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Advanced TOF Mass Spectrometry

Advanced TOF Mass Spectrometry. D.T. Young J. H. Waite and G. P. Miller Space Science and Engineering Division Southwest Research Institute San Antonio, TX. High Resolution Mass Spectrometry in Space Grenoble, France May 21-22, 2008. Outline. Selection of mass spectrometer technique

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Advanced TOF Mass Spectrometry

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  1. Advanced TOF Mass Spectrometry D.T. Young J. H. Waite and G. P. Miller Space Science and Engineering Division Southwest Research Institute San Antonio, TX High Resolution Mass Spectrometry in Space Grenoble, France May 21-22, 2008

  2. Outline • Selection of mass spectrometer technique • SwRI Multi-Bounce TOF (MBTOF) • Comparison with other TOF methods • Comments on general TOF MS design issues

  3. Key space MS measurement requirements • Choice of technique • Quadrupole, magnetic sector, time-of-flight, ion trap • Absolute sensitivity • ~10-4 A/Torr (~1 ion/1000 molecules) • Relative (species) sensitivity • <~ few ppb with 5% precision • For example CH4 and 136Xe at Mars • Duty cycle • ~100% (high sensitivity) • Dynamic range • ~105 in 1 s (Mars: [CO2] >> [36Ar]) • Mass resolution • 1000 min to ~106 max (nitriles at Titan; 87Sr/ 87Rb at Mars) • Mass range • 1 to >104 Da (Titan’s atmosphere & ionosphere) • Scan speed • >1 kHz (for GC x GC and high space/time resolution)

  4. Choice of techniques appropriate to space mass spectrometry

  5. Stable trapping Converging electrostatic mirrors Multi-bounce TOF (MBTOF) • Principle is analogous to optical resonator • Stable trapping conditions for ions: • Converging gridless mirrors • Overlapping focal lengths • Period of oscillation has to be stable (time coherence) • Stable ion energy and field strength

  6. Multi-bounce TOF spectrometry basics • TOF: t = L (m/2U)1/2 • Resolution: m/Δm = t/2Δt = ½ (L/Δt)*(m/2U)1/2 • Key to high resolution is L while Δt  0 • Increase path length: L = λ(N+1)where λ is mirror length and N is number of bounces • MBTOF equation: m/Δm = ½ (N+1) * λ/Δt * (m/2U)1/2 • In units: m/Δm≈ 11.5 (N+1) * λ[cm]/Δt[ns] * (m[amu]/U[keV])1/2 • MBTOF typical values: • N = 12 bounces, λ = 40 cm, Δt = 5 ns, U = 0.5 keV, M = 28 amu • M/ΔM ≈ 8950 8

  7. Detector MBTOF concept overview Mirror 2 Orthogonal Injection or Storage E-I sources Mirror 1

  8. Axially symmetric potential distribution • Absolute values of potentials can be modified

  9. Typical potential distribution Ion source exit plane Extraction Mirror Einzel Lens EL ET Grid

  10. ION SOURCE MIRROR 1 LENS 1 DRIFT SPACE LENS 2 MIRROR 2 DETECTOR (not shown) MBTOF-I raytracing (axial ray separations exaggerated)

  11. Ray tracing of ion distribution in plane of detector (18 mm dia.) • Flight tube is baffled to ~10 mm diameter to reduce scattering

  12. SwRI MBTOF-I prototype MIRROR 1 MIRROR 2 STORAGE SOURCE LENS 1 LENS 2 DETECTOR DRIFT SPACE 0.45 m

  13. INMS Conceptual Block Diagram ION PUMP

  14. MBTOF-I Performance Data (Next 9 slides) 0-bounce spectrum of test gas mixture: ~100 s accumulation using FastFlight

  15. MBTOF-I Prototype 12-bounce separation of CO/N2 At m/z = 28: m/Δm = 13,500 at 50% of peak

  16. Separation of 12C16O from 14N14N Comparison of MBTOF-I prototype vs. RTOF flight unit at 20% of peak height MBTOF-I Prototype M/ΔM = 8,500 at 20% RTOF Flight Unit M/ΔM = 2,800 at 20%

  17. Signal/noise ratio has slight dependence on # bounces Xe 4-bounce S/N = 5500 Xe 14-bounce

  18. 2 Bounce 0 Bounce 4 Bounce 6 Bounce 8 Bounce 10 Bounce 14 Bounce 12 Bounce 16 Bounce Dependence of ion transmission on number of bounces

  19. Isotope ratios are preserved within <1% error with 4 bounces

  20. Effect of increasing number of bounces on Xenon spectrum 136Xe

  21. Mirror operated in “bunch-mode” causes high level of time focusing • Spectrum taken with pulser rise times ~20 ns but peak width is <4 ns • m/Δm = t/2Δ = 445250/2*3.75 = 59,370 at FWHM

  22. Demonstrated MBTOF-I performance • Absolute sensitivity • ~10-4 A/Torr • Species sensitivity • ~100 ppb (system cleanliness & pressure limitations) • Storage source ions/packet • ~105 • Dynamic range (with MCP detector) • 5500 for Xe at 14 bounces • Mass resolution • 13,500 (FWHM); 8,500 at 20% • Bunch mode: 59,000 (FWHM) • Mass range • 1 to >10,000 (based on demonstrated Xe trapping time) • (Detector efficiency for high mass not tested) • Mirror trapping time • >10-3 s • Scan speed • 10,000 spectra/s

  23. SwRI MBTOF-2 Prototype

  24. SwRI MBTOF development status • INMS proposed for 2013 Mars Scout mission • Completed Phase A study for Mars Scout (decision Fall 2008) • By July 30 we will have a “flight-like” prototype incorporating • MBTOF-III with orthogonal and storage ion sources • Getters, manifold and gas storage volume • ETP multiplier and >1 Gs/s ADC • Fast-switch HV supplies • Comment on INMS resource tradeoffs • Roughly 20 kg and 35 W when getters are operating • INMS mass and power are resp. 30% and 100% higher than RTOF • Deliberate tradeoffs were made to reduce cost and risk and improve manufacturability • Cost of INMS is less than RTOF and Cassini INMS in today’s dollars • Experiences with Rosetta TOF failures suggest this may be a good trade

  25. Other TOF approaches: The multi-turn TOF MS MULTUM II (Okumura et al., NIM-A, 2004) “Proposed” 300 x 320 mm optics only

  26. COSAC Axial MBTOF (Wollnik and Casares Int. J. Mass Spec., 2003) m/Δm = 29,000 after 101 bounces m/Δm ~ 75 (Goesmann et al., SSR, 2006) PFTBL

  27. Planar multi-reflectron of Verentchikov et al., (Tech. Phys., Vol. 50, 2005)

  28. What are the critical tradeoffs for TOF? • Go back to TOF equation: m = 2U*(t/L)2 • Time (t) • Narrow mass peaks (pulse ~5 ns rise time over 1 kV) • Low capacitance optics • Power and electronics parts (MOSFETs) • Accurate peak shape measurement • 3 ~ 5 data points per peak • ADC with at least 10-bits and at least 1 Gs/s • Amplitude  quantitative abundance • Centroid  species identification • But: detailed shape needed for deconvolution of overlapping peaks • Injection energy and mirror potentials: U = U(x, t) • Variations in potentials (jitter, drift) introduce aberrations • Periodic jitter can add or cancel coherently • Random jitter will add stochastically • Instruments with large numbers of discrete electrodes and potentials are at greatest risk (e.g., multi-turn)

  29. What are the critical tradeoffs for TOF? • Path length: L(x, t) • Aberrations in trajectory angle and length depend on mechanical alignments • Because of path repetition, diffferential misalignments accumulate over many cycles (~N*ΔL) • Δt/t ~ 10-5 over measurement period  ΔL/L ~ 10-5 • For example MBTOF-II electrodes are parallel to 20μm over 400 mm ~ 5 x 10-5 • Total path length • Longer paths require higher vacuum which requires higher power (~10-10 Torr is preferred for high quality MS) • Longer paths subject measurements to more electronics jitter, background, noise, etc.  In general narrow peaks are better than longer paths

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