Ion ejection from a quadrupole ion trap

Abstract

A method of ejecting ions to be analyzed from a quadrupole ion trap in which a trapping field is created by one or more RF voltages applied to one or more electrodes of the trap, the method comprising the steps of cooling the ions to be analyzed within the quadrupole ion trap until the ions are thermalized, reducing the amplitude of one or more RF voltages applied to the quadrupole ion trap and applying the reduced amplitude RF voltages for one half cycle after the one or more RF voltages have reached a zero crossing point, turning off the RF voltages applied to the quadrupole ion trap, and ejecting the ions to be analyzed from the quadrupole ion trap.

Claims

1. An ion ejector system for a mass analyzer comprising a quadrupole ion trap for containing a buffer gas; a RF power supply with one or more outputs electrically connected to one or more electrodes of the quadrupole ion trap; an ejection power supply with one or more outputs electrically connected to one or more electrodes of the quadrupole ion trap; and a controller electrically connected to the RF power supply and the ejection power supply, the controller arranged to: (a) control the RF power supply to supply one or more RF voltages at a first amplitude to one or more electrodes of the ion trap for a first period of time, wherein the first period of time is sufficient for ions within the quadrupole ion trap to become thermalized due to collisions with the buffer gas; (b) control the RF power supply after the first period of time to supply one or more RF voltages of a second amplitude to one or more electrodes of the quadrupole ion trap for substantially one half cycle from where the one or more RF voltages at the first amplitude have reached a zero crossing point, the second amplitude being smaller than the first amplitude; and (c) control the RF power supply to turn off the RF voltages applied to the quadrupole ion trap after the one half cycle; the controller being arranged to perform (a) to (c) in that order.

2. The ion ejector system of claim 1 wherein the quadrupole ion trap is a linear trap comprising four electrodes extended generally parallel to an axis, the four electrodes comprising two opposing pairs of electrodes; a first opposing pair of electrodes being connected to a first output of the RF power supply and a second opposing pair of electrodes being connected to a second output of the RF power supply, the first and second RF outputs of the RF power supply being arranged to provide voltages of opposite polarities.

3. The ion ejector system of claim 1 wherein the quadrupole ion trap is a 3D trap comprising a ring electrode and two end-cap electrodes, the ring electrode being connected to a first output of the RF power supply and the end cap electrodes being connected to a second output of the RF power supply, the first and second RF outputs of the RF power supply being arranged to provide voltages of opposite polarities.

4. The ion ejector system of claim 1 wherein the quadrupole ion trap is a 3D trap comprising a ring electrode and two end-cap electrodes, the ring electrode being connected to a first output of the RF power supply and the end cap electrodes being connected to a steady state voltage supply.

5. The ion ejector system of claim 1 wherein in (b) the controller is arranged to control the RF power supply after the first period of time to supply a first RF voltage of the one or more RF voltages at a second amplitude, the second amplitude being a factor d of the first amplitude.

6. The ion ejector system of claim 5 wherein d is within the range 0.3 to 0.7.

7. The ion ejector system of claim 5 wherein d is within the range 0.4 to 0.6.

8. The ion ejector system of claim 5 wherein d is within the range 0.45 to 0.55.

9. The ion ejector system of claim 1 wherein in (b) the controller is arranged to control the RF power supply after the first period of time to supply first and second RF voltages at a second amplitude, the second amplitude being a factor d of the first amplitude.

10. The ion ejector system of claim 9 wherein d is within the range 0.3 to 0.7.

11. The ion ejector system of claim 9 wherein d is within the range 0.4 to 0.6.

12. The ion ejector system of claim 9 wherein d is within the range 0.45 to 0.55.

13. The ion ejector system of claim 1 wherein in (b) the controller is arranged to control the RF power supply after the first period of time to supply only a first RF voltage at a second amplitude, the second amplitude being substantially zero, and to supply a second RF voltage at the first amplitude.

14. The ion ejector system of claim 1 wherein in (b) the controller is arranged to control the RF power supply after the first period of time to supply a first RF voltage at a second amplitude and a second RF voltage at a third amplitude, the second amplitude being a factor e of the first amplitude and the third amplitude being a factor f of the first amplitude, where (e+f)/2 is smaller than 1.

15. The ion ejector system of claim 14 wherein (e+f)/2 is within the range 0.3 to 0.7.

16. The ion ejector system of claim 15 wherein (e+f)/2 is within the range 0.4 to 0.6.

17. The ion ejector system of claim 16 wherein (e+f)/2 is within the range 0.45 to 0.55.

18. The ion ejector system of claim 1 wherein in (c) the controller is arranged to control the RF power supply to turn off the RF voltages applied to the quadrupole ion trap and to switch all the trap electrodes to the same potential.

19. The ion ejector system of any of claim 1 wherein the controller is arranged to control the ejection power supply to supply one or more ejection voltages after a time delay from turning off the one or more RF voltages to ensure the voltages of trap electrodes have settled to a substantially steady state prior to application of the one or more ejection voltages.

20. The ion ejector system of claim 19 wherein the time delay is less than 30% of the period of oscillation of the RF power supply.

21. The ion ejector system of claim 1 wherein the buffer gas is at a pressure of between 10.sup.5-10.sup.2 mBar and the first period of time is between 104-102 RF cycles of the RF power supply.

22. The ion ejector system of claim 1 and a mass analyzer, the mass analyzer arranged to receive ions ejected from the quadrupole ion trap.

23. The ion ejector system and mass analyzer of claim 22 and an orthogonal ejector, the orthogonal ejector disposed between the quadrupole ion trap and the mass analyzer.

24. The ion ejector system and mass analyzer of claim 22 wherein the mass analyzer comprises a time-of-flight mass analyzer or an electrostatic trap mass analyzer.

25. The ion ejector system of claim 1 wherein the controller comprises a computer.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a schematic perspective view of a linear quadrupole ion trap for use with the present invention.

(2) FIGS. 2A-2C show examples of voltage waveforms plotted against time according to the method of the present invention, depicting three different embodiments of the invention suitable for ejecting positive ions from a quadrupole trap having reduced velocity distributions in the direction of ejection. FIG. 2A also includes a schematic figure depicting the orientation of ion ejection and voltages applied for an embodiment of a linear trap.

(3) FIG. 3 is a plot of R vs. Q, where R is the ratio of the effective temperature of ions in the ejection direction to the buffer gas temperature, and Q is the Mathieu stability parameter for the quadrupole ion trap. The figure provides data for a range of values d, where d=V.sub.1/V.sub.0.

(4) FIG. 4A is a plot of the voltage waveforms vs. time also showing points at particular phases. FIG. 4B shows the phase space in X from positively charged thermalized ions within a linear quadrupole ion trap as depicted in FIG. 1 having the voltage waveforms of FIG. 4A applied to the electrodes. The phase space plots of FIG. 4B correspond to the parameters of the ions at the phases noted in FIG. 4A.

(5) FIG. 5 is a phase space plot in X, showing the level lines of the ion ensemble's phase-space density function in the moment after time period t.sub.1 when the transition process starts (dashed ellipse) and after the further time period t.sub.2 one half an RF period later (solid ellipses).

(6) FIG. 6 is a simplified schematic diagram of an electronic arrangement suitable for providing RF trapping voltages and ejection voltages in accordance with an embodiment of the invention. The figure also includes a schematic figure depicting the orientation of a linear trap suitable for use with the electronic arrangement and voltages applied.

(7) FIG. 7 shows measured output from the electronic arrangement depicted schematically in FIG. 6, being a plot of voltages applied, V, vs. time. FIG. 7 shows three different amplitude waveforms superimposed (A, B, C), exemplifying three different trapping conditions able to be generated by the electronic arrangement as examples.

DETAILED DESCRIPTION OF THE INVENTION

(8) Various embodiments of the present invention will now be described by way of the following examples and the accompanying figures.

(9) FIG. 1 shows a schematic perspective view of a linear quadrupole ion trap for use with the present invention. The trap 100 comprises four electrodes, 101, 102, 103, 104. Electrodes 101 and 102 oppose one another in the X direction, and electrodes 103, 104 oppose one another in the Y direction. Electrodes 101 and 102 are oriented perpendicular to electrodes 103 and 104. Electrodes 101, 102, 103, 104 are shown as flat plates each having a length oriented parallel to axis Z, but may be round rods each with an axis parallel with axis Z. Alternatively the electrodes may comprise hyperbolic surfaces facing in towards axis Z. Other electrode shapes are contemplated. Electrode 101 has a slot 120 for ejection of ions 121 from the trap 100 in the X direction towards mass spectrometer 160, which may be a TOF mass spectrometer, or a FT mass spectrometer, or an EST mass spectrometer, for example.

(10) The ion trap is filled with a buffer gas, normally nitrogen, helium, or any other chemically inert gas, under the intermediate pressure 10.sup.4-10.sup.2 mBar. During ion accumulation, storage and cooling, the opposite pairs of electrodes 101, 102, and 103, 104, are activated by the radio frequency voltages RF.sub.1 and RF.sub.2 normally having the same frequency f and amplitude V.sub.0 but shifted by 180 degrees in phase relative to each other. Typical the RF amplitude may be 400-1000 V and the frequency 0.5-5 MHz.

(11) In prior art embodiments, at a certain moment of time, the RF generators 130 and 140 are switched off and a rapid bipolar voltage pulse is applied to the electrodes 101 and 102 from a DC voltage generator 150. The ions are accelerated by the electric field in the positive X direction and exit the ion trap through a slit aperture 120 in the electrode 101. In the present invention a different ejection process is utilized.

(12) Electrodes 101, 102 are connected electrically to RF drive circuit 130 which supplies voltage RF.sub.2 and also to extraction voltage supply 150 via switch 151. Extraction voltage supply 150 supplies voltage V.sub.eject across electrodes 101 and 102 when switch 151 is made conductive. Electrodes 103, 104 are connected electrically to RF drive circuit 140 which supplies voltage RF.sub.1. Trap 100 also comprises trapping electrodes at each end of the trap to confine the ions within the trapping volume 105 and prevent them escaping in directions generally along the Z axis, but for clarity these electrodes and their associated voltage supplies are omitted from the figure. Voltages RF.sub.1 and RF.sub.2 are periodically varying voltages in time (preferably sinusoidally), and are of opposite phases.

(13) In use, the trap 100 has a collision, or buffer, gas admitted within the trapping volume 105 and RF drive circuits 130 and 140 are switched on to provide RF trapping potentials to the trap electrodes 101, 102, 103, 104. Switch 151 is non-conductive so that no extraction voltages are supplied to the trap electrodes 101 and 102. Ions including, in this example, positive ions to be analyzed, are admitted to the trapping volume 105 and whilst held within the trap by the trapping field which is created by the trapping potentials, undergo collisions with the buffer gas molecules, losing excess energy. Once the ions have thermalized, i.e. substantially come into thermal equilibrium with the buffer gas under the influence of the trapping field, after a time delay t.sub.1 after ions were admitted to the trap, the ejection process may commence.

(14) Referring now also to FIG. 2A, in accordance with a preferred embodiment of the present invention, after time delay t.sub.1, just as voltage RF.sub.2 supplied by RF drive circuit 130 reaches a zero crossing point and is about to go to a positive voltage, the RF drive circuit 130 is turned off and electrodes 101 and 102 are held at the RF ground potential (RF 0V). RF drive circuit 140 is allowed to continue to operate, voltage RF.sub.1 passing from a zero crossing point at time t.sub.1 and going negative for a further half cycle during time period t.sub.2. After time period t.sub.2 has elapsed RF drive circuit 140 is also turned off, again at a zero crossing point, and electrodes 103 and 104 are held at the RF ground potential. At substantially the same time, extraction voltage supply 150 is switched by making switch 151 conductive so as to apply extraction potentials to electrodes 101 and 102. Extraction potentials are in practice developed on electrodes 101 and 102 very shortly after time period t.sub.2 has elapsed, preferably within one half RF cycle. Optionally a small delay, t.sub.3 (not shown in the figure), may occur between turning off RF drive circuit 140 and turning on extraction voltage supply 150 in order to ensure that the potentials on electrodes 103 and 104 have completely settled, though time period t.sub.3 should be less than 30% of one RF cycle. The extraction potential can also be applied shortly before the time period t.sub.2 ends, however the bunch of ejected ions must reach the ejection slot 120 after the RF field is completely stopped.

(15) Voltage supply 150 supplies voltage V.sub.eject such that electrode 101 has a negative ejection potential applied to it, and electrode 102 has a positive ejection potential applied to it. In this embodiment, electrodes 103 and 104 remain at the RF ground potential during ion ejection. Positive ions to be analyzed 121 are ejected from the trap 100 through slot 120, and travel to mass spectrometer 160. In this embodiment ions are ejected directly into an injection trajectory for the mass analyzer, and have reduced velocity spreads in the direction of ejection from the ion trap.

(16) A further embodiment of the invention may be utilized in a similar manner to that just described, but in accordance with FIG. 2B. In this case, after time delay t.sub.1, the RF drive circuit 140 is turned off at the zero crossing point and electrodes 103 and 104 are held at the RF ground potential (RF 0V). RF drive circuit 130 is allowed to continue to operate, voltage RF.sub.2 passing from a zero crossing point at time t.sub.1 and going positive for a further half cycle during time period t.sub.2. After time period t.sub.2 has elapsed RF drive circuit 130 is also turned off, again at a zero crossing point, and electrodes 101 and 102 are momentarily held at the RF ground potential. At substantially the same time, extraction voltage supply 150 is switched by switch 151 so as to apply extraction potentials to electrodes 101 and 102. Voltage supply 150 supplies voltage V.sub.eject such that electrode 101 has a negative ejection potential applied to it, and electrode 102 has a positive ejection potential applied to it. Positive ions to be analyzed are ejected from the trap 100 through slot 120, and travel to mass spectrometer 160. In this embodiment ions are ejected directly into an injection trajectory for the mass analyzer, and have reduced velocity spreads in the direction of ejection from the ion trap.

(17) An alternative embodiment of the invention may be utilized in accordance with FIG. 2C. In this case, after time delay t.sub.1, from the zero crossing point and for one half cycle thereafter RF drive circuits 130 and 140 provide reduced amplitude RF drive voltages RF.sub.2 and RF.sub.1 respectively, the peak to peak voltage changing from V.sub.0 to V.sub.1, where V.sub.1=dV.sub.0 (0<d<1). After a further time period t.sub.2 has elapsed, both RF drive circuits are turned off and electrodes 101, 102, 103, 104 are momentarily held at the RF ground potential. At substantially the same time, extraction voltage supply 150 is switched by making switch 151 conductive so as to apply extraction potentials to electrodes 101 and 102. Voltage supply 150 supplies voltage V.sub.eject such that, for positive ions to be analyzed, electrode 101 has a negative ejection potential applied to it, and electrode 102 has a positive ejection potential applied to it. Ions to be analyzed are ejected from the trap 100 through slot 120, and travel to mass spectrometer 160. In this embodiment ions are ejected directly into an analyzer injection trajectory, and have reduced velocity spreads in the direction of ejection from the ion trap.

(18) Embodiments described in relation to FIGS. 2A, 2B, and 2C are all arranged to eject ions of a positive polarity so that those ions have a minimum velocity distribution in the direction of ejection. If ions of negative polarity are to be ejected, the polarities of voltages RF.sub.1 and RF.sub.2 are reversed and upon ejection, electrode 102 has a negative ejection potential applied to it, and electrode 101 has a positive ejection potential applied to it.

(19) The moments after time periods t.sub.1 and t.sub.2 when the transition process correspondingly starts and ends, as well as the moment when the ejection voltage is applied, are defined with the accuracy up to a fraction of the RF period. Due to the limitation of the electronic circuits providing the RF and the pulsed ejection voltages, the transition from the full RF amplitude to the attenuated RF amplitude, switching the RF off, and the rise of the ejection voltage from zero to V.sub.eject take some time, which normally doesn't exceed one RF period. The moments after time periods t.sub.1 and t.sub.2 are considered herein as the time moments when the said changes start.

(20) Embodiments described in relation to FIGS. 2A and 2B have the additional advantage that they require complete termination of the RF voltages but not changing to lower, non-zero amplitudes. This is easier to implement provided that the two RF generators are individual but synchronized in phase, e.g. activated with one primary transformer coil. The method of fast termination of a RF voltage at the zero crossing point may be implemented in various ways, including those described in U.S. Pat. No. 7,498,571, U.S. Pat. No. 8,030,613, or WO2005/124821, for example.

(21) The present invention may also be used in an arrangement in which an orthogonal ejector is placed between the quadrupole ion trap and the mass spectrometer. In this case ions are ejected from the quadrupole ion trap with lowest velocity spread in a direction generally orthogonal to the ejection direction from the quadrupole ion trap, so that the lowest velocity spread lies in the direction of the analyzer injection trajectory. If positive polarity ions are to be ejected but with a minimum velocity distribution orthogonal to the direction of ejection, only the polarities of voltages RF.sub.1 and RF.sub.2 are reversed.

(22) As described in relation to FIG. 2A, in both the embodiments described in relation to FIGS. 2B and 2C, optionally a small delay, t.sub.3 (not shown in the figures), may occur after time delay t.sub.2 and before turning on extraction voltage supply 150 in order to ensure that the potentials on electrodes have completely settled, though time period t.sub.3 should be much shorter than one RF cycle.

(23) V.sub.1 may be selected from the range 0.3 V.sub.0 to 0.7 V.sub.0 with 0.45 V.sub.0 being a particularly preferred value. The inventors have found that the effective temperature of ions in the ejection direction falls below that of the buffer gas when the ions are at their maximum spatial extent in the ejection direction, and that by utilizing the present invention ions of approximately this lower effective temperature may be ejected from the quadrupole ion trap.

(24) FIG. 3 is a plot of R vs. Q, where R is the ratio of the effective temperature of ions in the preferred direction to the buffer gas temperature, and Q is the Mathieu stability parameter for the quadrupole ion trap. The figure provides data for a range of values d, where d=V.sub.1/V.sub.0. It can be seen that the effective temperature of ions in the preferred direction is equal to or below the temperature of the buffer gas for a wide range of stability values, Q, indicating that thermalized ions of a wide range of m/z may be simultaneously ejected from the trap using the present invention. Values for d of 0.4-0.5 produce ejected ions with the lowest effective temperatures. Lowest effective temperatures achieved for these values of d are found at highest values of Q. The effective temperature is defined by the formula T.sub.eff=m<v.sup.2>/k.sub.b where the angle brackets denote averaging over the ion ensemble and v is the velocity component in the preferred direction. The values of attenuation coefficients in the range 0.3<d<0.6 correspond to the effective temperature below the temperature of the buffer gas T over a wide range of the Mathieu parameter Q. The optimal attenuation parameter was found to be 0.45.

(25) FIG. 4A is a plot of the voltage waveforms also showing points at particular phases. FIG. 4B shows the phase space in X from positively charged thermalized ions within a linear quadrupole ion trap as depicted in FIG. 1 having the voltage waveforms of FIG. 4A applied to the electrodes. The phase space plots of FIG. 4B correspond to the parameters of the ions at the phases noted in FIG. 4A. The phase space plots of FIG. 4B illustrate typical phase-volume distributions of an ion ensemble in a RF quadrupole ion trap in the state of dynamic equilibrium with a buffer gas. The solid and dashed lines 1-4 schematically show the level lines of the probability density function in coordinates x and v=dx/dt. The biggest spatial spread (the distribution 1) is attained in the RF phase =.sub.1 characterized with the maximal span of RF voltages RF.sub.1 and RF.sub.2, with the voltage on the electrodes separated in the x direction (RF.sub.2 in accordance with FIG. 1) being retarding for the ions, i.e. positive in case of positively charged ions or negative for the negatively charged ions. In the RF phase .sub.2 when the polarity of voltages is reversed, the spatial spread attains its minimum as shown by the lines 2. The velocity spread is accordingly bigger than in the phase .sub.1. In the intermediate phases .sub.3 and .sub.4 the RF voltages cross the zero line. These phases correspond to the transition from the biggest spatial spread to the smallest spatial spread (.sub.3) and vice versa (.sub.4). The ion ensemble is characterized by extra collective velocity as shown by lines 3 and 4, correspondingly.

(26) Table 1 provides values for R, the ratio of the effective temperature of ions to the buffer gas temperature, for different mass ions within the trap (m/z, where z=1), and at different moments of time corresponding to the different phase conditions, .sub.1, .sub.2, .sub.3, .sub.4 referred to in relation to FIG. 4. The tabulated values are for a linear quadrupole ion trap having r.sub.0=2.2 mm and being operated with V.sub.0=800V, f=2.8 MHz.

(27) TABLE-US-00001 TABLE 1 Ion mass m, Da (z = 1) 1522 254 195 Q 0.07 0.55 0.7 R (Effective In the maximum of the RF amplitude 0.93 0.68 0.49 temperature span .sub.1 T.sub.eff/T) In the maximum of the RF amplitude 1.1 1.7 3.0 span .sub.2 In the point of zero-crossing (without 3.0 3.6 4.3 the invention) .sub.3, .sub.4 In the moment of the second zero- 0.90 0.56 0.49 crossing and ejection according to the invention (d = 0.5)

(28) Table 1 shows that for ions ejected at a zero crossing point (.sub.3, .sub.4), as in prior art arrangements (i.e. without the benefit of the present invention), the ions possess an effective temperature between 3.0 and 4.3 times larger than the buffer gas temperature. In contrast, when the present invention is utilized, with an attenuation parameter d=0.5, the same ions possess an effective temperature between 0.90 and 0.49 times that of the buffer gas temperature. The present invention thus affords an improvement in effective temperature of a factor 3.3-8.6 depending upon the mass of the ions. The table also shows that with the present invention the ions attain almost the same temperature at a zero-crossing moment as they possessed at .sub.1 when the RF voltages were at their maximum amplitude, demonstrating that the reduced RF voltage amplitude for one half cycle causes the ions to retain their minimum temperature.

(29) FIG. 5 shows the level lines of the ion ensemble's phase-space density function in the moment t.sub.1 when the transition process starts (dashed ellipse) and in the moment t.sub.2 one half an RF period later (solid ellipses). In the moment t.sub.1, the ions had distribution corresponding to the phase .sub.4 as shown in FIG. 4. Evolution of the ion ensemble during the transition process t.sub.1<t<t.sub.2 depends on the attenuation parameter value, d. The attenuation parameter value d=0 corresponds to complete stop of the RF voltages in the moment t.sub.1, so that the ions experience no electric forces and continue the motion with velocities they had in the moment t.sub.1. The opposite case, d=1, corresponds to no attenuation effectively applied, and the phase-space density function turns to coincide with that in the RF phase .sub.3 after one half of the period. The intermediate value of the attenuation parameter in accordance with this invention, d=0.5, brings the phase-space density to the state with substantially less velocity spread and small correlation between the spatial coordinate x and the corresponding velocity. As already noted, a preferred range for d is between 0.45 and 0.55.

(30) FIG. 6 is a simplified schematic diagram of an electronic arrangement suitable for providing RF trapping voltages and ejection voltages in accordance with an embodiment of the invention. A two-fold chopper generator G drives the primary coil P. The set of secondary coils comprises a pair of three-fold coils L1 and L2, which provides the ion trap with both RF polarities, RF.sub.1 and RF.sub.2, with the 180 degrees phase shift between them. Each of the three-fold coils L1 and L2 is strongly magnetically coupled, but decoupled from the other three-fold coil. The coils L1 and L2 constitute LC tanks together with the capacitances of corresponding trap's electrodes.

(31) Two coils, one from L1 and one from L2, are incorporated with a half-wave rectifier that comprises high-voltage diodes D1 and D2. When at least one of the diodes is forward-biased, a capacitor C is charged periodically to the RF peak voltage. The derived voltage is used to control the output RF amplitude. A high-voltage switch S is connected in parallel with the capacitor C. The switch is implemented with MOSFET transistor(s) and is controlled by a voltage Us, which is kept zero (the switch is non-conductive) during the time period t.sub.1 during the ion's accumulation and cooling. After time period t.sub.1 has elapsed, which is synchronized with the RF phase as shown in FIG. 2A, the control voltage Us is turned positive and turns the switch S into the conductive mode. The phase RF.sub.2 is going positive with respect to the high-voltage ground (HVGND) and the diode D2 allows the three-fold coil L2 to be shortcut, thus suppressing the following positive semi-period of RF.sub.2. The other phase RF.sub.1 stays negative for another semi-period, so that the diode D1 remains reverse-biased and the switch S has no effect on the coil L1 until the time period t.sub.2 has elapsed. The phase of RF.sub.1 performs a semi-period swing with its stored energy until the time period t.sub.2 has elapsed when the diode D1 becomes forward-biased and shortcuts the coil L1 in its turn. Both RF voltages become zero after time period t.sub.2.

(32) Finally, two eject voltage pulse generators V.sub.eject apply ejection voltages to the corresponding coils of L2 in opposite polarities, resulting in the voltage difference between RF.sub.2 and RF.sub.2 that drives the stored ion out of the trap.

(33) After ejection, the control voltage Us can be switched back to zero thus allowing the RF energy to be accumulated in the LC tanks composed by the coils L1 and L2 and capacitances of corresponding trap' electrodes. The ion trap is then capable of storing ions for another duty-cycle. The schematic solution as described above allows accumulation, cooling, and ejection of positively charged ions. In case of negatively charged ions, the moment t.sub.1 when the switch S is turned on (made conductive) should be shifted by one half of RF period and the ejection voltage generators of reversed polarities should be used.

(34) FIG. 7 shows measured output from the electronic arrangement depicted schematically in FIG. 6, being a plot of voltages applied, V, vs. time. FIG. 7 shows three different amplitude waveforms superimposed (A, B, C), exemplifying three different trapping conditions able to be generated by the electronic arrangement as examples. After time period t.sub.1, voltage RF.sub.2 is terminated to 0V and RF.sub.1 continues for one half cycle during a further time period t.sub.2. After time period t.sub.2 RF.sub.1 is terminated and ejection voltages V.sub.eject are applied.

(35) As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as a or an means one or more.

(36) Throughout the description and claims of this specification, the words comprise, including, having and contain and variations of the words, for example comprising and comprises etc., mean including but not limited to, and are not intended to (and do not) exclude other components.

(37) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(38) The use of any and all examples, or exemplary language (for instance, such as, for example and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(39) It will also be understood that the present invention is not limited to the specific combinations of features explicitly disclosed, but also any combination of features that are described independently and which the skilled person could implement together.