The present invention relates generally to ion trap mass analyzers and more particularly to methods for carrying out ion excitation.
Mass spectrometry (MS) has been widely used in a range of fields such as chemistry, biology, food safety, industrial pharmaceutical manufacturing, environmental monitoring and homeland security because of its excellent ability to identify and quantify chemical compounds. Tandem mass spectrometry (MS/MS) is the key technology for primary structural characterization of molecules in mass spectrometry. There are several examples of MS/MS techniques that are used in ion trap instruments, such as collision-induced dissociation (CID), electron transfer dissociation (ETD), infrared multiphoton dissociation (IRMPD), proton transfer disassociation and electron capture dissociation (ECD). Among these, CID has been widely used owing to its easy implementation and high predictability.
Linear ion trap (LIT) is an important device in mass spectrometry. LITs have been widely used as standalone mass spectrometers or ion storage devices, and they can also be combined with other mass analyzers such as time-of-flight, quadrupole, etc. to form more complex and versatile analytical systems. In these systems, LITs can be used to select ions of a certain mass-to-charge ratio, study ion-molecular reactions, and enhance other aspects of mass spectrometry performance. As mass analyzers, they have some significant advantages, such as relatively simple structure, small volume, high working pressure and capability of performing tandem mass spectrometry in a single chamber.
First, the parent ion is isolated and ions of interest are selected by Fourier transform waveform or a forward or reverse scan or a stable map vertex isolation. Then, CID in a QIT is implemented by applying an alternating current (AC) potential that matches the ion duration frequency to increase the amplitude of ion vibration close to the edge of the electrode. The mass-selected ions (precursor ions) move close to the electrodes and thereby increase energy. Collisions of the ion with the buffer gas will convert the energy into ion internal energy continually to achieve dissociation.
In the CID process, internal energy must be sufficient to break chemical bonds and produce efficient ion fragmentation. The qu value determines the maximum amount of energy which can be imparted to the ion upon excitation. It is defined by the following equation:
where q is a relationship proportional to the amplitude of the RF voltage, m is the ion mass, e is the ion charge, Ω is the main radio frequency (RF) power angular frequency, V is the main RF power amplitude, r0 is the field radius.
The movement of ions in the trap can be thought of as moving in a potential trap whose function cannot exceed the depth of the potential trap. Since the movement of ions in the x and y directions does not affect each other in the ideal quadrupole field, it is only necessary that the function in each direction does not exceed the depth of the trap. For q<0.4, the approximate expression of the trap depth is:
To achieve collision-induced dissociation, the value of q must be suitable. At a low q value, the precursor ions are easily ejected because of a shallow potential trap depth. Additionally, energy deposited into the precursor ions is not enough to form product ions. However, if a large q value is chosen, more fragment ions will be lost as a result of the low mass cut-off (LMCO) effect, which is referred to the fact that those ions below the mass-to-ratio trapped at q=0.908 cannot be trapped. As a compromise, CID is usually performed between q value of 0.2 and 0.4 to obtain sufficient dissociation the smaller the q value the better.
There are mainly two methods for redeeming LMCO effects. The first is to decrease q value by increasing the initial internal energy or improving the conversion efficiency of ion kinetic energy to the ion internal energy, for example, thermally assisted collision-induced dissociation (TA-CID)(Ref 1). Secondly, ions are activated at a high q value for a very short time followed by a period with a low q value so that ion disassociation happens at a low q value, such as pulsed q dissociation (PQD) (Ref 2)
Presently, the conventional collision-induced dissociation method in linear ion traps only applies an auxiliary excitation signal on a pair of electrodes of the trap, because the two directions in the ideal quadrupole field do not affect each other, mainly causing the ions to be excited in the x or y direction. If the ions can be excited simultaneously in both directions, the average kinetic energy and maximum function of the ions are improved, compared to the one-way excitation, so that the q value can be lowered.
The invention provides a method for ion excitation for collision-induced dissociation in linear ion trap that is capable of increasing the average kinetic energy and maximum function of ions. In the present invention, ions are excited simultaneously in two radial coordinate directions (X and Y), thereby increasing the average kinetic energy and maximum function of ions. In a specific implementation, an auxiliary AC voltage is applied in a dipole manner on the pair of electrodes x and y, that is, the phase of the alternating signal applied on the opposite pair of electrodes is 180 degrees out of phase: or, in x and y auxiliary alternating voltage, is applied to the electrodes in a unipolar manner, i.e., an alternating current signal is applied only to a single electrode.
In linear ion traps, the traditional ion excitation method mainly causes the ions to increase in amplitude in a single coordinate direction. In a pure quadrupole field, the ion amplitude has the following relationship with the average kinetic energy:
where m is the ion mass, Ω is the main radio frequency (RF) power circular frequency, ri is the maximum particle motion radius,
is me most probable ion thermal speed, and v212(0,qu) and v222(0,qu) are the dimensionless parameters that depend only on the RF field properties. The first variable is the main determining factor to the ion kinetic energy, and it is proportional to ri2.
r
i
2
=x
t
2
+y
t
2
Therefore, when ions are excited in both x- and y-directions, ions will move in the xy plane, rather than along x or y axis. Thus, the kinetic energy can be increased due to the larger ion moving radius and the addition of another excitation dimension.
The potential trap depth for ions moving in the xy plane is
where D is the trap depth in one dimension, Φ is the angle between the x-axis and the straight line between the origin and the ion position (less than 90 degrees). It can be seen that moving along a certain angle, the potential trap is deeper, and the functions that can be achieved are also greater.
In addition, the x and y directions move simultaneously, and the ions are in a higher function for a longer period of time. Since the unidirectional excitation kinetic energy changes approximately sinusoidally with time, in a period, even if the x-direction kinetic energy is at the lowest value, the y-direction kinetic energy can be at a high value in comparison, which increases the average kinetic energy.
In this method, two AC excitation signals are typically applied to the two pairs of LIT electrodes in dipolar fashion. They can also be applied in monopolar fashion, where each of the two AC signals is only applied to one electrode. The auxiliary AC waveforms can only have one frequency component, such as sinusoidal wave, rectangular wave or triangular wave. The auxiliary AC waveforms can also be the sum of multiple frequency components, such as noise signal, stored waveform inverse Fourier Transform (SWIFT) signal. The type of the two AC waveforms can be the same and for the same type the frequency, amplitude and phase difference can be the same or different. The type of the two AC excitation waveforms can be different, for example, the phase difference can vary from 0 to 360 degrees, but not 90 degrees. In the present invention, the two alternating current signals applied to the x and y electrodes may be completely different in type, such as a sine wave on the x electrode and a sine wave containing a plurality of frequencies on the y electrode.
The two AC signals are applied to the electrodes in the ion dissociation stage.
This method can be used in various types of linear quadrupole devices, such as linear quadrupoles, linear ion trap with the hyperbolic electrodes, rectilinear ion trap, and triangular-electrode linear ion trap, dual plane linear ion trap and other linear traps of various shapes and configurations.
In this method, ions are excited in both x- and y-directions, while ions are only excited in one direction in the conventional method. More energy can be deposited into the mass selected precursor ions, which arises from the following two factors: the addition of an excitation dimension and larger ion motion amplitude in either x- or y-direction. As a result, the ion dissociation rate constant and fragmentation efficiency are all increased, and the LMCO effect is redeemed.
The present invention will now be described in further detail with reference to the accompanying embodiments and drawings.
In the present invention, ions in the linear quadrupole devices are excited in the two-ways by applying auxiliary excitation signals in the x and y directions. Compared to the conventional single-way excitation method, the ion kinetic energy is increased and therefore more internal energy will be deposited into ions during collision-induced dissociation.
Embodiment 1,
The experimental conditions in
the mass spectrum of the two-way excitation of the present invention, q is 0.43. It can be seen that this method can observe more fragment ions.
Number | Date | Country | Kind |
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201510099378.6 | Mar 2015 | CN | national |
Filing Document | Filing Date | Country | Kind |
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PCT/CN2015/095259 | 11/23/2015 | WO | 00 |