Patent Application
20080035842
This invention relates to ion trap and time-of-flight mass spectrometry, and more particularly is concerned with a method of ejecting ions out of the trap into time-of-flight mass spectrometer.
Time-of-flight (TOF) mass spectrometers distinguish ions of different mass-to-charge ratio by the difference of there flight time from the ion source to detector. Thus TOF method essentially requires the ion source from which ions can be pulsed out having the same initial position and energy. In practice this is not possible due to inherent thermal energy spread and position spread of ions inside the ion source. Modern ToF mass spectrometers use acceleration of ions by high voltage pulse out of the pulsar region. Before ejection the ion cloud occupy comparatively wide volume and have substantial energy spread. After ejection out of the pulsar different ions of the same mass-charge ratio have different energy partly due to difference in initial position and partly due to initial velocity spread. Both factors introduce spread into the time of ion arrival to detector, thus limiting the resolution of ToF mass measurement. Energy distribution of ions, which is introduced by position spread, can be corrected by energy focusing devices like reflectron. The energy distribution, which results from velocity spread cannot be corrected by any combination of electrostatic fields and appears as a major factor limiting mass resolution of ToF. Imagine, for example, two ions of identical mass-to-charge ratio (m/z) positioned in the same point in the ion trap, but having different velocity. Both ions have the same absolute velocity V but velocity of the first ion is directed towards ToF while second ion has velocity in opposite direction. Upon application of the extraction field both ions have the same acceleration a=E/(m/z), where E—is strength of the extraction electrical field. While the first ion starts moving towards ToF, the second ion has to move in opposite direction until its velocity becomes zero and reversed. After the time δt=2V/a the second ion arrives into original position having the same velocity as first ion in the beginning of ejection. At this moment second ion is undistinguishable from the first ion. Time δt elapsed from the beginning of ejection towards reverse of second ion is called “turn-around-time”. First ion has started earlier by the time δt and will arrive at the detector earlier. Mass measurement in ToF is essentially based on the measurement of the time of ions arrival to detector, hence the two ions of identical m/z will arrive to detector within time difference of δt and this cannot be corrected by any electrostatic field configuration. In real devices ions always have thermal velocity spread δV and turn-around-time δt due to thermal spread limits the resolution of ToF spectra by theoretical value R=Ttof/2 δt, where Ttof is the total flight time. Thermal energy spread per one degree of freedom at 300 K equals 0.013 eV. For example, for singly charged ions of mass 1000 Da corresponding velocity spread equals 100 m/s. Upon application of 10 kV acceleration voltage on distance of 10 mm the total turn-around-time equals δt=1.1 ns. Assuming the total flight path of 4 m the time-of-flight equals 91 μs. Hence theoretical resolution limit due to turn-around-time in this case is 41.000.
It is known in the art of mass spectrometry, that ion traps can provide an improved ion source for TOF mass spectrometer [1]. By using momentum-dissipating collisions of ions with light buffer gas the ion cloud can be collected near the centre of the trap to a size of less than 1 mm. Kinetic energy spread of ions in such cloud is believed to be close to thermal. Modern methods of ion trapping are based on the use of harmonic periodical voltages (trapping RF) applied to one or several electrodes of the ion trap. Voltage power supply for such RF contains high-Q resonator, which keeps all the energy of the RF field during ion trapping period. Typical ion trap device with internal size of 10 mm can require voltages up to 10 kVo-p for ion trapping. In order to optimise ejection conditions the RF voltage should be switched off upon ejection of ions into TOF [2]. In practice it is extremely difficult due to huge amount of energy in RF resonator. Extraction pulse should be applied not later than few μs after RF switch off, otherwise ions will be lost on the electrodes. It follows that residual “ringing” RF is still present in the trapping volume upon application of extraction pulse. Such ringing deteriorates accuracy and resolution of TOF mass spectra by introducing hardly predictable acceleration fields during ejection. Assuming, that residual RF ringing is only 0.1% of original voltage the magnitude of oscillating voltage is several Volt. The energy spread of ions introduced by this voltage difference is of the order of several electron-volt, which is by two orders of magnitude bigger than thermal energy spread of ions at 300 K. The objective of present invention is to improve performance of TOF mass analysis in terms of resolution and mass accuracy by eliminating the energy spread of ions, which is introduced by residual RF during ejection of ions out of the ion trap.
The use of ion trap as an ion source for TOF has been discussed in a number of patents. A 3D ion trap with ejection and post acceleration of ions into the TOF flight path is disclosed in U.S. Pat. No. 5,569,917[3]. The method described in this patent uses comparatively low extraction voltages (below 500V) and hence does not allow efficient eliminating of the turn-around time. Improved method of extraction from 3D ion trap is disclosed by Kawato in U.S. Pat. No. 6,380,666[4]. This method uses extraction by high voltage pulse (5 kV and more) and specific combination of voltages on extraction electrodes to achieve almost parallel beam of ions. Both patents suggest that RF is not present during ejection process, but do not teach how this can be done in practice. Ejection of ions out of the trap into a pulsar and orthogonal (with respect to extraction flight-path) post acceleration into TOF is described in EP 1 302 973 A2 [5]. In this case the turn-around-time in the direction of ion extraction does not affect TOF resolution. Ions can be extracted using relatively low voltages, while HV pulse is applied afterwards when ions arrive into pulsar. Such ejection method should be optimised in order to give smallest velocity spread in the orthogonal direction, because it will decide the turn-around-time for orthogonal acceleration from the pulsar. This kind of optimisation was not used in a method of cited patent application [5]. It is obvious that such optimisation is hardly possible if sinusoidal RF inside the trap is still running during the ejection process.
Over the last few years there was an effort to increase the number of ions that can be stored inside the trap and used for mass analysis. It is known that a typical 3D trap can hold up to 107 ions, but high resolution manipulations with ions by using supplementary AC signals is only possible if the total amount of elementary charges inside 3D trap is below few thousands. Talking into account typical time of 100 ms for ion manipulations inside the trap one arrives with total throughput of 10.000 charges per second or analytical current of 0.0016 pA. Such throughput is not acceptable for most applications as modern ion sources can provide total ion current of several nA. Influence of space charge is significantly smaller for a linear ion trap (LIT). Electrode structure of LIT is based on quadrupole with four parallel electrodes elongated along the same axis. In such ion trap ions are confined in radial direction by periodical high frequency (typically 0.5-3 MHz) electrical field. Motion of ions along the axis is restricted by DC voltage applied to the entrance and exit electrodes of the LIT. In equilibrium conditions ions in such trap tend to collect along z-axis in a cigar-like cloud. Assuming that the radial size of the cloud is the same as in 3D trap (typically 0.2-1.0 mm) and the length of the cloud is 10 mm, the total number of ions is at least 10 times bigger before the space charge becomes significant [6].
The use of a linear ion trap in combination with TOF is discussed in a number of patents. D. Douglas in WO 99/30350[7] describes a tandem LIT-TOF instrument in which ions are manipulated within the LIT and then released along the axis of the trap. A pulsar is positioned inline with the ion trap and ions are pulsed into TOF upon arrival into pulsar. TOF axis is orthogonal to the LIT axis and in order to achieve high resolution the velocity spread of ions in orthogonal direction (with respect to the LIT axis) should be minimised. Collimating the ion beam from the LIT using small diaphragm can do this. In general the method suffers from mass discrimination of ions. At the moment when extraction pulse is applied the pulsing region contains only ions of certain mass range—ions that just arrived and not gone yet. Only a limited portion of mass range can be extracted into TOF at a time. The wide mass range of ions can be analysed by obtaining mass spectra of several sub ranges. Analysis of each sub range requires refill of the ion trap with ions and repeat all manipulations. As a result, such instrument has low throughput.
J. Franzen in U.S. Pat. No. 5,763,878[8] describes a method of ion ejection directly from a linear trap into the ion path of ToF. According to the method ions are passed from the ion source into the LIT, cooled down by collisions with buffer gas and collected along the axis of the trap. The voltage supply for LIT is capable of supplying at least two voltage configurations one for ion trapping and another for extraction. Upon application of extraction voltages ions are pulsed out in the direction orthogonal to the axis of the trap. They pass between the rods and appear on the flight path of ToF. The method suggests that the RF is completely switched off upon extraction and replaced by certain combination of DC voltages on the electrodes of the trap. The patent does not teach how to switch off the RF field although it is mentioned as a difficult practical problem. The optimum voltage configuration on the electrodes and the timing of extraction are also not described.
Recently a 3D ion trap with so-called “digital drive” was proposed [9]. In this device the voltage of the ring electrode is switched every cycle from positive to negative discrete DC levels. A computer controls switching time with high precision and capable of generating any given switching sequence. It was found that ions of a wide mass range can be trapped inside such trap by switching between only two discreet DC levels (positive and negative) periodically with equal time for each level. Such waveform is referred as square waveform with 50% duty cycle. All traditional modes of ion trap operation are possible with the use of such a trapping method [10]. A method to combine digital ion trap with TOF analysis and benefits of such tandem was not described so far.
According to the invention there is provided a tandem linear ion trap and time-of-flight mass spectrometer, the ion trap having a straight central axis orthogonal to the flight path of said time-of-flight mass spectrometer and comprising; a set of electrodes, at least one said electrode having a slit for ejecting ions towards said time-of flight mass spectrometer; a set of DC voltage supplies to provide discrete DC levels and a number of fast electronic switches capable of connecting and disconnecting said DC supplies to at least two said electrodes of said ion trap; a neutral gas filling the volume of said ion trap in order to reduce the kinetic energy of trapped ions towards equilibrium; a digital controller to provide a switching procedure for ion trapping, manipulations with ions, cooling and including one state at which all ions are ejected from said ion trap towards said time-of-flight mass spectrometer.
According to the invention there is also provided a method of extracting ions from a linear ion trap, said ion trap being driven by a set of digital switches, said method comprising the following steps; trapping said ions in said ion trap by switching between a set of trapping states on the electrodes of said ion trap; cooling said trapped ions by collisions with a buffer gas down to equilibrium: and switching from a pre-selected trapping state to a final ejection state in a pre-selected time.
The present inventors have realised, that the combination of a digital ion trap with TOF provides a tandem mass spectrometer with improved performance. The quality of TOF mass analysis such as resolution and mass accuracy can be improved by optimising conditions of ion ejection into TOF, which is only possible if fields are constant during the ejection process. In order to achieve such conditions authors propose to use the ion trap with digital drive, so that the voltages within the trap remain constant with high precision on application of extraction pulses. Thus the extraction voltages and switching time can be optimised in such way, that ion cloud leaves the ion trap having optimum phase-space distribution for further processing. Further processing can include mass analysis using TOF, or post acceleration stage of TOF mass spectrometer, or it can be any other ion optical device that requires pulses of ions. In each case the distribution of ion positions and velocities can be optimised for each particular purpose. After ejecting of ions out of the trap the trapping waveform is returned to original state allowing the next cycle of ion introduction, manipulation and mass analysis.
In preferred embodiments the invention includes an ion source with transmission ion optics including storing and pulsing ion guide, a linear ion trap filled with neutral gas of mTorr or higher pressure and a time-of-flight analyser. Ion trap is driven by a digital switches connected to all four main electrodes in order to provide periodic trapping potential consisting of at least 2 discreet DC levels. A square wave with equal positive and negative DC levels is preferable as the most simple trapping waveform allowing to trap a wide mass range of ions. Ions from the ion source are transmitted into a linear ion trap and injected into the trapping volume from a region of low field near the central axis of the trap. Ions are manipulated within the trap in a desired manner. These manipulations can include several stages of cooling, isolation of selected ion species by removing all ions with other mass-to-charge ratio and fragmentation of ions by using any of methods known in the art such as collisionally induced dissociation (CID), surface induced dissociation (SID), electron assisted dissociation, photon induced dissociation or other. Finally, remaining ions are cooled down by collisions with light buffer gas and collected near the central axis of the trap in a cigar-like cloud. At appropriate time the period of the trapping square wave is changed to a longer value and the extraction pulse is applied shortly after that. At least one of the electrodes of a linear ion trap has a slit through which the ions are ejected out of the trap. Digital signal generator (DSG) allows controlling the actual voltage state on the electrodes of the trap before the period change applies (switching state). The switching state, the duration between the start of last state and the start of the extraction pulse (duration of the last state prior to ejection) is adjusted in such way as to produce the best distribution of ions for further processing in TOF mass analyser.
In preferred embodiments the TOF has a flight path orthogonal to the axis of linear ion trap and equipped with an ion mirror (reflectron).
In the first preferred embodiment the ions are ejected out of the trap into a pulsar, which is located parallel to the axis of ion trap and orthogonal to the TOF axis. On arrival of ions into the pulsar a high voltage pulse is applied to the electrodes of the pulsar in order to accelerate ions into the ion path of TOF. Acceleration voltages in the pulsar are as big as possible in order to reduce the turn-around time of ions. Ions are reversed in the TOF by an ion mirror and focused to the detector in such way that ions of the same mass-to-charge ratio arrive as close to each other in time as possible. A wide multi channel plate can be used as a detector. Arriving at the detector ions produce electrical pulses in the circuit, which are registered by a recording system. A digitiser with high sampling speed (1 Gsample/s or over) and high dynamic range (12 bit or over) is preferable.
In another preferred embodiment ions are ejected out of the trap directly into the flight path of the TOF, which is positioned orthogonal with respect to the ion trap axis and almost inline with the ejection flight path of ions. A small angle between the flight path of ejected ions and the TOF flight path can be introduced in order to allow deflection of ions into detector. Operation of the TOF and detector system is the same as in previous case. Power supply for an ion trap is different from previous case in respect of the voltages applied upon extraction of ions. In this case ions are ejected directly into the flight path and extraction voltages should be as high as possible. Power supply for extraction electrodes allows at least 3 DC levels—positive and negative voltages for ion trapping and high voltage for extraction. Additional switch is required for protecting comparatively low-voltage trapping circuit from high extraction voltage.
In yet another preferred embodiment the trapping of ions is achieved by driving only one set of the rods of the linear ion trap (Y electrodes) by switching between positive and negative DC levels. The high voltage switches for extraction are connected to another pair of rods (X electrodes) at least one of which has a slit for ejecting ions towards TOF. This kind of power supply is referred as “two-pole” digital trapping waveform. An advantage of this configuration is the possibility of separating the high voltage and trapping voltage supply from each other, which simplifies electronics and reduces the overall cost of instrument. As a result of such separation, the digital driving waveform on Y electrodes is not switched off during ejection. Only the switching period is changed to a longer value allowing all ions to be ejected from the trap with assistance of high voltage pulse.
The above and further advantages of this invention can be better understood by the following description taking in conjunction with the accompanying drawings, in which
Referring to
IT-TOF tandem can be built on the basis of 3D trap. Configuration of such instrument with ejection of ions out of the trap directly into the TOF flight path is presented on
Referring to
For further discussion of preferred embodiments the preparation of the ion cloud within the ion trap is of importance. Modern ion traps operate under elevated pressure conditions (1-0.1 mTorr). Typically He buffer gas is used in order to provide momentum-dissipating collisions for ions. Such collisions assists with removing of excess kinetic energy during ion introduction process and provide means for cooling of the ion cloud. In some configurations a pulsed introduction of heavy gases (Ar, Xe, . . . ) is used in order to provide more energetic collisions during ion fragmentation step. Preparation steps can include several stages of ion cooling, selection of the ions of interest by removing ions of other mass-to-charge ratio out from the trap and fragmentation of selected ions. Isolation and fragmentation can be implemented by several methods known in the art. All the way of preparation of the ion cloud the ion trap operation can be very complicated. The trapping waveform (voltage or/and frequency) can be modified many times including slow scan and application of additional low voltage AC signals to the electrodes of the trap. Finally, ions are cooled down within the ion trap and prepared for extraction into TOF.
Resolution and mass accuracy of final mass analysis is determined by TOF properties itself, but the process of ion ejection out of the trap is the most important factor in this. The core of invention is to create optimum conditions of ion ejection out of the trap, so that with any given TOF mass spectrometer the resolution can reach maximum possible value. This is achieved by creating conditions of electrostatic field inside the trap all the way during ejection, which is possible using “digital drive” for ion trap. Such driving method is described in patent application [9] the entire content of which is included here by reference. Unlike in conventional sinusoidal RF supply the voltages on the electrodes of the ion trap with digital drive are switched between discreet DC levels. In the simplest case the voltages are switched between two levels—positive and equal negative with the same duration of each level (square waveform with 50% duty cycle). A precise control of the period can be achieved with the help of digital controller. Using this method the period of waveform can be switched at any given time to a longer period.
Further discussion of preferred embodiments is based on optimisation of the ejection process. For this the distribution of ion positions and velocity was investigated in detail. After sufficient cooling time ions are collected near the centre of the ion trap along the axis in a cigar-like cloud. Due to inherent nature of RF trapping, the energy spread of the ions in radial direction is phase dependant. This phenomena was investigated by using simulations of a big population of ions trapped in presence of He buffer gas at temperature 323 K.
For minimising the velocity spread of the ions prior to ejection into TOF the extraction pulse should be applied when the energy spread is minimal. For example for IT-TOF with extraction into a pulsar (
For a configuration with ejection directly into the TOF flight path (X direction) the moment of applying extraction pulse should be close to 0.75 phase as it provides minimum velocity spread of ions in X direction. Phase space distribution of initial positions of ions in X phase space at phase 0.75 (middle of negative voltage on Y electrodes) is presented in
It worth mentioning that the resolution can be further optimised by the adjusting of the extraction voltages and by using traditional ion optics on the flight path of ions from ion trap into TOF. Such methods are known in the art and are included within the scope of current invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0404285.9 | Feb 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB05/00671 | 2/23/2005 | WO | 4/20/2007 |
