Ion Trap with Elongated Electrodes

Abstract

An ion trap 1 comprises one ejection electrode 2 for ion trapping having an opening 4, through which ions in the ion trap 1 can be ejected in an ejection direction E and further electrodes 3 for ion trapping, wherein the ejection electrode 2 and the further electrodes 3 are elongated in a longitudinal direction L. The angle between the longitudinal direction L and the ejection direction E is nearly 90. The ion trap 1 comprises a primary winding 5 connected to an RF power supply 6, a secondary winding 7 coupling with the primary winding 5 for transforming the RF voltage of the RF power supply 6 supplying the transformed RF signals to the ejection electrode 2 and secondary windings 7 coupling with the primary winding 5 for transforming the RF voltage of the RF power supply 6 supplying the transformed RF signals to the further electrodes 3. The ion trap 1 comprises a first DC supply 8, a second DC supply 9 and a controller 50, which is applying in a time period a first DC voltage provided by first DC supply 8 via the secondary winding 7 to the ejection electrode 2 to pull ions in the ion trap to the opening 4 of the ejection electrode 2 and a second DC voltage provided by the second DC supply 9 via the secondary windings 7 to the at least 70% of the further electrodes 3 to push ions in the ion trap to the opening 4 of the ejection electrode 2.

Claims

1. An ion trap comprising: an ejection electrode for ion trapping having an opening, through which ions in the ion trap can be ejected in an ejection direction E; electrodes for ion trapping a primary winding connected to an RF power supply; a secondary winding coupling with the primary winding for transforming the RF voltage of the RF power supply and supplying the transformed RF signals to the ejection electrode; secondary windings coupling with the primary winding for transforming the RF voltage of the RF power supply and supplying the transformed RF signals to the further electrodes; a first DC supply; a second DC supply; and a controller, wherein the ejection electrode and the further electrodes are elongated in a longitudinal direction L, the angle between the longitudinal direction L and the ejection direction E deviates from 90 not more than 15, the controller is configured for applying in a time period a first DC voltage provided by first DC supply via the secondary winding to the ejection electrode to pull ions in the ion trap to the opening of the ejection electrode and a second DC voltage provided by the second DC supply via the secondary windings to the at least 70% of the further electrodes to push ions in the ion trap to the opening of the ejection electrode.

2. The ion trap according to claim 1, wherein the ion trap is comprising 3 further electrodes.

3. The ion trap according to claim 1, wherein the ion trap is comprising 5 further electrodes.

4. The ion trap according to claim 1, wherein the ion trap is comprising 7 further electrodes.

5. The ion trap according to claim 1, wherein the ion trap is a curved ion trap.

6. The ion trap according to claim 1, wherein the angle between the longitudinal direction L and the ejection direction E deviates from 90 not more than 7, preferably not more than 3.

7. The ion trap according to claim 1, wherein the controller is applying in the time period the second DC voltage provided by the second DC supply via the secondary windings to the at least 80% of the further electrodes to push ions in the ion trap to the opening of the ejection electrode.

8. The ion trap according to claim 7, wherein the controller is applying in the time period the second DC voltage provided by the second DC supply via the secondary windings to all further electrodes to push ions in the ion trap to the opening of the ejection electrode.

9. The ion trap according to claim 1, wherein the control is applying at the same time a first DC voltage provided by first DC supply via the secondary winding to the ejection electrode to pull ions in the ion trap to the opening of the ejection electrode and a second DC voltage provided the second DC supply via the secondary windings to the at least 70% of the further electrodes to push ions in the ion trap to the opening of the ejection electrode

10. The ion trap according to claim 1, wherein voltage difference between the first DC voltage applied to the ejection electrode and the second DC voltage applied to the further electrodes is between 50 V and 800 V, preferably between 100 V and 600 V and particular preferably between 200 V and 400 V.

11. The ion trap according to claim 1, wherein the ion trap is comprising a focusing lens, which is arranged for the ejected ions downstream of the of the opening of the ejection electrode and is focusing the ejected ions.

12. The ion trap according to claim 11, wherein the focusing lens has an opening into which the ejected ions are directed which is larger than the opening of the ejection electrode.

13. The ion trap according to claim 11, wherein the focusing lens is an electrostatic lens to which a DC voltage is applied, so that the voltage difference between the DC voltage of the focusing lens and the first DC voltage of the ejection electrode is between 250 V and 1,500 V, preferably between 400 V and 1,000 V and particular preferably between 600 V and 800 V.

14. The ion trap according to claim 11, wherein the focusing lens is an electrostatic lens to which a DC voltage is applied and the ratio of the voltage difference between the DC voltage of the focusing lens and the first DC voltage of the ejection electrode and the voltage difference between the DC voltage applied to the ejection electrode and the DC voltage applied to the further electrodes is between 1.5 and 6, preferably between 2.0 and 4 and particular preferably between 2.2 and 3.

15. The ion trap according to claim 11, wherein the ion trap is comprising an acceleration lens, which is arranged for the ejected ions downstream of the focusing lens.

16. The ion trap according to claim 15, wherein the acceleration lens has an opening into which the ejected ions are directed which is smaller than the opening of focusing lens.

17. The ion trap according to claim 15, wherein the acceleration lens is an electrostatic lens to which a DC voltage is applied, so that the voltage difference between the DC voltage of the acceleration lens and the DC voltage of the focusing lens is between 800 V and 5,000 V, preferably between 1,500 V and 3,500 V and particular preferably between 2,000 V and 2,700 V.

18. The ion trap according to claim 15, wherein the acceleration lens is an electrostatic lens to which a DC voltage is applied, and ratio of the voltage difference between the voltage difference between the DC voltage of the acceleration lens and the first DC voltage of the ejection electrode and the voltage difference between the first DC voltage applied to the ejection electrode and the second DC voltage applied to the further electrodes is between 2 and 12, preferably between 4 and 9 and particular preferably between 5 and 7.

19. The ion trap according to claim 15, wherein the acceleration lens is an electrostatic lens to which a DC voltage is applied, and ratio of the voltage difference between the first DC voltage applied to the ejection electrode and the second DC voltage applied to the further electrodes and the voltage difference between the voltage difference between the DC voltage of the acceleration lens and the second DC voltage applied to the further electrodes and is between 0.05 and 0.4, preferably between 0.1 and 0.25 and particular preferably between 0.12 and 0.2.

20. The ion trap according to claim 1, wherein the secondary winding supplying the transformed signal to the ejection electrode and the secondary winding supplying the transformed signal to one of the further electrodes are a pair of secondary windings connected in series.

21. The ion trap according to claim 1, wherein the secondary windings supplying the transformed signal to two of the further electrodes are a pair of secondary windings connected in series.

22. The ion trap according to claim 1 at least one of claims 1 to 21, wherein by tapping RF signals from the RF supply of the ejection electrode and further electrodes of the ion trap further components of a mass spectrometer, in particular a HCD cell or a transport multipole, are supplied with a RF voltage, wherein preferably an inductance divider is used.

23. Method of ejecting ions from an ion trap, which is comprising one ejection electrode and further electrodes elongated in a longitudinal direction L for ion trapping, wherein the ejection electrode comprising an opening, through which ions in the ion trap can be ejected in an ejection direction E, wherein an angle between the longitudinal direction L and the ejection direction E deviates from 90 not more than 15, wherein RF voltage is supplied to the ion trap by a primary winding connected to an RF power supply, a secondary winding coupling with the primary winding transforming the RF voltage of the RF power supply and supplying the transformed RF voltages to the ejection electrode and secondary windings coupling with the primary winding transforming the RF voltage of the RF power supply and supplying the transformed RF voltages to the further electrodes, a first DC supply and a second DC supply, comprising the steps: switching off the RF voltage supplied to the one ejection electrode and the further electrodes of the ion trap; and applying in a time period a first DC voltage via secondary winding provided by the first DC supply to the ejection electrode to pull ions in the ion trap to the opening of the ejection electrode and a second DC voltage provided by the second DC supply via the secondary windings to the at least 70% of the further electrodes to push ions in the ion trap to the opening of the ejection electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 shows the ejection of ions from an ion trap according to the prior art.

[0047] FIG. 2 shows the ejection electrode of an ion trap with elongated electrodes.

[0048] FIG. 3 shows more detailed electrical circuit of the voltage supply of another ion trap according to the prior art, which is a linear ion trap.

[0049] FIG. 4 shows more detailed electrical circuit of the voltage supply of another ion trap according to the prior art, which is a curved ion trap.

[0050] FIG. 5 shows detailed electrical circuit of the voltage supply of a first embodiment of an inventive ion trap.

[0051] FIG. 6 shows the ejection of ions from a second embodiment of an inventive ion trap.

[0052] FIG. 7 shows the detailed electrical circuit of the voltage supply of a first embodiment of an inventive ion trap shown in FIG. 5 including also an RF voltage supply for HCD cell and transport multipole.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0053] FIG. 1 shows the ejection of ions from an ion trap according to the prior arthaving elongated electrodes in the longitudinal direction L. A cross section of the ion trap perpendicular to the longitudinal direction L is shown. The ion trap is comprising an elongate ejection electrode 2 and three further elongate electrodes 3. The ejection electrode 2 has an opening 4 through which ions stored in the ion trap 1 can be ejected in an ejection direction E. In this drawing further is shown the DC voltage supply to the electrodes of the ion trap when ions shall be ejected. The ejected ions are accelerated from the opening 4 of the ion trap to an acceleration lens 12 having an opening 13. At least an RF voltage is applied to the four electrodes 2,3 of the ion trap, when ions are trapped. It is further possible to apply a small DC voltage to at least one of the electrodes 2,3 of the ion trap to improve the trapping by potential wells. Axial trapping is enabled by applying DC voltages to the end apertures of the trap (not shown).

[0054] When the trapped ions are to be ejected, the RF voltage and if present also the small DC voltage is switched off. Then an offset voltage V.sub.acc is applied via an offset DC source which is positioned between the lower further electrode 3 and the acceleration lens 12, which is grounded. The same offset voltage V.sub.acc (now shown) is also supplied to the upper further electrode 3. A typical value for the applied offset voltage is 2,200 V. Via a first DC supply 8 a first DC voltage V.sub.eject is applied to ejection electrode 2. This first DC voltage V.sub.eject is applied between the lower further electrode 3 and the ejection electrode 2 by the first DC supply 8. A typical value for the first DC voltage V.sub.eject is 300V, wherein the negative polarity is applied to the ejection electrode 2. Due to the ejection electrode 2 also being connected to the offset DC source that supplies voltage V.sub.acc, relative to ground a voltage of 1,900 V is applied to the ejection electrode 2. Via a second DC supply 9 a second DC voltage is applied to left further electrode 3, which is arranged opposite to the ejection electrode 2 in the ion trap. In the shown example second DC voltage has the same value V.sub.eject as the first DC voltage. This second DC voltage is applied between the lower further electrode 3 and the left further electrode 3 by the first DC supply 9. Then the value of the second DC voltage is also 300V, wherein the positive polarity is applied to the left further electrode 3. Due to the ejection electrode 2 also being connected to the offset DC source that supplies voltage V.sub.acc, relative to ground a voltage of 2,500 V is applied to the left further electrode 3. When these voltages are applied to the electrodes 2,3 of the ion trap, positively charged ions are pushed by the voltage applied to the left further electrode 3 in the direction of the ejection electrode 2 and pulled by the voltage applied to the ejection electrode 2 to the ejection electrode 2. This effect on the positively charged ions is created by the electric field in the ion trap, which is provided by the voltage difference between the left further electrode 3 and the ejection electrode 2. This electric field has in particular a component directed to the ejection electrode 2 and as shown by the ion beam 32 of the ions in the ion trap is directed to the opening 4 of the ejection electrode 2. But not all ions are ejected through the opening and are further accelerated by the acceleration lens 12. A portion of ions strike the edge of the opening 4. This leads to a reduced efficiency of the ion ejection and to a contamination of the ejection electrode 2 at the edge of its opening. Further, a small dotted circle shows the central region from which ions are ejected from the ion trap 1, when the DC voltages are applied as described before.

[0055] FIG. 2 shows the ejection electrode 2 of an ion trap with elongated electrodes. The ejection electrode in elongated in the longitudinal direction L. Also shown is opening 4 provided in the ejection electrode. Through this aperture 4 ions trapped in the ion trap 1 can be ejected by applying the DC voltages to the ejection electrode 2 and also further electrodes 3 of the ion trap 1. It is shown that the ions are ejected from the ion through the opening 4 into an ejection direction E. It is shown, that the ejection direction E of the ejected ions is substantially perpendicular to the longitudinal direction L of the electrodes. In general, the ejection direction E of the ejected ions is at least nearly perpendicular to the longitudinal direction L of the electrodes. The angle between the longitudinal direction L and the ejection direction E deviates from 90 typically not more than 15, preferably not more than 10 and particular preferably not more than 5.

[0056] FIG. 3 shows in more detail an electrical circuit of the voltage supply of the ion trap 1 according to the prior art disclosed in US 2011/0315873 A1. In this Figure is the voltage supply for a linear ion trap shown.

[0057] It is shown the ejection of ions stored in the ion trap 1 in ejection direction E. The ion trap is comprising an ejection electrode 2 and the further electrodes 3, 3. To facilitate ejection, a opening 4 is provided in the ejection electrode 2.

[0058] A RF power supply 6 is shown which is connected with a primary winding 5. Further three pairs of symmetrical secondary windings 7,7 are shown, which are coupled with the primary winding 5. A RF switch 20 is shown to switch off the RF power supplied to secondary windings explained below. A first pair of secondary symmetrical windings 21 is shown which are connected to the full-wave rectifier 42 to rapidly reduce the RF voltage in the further secondary winding after the switch of the supplied RF voltage.

[0059] A first and a second winding 7 of a second pair of secondary windings supplies the further electrodes 3, of the ion trap, which are above the middle and at the right side of the ion trap 1. A first winding 7 of a third pair of secondary windings supplies the other further electrode 3 of the ion trap below the middle of the ion trap. The second winding 7 of a third pair of secondary windings supplies the other ejection electrode 2 of the ion trap. As can be seen from FIG. 3, all of the first windings of the first, second and third pair of secondary windings are connected together at the central tap 22 of the first pair of windings. However, only the second winding of the first pair is also connected to the central tap 22. The ends of the second of the windings 7 and 7 of the second and third pairs of secondary windings close to the central tap 22 are instead connected to a DC offset supply.

[0060] With the circuit shown in FIG. 3, positive or negative offsets (depending on the polarity of the ions trapped in the ion trap) can be set from DC voltage supplies 24, 25 that are selectable through a DC offset switch 23. However, rather than simply supply these selected DC offset voltages directly to secondary windings 7,7, they are routed through further high voltage supply switches 26 and 27. These switches 26 and 27 that preferably have low internal resistance may be set such that the DC offsets are delivered direct to the secondary windings 7,7. In an alternative configuration, the switches may be set so that independent HV offsets can be applied to the two secondary windings 7, 7 respectively supplying the ejection electrode 2 and the right further electrode 3. In the case of positive ions in the ion trap, a DC push voltage supply 9 supplies a large positive voltage through push switch 27 that can be set on secondary winding 7 thereby applying a large positive potential to the right further electrode 3. This large positive potential repels ions stored in the ion trap towards the aperture 4 provided in opposite ejection electrode 2. A corresponding pull DC voltage supply 8 supplies a large negative potential through pull switch 8 and onto secondary winding 7, thereby applying a large negative potential on the ejection electrode 2 that will attract ions towards its opening 4. Accordingly, this arrangement allows either a small DC offset to be applied to the electrodes 2 and 3 that may be used, for example, to provide a potential well for trapping ions within the ion trap. This potential may even, for example, be supplied at the same time as the RF potential being supplied to the electrodes 2 and 3. When the RF potential is switched off using switch 20, ions may be ejected orthogonally from the ion trap by applying the DC push voltage supply 9 and pull DC voltage supply 8 to the right further electrode 3 and the ejection electrode 2 respectively in addition to applying the DC offset voltage from 24 or 25 to all of the electrodes.

[0061] FIG. 4. shows detailed electrical circuit of the voltage supply of a curved ion trap 1 (C-trap) according to the prior art. The electrical circuit for providing the RF voltages and DC voltages is essentially the same as shown in FIG. 3. The ions are now supplied to the C-trap 1 by a transport multipole 31 to which ions are supplied from an ion source (not shown). The C-trap 1 ejects the trapped ions through the opening 4 of the ejection electrode 2 into an Orbitrap mass analyser.

[0062] FIG. 5 shows in detail an electrical circuit of the voltage supply of a first embodiment of an inventive ion trap 1. The ion trap 1 has elongated electrodes 2,3 in a longitudinal direction L and can be a linear ion trap or a curved ion trap. A cross section of the ion trap 1 perpendicular to the longitudinal direction L is shown. The ion trap is comprising an ejection electrode 2 and three further electrodes 3. The ejection electrode 2 has an opening 4 through which ions stored in the ion trap 1 can be ejected in an ejection direction E.

[0063] In this drawing further is shown the RF voltage supply to the electrodes 2,3 when ions are being trapped and the DC voltage supply to the electrodes 2,3 of the ion trap when ions are being ejected.

[0064] Normally at least an RF voltage of two opposite phases is applied to the four electrodes 2,3 of the ion trap, when ions are trapped. It is further possible to apply a small DC voltage to at least one of the electrodes 2,3 of the ion trap to improve the trapping by potential wells (supplied as the LO OFFSET voltage from supply 9).

[0065] An RF generator is shown as RF power supply 6, which is connected with a primary winding 5 of a transformer. This primary winding 5 in the transformer arrangement is coupled with two pairs of secondary windings 34, 35. The first pair of secondary windings 34 is supplying via the two windings 7 a transformed RF voltage to the lower further electrode 3 and the left further electrode 3 arranged in the ion trap 1 opposite to the ejection electrode 2. The second pair of secondary windings 35 is supplying via the winding 7 a transformed RF voltage to the ejection electrode 2 and via the winding 7 a transformed RF voltage to the upper further electrode 3. Further a low offset DC voltage is optionally applied to all electrodes 2, 3 when ions are trapped in the ion trap. In this situation a pull switch 26 is opened (lower switch position) and a push (offset) switch 27 is switched on to provide the low offset voltage (OFFSET_LO).

[0066] When the controller 50 of the ion trap is switching the voltage supply of the ion trap 1 to eject ions, the control is switching the RF switch 36, the pull switch 26 and the push switch 27. At first the RF switch 36 is activated so as to switch off the RF voltage supplied to all electrodes 2, 3 of the ion trap. Then with a very short delay of 0-1000 ns the push switch 27 is activated so as to provide a second DC voltage, a high push voltage (OFFSET_HI) to the further electrodes 3 and the ejection electrode 2 via the opened pull switch 26. The value of the push voltage is typically between 1,500 and 2,500 V, preferably between 1,800 V and 2,200 V. Then with a very short delay of 0-1000 ns the pull switch 26 is activated (upper switch position) so as to provide a first DC voltage (PULL_DC) to the ejection electrode 2 in addition to the high push voltage. Due to this DC voltage supply the ions in the ion trap 1 are ejected through the opening 4 of the ejection electrode. More details about the ion ejection are explained in FIG. 6.

[0067] FIG. 6 shows the ejection of ions from a second embodiment of an inventive ion trap 1. The ion trap has elongated electrodes 2, 3 in the longitudinal direction L. A cross section of the ion trap perpendicular to the longitudinal direction L is shown. The ion trap comprises an ejection electrode 2 and three further electrodes 3. The ejection electrode 2 has an opening 4 through which ions stored in the ion trap 1 can be ejected in an ejection direction E. In this drawing further is shown the DC voltage supply to the electrodes of the ion trap when ions are ejected. The ejected ions are accelerated to an acceleration lens 12 having an opening 13. Further is shown a focusing lens 10 with an opening 11, which is arranged between the opening of the ejection electrode 2 and the acceleration lens 12. Ejection electrode 2, focusing lens 10 and acceleration lens 12 are arranged along the ejection direction E, wherein ejected ions first pass the opening 4 of the ejection electrode 2, then the opening 11 of the focusing lens 10 and finally the opening 13 of the acceleration lens 12.

[0068] Normally at least an RF voltage is applied to the four electrodes 2,3 of the ion trap, when ions are trapped, for example in the manner shown in FIG. 5. It is further possible to apply a small DC voltage to at least one of the electrodes 2,3 of the ion trap to improve the trapping by potential wells.

[0069] When the trapped ions are to be ejected, the RF voltage and if present also the small DC voltage is switched off. Then a second DC voltage V.sub.acc is applied via second DC supply 9 which is connected with the three further electrode 3 and the acceleration lens 12, which is grounded. The three further electrode 3 are connected with the second DC supply 9 via secondary windings 7 of a transformer supplying the RF voltage to the three further electrodes 3. A typical value for the applied second voltage is 2,000 V. The second DC supply 9 is also connected to the ejection electrode 2 via a first DC supply 8, whereby a first DC voltage V.sub.eject is applied to ejection electrode 2. This first DC voltage V.sub.eject is applied between the further electrodes 3 and the ejection electrode 2 by the first DC supply 8. A typical value for the first DC voltage V.sub.eject is 300V, wherein the negative polarity is applied to the ejection electrode 2. Due to this, relative to ground, a voltage of 1,700 V is applied to the ejection electrode 2. When these voltages are applied to the electrodes 2,3 of the ion trap, positively charged ions are pushed by the voltage applied to the further electrodes 3 in direction of the ejection electrode 2 and pulled by the voltage applied to the ejection electrode 2 to the ejection electrode 2. This effect on the positively charged ions is created by the electric field in the ion trap, which is provided by the voltage difference between the further electrode 3 and the ejection electrode 2. This electric field has in particular an improved component directed to the ejection electrode 2 and in particular an improved component directed to the opening 4 of the ejection electrode 2. Providing only the ejection electrode 2 with a different voltage to the further electrodes 3 creates a more non-uniform electric field than in the prior art. Thus by the curcature of the equi-potentials of this electric field, stronger focusing of ions through the opening 4 of the ejection electrode 2 may be achieved. Such non-uniformity of the electric field within the volume of the ion trap creates a converging lens that may bring ions from a wide region (dashed circle) through the narrow ejection opening 4. Thus, advantages are that a higher amount of ions may be extracted from a region substantially wider that the ejection opening and contamination of the ejection electrode around the opening may be reduced. Accordingly, as shown in the Figure, the ion beam 32 of the ions in the ion trap is directed to the middle of opening 4 of the ejection electrode 2. At least nearly all ions are passing the opening 4 and are further accelerated by the grounded acceleration lens 12. In the inventive ion trap 1, preferably ions do not strike the edge of the opening 4 or only a small portion of ions do so. This leads to an improved efficiency of the ion ejection: a contamination of the ejection electrode 2 at the edge of its opening 4 can be avoided. Further the dotted circle shows the region from which ions are ejected from the ion trap 1, when the DC voltage is applied as decribed before. In comparison to FIG. 1 the diameter of the circle is bigger and therefore ions of a larger region in the ion trap are ejected by the inventive ion trap. Further is shown that the ion beam 32 leaving the opening 4 is strongly diverging. To reduce this the focusing lens is positioned between the ejection electrode 2 and the acceleration lens 12. The focusing lens 10 has a wide opening 11 with a diameter larger than the opening 13 of the acceleration lens 12 and larger than the opening 4. Typically, voltages Viens between 800 V and 5,000 V are applied to the focusing lens 10 by a third DC voltage supply 40, wherein this voltage is applied between the focusing lens 10 and the grounded acceleration lens 12. In the shown embodiment a voltages V.sub.lens of 2,400 V is applied. Preferably the voltages V.sub.lens is between 1,500 V and 3,500 V. By the application of the voltage V.sub.lens to the focusing lens a collinear ion beam 32 of the ejected ions can be formed.

[0070] FIG. 7 shows the detailed electric circuit of voltage supply shown in FIG. 5 wherin further RF voltages are tapped from the RF voltage supply to provide RF voltages to further components of a mass spectrometer, in this embodiment a transport multipole and a HCD cell. Still other components of a mass spectrometer can be supplied in the same way with a RF voltage. The common supply of the electrodes of the ion trap and other components has the advantage to reduce the number of RF sources and avoid synchrosation problems between the ion trap and other components which are influencing the performance of mass spectrometer. Several of the processes of the inventive method can be supported or implemented using one or more computers and processesors, being stand alone or connected or in a cloud system, and by software to execute the processes.

[0071] The embodiments described in this application represent examples of the inventive ion traps and inventive methods. Accordingly, the invention can be realised by each embodiment alone or by a combination of several or all features of the described embodiments without any limitations.