electrostatic sector field ion trap; or a combination thereof. In one example, the quadrupole lens is configured to isolate the precursor ion using a focusing voltage, a defocusing and/or a deflection voltage, to selectively focus the precursor ion to be isolated, selectively defocus ions other than the precursor ion to be isolated and/or selectively deflect the precursor ion to be isolated or the ions other than the precursor ion to be isolated, respectively. In one example, the einzel lens is configured to isolate the precursor ion using a focusing voltage, to selectively focus the precursor ion to be isolated. In one example, the, deflection plate is configured to selectively deflect the precursor ion to be isolated or to selectively deflect the ions other than the precursor ion to be isolated. In one example, the electrostatic sector field ion trap is configured to isolate the precursor ion by an oscillating voltage applied thereto according to a mass to charge m/z thereof, to cause ions other than the precursor ion to be selected to be unstable therein. In this way, a single precursor ion or a plurality of precursor ions all having the mass to charge m/z or an harmonic thereof is or are isolated, respectively. In one example, the ion isolating optical element is configured to isolate the precursor ion by applying an electrical field, according to a mass to charge m/z thereof, according to an oscillation frequency, for example a harmonic thereof, of the precursor ion in the electrostatic sector field ion trap, for example as described by Verenchikov in [19], incorporated herein by reference in entirety. In more detail, Verenchikov [19] proposed a device entitled Resonance Mass Spectrometer whereby ions are passed into isochronous open trap. An open trap means that ions undergo a number of oscillations within the trap in a first direction. The device utilises two dimensional electric fields to trap ions in these two dimensions, while ions all allowed to progress in a third dimension to exit the device to a destructive detector. The ion path is not closed, hence the term open trap. A set of n ions of differing mass to charge ratios m/z have unique oscillation frequencies {f1,f2, . . . , fn} each frequency being the reciprocal of the isochronous transit time through the analyser. By applying a high frequency deflection field of chosen frequency, F, only ions that have a high number harmonic of that frequency F are allowed (i.e. selected) to pass through the open trap to the detector. That is to say an ion having an unique frequency fi must satisfy the condition:
The Verenchikov patent utilises substantially 2-dimensional fields necessary for an open trap to allow ions to be modulated as they progress through the device in the orthogonal direction before they are detection by a destructive detector such as an Electron Multiplier (EMT) or Micro Channel Plate (MCP). In contrast, the CDMS according to the first aspect and the third aspect is a closed trap, particularly a closed 3-dimensional ion trap with inductive detection. Ions may be modulated by a deflection field placed in a field free region of the electrostatic sector field trap, allowing selection of unique species (i.e. a precursor ion) in the same principle as [19]. After isolation, the selected species may be passed out of the electrostatic sector field trap for subsequent fragmentation in the fragmentation device, as described previously. The resulting fragment or product ion population may be then reaccelerated back in to the electrostatic sector field trap for tandem mass analysis. As described with respect to the first aspect, the electrostatic sector field ion trap has a relatively high space charge capacity, enabling accurate simultaneous mass determination of the fragment population without distortion.
In one example, the CDMS is configured to perform MSn experiments of ions by repeatedly isolating and fragmenting nth generation product ions for further structural elucidation.
In one example, determination of masses comprises real time Fourier Transform processing. In this way, early indication of required resonant frequency for isolation can be determined from examination of the signal, and isolation can occur more quickly. Efficient isolation of the selected species allows speeds up experimental cycle times and therefore reduces the possibility of loss of desired species due to collision with residual gas molecules within the CDMS chamber.
At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as ‘computer’, ‘component’, ‘module’ or ‘unit’ used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the computers, components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others.
For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:
The CDMS described herein are according to exemplary embodiments, comprising:
The inventor of the present invention recognised that a better energy focussing characteristic may be achieved by employing a geometry first proposed by Poschenrieder [4], who considered time-of-flight energy focusing by electrostatic fields. Particularly, Poschenrieder considered configurations of linear drift spaces and fields in which the time-of-flight is no longer a function of the initial energy to first order but of the mass to charge ratio m/z only. Such configurations are also known as isochronous. For ions of about equal energy, electrostatic fields should be used since trajectories should be identical for all masses. While Poschenrieder limited treatment to toroidal sector fields, other configurations are possible, as described below.
Poschenrieder's work proposed the use of toroidal sector fields in special arrangements to compensate for initial ion conditions for time-of-flight mass spectrometry.
Ion velocity v of an ideal ion, having a mass m, after entering an electrostatic sector field, with ion energy (1+β)mv02/2, is given to first order by Equation (7) of Poschenrieder [4]:
v=v0
The time-of-flight te through the electrostatic sector field is then obtained by integration of
Elimination of the dependence of te on the entrance angle α0 requires Equation (9) of Poschenrieder [4]:
An electrostatic sector field obeying Equation (9) of Poschenrieder [4] is simultaneously free of first-order chromatic aberrations at the second image. However, large lateral energy dispersion may be found at the intermediary image, though the transmitted energy spread may be limited here by a suitable stop.
The dispersion in time-of-flight Δte due to the fractional ion energy spread β=ΔE/Ea within the electrostatic sector field is given by Equation (10) of Poschenrieder [4]:
The dispersion ΔtD along a linear drift tube of length D is given simply by Equation (11) of Poschenrieder [4]:
For an electrostatic sector field to be free of this person requires that Δte+ΔtD=0, which leads to the focusing condition given by Equation 12 of Poschenrieder [4]:
The actual linear drift length D can comprise gr on the entrance side, gr′ on the exit side and some additional drift range d.
Hence, an electrostatic sector field obeying Equation 12 of Poschenrieder [4] will be free of any energy-dependent dispersion in time-of-flight (isochronous) for any two points comprising a linear drift range of total length D. In addition, this electrostatic sector field will provide achromatic radial imaging for a point G at a distance gr from the field edge.
Consider an electrostatic sector field with stigmatic imaging in which a radial and an axial intermediary image coincide at ψi=ψ0/2. If ga represents the distance of the entrance from the field slit for axial focusing, from the directional focusing properties of toroidal sector field, the distance gr of the source point form the field edge is given by Equation 20 of Poschenrieder [4]:
Hence, it follows that h=k and c=1, which corresponds to a spherical condenser field. From Equation 12 of Poschenrieder [4] and setting D=2gr=2ga, Equation 21 of Poschenrieder [4] is obtained:
A graphical solution yields the values:
ψ0=199.2°
gr=5.9r
For this electrostatic sector field, the source and its image exactly coincide, as schematically depicted in
Poschenrieder paper [4] concentrates on isochronicity (time aberrations) with respect to beam initial conditions (all zero to first order) but its treatment of stigmatic (spatial) aberrations was much less expansive. Poschenrieder [4] does not seem to consider the spatial aberrations of ‘angle with respect to position’ or ‘angle with respect to energy’. However, the stigmatic (imaging) requirements of the CDMS are secondary compared with the isochronicity requirements of the CDMS: the stigmatic requirements being sufficiently stable ion trajectories for inductive charge detection, preferably to permit use of relatively narrow charge tubes as described herein, and to avoid ion loss arising from ions wandering away from the ion path.
In this example, the electrostatic sector field ion trap 30 comprises a set of electrostatic sectors 31, including a first electrostatic sector 31A and a second electrostatic sector 31B. In this example, the first electrostatic sector 31A is a spherical electrostatic sector. In this example, the first electrostatic sector 31A and the second electrostatic sector 31B are mutually opposed. In this example, the set of electrostatic sectors 31 includes only the first electrostatic sector 31A and the second electrostatic 31B. In this example, the first electrostatic sector 31A comprises a set of shunts 32, including a first shunt 32A, arranged to delimit a field due to the first electrostatic sector 31A. In this example, the electrostatic sector field ion trap 30 is isochronous. In this example, the electrostatic sector field ion trap 30 is configured to define the ion path IP in three mutually-orthogonal dimensions. In this example, the ion path defined by the electrostatic sector field ion trap 30 includes a crossover. In this example, the electrostatic sector field ion trap 30 comprises an ion inlet 33 for introduction of ions therethrough into the ion path, particularly provided in the outer electrode of the first electrostatic sector 31A. In this example, the first electrostatic sector 31A has a deflection angle of ψ0=199.2°. In this example, the field-free region has a length of gr=5.9r to the central cross over point (i.e. the origin, at which point focus is achieved). In this example, the electrostatic sector field ion trap 30 has rotational symmetry about the x axis through the origin. In this example, the electrostatic sector field ion trap 30 is symmetric in the y-z plane through the origin. In this example, the basic unit comprises two drift spaces (i.e. two field-free regions each having a length of gr=5.9r) and a spherical electrostatic sector 31A, 31B. In this example, the outer electrode of the first electrostatic sector 31A has an internal radius of 23 mm and the inner electrode of the first electrostatic sector 31A has an external radius of 17 mm such that there is a spherically radial gap of 6 mm therebetween. The first shunt 32A has a toroidal aperture of width 4 mm, thereby presenting a relatively large entrance aperture into the first electrostatic sector 31A. The second electrostatic sector 31B is generally as described with respect to the first electrostatic sector 31A, while not including the ion inlet 33.
In the arrangement of
In this example, the CDMS 4, comprises: an electrostatic sector field ion trap 40 and an inductive charge detector 400; wherein the electrostatic sector field ion trap 40 is configured to define, at least in part, an ion path via the inductive charge detector 400.
The electrostatic sector field trap 40 is as described with respect to the electrostatic field ion trap 30, as described with respect to
In this example, the electrostatic sector field ion trap 40 comprises a set of electrostatic sectors 41, including a first electrostatic sector 41A and a second electrostatic sector 41B. In this example, the first electrostatic sector 41A is a spherical electrostatic sector. In this example, the basic unit comprises two drift spaces and a spherical electrostatic sector. In this example, the first electrostatic sector 41A and the second electrostatic sector 41B are mutually opposed. In this example, the set of electrostatic sectors 41 includes only the first electrostatic sector 41A and the second electrostatic 41B. In this example, the first electrostatic sector 41A comprises a set of shunts 42, including a first shunt 42A, arranged to delimit a field due to the first electrostatic sector 41A. In this example, the electrostatic sector field ion trap 40 is isochronous to first order with respect to energy. In this example, the electrostatic sector field ion trap 40 is configured to define the ion path in three mutually-orthogonal dimensions. In this example, the ion path defined by the electrostatic sector field ion trap 40 includes a crossover. In this example, the electrostatic sector field ion trap 40 comprises an ion inlet 43 for introduction of ions therethrough into the ion path. In this example, the first electrostatic sector 41A has a deflection angle of ψ0=199.2°. In this example, the field-free region has a length of gr=5.9r to the central cross over point (i.e. the origin). In this example, the second electrostatic sector 41B is as described with respect to the first electrostatic sector 41A. In this example, the electrostatic sector field ion trap 40 has rotational symmetry about the x axis through the origin. In this example, the electrostatic sector field ion trap 40 is symmetric in the y-z plane through the origin. In this example, the inductive charge detector 400 comprises a first set of charge detector tubes 410, including a first charge detector tube 410A and a second charge detector tube 410B. In this example, the first charge detector tube, having a length L and a width W, has a ratio of the length L to the width W in a range from 3:2 to 5:2, for example 2:1. In this example, a portion of the ion path via the inductive charge detector 400 is about 50% of the ion path defined by the electrostatic sector field ion trap 40.
SIMION [10] simulations were performed with the geometry shown in
The CDMS 5 is generally as described with respect to the CDMS 4, as described with respect to
In this example, the electrostatic sector field ion trap 50 comprises a set of electrostatic sectors 51, including a first electrostatic sector 51A and a second electrostatic sector 51B. In this example, the first electrostatic sector 51A is a spherical electrostatic sector. In this example, the first electrostatic sector 51A and the second electrosatatic sector 51B are mutually opposed. In this example, the set of electrostatic sectors 51 includes only the first electrostatic sector 51A and the second electrostatic 51B. In this example, the first electrostatic sector 51A comprises a set of shunts 52, including a first shunt 52A, arranged to delimit a field due to the first electrostatic sector 51A. In this example, the electrostatic sector field ion trap 50 is isochronous with respect to energy, to first order. In this example, the electrostatic sector field ion trap 50 is configured to define the ion path in three mutually-orthogonal dimensions. In this example, the ion path defined by the electrostatic sector field ion trap 50 includes a crossover. In this example, the electrostatic sector field ion trap 50 comprises an ion inlet 53 for introduction of ions therethrough into the ion path. In this example, the first electrostatic sector 51A has a deflection angle of θ0=199.2°. In this example, the field-free region has a length of gr=5.9r to the central cross over point (i.e. the origin). In this example, the second electrostatic sector 51B is as described with respect to the first electrostatic sector 51A. In this example, the electrostatic sector field ion trap 50 has rotational symmetry about the x axis through the origin. In this example, the electrostatic sector field ion trap 50 is symmetric in the y-z plane through the origin. In this example, the inductive charge detector 500 comprises a first set of charge detector tubes 510, including a first charge detector tube 510A and a second charge detector tube 510B. In this example, the first set of charge detector tubes 510 comprises an axially segmented charge detector tube, including 10 segments. In this example, the first charge detector tube, having a length L and a width W, has a ratio of the length L to the width W in a range from 3:2 to 5:2, for example 2:1. In this example, a portion of the ion path via the inductive charge detector 500 is about 50% of the ion path defined by the electrostatic sector field ion trap 50.
Referring again to
The performance advantages of the present invention over the state of the art ELIT of reference [3] is shown in Table 1. The CDMS 5 has equivalent angular and spatial acceptance when compared to the ELIT of reference [3] but offers more transients per unit time and superior energy acceptance. This larger energy acceptance can lead to a better resolution/sensitivity characteristic. With prudent upstream beam collimation and an energy spread of 0.5 eV/z, single pass mass resolutions of several thousand are expected for this embodiment.
In more detail, a further advantage of the present invention is afforded by the stigmatic or quasi-stigmatic focussing properties of the electrostatic sector field ion trap. These properties mean that the ion beam is confined to a narrow arc as it traverses the analyser. A narrow ion beam means that it is possible to use a similarly narrow charge tube for detection of ions.
The relatively long charge tubes afforded by the geometry of the CDMS 4 and the CDMS 5, for example, allow axial charge tube segmentation. As a general rule, the induced signal by the moving ion is negligible after it has passed a length of twice the tube width into said tube making an overly long tube wasteful of useful signal.
Furthermore, the three-dimensional figure of eight path of the electrostatic sector field ion trap and even the constrained three-dimensional figure of eight path of the electrostatic sector field ion trap 40 additionally and/or alternatively allow radial charge tube segmentation, as described previously.
In this example, the electrostatic sector field ion trap 100 is as described with respect to the MULTUM of reference [15]. In this example, the electrostatic sector field ion trap 100 comprises a set of electrostatic sectors 101, including a four similar cylindrical electrostatic sectors 101A to 101D, each having a deflection angle of ψ0=156.87° and a deflection radius of 50 mm. In this example, the electrostatic sector field ion trap 100 comprises a set of electric quadrupole lenses 102, including eight electric quadrupole lenses 102A to 102H. In this example, the basic unit comprises four drift spaces, two electric quadrupole lenses and a cylindrical electrostatic sector. In this example, the electrostatic sector field ion trap 100 is isochronous to first order with respect to energy. In this example, the ion path defined by the electrostatic sector field ion trap 100 includes a crossover and has three fold planar symmetry therethrough. Ion injection may be by deflection, as described with respect to reference [14], or via an ion inlet.
In this example, the inductive charge detector 1000, generally as described with respect to the inductive charge detectors 400, 500 and 800, comprises a first set of segmented charge detector tubes 1010, including four segmented charge detector tubes 1010A, 1010B, 1010C, 1010D.
In this example, the electrostatic sector field ion trap 110 is as described with respect to the MULTUM Linear plus of reference [15] and is generally as described with respect to the electrostatic sector field ion trap 100, having additional electric quadrupole lenses to enable ion injection (and ejection).
In this example, the inductive charge detector 1100, generally as described with respect to the inductive charge detector 1000, comprises a first set of segmented charge detector tubes 1110, including four segmented charge detector tubes 1110A, 1110B, 1110C, 1110D.
In this example, the electrostatic sector field ion trap 120 is as described with respect to the MULTUM of reference [15]. In this example, the electrostatic sector field ion trap 120 comprises a set of electrostatic sectors 121, including four similar toroidal electrostatic sectors 121A to 121D, each having a deflection angle of θ0=157.10°, a deflection radius of 50 mm and a C1 value of 0.0337. In this example, the electrostatic sector field ion trap 120 does not comprise electric quadrupole lenses. In this example, the basic unit comprises two drift spaces and a toroidal electrostatic sector. In this example, the electrostatic sector field ion trap 120 is isochronous to first order with respect to energy. In this example, the ion path defined by the electrostatic sector field ion trap 120 includes a crossover and has three fold planar symmetry therethrough. Ion injection may be by deflection, as described with respect to reference [14], or via an ion inlet.
In this example, the inductive charge detector 1200, generally as described with respect to the inductive charge detector 1000, comprises a first set of segmented charge detector tubes 1210, including four segmented charge detector tubes 1210A, 1210B, 1210C, 1210D.
In this example, the electrostatic sector field ion trap 130 is as described with respect to the planar figure of eight of reference [15]. In this example, the electrostatic sector field ion trap 130 comprises a set of electrostatic sectors 131, including two similar cylindrical electrostatic sectors 131A to 131B, each having a deflection angle of ψ0=227.95° and a deflection radius of 50 mm. In this example, the electrostatic sector field ion trap 130 comprises a set of electric quadrupole lenses 132, including eight electric quadrupole lenses 132A to 132H. In this example, the basic unit comprises six drift spaces, four electric quadrupole lenses and a cylindrical electrostatic sector. In this example, the electrostatic sector field ion trap 130 is isochronous to first order with respect to energy. In this example, the ion path defined by the electrostatic sector field ion trap 130 includes a crossover and has three fold planar symmetry therethrough. Ion injection may be by deflection, as described with respect to reference [14], or via an ion inlet. Since the electrostatic sector field ion trap 130 already has a planar figure of eight geometry, in contrast with the three dimensional figure of eight geometry of the electrostatic sector field ion traps 30, 40 and 50, the topological problem as described previously does not arise.
In this example, the inductive charge detector 1300, generally as described with respect to the inductive charge detector 1000, comprises a first set of segmented charge detector tubes 1310, including four segmented charge detector tubes 1310A, 1310B, 1310C, 1310D.
In this example, the electrostatic sector field ion trap 140 is as described with respect to the rhomboid of reference [16]. In this example, the electrostatic sector field ion trap 140 comprises a set of electrostatic sectors 141, including two double toroidal electrostatic sectors 142A, 142B, each including a first toroidal electrostatic sector 141A having a deflection angle of ψ0=156.2° and a second toroidal electrostatic sector 141A having a deflection angle of ψ0=23.8°. In this example, the electrostatic sector field ion trap 120 does not comprise electric quadrupole lenses. In this example, the basic unit comprises three drift spaces and two toroidal electrostatic sectors. In this example, the electrostatic sector field ion trap 140 is isochronous to first order with respect to energy. In this example, the ion path defined by the electrostatic sector field ion trap 140 does not include a crossover and has one plane of symmetry. Ion injection may be by deflection, as described with respect to reference [14], or via an ion inlet.
In this example, the inductive charge detector 1400, generally as described with respect to the inductive charge detector 1000, comprises a first set of segmented charge detector tubes 1410, including four segmented charge detector tubes 1410A, 1410B, 1410C, 1410D.
It should be understood that any of the CDMS 4 to 9 interface with the hexapole 3 of region III of the CDMS 10, thereby replacing region IV of CDMS 10, or interface with focusing lens 4 of region IV, thereby replacing the HAD 6 and modified cone trap with image charge detector 7 of region IV, wherein the orthogonal TOF-MS 5 is optionally removed.
The apparatus 100 includes an ionization source 105 that generates ions from a sample to be analyzed. As used herein, the term “ion(s)” refers to any charged molecule or assembly of molecules, and is specifically intended to embrace high molecular weight entities sometimes referred to in the art as macro-ions, charged particles, and charged aerosols. Without limiting the scope of the invention, ions that may be analyzed by apparatus 100 include proteins, protein complexes, antibodies, viral capsids, oligonucleotides, and high molecular weight polymers. Source 105 may take the form of an electrospray ionization (ESI) source, in which the ions are formed by spraying charged droplets of sample solution from a capillary to which a potential is applied. The sample may be delivered to source 105 as a continuous stream, e.g., as the eluate from a chromatographic column.
Ions generated by source 105 are directed and focused through a series of ion optics disposed in vacuum chambers of progressively reduced pressures. As depicted in
Apparatus 100 may additionally include a quadrupole mass filter (QMF) 110 that transmits only those ions within a selected range of values of m/z. The operation of quadrupole mass filters is well known in the art and need not be discussed in detail herein. Generally described, the m/z range of the selectively transmitted ions is set by appropriate adjustment of the amplitudes of the RF and resolving direct current (DC) voltages applied to the electrodes of QMF 110 to establish an electric field that causes ions having m/z's outside of the selected range to develop unstable trajectories. The transmitted ions may thereafter traverse additional ion optics (e.g., lenses and RF multipoles) and enter ion store 115. As is known in the art, ion store 115 employs a combination of oscillatory and static fields to confine the ions to its interior. In a specific implementation, ion store 115 may take the form of a curved trap (referred to colloquially as a “c-trap”) of the type utilized in Orbitrap mass spectrometers sold by Thermo Fisher Scientific. The curved trap is composed of a set of generally parallel rod electrodes that are curved concavely toward the ion exit. Radial confinement of ions within ion store 115 may be achieved by applying oscillatory voltages in a prescribed phase relationship to opposed pairs of the rod electrodes, while axial confinement may be effected by applying static voltages to end lenses positioned axially outwardly of the rod electrodes.
Ions entering ion store 115 may be confined therein for a prescribed cooling period in order to reduce their kinetic energies prior to introduction of the ions into electrostatic trap. Confinement of the ions within the ion store for a prescribed period may also assist in desolvation of the ions, i.e., removal of any residual solvent moieties from the analyte ion. As discussed hereinabove, the presence of residual solvent may result in mass shifts during analysis which interfere with the ability to accurately measure m/z and charge. To facilitate kinetic cooling and desolvation of the ions, an inert gas such as argon or helium may be added to the ion store internal volume; however, the cooling gas pressure should be regulated to avoid unintended fragmentation of the analyte ions and/or excessive leakage of the gas into electrostatic trap 120. The duration of the cooling period will depend on a number of factors, including the kinetic energy of ions entering ion store 115, the inert gas pressure, and the desired kinetic energy profile of ions injected into electrostatic trap 120. After the cooling period has been completed, ions confined in ion store 115 may be radially ejected from ion store toward entrance lenses 125, which act to focus and direct ions into inlet 130 of electrostatic trap 120. Rapid ejection of ions from ion store 115 may be performed by rapidly collapsing the oscillatory field within the ion store interior and applying a DC pulse to the rod electrodes positioned away from the direction of ejection.
To reliably measure ion charge using the CDMS technique, only individual ions of a particular ion species can be present in electrostatic trap 120 during a measurement event. As used herein, the term “ion species” refers to an ion of a given elemental/isotopic composition and charge state; ions of different elemental/isotopic compositions are considered to be different ion species, as well as are ions of the same elemental composition but different charge states. The term “ion species” is used interchangeably herein with the terms “analyte ion(s)” and “ion(s) of interest”. If multiple ions of the same ion species are present during a measurement event, then the measured charge state (determined from the amplitude of the signal generated by image current detector 132, as described below) will be a multiple of the actual charge state of an individual ion. To avoid this type of mismeasurement, the ion population within ion store 115 should be kept sufficiently small such that the likelihood that two ions of the same ion species are confined within the ion store is maintained at an acceptable minimum. This may be accomplished by attenuation of the ion beam generated by source 105 (more specifically, by “detuning” ion optics located in the upstream ion path such that high losses of ions occur) and/or via regulation of the fill time (the period during which ions are accepted into ion store 115). To control the fill time, one or more ion optic components located upstream in the ion path of ion store may be operated as a gate to selectively allow or block passage of ions into the internal volume of ion store 115.
Electrostatic trap 120 may take the form of an orbital electrostatic trap, of the type commercially available from Thermo Fisher Scientific under the trademark “Orbitrap” and depicted in cross-section in
Ions may be introduced tangentially into trapping region 145 through inlet aperture 130 formed in outer electrode 240. Inlet aperture 130 is axially offset (along the z-axis) from the transverse plane of symmetry, such that, upon introduction into trapping region 145, the ions experience a restorative force in the direction of the plane of symmetry, causing the ions to initiate longitudinal oscillation along the z-axis while orbiting inner electrode 135, as illustrated in
Measurement of charge state and m/z, and consequent calculation of the product mass, proceeds by the acquisition and processing of the transient. Transient acquisition by detector 132 is initiated promptly after injection of the analyte ion(s), and continued for a predetermined transient length. The transient length required for accurate measurement of m/z and charge state will vary according to the analyte, as well as the physical and operational parameters of electrostatic ion trap 120. In general, the transient will need to be of adequate duration to allow the signal to be reliably distinguished from noise. For a typical analyte ion, it is anticipated that a satisfactory signal-to-noise ratio may be achieved using a commercially-available orbital trapping mass analyzer at a transient length of 500 milliseconds. It will be understood that the maximum transient length will be limited by the duration for which the analyte ion is stably trapped within trapping region 145 without colliding with background gas atoms/molecules or other ions, which is in part a function of the trapping region pressure.
Electrospray of very large species (>1 MDa) such as virus molecules yields a continuum of possible charge states meaning that conventional mass analysers that only detect m/z ratios fail to give distinct spectral peaks providing little useful analytical data. CDMS overcomes this limitation by injecting individual (or small populations) of ions and measuring both m/z and z, the product of these two parameters allow true mass (m) measurements. Mass resolution depends on both m/z resolution and charge (z) accuracy. Presented herein are the first CDMS spectra from a dual 199.2° spherical electric sector ion trap configured for such measurements. This device has the advantages of simple construction with high space charge capacity for higher experimental throughput. The instrument (
Referring to
A refined potential array of the whole CDMS analyser was generated using SIMION [Ref 4](
Data were acquired using a charge sensitive preamplifier and a Spectroswiss FTMS Booster X2 which employs a signal conditioning amplifier and High-Performance Digitiser with FPGA chip and Spectroswiss software (
Proof of operation of the analyser was confirmed by analysing Myoglobin ion packets in conventional m/z mode (
Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.
Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.
Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
Soc. Mass Spectrom. (2018) October; 29(10):2086-2095
| Number | Date | Country | Kind |
|---|---|---|---|
| 2308320.7 | Jun 2023 | GB | national |
