GB2588292A

GB2588292A - Ion transfer interface for TOF MS

Info

Publication number: GB2588292A
Application number: GB2014485.3A
Authority: GB
Inventors: Verenchikov Anatoly
Original assignee: Micromass UK Ltd
Current assignee: Micromass UK Ltd
Priority date: 2018-07-27
Filing date: 2019-07-24
Publication date: 2021-04-21
Anticipated expiration: 2039-07-24
Also published as: GB2605116B; WO2020021255A1; GB202209542D0; GB2588292B; GB202014485D0; GB2605116A; GB201812329D0

Abstract

A combination (200) of protruding RF ion guide (41) and of periodic lens (51) is proposed for non distorting transferring of ions from a region of effective collisional dampening into an orthogonal accelerator (61) of singly or multi-reflecting or multi -turn TOF mass spectrometer (60). The system allows substantial reduction of ion beam phase space and energy spread, in turn providing for an improved combination of time and energy spread of ion packets in TOF MS. In one embodiment (700), the transfer interface is arranged spiral around multi-port turbo-molecular pump for significant reduction of the interface length.

Claims

14285802n1

1. A mass or mobility spectrometer comprising: a first pumping stage and a second pumping stage that is separated from the first pumping stage by a wall having an orifice; at least one vacuum pump for differentially pumping the pumping stages such that the second pumping stage is at a lower pressure than the first pumping stage; an ion guide extending continuously from the first pumping stage to the second pumping stage, through the orifice, such that an upstream end of the ion guide is in the first pumping stage and a downstream portion of the ion guide is in the second pumping stage; wherein the ion guide comprises a plurality of electrodes for radially confining ions and wherein the electrodes define radially extending gaps therebetween; wherein, in the downstream portion of the ion guide, the gaps are open such that gas is evacuated, in use, radially out of the downstream portion through said gaps; and wherein, at the upstream end of the ion guide, the gaps are either: (i) blocked such that gas cannot be evacuated radially at the upstream end; or (ii) restricted relative to the gaps in said downstream portion of the ion guide for restricting the evacuation of gas radially at the upstream end.

2. The spectrometer of claim 1, wherein the gaps at the upstream end are blocked or restricted such that, in use, the pressure within the upstream end is higher than the pressure within the rest of the ion guide and causes the ions to be collisionally dampened, whereas the gaps in the downstream portion are open such that the gas is radially evacuated and the pressure therein is such that ions passing therethrough substantially do not collide with gas molecules or collide with gas at a lower rate than in the upstream end portion.

3. The spectrometer of claim 1 or 2, wherein each of the plurality of electrodes of the ion guide extend continuously from an upstream distal end of the ion guide to a downstream distal end of the ion guide.

4. The spectrometer of claim 1, 2 or 3, wherein the ion guide further comprises: an intermediate sealed portion between the upstream end and the downstream portion, wherein the gaps of the intermediate sealed portion are blocked such that gas cannot be radially evacuated from within the intermediate sealed portion; and an intermediate open portion between the upstream end and the intermediate sealed portion, wherein the gaps of the intermediate open portion are open such that gas is evacuated, in use, from within the intermediate open portion through said gaps.

5. The spectrometer of claim 4, wherein the intermediate sealed portion extends axially upstream and/or downstream from said orifice.

6. The spectrometer of claim 4 or 5, wherein the intermediate sealed portion has an axial length selected from the group of: >lOD, where D is the inscribed diameter of said ion guide; > 20 mm; > 30 mm; > 40 mm; > 50 mm; and > 60 mm; and/or wherein the intermediate sealed portion has an axial length Lc, wherein the ion guide has an inscribed diameter D, and wherein D3/Lc < 1 mm2.

7. The spectrometer of any preceding claim, wherein the upstream distal end of the ion guide is spaced from the upstream wall and downstream wall of the first pumping stage.

8. The spectrometer of any preceding claim, wherein the ion guide does not pass through or reside within a differential pumping aperture in an upstream wall of the first pumping stage.

9. The spectrometer of any preceding claim, configured such that in use a pressure within the upstream end is > 10 mTorr and/or a pressure within the downstream end of the ion guide is < 1E-6 Torr; and/or wherein the ratio of the pressure within the upstream end PA to the pressure within the downstream end portion PD is RA/RO³ 1E+4.

10. The spectrometer of any preceding claim, wherein the portion of the ion guide at the upstream end having said blocked or restricted gaps has an axial length selected from the group of: > 5 mm; > 10 mm; > 15 mm; > 20 mm; > 30 mm; > 40 mm; > 50 mm; and > 60 mm.

11. The spectrometer of any preceding claim, wherein the downstream portion has an axial length of >l0D, where D is the inscribed diameter of said ion guide; and/or wherein the downstream portion has an axial length selected from the group of: > 20 mm; > 30 mm; > 40 mm; > 50 mm; and > 60 mm.

12. The spectrometer of any preceding claim, wherein the radial length of each gap defined between adjacent electrodes of the ion guide, in at least the upstream end and/or intermediate sealed portion, is at least three times the minimum width h of the gap, perpendicular to the radial direction of the ion guide.

13. The spectrometer of any preceding claim, wherein the inscribed diameter D of said ion guide is between 2 and 5 mm, or about 3 mm.

14. The spectrometer of any preceding claim, wherein the gaps in the upstream end of the ion guide are blocked by a seal extending circumferentially around the outside of the electrodes such that gas cannot be radially evacuated from within the upstream end; or wherein the gaps in the upstream end of the ion guide are blocked by plugs located in the gaps between the electrodes.

15. The spectrometer of any preceding claim, wherein the ion guide is a multipole RF ion guide having rod electrodes, such as a quadrupole ion guide.

16. The spectrometer of any preceding claim, further comprising a lens system arranged downstream of the ion guide for shaping the ion beam received therefrom, wherein the ion path from the ion guide into the lens system is free from apertures having a diameter that is less than the inscribed diameter of said ion guide.

17. The spectrometer of claim 16, wherein the lens system comprises electrodes defining an inscribed diameter that is at least twice as large as the inscribed diameter of the ion guide.

18. The spectrometer of claim 16 or 17, wherein the lens system comprises a plurality of DC electrodes spaced along a longitudinal axis on which ions are received from the ion guide, and DC voltage supplies configured to apply different DC potentials to different ones of said DC electrodes such that when ions travel through the lens system along the longitudinal axis they experience an ion confining force, generated by the DC potentials, in at least one dimension orthogonal to the longitudinal axis.

19. The spectrometer of claim 18, wherein the plurality of DC electrodes are apertured electrodes having apertures through which the ions travel as they pass along the longitudinal axis.

20. The spectrometer of claim 18 or 19, wherein the DC voltage supplies are configured to maintain adjacent DC electrodes at different DC potentials, and alternating DC electrodes at the same DC potential.

21. The spectrometer of claim 20, wherein one of the DC potentials is a ground potential; optionally wherein the DC electrodes at the distal ends of the lens system are maintained at ground potential in use.

22. The spectrometer of any one of claims 18-21, wherein the DC electrode at one or both longitudinal ends of the lens system has a length, in the longitudinal direction, that is longer than the length of each DC electrode arranged between the end electrodes.

23. The spectrometer of any one of claims 16-22, wherein the lens system passes between at least two differentially pumped stages of the spectrometer.

24. The spectrometer of any one of claims 16-23, comprising a heater for heating electrodes of the lens system.

25. The spectrometer of any preceding claim, wherein the ion guide and/or lens system has a curved longitudinal axis for guiding ions in a curved path.

26. The spectrometer of any preceding claim, comprising a mass or mobility analyser arranged to receive ions from the ion guide or lens system.

27. The spectrometer of claim 26, wherein the mass analyser is a time of flight mass analyser comprising an orthogonal ion accelerator arranged to receive the ions from the ion guide or lens system.

28. The spectrometer of claim 27, wherein the time-of-flight mass analyser is: (i) a multi-reflecting time of flight mass analyser having two ion mirrors that are elongated in a drift direction and configured to reflect ions multiple times in an oscillation dimension that is orthogonal to the drift direction, wherein the orthogonal ion accelerator is arranged to receive ions and accelerate them into one of the ion mirrors; or (ii) a multi-turn time of flight mass analyser having at least two electrostatic sectors configured to turn ions multiple times in an oscillation plane, wherein the orthogonal accelerator is arranged to receive ions and accelerate them into one of the sectors.

29. A method of mass spectrometry or ion mobility spectrometry comprising: providing a spectrometer as claimed in any preceding claim; operating the at least one vacuum pump so as to evacuate gas from the ion guide such that the pressure within the upstream end portion of the ion guide is higher than the pressure within the rest of the ion guide; transmitting ions through the ion guide, wherein the ions are collisionally dampened by gas in the upstream end portion but substantially do not collide with gas molecules in the downstream portion, or collide with gas at a lower rate in the downstream portion than in the upstream end portion; and mass analysing or ion mobility analysing ions downstream of the ion guide.

30. A time of flight mass spectrometer comprising: first, second and third interconnected pumping stages; at least one vacuum pump for evacuating the pumping stages; an ion guide continuously extending from first pumping stage to the second pumping stage; an orthogonal ion accelerator and at least one ion mirror or electrostatic sector arranged in the third pumping stage, wherein the orthogonal ion accelerator is configured to pulse ions into the ion mirror or electrostatic sector; and a lens system between the ion guide and orthogonal ion accelerator, wherein the ion path from the ion guide into the lens system is free from apertures having a diameter that is less than the inscribed diameter of said ion guide.

31. The spectrometer of claim 30, configured such that the at least one vacuum pump pumps the first and second pumping stages such that, in use, the pressure within the upstream end portion of the ion guide is higher than the pressure within a downstream end portion of the ion guide and such that the ions are collisionally dampened in the upstream end portion but substantially do not collide with gas molecules in the downstream end portion, or collide with gas at a lower rate than in the upstream end portion.

32. The spectrometer of claim 30 or 31, wherein the lens system comprises a plurality of DC electrodes spaced along a longitudinal axis on which ions are received from the ion guide, and DC voltage supplies configured to apply different DC potentials to different ones of said DC electrodes such that when ions travel through the lens system along the longitudinal axis they experience an ion confining force, generated by the DC potentials, in at least one dimension orthogonal to the longitudinal axis;

33. A time-of-flight mass spectrometer, comprising conventional components: a gaseous ion source, generating an ion beam; a multi-stage differentially pumped ion transfer interface with stages separated by differential apertures; a gas filled radio-frequency (RF) ion guides for collisional dampening and transfer of said ion beam; a lens system for transferring and shaping said ion beam past said ion guide; an orthogonal accelerator for pulsed extracting of ion packets from the ion beam past said lens system; and a singly reflecting, or multi- reflecting, or multi-turn electrostatic analyzer for mass separation of said ion packets; wherein for the purpose of non distorting ion transfer of collisional dampened ion beam into said orthogonal accelerator, one of said RF ion guides satisfies the following range of parameters: a) said RF ion guide is quadrupolar, i.e. composed of four parallel rods, wherein the inner bore between rods with the inscribed diameter d is continuous and non distorted at the entire guide length; b) the guide comprises at least four segments A to D, formed by electrically insulating radial seals around said rods, alternated with open rod areas having gaps between rods for gas evacuation; c) the entrance segment A of said RF ion guide has said radial seal, which either serves as a differential aperture, or the segment A resides past a differential aperture and said radial seal has length L i>l mm//-â i(Torr), where P \ - is the gas pressure within segment A, accounting local raise of the gas pressure at a limited gas conductance along the guide rods; d) the next segment B is open (no radial seal) and is arranged for sufficient radial gas evacuation, achieved at segment length g>l0d and at the gap width h between the guide rods being /i><7/4; e) the next segment C has said radial seal which protrudes through the differential pumping wall, thus forming a differential channel of length Lc>\ 0d; f) the gas pressure Pc at the entrance of segment C is arranged low enough for gas mean free path g>d for suppression of gas conductance of said channel by factor Lc/d g) the exit segment D is open (no radial seal), it has length LD>d2!2h for sufficient gas evacuation, and the gas pressure PD at the segment end is arranged Pi><\ E-6Torr for collisional free ion beam formation in the subsequent lens system; and h) the exit of said ion guide is open and aligned with the entrance of said lens system without using any aperture with diameter less than d.

34. A spectrometer as in claim 33, wherein said lens system is a periodic lens, energized by at least two distinct DC potentials with one potential optionally being at ground; wherein inscribed diameter of said periodic lens is at least twice larger than D;

35. A spectrometer as in claim 33 or 34, wherein RA/RÏ >1E+4; and wherein the inscribed diameter D of said ion guide is between 2 and 5mm, preferably 3mm; and wherein D3/Lc<lmm2.

36. A spectrometer as in claims 33-35, wherein to prevent charging of said radial seals, the gap between electrodes of said quadrupole is at least three calibers long H: H/h>3.

37. A spectrometer as in claims 33-36, wherein electrode shape at the entrance of said periodic lens attenuate the filed of said periodic lens to provide for acceleration of continuous ion beam at the exit cross section of said ion guide being less than 10% the beam mean energy at the entrance of said orthogonal accelerator.

38. A spectrometer as in claims 33-37, wherein said periodic lens is a rigid structure made by cutting conductive tube with electro erosion into two combs; and wherein ends of said periodic lens are aligned with axis of said RF ion guide and of said orthogonal accelerator.

39. A spectrometer as in claims 33-38, wherein said periodic lens is heated to at least l50°C for reducing oil deposits on the hot lens surface, this way avoiding surface charging by ions.

40. A spectrometer as in claims 33-39, wherein said periodic lens passes between at least two differentially pumped stages.

41. A spectrometer as in claims 33-40, wherein said periodic lens continues through fringing fields and electrode boundaries of said orthogonal accelerator.

42. A spectrometer as in claims 33-41, wherein at least one - said periodic lens or said RF ion guide is curved.

43. An ion transfer interface between a gaseous ion source and a mass spectrometer comprising a) a cartridge multi-stage turbo-molecular pump; b) at least one ion optical component of the list: (i) RF ion guides; (ii) analytical quadrupole; (iii) ion mobility separator; (iv) CID cell; and (iv) periodic lens; and c) wherein said ion optical component are arranged spiral, thus forming a spiral ion path, surrounding said cartridge pump.