Mass spectrometer

1. A mass spectrometer comprising:

an ion source;

an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween;

an ion gate upstream of said electrode; and

control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means:

(i) switches said ion gate from said first mode to said second mode at a time T 1 ; and

(ii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a later time T 1 +ΔT 1 ;

wherein ΔT 1 is set such that ions having a mass to charge ratio ≦ a value M1 are not substantially injected or orthogonally accelerated into said drift region by said electrode.

2. A mass spectrometer as claimed in claim 1 , wherein ions having a mass to charge ratio ≧ a value M1′ are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency and ions having a mass to charge ratio in the range M1-M1′ are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M1<M1′ .

3. A mass spectrometer as claimed in claim 2 , wherein M1′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

4. A mass spectrometer as claimed in claim 1 , wherein ΔT 1 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

5. A mass spectrometer as claimed in claim 1 , wherein M1 falls within a range selected from the group consisting of: (i) 1-5; (ii) 5-10; (iii) 10-15; (iv) 15-20; (v) 20-25; (vi) 25-30; (vii) 30-35; (viii) 35-40; (ix) 40-45; (x) 45-50; (xi) 50-55; (xii) 55-60; (xiii) 60-65; (xiv) 65-70; (xv) 70-75; (xvi) 75-100; (xvii) 100-150; (xviii) 150-200; (xix) 200-250; (xx) 250-300; (xxi) 300-350; (xxii) 350-400; (xxiii) 400-450; (xxiv) 450-500; (xxv) 500-550; (xxvi) 550-600; (xxvii) 600-650; (xxviii) 650-700; (xxix) 700-750; (xxx) 750-800; (xxxi) 800-850; (xxxii) 850-900; (xxxiii) 900-950; (xxxiv) 950-1000; and (xxxv) >1000.

6. A mass spectrometer as claimed in claim 1 , wherein M1 is selected from the group consisting of: (i) 4; (ii) 17; (iii) 18; (iv) 28; (v) 29; (vi) 40; (vii) 41; (viii) 93; (ix) 139; (x) 185; (xi) 379; and (xii) 568.

7. A mass spectrometer as claimed in claim 1 , wherein immediately after said control means has caused said electrode to inject or orthogonally accelerate ions into said drift region at time T 1 +ΔT 1 said control means switches said ion gate from said second mode to said first mode.

8. A mass spectrometer comprising:

an ion source;

an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween;

an ion gate upstream of said electrode; and

control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means:

(i) switches said ion gate from said second mode to said first mode at a time T 2 ; and

(ii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a later time T 2 +ΔT 2 ;

wherein ΔT 2 is set such that ions having a mass to charge ratio ≧ a value M3 are not substantially injected or orthogonally accelerated into said drift region by said electrode.

9. A mass spectrometer as claimed in claim 8 , wherein ions having a mass to charge ratio ≦ a value M3′ are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency and ions having a mass to charge ratio in the range M3′-M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M3′<M3.

10. A mass spectrometer as claimed in claim 9 , wherein M3′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

11. A mass spectrometer as claimed in claim 8 , wherein ΔT 2 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

12. A mass spectrometer as claimed in claim 8 , wherein M3 falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

13. A mass spectrometer as claimed in claim 8 , wherein immediately after said control means has caused said electrode to inject or orthogonally accelerate ions into said drift region at time T 2 +ΔT 2 said control means switches said ion gate from said first mode to said second mode.

14. A mass spectrometer comprising:

an ion source;

an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween;

an ion gate upstream of said electrode; and

control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means:

(i) switches said ion gate from said second mode to said first mode at a time T 3 ;

(ii) switches said ion gate from said first mode to said second mode at a later time T 3 +δT 3 ; and

(iii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a yet later time T 3 +δT 3 +ΔT 3 ;

wherein δT 3 and ΔT 3 are set such that ions having a mass to charge ratio ≦ a value M1 are not substantially injected or orthogonally accelerated into said drift region by said electrode and such that ions having a mass to charge ratio ≧ a value M3 are not substantially injected or orthogonally accelerated into said drift region by said electrode, wherein M1<M3.

15. A mass spectrometer as claimed in claim 14 , wherein ions having a mass to charge ratio M2 are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency and other ions having a mass to charge ratio in the range M1-M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M1<M2<M3.

16. A mass spectrometer as claimed in claim 15 , wherein M2 falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

17. A mass spectrometer as claimed in claim 14 , wherein ions having a mass to charge ratio in a range M1′-M3′ are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency and ions having a mass to charge ratio in the range M1-M1′ and M3′-M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M1<M1′<M3′<M3.

18. A mass spectrometer as claimed in claim 17 , wherein M1′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

19. A mass spectrometer as claimed in claim 17 , wherein M3′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

20. A mass spectrometer as claimed in claim 14 , wherein δT 3 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

21. A mass spectrometer as claimed in claim 14 , wherein ΔT 3 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

22. A mass spectrometer as claimed in claim 14 , wherein M1 falls within a range selected from the group consisting of: (i) 1-5; (ii) 5-10; (iii) 10-15; (iv) 15-20; (v) 20-25; (vi) 25-30; (vii) 30-35; (viii) 35-40; (ix) 40-45; (x) 45-50; (xi) 50-55; (xii) 55-60; (xiii) 60-65; (xiv) 65-70; (xv) 70-75; (xvi) 75-100; (xvii) 100-150; (xviii) 150-200; (xix) 200-250; (xx) 250-300; (xxi) 300-350; (xxii) 350-400; (xxiii) 400-450; (xxiv) 450-500; (xxv) 500-550; (xxvi) 550-600; (xxvii) 600-650; (xxviii) 650-700; (xxix) 700-750; (xxx) 750-800; (xxxi) 800-850; (xxxii) 850-900; (xxxiii) 900-950; (xxxiv) 950-1000; and (xxxv) >1000.

23. A mass spectrometer as claimed in claim 14 , wherein M3 falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

24. A mass spectrometer comprising:

an ion source;

an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween;

an ion gate upstream of said electrode; and

control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means:

(i) switches said ion gate from said first mode to said second mode at a time T 4 ;

(ii) switches said ion gate from said second mode to said first mode at a later time T 4 +δT 4 ; and

(iii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a yet later time T 4 +δT 4 +ΔT 4 ;

wherein δT 4 and ΔT 4 are set such that ions having a mass to charge ratio equal to a value M2 are not substantially injected or orthogonally accelerated into said drift region by said electrode.

25. A mass spectrometer as claimed in claim 24 , wherein M2 falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

26. A mass spectrometer as claimed in claim 24 , wherein ions having a mass to charge ratio ≦ a value M1 and ions having a mass to charge ratio ≧ a value M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency, and wherein ions having a mass to charge in the range M1-M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M1<M2<M3.

27. A mass spectrometer comprising:

an ion source;

an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween;

an ion gate upstream of said electrode; and

control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means:

(i) switches said ion gate from said first mode to said second mode at a time T 4 ;

(ii) switches said ion gate from said second mode to said first mode at a later time T 4 +δT 4 ; and

(iii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a yet later time T 4 +δT 4 +ΔT 4 ;

wherein δT 4 and ΔT 4 are set such that ions having a mass to charge ratio in a range M1′-M3′ are not substantially injected or orthogonally accelerated into said drift region by said electrode, wherein M1′<M3′.

28. A mass spectrometer as claimed in claim 27 , wherein M1′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

29. A mass spectrometer as claimed in claim 27 , wherein M3′ falls within a range selected from the group consisting of: (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

30. A mass spectrometer as claimed in claim 27 , wherein ions having a mass to charge ratio ≦ a value M1 and ions having a mass to charge ratio ≧ a value M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a first transmission efficiency and ions having a mass to charge ratio in the range M1-M1′ and ions having a mass to charge ratio in the range M3′-M3 are substantially injected or orthogonally accelerated into said drift region by said electrode with a second transmission efficiency lower than said first transmission efficiency, wherein M1<M1′<M3′<M3.

31. A mass spectrometer as claimed in claim 30 , wherein M1 falls within a range selected from the group consisting of: (i) 1-5; (ii) 5-10; (iii) 10-15; (iv) 15-20; (v) 20-25; (vi) 25-30; (vii) 30-35; (viii) 35-40; (ix) 40-45; (x) 45-50; (xi) 50-55; (xii) 55-60; (xiii) 60-65; (xiv) 65-70; (xv) 70-75; (xvi) 75-100; (xvii) 100-150; (xviii) 150-200; (xix) 200-250; (xx) 250-300; (xxi) 300-350; (xxii) 350-400; (xxiii) 400-450; (xxiv) 450-500; (xxv) 500-550; (xxvi) 550-600; (xxvii) 600-650; (xxviii) 650-700; (xxix) 700-750; (xxx) 750-800; (xxxi) 800-850; (xxxii) 850-900; (xxxiii) 900-950; (xxxiv) 950-1000; and (xxxv) >1000.

32. A mass spectrometer as claimed in claim 30 , wherein M3 falls within a range selected from the group consisting of (i) 1-50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1500; (xxii) 1500-2000; (xxiii) 2000-2500; (xxiv) 2500-3000; and (xxv) >3000.

33. A mass spectrometer as claimed in claim 27 , wherein δT 4 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

34. A mass spectrometer as claimed in claim 27 , wherein ΔT 4 falls within a range selected from the group consisting of: (i) 0.1-1 μs; (ii) 1-5 μs; (iii) 5-10 μs; (iv) 10-15 μs; (v) 15-20 μs; (vi) 20-50 μs; (vii) 50-100 μs; (viii) 100-500 μs; and (ix) 500-1000 μs.

35. A mass spectrometer as claimed in claim 27 , wherein said electrode comprises a pusher and/or puller electrode.

36. A mass spectrometer as claimed in claim 27 , wherein said ion gate comprises one or more electrodes for altering, deflecting, reflecting, defocusing, attenuating or blocking a beam of ions.

37. A mass spectrometer as claimed in claim 27 , wherein in said second mode said ion transmission efficiency is substantially 0%.

38. A mass spectrometer as claimed in claim 27 , wherein in said second mode said ion transmission efficiency is ≦×% of the ion transmission efficiency in said first mode, wherein x falls within a range selected from the group consisting of; (i) 0.001-0.01; (ii) 0.01-0.1; (iii) 0.1-1 (iv) 1-10; and (v) 10-90.

39. A mass spectrometer as claimed in claim 27 , wherein said electrode is repeatedly energised with a frequency selected from the group consisting of: (i) 100-500 Hz; (ii) 0.5-1 kHz; (iii) 1-5 kHz; (iv) 5-10 kHz; (v) 10-20 kHz; (vi) 20-30 kHz; (vii) 30-40 kHz; (viii) 40-50 kHz; (ix) 50-60 kHz; (x) 60-70 kHz; (xi) 70-80 kHz; (xii) 80-90 kHz; (xiii) 90-100 kHz; (xiv) 100-500 kHz; (xv) 0.5-1 MHz; and (xvi) >1 MHz.

40. A mass spectrometer as claimed in claim 27 , wherein said ion source comprises a continuous ion source.

41. A mass spectrometer as claimed in claim 40 , wherein said ion source is selected from the group consisting of: (i) an Electron Impact (“EI”) ion source; (ii) a Chemical Ionisation (“CI”) ion source; (iii) a Field Ionisation (“FI”) ion source; (iv) an Electrospray ion source; (v) an Atmospheric Pressure Chemical Ionisation (“APCI”) ion source; (vi) an Inductively Coupled Plasma (“ICP”) ion source; (vii) an Atmospheric Pressure Photo Ionisation (“APPI”) ion source; (viii) a Fast Atom Bombardment (“FAB”) ion source; and (ix) a Liquid Secondary Ions Mass Spectrometry (“LSIMS”) ion source.

42. A mass spectrometer as claimed in claim 27 , wherein said ion source is a pseudo-continuous ion source.

43. A mass spectrometer as claimed in claim 42 , wherein said ion source is selected from the group consisting of: (i) a Matrix Assisted Laser Desorption Ionisation (“MALDI”) ion source; and (ii) a Laser Desorption Ionisation (“LDI”) ion source.

44. A mass spectrometer as claimed in claim 43 , further comprising an RF ion guide comprising a collision gas for dispersing a packet of ions emitted by said ion source.

45. A mass spectrometer as claimed in claim 27 , wherein said ion source is coupled to a liquid chromatography source.

46. A mass spectrometer as claimed in claim 27 , wherein said ion source is coupled to a gas chromatography source.

47. A method of mass spectrometry, comprising:

switching an ion gate from a first mode to a second mode at a tune T 1 , said second mode having a lower ion transmission efficiency than said first mode; and

injecting or orthogonally accelerating ions into a drift region of an orthogonal acceleration Time of Flight mass analyser at a later time T 1 +ΔT 1 ;

wherein ΔT 1 is set such that ions having a mass to charge ratio ≦ a value M1 are not substantially injected or orthogonally accelerated into said drift region.

48. A method of mass spectrometry, comprising:

switching an ion gate from a second mode to a first mode at a time T 2 , said second mode having a lower ion transmission efficiency than said first mode; and injecting or orthogonally accelerating ions into a drift region of an orthogonal acceleration Time of Flight mass analyser at a later time T 2 +ΔT 2 ;

wherein ΔT 2 is set such that ions having a mass to charge ratio ≧ a value M3 are not substantially injected or orthogonally accelerated into said drift region.

49. A method of mass spectrometry, comprising:

switching an ion gate from a second mode to a first mode at a time T 3 , said second mode having a lower ion transmission efficiency than said first mode;

switching said ion gate from said first mode to said second mode at a later time T 3 +δT 3 ; and

injecting or orthogonally accelerating ions into a drift region of an orthogonal acceleration Time of Flight mass analyser at a yet later time T 3 +δT 3 +ΔT 3 ;

wherein δT 3 and ΔT 3 are set such that ions having a mass to charge ratio ≦ a value M1 are not substantially injected or orthogonally accelerated into said drift region and such that ions having a mass to charge ratio ≧ a value M3 are not substantially injected or orthogonally accelerated into said drift region, wherein M1<M3.

50. A method of mass spectrometry, comprising:

switching an ion gate from a first mode to a second mode at a time T 4 , said second mode having a lower ion transmission efficiency than said first mode;

switching said ion gate from said second mode to said first mode at a later time T 4 +δT 4 ; and

injecting or orthogonally accelerating ions into a drift region of an orthogonal acceleration Time of Flight mass analyser at a yet later time T 4 +δT 4 +ΔT 4 ;

wherein δT 4 and ΔT 4 are set such that ions having a mass to charge ratio equal to a value M2 are not substantially injected or orthogonally accelerated into said drift region.

51. A method of mass spectrometry, comprising:

switching an ion gate from a first mode to a second mode at a time T 4 , said second mode having a lower ion transmission efficiency than said first mode;

switching said ion gate from said second mode to said first mode at a later time T 4 +δT 4 ; and

injecting or orthogonally accelerating ions into a drift region of an orthogonal acceleration Time of Flight mass analyser at a yet later time T 4 +δT 4 +ΔT 4 ;

wherein δT 4 and ΔT 4 are set such that ions having a mass to charge ratio in a range M1′-M3′ are not substantially injected or orthogonally accelerated into said drift region, wherein M1′<M3′.