Method and apparatus for injection of ions into an electrostatic ion trap

Abstract

A method of injecting ions into an electrostatic trap, comprising: generating ions in an ion source; transporting the ions from the ion source to an ion store downstream of the ion source; releasing the ions from the ion store to an ion guide downstream of the ion store; and accelerating the ions from the ion guide as a pulse into an orbital electrostatic trap for mass analysis, wherein the average velocity of the ions as the ions exit from the ion guide is substantially higher than the average velocity of the ions as they exit from the ion store, wherein there is a delay between releasing the ions from the ion store and accelerating the ions from the ion guide. Also an apparatus suitable for the method.

Claims

1. A method of injecting ions into an electrostatic trap, comprising: generating ions in an ion source; transporting the ions from the ion source to an ion store downstream of the ion source; releasing the ions from the ion store to a non-trapping ion guide downstream of the ion store; and accelerating the ions from the ion guide as a pulse into an orbital electrostatic trap for mass analysis, wherein the average velocity of the ions as the ions exit from the ion guide is substantially higher than the average velocity of the ions as they exit from the ion store, wherein there is a delay between releasing the ions from the ion store and accelerating the ions from the ion guide such that for ions of the same m/z forming an ion packet, the duration of the ion packet as it enters the electrostatic trap is substantially shorter than when the ion packet enters the ion guide from the ion store.

2. The method as claimed in claim 1 wherein the electrostatic trap separates the ions along a direction z according to their mass-to-charge ratio; and the initial velocity of the ions in the ion guide in the direction z prior to acceleration is substantially smaller than the velocity of the ions in the direction z during ion detection in the electrostatic trap.

3. The method as claimed in claim 2 wherein the ions are accelerated from the ion guide to the electrostatic trap either: a. substantially in the same direction as the ions were released from the ion store, which is a direction substantially orthogonal to direction z; or b. along a direction that is substantially orthogonally to the direction in which the ions were released from the ion store and is substantially parallel to direction z.

4. The method as claimed in claim 1 wherein the ions are transported from the ion source to the ion store via at least one ion optical device.

5. The method as claimed in claim 1 further comprising separating the ions according to mass-to-charge ratio or ion mobility upstream of the ion store.

6. The method as claimed in claim 1 wherein the ion store is a linear or 3D RF ion trap.

7. The method as claimed in claim 1 wherein the ion store is gas-filled, optionally to a pressure 1 ×10.sup.−3 mbar to 5 ×10.sup.−3 mbar.

8. The method as claimed in claim 1 wherein the ions are slowly released from the ion store to the ion guide with energies less than 1V.

9. The method as claimed in claim 1 wherein the ions are released from the ion store over a period of 10 to 100 microseconds.

10. The method as claimed in claim 1 wherein the ions are released from the ion store by applying a DC voltage pulse to generate an axial field gradient in the ion store.

11. The method as claimed in claim 1 wherein the ion guide is gas-free, optionally wherein the pressure is less than or equal to 10.sup.−3 mbar.

12. The method as claimed in claim 1 wherein the ions are accelerated from the ion guide by applying a DC voltage pulse to generate an axial field gradient in the ion guide, whereby an energy increase of the ions depends on their initial position within the guide.

13. The method as claimed in claim 12 wherein the energy range of the accelerated ions is 1% to 30% of the final energy of the ions in the electrostatic trap.

14. The method as claimed in claim 1 wherein at the same time as applying the axial field gradient, any RF field in the ion guide is switched off.

15. The method as claimed in claim 1 wherein the energy spread of the ions accelerated by the ion guide is significantly smaller than the final energy of the ions within the electrostatic trap, optionally wherein the energy spread of the ions is not more than 30% or not more than 20% or not than 10%, of the final energy of the ions within the electrostatic trap.

16. The method as claimed in claim 1 wherein the ion guide focuses the ions at a focal point within the electrostatic trap and wherein the ions are focused into ion packets that are sufficiently narrow in the z-direction of the electrostatic trap when they pass near one or more detection electrodes of the electrostatic trap so as to maintain coherence of the ion packets during detection.

17. The method as claimed in claim 16 further comprising adjusting the position of the focal point using an ion lens located downstream of the ion guide.

18. The method as claimed in claim 1 wherein the ion guide is a linear RF multipole ion guide, optionally wherein the ion guide axis is substantially orthogonally to the direction z of mass separation in the electrostatic trap.

19. The method as claimed in claim 1 wherein the ion guide is a helical trajectory ion guide wherein the ions move with both axial and rotational motion, optionally wherein the ion guide axis is substantially parallel to the direction z of mass separation in the electrostatic trap.

20. The method as claimed in claim 19, further comprising providing a potential step within the ion guide that reduces the velocity of the ions in the direction z.

21. The method as claimed in claim 19 wherein the ions are accelerated out of the ion guide at a fixed radius to the ion guide axis, which lies parallel to the direction z.

22. The method as claimed in claim 19 wherein after accelerating the ions out of the ion guide by means of applying a DC axial field gradient in the ion guide, a further DC axial field gradient is applied in the ion guide that stays on whilst ions are being detected in the electrostatic trap, wherein the further DC axial field gradient is chosen such that perturbations of an ideal electric field within the electrostatic trap are minimized.

23. The method as claimed in claim 22 wherein the further DC axial field gradient is chosen such that ion oscillations in the EST in the direction z remain as close to harmonic as possible.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically a first embodiment of the invention employing an RF linear ion guide to inject ions into an EST.

(2) FIG. 2 shows schematically a second embodiment of the invention employing a helical means for injecting ions into an EST.

(3) FIG. 3 shows schematically a cross sectional view of the ion guide in the FIG. 2 embodiment looking in the direction of the arrow A shown in FIG. 2 with the EST visible.

(4) FIG. 4 shows schematically the potential distribution in the FIG. 3 embodiment along the axis z extending over the guide 30 and the EST 60.

DESCRIPTION OF PREFERRED EMBODIMENTS

(5) In order to enable a more detailed understanding of the invention, numerous embodiments will now be described by way of example and with reference to the accompanying drawings.

(6) In a first type of embodiment, it is preferred to store ions in a gas-filled linear trap and then release the ions as a slow stream into an RF or electrostatic ion guide preferably orthogonally to the direction z of ion dispersion in the EST (i.e. the direction along which ions are separated according to m/z in the EST). Once the ion population has left the trap, an electrical pulse is applied to the guide so that the ions become bunched into packets that are sufficiently narrow in direction z when they pass near to the detection electrode(s) of the EST.

(7) Referring to FIG. 1, there is shown schematically a first preferred embodiment with linear ion store/ion guide geometry. The ions are introduced from an ion source or previous stage of mass analysis 10 into an ion storage device 20, which is preferably an RF-only linear ion trap. This trap is filled with gas such as nitrogen or argon at pressures preferably from 1×10.sup.−3 to 5×10.sup.−3 mbar. Once the required ion population is stored in trap 20, the ions are slowly released, typically on the timescale of hundreds of microseconds, with the use of an axial field gradient within the trap 20 into an RF-only linear ion guide 30. The ions are preferably released from the trap 20 into the linear guide 30 at energies less than 1 V.

(8) Once all ions of m/z of interest have entered the linear guide 30, i.e. after a time delay following their slow release from the ion store, the RF field is preferably switched off in the guide 30 and a pulser (not shown) provides that an axial electric field pulse is applied to the guide to pulse the ions out of the guide 30 into the EST 60 wherein the ions are increased in energy dependent on their initial positions within the guide. Thus, the average velocity of the ions as the ions exit from the ion guide is substantially higher than the average of the ions velocity as they enter the ion guide. A DC axial electric field gradient may be created by use of external, auxiliary electrodes, i.e. outside the guide, e.g. which may be angled to the ion guide axis or segmented in the direction of the ion guide axis, or by arranging the RF linear guide 30 to have multipole electrodes comprising multiple segments. The voltage pulse from the pulser could be fed to all segments of the electrodes via a capacitive or capacitive/resistive divider as known in the art. In the embodiment shown, segmented auxiliary electrodes 32 provide the axial field gradient.

(9) The axial electric field gradient provides focusing of the ions to the entrance of an EST 60 of orbital type, such as an ORBITRAP mass analyzer. Importantly, for ions of the same m/z that form a respective ion packet, the duration of the ion packet (i.e. of each packet of a particular m/z) as the packet enters the EST is substantially shorter than when the packet of ions enters the ion guide after release from the ion store. The spatial divergence of the ions in radial direction is compensated by ion optics 40 comprising a lens. The ion injection is finally facilitated by use of ion deflector 50 so that ions commence their orbital flight 65 within the EST about central electrode 63 whilst oscillating back and forth in the direction z, which is perpendicular to the plane of the page. The ions disperse according to their m/z along the direction z, since ions of different m/z have different oscillation frequencies in direction z.

(10) It can be seen from FIG. 1 that in the first preferred embodiment the release of ions from the ion trap 20 is in a direction x orthogonal to direction z and the pulsed extraction of the ions from the ion guide 30 to the EST 60 is in the same direction x as the release of ions from the ion trap 20, i.e. orthogonal to z.

(11) Direct line-of-sight gas carryover from the ion trap 20 to the EST 60 can be avoided by introducing a slight bend of the ion beam by the ion optics 40, thereby separating the ion beam from the gas stream. Furthermore, the ion guide 30 and ion optics 40 may be housed in regions subject to differential pumping.

(12) In the case of a linear potential distribution in ion guide 30 for accelerating the ions, optimum bunching from the centre of the guide to any desired point downstream takes place if the time-of-flight (TOF) to that point from the exit of the guide (x=x.sub.0) is substantially equal to the TOF from the ion beam centre to x.sub.0. Generally, the potential distribution in guide 30 is given by U=C*(x−x.sub.0).sup.n, where C is a constant, x is the axial position in the ion guide and n is an integer. The case of linear potential corresponds to n=1. The case of n>1 would result in a relatively shorter TOF after x.sub.0 and n<1 in would result in a relatively longer TOF.

(13) By providing acceleration in optics 40, the actual location of the minimum spread of TOFs (i.e. focal point) can be adjusted to reside inside the EST 60. Generally, the energy spread introduced within the guide 30 should be significantly smaller than the final energy of ions within the EST, preferably not more than 10-30% of the final energy. In practice, ion guide 30 is typically 0.05 to 0.2 m long for an EST with axial amplitude of ion oscillations during detection of 5-10 mm.

(14) For a standard ORBITRAP mass analyzer having two detection electrodes, the preferred location of the focal point for the ions is near the centre of the EST. If ions are excited by injection at coordinate z=h (typically the axial amplitude of ion oscillations during detection), as described in U.S. Pat. No. 7,714,283, there is an additional effective path length ΔL given approximately by

(15) $\Delta L=h\frac{\pi }{2}\frac{{\omega }_{\phi }}{\omega }$
where ω.sub.φ is the angular velocity of rotation and ω the angular velocity of axial oscillations. Where multiple detection electrodes are used, as shown in FIGS. 5-7 of U.S. Pat. No. 7,714,283 for example, the energy spread introduced within the ion guide 30 should be sufficiently small to allow limiting the TOF spread of the ions on the way into the EST at a level substantially below the TOF along the length of each of these electrodes (in practical terms, <5-20%). This condition is necessary for the coherence of the ion packets and its violation will lead to a loss of sensitivity and, in some cases, resolution (since it increases the influence of natural decay of the signal on de-phasing of the packets).

(16) FIG. 2 shows a second preferred embodiment of a helical motion type of ion injection, wherein the ion guide 30 in this case is a multi-turn electrostatic sector located external to the EST 60. FIG. 3 shows schematically a cross sectional view of the ion guide in the FIG. 2 embodiment looking in the direction of the arrow A. Generally, the same or similar components in FIGS. 2 and 3 are given the same numerals as in FIG. 1. Referring to both FIGS. 2 and 3, the guide 30 comprises a pair of coaxial electrode sets 80 and 90 generally of cylindrical form. As shown in FIG. 3, each of the electrode sets contains at least two electrodes, 81/82 and 91/92, respectively. The at least two electrodes, 81/82 and 91/92, of each electrode set are axially spaced apart. In some other embodiments, more than one pair of coaxial electrode sets may be used. In some other embodiments, more than two electrodes may be used in each electrode set. The guide 30 also comprises an entrance unit 70 through which ions enter guide 30. The entrance unit 70 lies in place of a sector taken out from the outer circular electrode 91 and allows the ion beam to enter the space between electrodes 81 and 91 of the electrode sets via an entrance aperture 71 in unit 70. Unit 70 can be arranged to have field-sustaining elements on its side so as to reduce potential perturbations within the space between the electrode sets.

(17) The ions are released from the ion trap 20 in the same way as in FIG. 1, although it is preferable in this embodiment to complete ejection of ions from the trap 20 in less than 100 microseconds due to the high ion energy in the device 30. As ions enter the space between the electrode sets at a small angle to the axis z, they start to move along a helical or spiral trajectory rotating around the central electrode set 80 with a step that clears them from the side of entrance unit 70 as they complete their first rotation. As the ions proceed to move away from the unit 70, preferably at around 1-2 times the gap between electrodes 81 and 91, they are subjected to a potential step formed between first and second electrodes 81 and 82 of the inner electrode set 80 and between first and second electrodes 91 and 92 of the outer electrode set 90, so that their velocity in direction z becomes reduced without affecting the ions' rotational movement. This is similar to the step described with reference to FIG. 3 of U.S. Pat. No. 5,886,346. This allows reduction of the step or pitch of the ion beam helix dramatically, preferably down to 1-2 times the ion beam diameter, and allows storing many microseconds of duty cycle within just a few millimetres along direction z. This is illustrated in FIG. 3 by ion trajectories 100, where the bold dots illustrate ions flying out of the plane of the drawing and crosses illustrate ions flying into the plane of the drawing.

(18) The exit of the electrostatic sector guide 30 is coupled to the EST 60 which in this case is preferably an ORBITRAP mass analyzer as shown, with two outer detection electrodes 61, 62 and a central spindle-shaped electrode 63. Typical representative equipotentials present in the EST as ions are injected are shown by lines 110 and 111 in FIG. 3 and by line 120 in FIG. 4. It can be seen from FIGS. 2 and 3 that the release of ions from the ion trap 20 in the second preferred embodiment is in a direction x orthogonal to direction z and the pulsed extraction of the ions from the ion guide 30 to the EST 60 is in the direction z, i.e. orthogonally to the direction in which the ions were released from the ion trap 20.

(19) Ions are injected into the EST 60 from the electrostatic sector guide 30 not in the trap centre along a tightening radius as described in U.S. Pat. No. 5,886,346, but rather at a fixed radius and only in the axial direction z (the direction of mass separation). This is achieved by applying a voltage pulse to the first electrodes 81 and 91 of each electrode set in FIG. 3 (e.g. a capacitively coupled voltage from an external pulser) so that the potential distribution changes from 121 to 122 as shown in FIG. 4. As the ions start moving towards the centre of the EST (shown by the position of axis y), each m/z gets bunched into a shorter packet 101. By the time the ions return back to the second electrodes 82 and 92 at the end of their first oscillation, the voltage on both electrodes 82 and 92 is changed so that the potential distribution changes again to 123 in FIG. 4 (e.g. again by applying a voltage pulse but in this case a DC-coupled voltage because it needs to stay switched high (on) for the entire duration of ion detection in the EST 60). As the time of the ions' return to guide 30 depends strongly on m/z, a delay of switching the potential to 123 can be used to define the mass range to be trapped in the EST, for example in the simplest case where the maximum m/z to minimum is of the order of about 10 (e.g. the heaviest m/z are approaching the trap centre as the lightest m/z are already returning in the direction towards the guide 30). This mass range may be extended significantly by utilizing synchronised time-dependent voltages on the first electrodes 81/91, and second electrodes 82/92 so that a few percent of voltage change during the injection enables ions of lighter m/z (which reach the gap between those first and second electrodes first) to receive a smaller increase of velocity in the z direction than ions of heavier m/z. The final voltages on electrodes 82 and 92 are chosen in such a way that perturbations of the ideal field in the area of ion motion in the EST are minimised by the potential distribution 123. Therefore, the ions are forced to continue oscillating back and forth in the direction z within the EST (whilst orbiting about central electrode 63) until ion detection has finished, with the bunching into packets enforcing coherence of the axial oscillations along z. In this way, deviations of the ion oscillations from harmonic oscillation are minimised, i.e. ion oscillations in the axial direction z remain as close to harmonic as possible.

(20) 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.

(21) 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.

(22) 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”.

(23) 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.

(24) Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise.

(25) All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).