The physics and technology of QMS

1. The physics and technology of QMS J H Batey Workshop on measurement characteristics and use of quadrupole mass spectrometers for vacuum applications Bled, Slovenia, April 10–13, 2012

2. Paul & Steinwedel 1956 DE 944900

3. Examples of quadrupole construction

4. Typical quadrupole RGAs from c. 1982 SX200 Anavac

5. Modern RGAs

6. Typical analytical quadrupole from c. 1990

7. A novel geometry: circular axis to make a compact instrument

8. Liverpool microquadrupole mass filter Rods 0.5 mm diameter r0 0.22 mm Length 20 mm 1 2

9. Mass filters come in a wide range of sizes … 1: ICP-MS L: 230mm r0: 5.5mm 2: RGA (SX200) L: 125mm r0: 2.7mm 3: RGA (Anavac) L: 50mm r0: 2.7 mm 4: Microquad L: 20mm r0: 0.22 mm 1 2 3 4

10. Isotope separators: quadrupoles on an altogether different scale! Finlan, Sunderland & Todd, Nucl. Inst & Methods, 195 (1982), 447-456 Von Zahn, Zeitschrift fur Physik, 168 (1962), 129-142 r0 : 35mm L : 5.86 metres r0: 13.5mm L : 3 metres

11. Early mass spectrometer: Dempster 1918 • Recognizable components: • Vacuum system • Source • Mass analyzer • Detector Isotope studies on alkali metals

12. Main components of a mass spectrometer Main components can be identified in Dempster’s system

13. Quadrupole mass spectrometer

14. Ion source Electron-impact source is the commonest. The design can be quite complex for analytical mass spectrometers. Filament; source electrode; extraction optics Source voltage; electron energy Repeller; collimating magnets

15. RGA source For an RGA the source is of relatively simple construction – it resembles an extractor ion gauge.

16. Source • General requirements: • Physical size • - usually “small enough” • Sensitive • - typically 10-4A/mbar • Robust • Linear • - beware of log/log plots • Reproducible • Serviceable • - easy to dismantle/reassemble • Low power; low voltage • Non-invasive; • - that is, operating the mass spectrometer should not alter the vacuum composition • Desirable features • Keep electrons confined to source • - avoid electron background signal • Variable electron energy • - helps separate some species • Low outgassing • - minimise materials • Avoid trapped volumes • - memory effects • Closed or open? • - depends on application • Choice of filament material • - tungsten, thoria, yttria

17. Linearity: beware of “log-log” plots! Which would you rather have?

18. Source • General requirements: • Physical size • - usually “small enough” • Sensitive • - typically 10-4A/mbar • Robust • Linear • - beware of log/log plots • Reproducible • Serviceable • - easy to dismantle/reassemble • Low power; low voltage • Non-invasive; • - that is, operating the mass spectrometer should not alter the vacuum composition • Desirable features • Keep electrons confined to source • - avoid electron background signal • Variable electron energy • - helps separate some species • Low outgassing • - minimise materials • Avoid trapped volumes • - memory effects • Closed or open? • - depends on application • Choice of filament material • - tungsten, thoria, yttria

19. Electron energy Reduce electron energy to 40eV: eliminates interferences due to Ar2+ Better detection limit for water in argon

20. Source • General requirements: • Physical size • - usually “small enough” • Sensitive • - typically 10-4A/mbar • Robust • Linear • - beware of log/log plots • Reproducible • Serviceable • - easy to dismantle/reassemble • Low power; low voltage • Non-invasive; • - that is, operating the mass spectrometer should not alter the vacuum composition • Desirable features • Keep electrons confined to source • - avoid electron background signal • Variable electron energy • - helps separate some species • Low outgassing • - minimise materials • Avoid trapped volumes • - memory effects • Closed or open? • - depends on application • Choice of filament material • - tungsten, thoria, yttria

21. Filaments • Tungsten • Simple • Mechanically robust • Affected by oxidising/reducing gas • Runs hot, so outgassing problems • Rapid burn-out if vacuum leak • OK with halogens • Thoria-coated iridium • Coating is delicate • More stable in oxidizing/reducing gas • Cooler, so less outgassing • Resistant to burn-out • Not good for halogens • Weak a emitter – possible health issues? • Yttria-coated iridium • Generally similar to thoria, with no radiation worries.

22. Detector Faraday plate/collector Simple and robust. Electron background and/or secondary electron emission may be a problem (easily prevented). Electron multiplier Higher sensitivity; needs high voltage supply; more prone to calibration drift; not suitable for coarse vacuum Discrete dynode multiplier; SCEM; micro-channel plate

23. QUADRUPOLE Hyperbolic electrodes to give 2D hyperbolic field. Though in practice round rods are often used. F(x,y,z) = F0 . (x2 – y2) 2r02 Here F0 is 20V

24. QUADRUPOLE “Saddle” shaped 3D field plot. X field is proportional to the X co-ordinate Y field is proportional to the Y co-ordinate.

25. QUADRUPOLE The quadrupole structure can be used as a static device (that is, one in which the applied voltage F0is constant) for steering and shaping an ion beam, with no mass selection. But for a mass filter, the potential F0consists of a constant and an alternating component. Specifically F0 = U – V cos (2 p f (t-t0) ) where U is the constant (“DC”) potential V is the alternating (“RF”) potential f is the frequency of the RF supply t is the time t0 is the initial phase of the RF component

26. QUADRUPOLE Influenced by this field, the ions travel on complex trajectories in the X and Y directions, with a constant drift along the Z axis.

27. QUADRUPOLE Mathieu equation

28. QUADRUPOLE The significance of the stability region becomes clearer when it is plotted in terms of V and U for a particular case r0= 6 mm f = 2x106Hz (typical values for a quadrupole ICP-MS)

29. QUADRUPOLE Conceptual mass spectra, deduced from the stability diagram.

30. QUADRUPOLE These peak shapes have been calculated using numerical integration of the Mathieu equation. Field radius (r0): 6 mm Radio frequency: 2 MHz Field length: 200 mm Input radius: 1 mm Exit radius: 6 mm Ion energy: 5 eV Beam divergence 5 degrees Ion masses 1, 2, 3, 4 & 5 amu

31. QUADRUPOLE A basic quadrupole model is provided with the Simion package. The dynamic voltages are programmed using the Lua language.

32. SIMION QUADRUPOLE Round rods give a field that is essentially hyperbolic near the axis, but well away from the axis, the field is quite different. Potential contours at intervals of 2V Gradient contours, at intervals of 1V/mm -10V -10V +10V +10V +10V +10V -10V -10V -10V -10V +10V +10V +10V +10V -10V -10V

33. Quadrupole field in X and Y directions r0 = 2.76 mm DC constant +20V. No RF applied.

34. Ion motion in RF & DC quadrupole field X component of ion motion. Vary RF amplitude. DC + 20V r0 = 2.76 mm F = 2 MHz M = 40 amu DC - 20V DC - 20V DC - 20V

35. SIMION QUADRUPOLE Plot the values of RF and DC that give stable and unstable X trajectories.

36. SIMION QUADRUPOLE Now add stability for Y trajectories (mirror image about DC = 0 axis). The ion motion is stable for RF and DC values within the region bounded by the four coloured lines.

37. SIMION QUADRUPOLE A Simion model, using parameters as listed by Taylor & Gibson. Hyperbolic rods (but note T&G used round rods). S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609–616 S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609–616

38. SIMION QUADRUPOLE Mathieu stability region and scan line Peak Hyperbolic electrodes. The 50% peak with is 0.117 amu, corresponding to a resolution of 343. The peak is shifted to lower mass by 0.015 amu; presumably a smaller grid size would give a smaller shift.

39. SIMION QUADRUPOLE Now we change to round rods … S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609–616

40. SIMION QUADRUPOLE Mathieu stability region and scan line Peak

41. SIMION QUADRUPOLE: 3D 2D 3D 3D model with fringing field: transmission is increased and the low-mass tail is reduced

42. SIMION QUADRUPOLE: 3D The previous slide showed an unusually narrow peak. Usually a quadrupole is tuned to give a wider peak. This is data from the same Simion model, but with the scan line set to give a peak width 1 amu at 50% height. The peak is much smoother, and there is no low-mass tailing. This would be an excellent performance for an analytical quadrupole, such as an ICP-MS, for which abundance sensitivity of 1 ppm or better is needed. The flat peak top is rarely seen in practice, though examples have been reported.

43. Flat-topped peaks! Some very early quadrupole papers showed flat-topped peaks. Is there still room for improvement from 21st century manufacturers? W Paul, HP Reinhard & U von Zahn, Zeitschrift fur Physik,152 (1958), 143-182 Brubaker, Recent developments in Mass Spectrometry, Proc. Int. Conf. on Mass Spectrosc., Kyoto, Japan, 1969, Pub Univ. of Pank, Baltimore, 1970 R = 1.16 R0 (for round comparison) L = 25.4 cm; r0 = 6.55 mm Hyperbolic, 1.414 MHz, 1 eV, aperture 1.27 mm

44. SUMMARY • Quadrupole: versatile – wide range of design possibilities • The mechanical design of current RGAs mostly follows long-established design principles … • … but there is increasing interest in smaller devices • Simulation (e.g. with Simion) allows theoretical performance to be investigated in considerable detail. AREAS NOT COVERED (in this talk). • Electronics • Data systems • Calibration