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Acknowledgements

Acknowledgements. Slides and animations were made by Dr. Jon Karty Mass Spectrometry Facility Indiana University, Bloomington. TOF Concept. A packet of ions is accelerated to a defined kinetic energy. The time required to move through a fixed distance is measured

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Acknowledgements

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  1. Acknowledgements Slides and animations were made by Dr. Jon Karty Mass Spectrometry Facility Indiana University, Bloomington

  2. TOF Concept • A packet of ions is accelerated to a defined kinetic energy. The time required to move through a fixed distance is measured • First TOF design published in 1946 by W.E. Stephens Detector

  3. TOF advantages • Theoretically unlimited mass range • Ions are not trapped (quad, IT, FTICR) nor are their flight paths curved (BE sectors) • Quadrupole devices have an upper limit as to what mass can actually be trapped • Detection efficiencies induce practical limits of a few hundred kDa (M+H)+ • Instrument is not scanning (it is dispersive) • Analysis is very rapid compared to other mass analyzers • All ions in source are analyzed simultaneously • Wide range of m/z’s can be measured with good sensitivity and resolution • Moderate to high resolving powers (5,000-40,000+) • Accurate mass (sub 3 ppm) is attainable • Moderate cost ($100k to $500k) • Couples extremely well with pulsed ion sources (e.g. MALDI)

  4. TOF Disadvantages • Requires high vacuum (<10-6 torr) • High pressures lead to peak broadening • Requires complex and high speed electronics • High acceleration voltages (5-30 kV) with excellent stability • Fast detectors (ns or faster) • GHz analog to digital conversion • Large volumes of data can be generated quickly • Limited dynamic range • Often 102 or 104 at most • Highest resolving power instruments can get rather large • Bruker Maxis (RP>40,000) is nearly 8’ tall • Calibration of TOF’s tends not to be very robust • Temperature changes alter flight tube length • Small fluctuations in power supply voltages affect ion kinetic energy

  5. The Ion Source • The source in a TOF instrument serves two purposes • Ionize neutral species • Give the ions the kinetic energy for time-of-flight analysis • Equation for an ion in an electric field: • KE is kinetic energy of the ion (J or eV), z is the charge of the ion (C or e),Eis electric field strength (V/m), ds is distance traveled through the field, E in (m) • Energy gained through electrostatic field is independent of mass • If ions of different m but same z are accelerated by an electric field, the kinetic energies of all ions are the same • Assuming all start from same position in the field • High acceleration potential minimizes effect of initial energy distribution

  6. Real TOF Ion Sources • Ions are NOT formed in the exact same position in real ion sources • These differences in initial position have profound effects on the mass spectrum • Ions are also formed with a distribution of kinetic energies and velocity vectors • Ions spend some time in the source prior to cruising through the flight tube • Observed flight time is sum of time spent in source AND flight tube • TOFtotal = TOFsource + TOFflight_tube • A more complete understanding of the TOF mass spectrometer requires that one consider where the ions start in the source

  7. 100V  0 V +100 V Influence of Initial Position on Final Ion Kinetic Energy • Consider two stationary +1 ions between two plates, 1 cm apart. • The red ion is 6 mm from right plate, blue ion is 5 mm from right plate • What are the energies of these ions after they exit the source • Red ion: KE = z * E * ds = 1 e * 104 V/m * 0.006 m = 60 eV • Blue ion: KE = 50 eV • Two ions have different energies exiting the source • Problem for drift tube analysis TOF experiment starts when right plate is pulsed from 100 V to 0 V

  8. Simulation of the Flight TimesDue to Differences in Where Ion Forms in Source • Both ions are 100 m/z • Red ion is 6 mm from 0 V plate, blue ion is 5 mm from 0 V plate • s for red ion is 0.006 m; s for blue ion is 0.005 m • Distance between plates is 1 cm • Electric field is 10,000 V/m • Detector is 1 m from 2nd grid (D =1 m) • TOFred = 94 µsec TOFblue = 103 µsec Detector 0 V +100 V

  9. Reflectron • In 1966, B. Mamyrin patented an ion mirror device for energy focusing and resolution improvement • A reflectron is a series of electrodes that create an electric field to reverse the direction the ions travel • Reflectron serves two main purposes 1) Ions can make two passes down the flight tube • Get resolution of a 2 m flight tube for a 1 m length of pipe 2) Arrival time distribution due to kinetic energy spread of the ions is reduced

  10. ***Red and blue ions have same m/z 1-Stage Reflectron Diagram 0 V +110 V 0 V +100 V Detector Ions with same m/z but slightly different KE’s can be made to arrive at a detector simultaneously. Higher energy ions of same m/z go deeper into reflectron than lower energy ions of same m/z. Thus higher energy ions will take a little longer to exit reflectron than lower energy ions This focuses the energy spread of a population of ions

  11. TOF Animation 337 nm Nitrogen laser Target +20 kV Reflectron +22 kV Lens Extraction Plate +15 kV Detector Flight Tube Entrance 0 V

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