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Mass Analyzers : Time-of-flight Mass Spectrometry

Mass Analyzers : Time-of-flight Mass Spectrometry. CU- Boulder CHEM 5181 Mass Spectrometry & Chromatography Joel Kimmel Fall 2007. Resources for Journal Skims Web of Science http://apps.newisiknowledge.com/ VPN for Off-campus Access to Electronic Journals and Web of Science

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Mass Analyzers : Time-of-flight Mass Spectrometry

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  1. Mass Analyzers: Time-of-flight Mass Spectrometry CU- Boulder CHEM 5181 Mass Spectrometry & Chromatography Joel Kimmel Fall 2007

  2. Resources for Journal Skims Web of Science http://apps.newisiknowledge.com/ VPN for Off-campus Access to Electronic Journals and Web of Science http://www.colorado.edu/its/vpn/ Restricted pdfs on course website chem5181 and pass5181

  3. An ion with m/z = m has charge equal: (a) 1 Coulombs (b) m Coulombs (c) 1.6022 × 10−19 Coulombs (d) e Coulombs

  4. An ion with m/z = m has charge equal: (a) 1 Coulombs (b) m Coulombs (c) 1.6022 × 10−19 Coulombs (d) e Coulombs

  5. A mass spectrometer with a resolution of 5000 should be capable of resolving isotopic peaks for singly charged species with m (a) Of any value (b) Less than 5000 u (c) Greater than 5000 u (d) It depends on the type of mass spectrometer

  6. (a) Of any value (b) Less than 5000 u (c) Greater than 5000 u (d) It depends on the type of mass spectrometer

  7. Calculate resolution and accuracy? Argon Atomic Weight (Da): 39.948 39.9600 0.015934

  8. Which of the following pairs requires the greatest mass resolving power to distinguish: • Ar+ from Ar2+ • CO+ from N2+ • CH3+ from CDH2+ • Ar = 39. 9623837 u, C = 12 u, O = 15.9949 u, N = 14.003 u, H = 1.007 u, and D = 2.0141 u

  9. Which of the following requires the mass greatest resolving power, distinguishing: • Ar+ from Ar2+ • CO+ from N2+ • CH3+ from CDH2+ • Ar = 39. 9623837 u, C = 12 u, O = 15.9949 u, N = 14.003 u, H = 1.007 u, and D = 2.0141 u m/Δm = R 40/20 = 2 28/.01 = 2800 15/1 = 15 Note that requirements for isotopes with grow with m/z!

  10. Electric Field Lines + A - - - - - - - + + + B + + Ion Motion in Electrostatic Fields • Electrical force on an ion: Electric field is the force on a unit positive charge Q: What effect does this field have on a positive ion? Q: How does the voltage vary across the space between the + and – electrodes? Q: Would ions originating at A and B be affected equally? (Same E? Same V?)

  11. Time-of-Flight Mass Spectrometry To determine m/z values A packet of ions is accelerated by a known potential and the flight times of the ions are measured over a known distance. Q: What are V, e, and z? • Key Performance Notes • Based on dispersion in time • Measures all m/z simultaneously, implying potentially high duty cycle • “Unlimited” mass range • DC electric fields • Small footprint • Relatively inexpensive

  12. Electric Field Lines + A - - - - - - - + + + B + + Force: F = qE Effective Voltage: V = Es Potential: U = Vq Potential: U = (Es)q sA sB

  13. a TOFMS Drift Region, D Source, S Detector V E = 0 E = V/S Ions accelerated by strong field, E, within short source region, S. Drift times recorded across long, field-free drift region, D. vDdepends on starting position of ion – ideally all ions start from same plane. Q: What else is ideally assumed (Uo, E, D, …)? Q: What figures of merit will non-ideality affect? Drawing adapted from p20 of Cotter reference on next slide.

  14. Actual Picture More Complex TOF = total recorded flight time of an ion to = Ion formation time after T0 of TOF measurement ta = Time in acceleration region, which depends on initial position and initial energy tD = Time in drift region, which depends on initial position and initial energy td = Response time of detector For detailed discussion see: Guilhaus, J. Mass. Spec, 1519, 1995. Cotter, “Time-of-flight Mass Spectrometery: Instrumentation and Applications in Biological Research,” ACS, 1997.

  15. Resolution For any m/z in a time-of-flight mass spectrum, the recorded peak will be the sum of signals corresponding to multiple, independent, ion arrival events Each ion arrival will be recorded at a unique TOF, as determined by expression on previous slide TOF’, which is the center of the peak in the mass spectrum, will be an average of all individual ion arrival TOFs The width of TOF’, Δt, will depend on the distribution of the individual ion arrival TOFs (and other factors …)

  16. Improving Resolution • TOFMS was first commercialized in 1950s • Early instruments had limited resolution • Speed of electronics • Energy distribution • Recent “Renaissance ….”

  17. Delayed Ion Extraction / Time-Lag Focusing x V0 V1 V0 V1 V2 V2 Modified from Cotter: http://www.hopkinsmedicine.org/mams/MAMS/middleframe_files/teaching_files/ME330.884/2005/MS2005-Lecture-5-Instrumentation.pdf From De Hoffmann Delay between ionization and extraction events. At ionization: U = U0 (Initial Energy of Ion) At exit of extraction: U = U0 + Eextxq At beginning of drift: U = U0 + Eextxq + (V1-V2)q Tune source voltages and/or delay to compensate for ΔU0 and create space focus at detector. Mass dependent. From Guilhaus, J. Mass. Spec, 1519, 1995.

  18. Reflectron Reflectron consists of a series of electrodes, forming a linear field in direction opposite of initial acceleration. Ions are slowed by this field, eventually turning around and accelerating back in direction of detector. Penetration depth depends on Us, which is function of U0 and acceleration field, E. Reflectron voltages are tuned to create a space focus at the plane of the detector. From: http://www.chemistry.wustl.edu/~msf/damon/reflectrons.html

  19. In Delayed Extraction, we give ions different U to achieve same TOF. • In Reflectron, ions possess different U. We force them to travel different D to achieve same same TOF

  20. An Inherent Dilemma hv TOFMS is an ideal detector for pulsed ionization methods If ionization event is synchronized with time zero, high duty cycle is achieve Laser Desorption: Static, solid sample probed with a pulsed laser Because of pulsing, ions are wasted when TOFMS is applied to a continuous source & Increased efficiency comes at the expense of mass range and mass resolution Still, figures of merit and cost make the technique desirable ESI: Sample is continuously flowing towards the mass analyzer

  21. Sampling Time Duty Cycle  Sampling Time +Drift Time Performance Trade-offs: On Axis Gating Function t Drift Time Sampling Time Δt proportional to sampling time Mass Range proportional to drift time Ion Beam GATE

  22. Orthoganol Extraction t GATE Ions are extraction in a direction orthogonal to source trajectory Extraction event is still rapid (Δt), but extraction volume is determined by length of gate region. t

  23. oTOFMS • Able to reduce average initial energy in ToF direction to 0 (resolution and accuracy). • Independent control of beam energy and drift energy, allows maximum duty cycle. • Want tightly collimated beam in extraction region See: Guilhaus, et al. Mass Spec Rev, 2000, 65-107

  24. ToF-AMS Animation from Agilent: http://www.chem.agilent.com/scripts/pds.asp?lpage=10175

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