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
Data sampling as fast 8 or 10 GHz has been demonstrated
1-4 GHz is typical in commercial instruments

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), E is 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. 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 * m = 60 eV
Blue ion: KE = 50 eV
Two ions have different energies exiting the source
Problem for drift tube analysis
+100 V
100V  0 V
TOF experiment starts when right plate is pulsed from 100 V to 0 V

8. Simulation of the Flight Times Due 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 m; s for blue ion is 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
Draw effect on peak shape (peak has a tail on it)
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. 1-Stage Reflectron Diagram
***Red and blue ions have same m/z
1-Stage Reflectron Diagram
0 V
0 V
+110 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 Lens
Reflectron +22 kV
Extraction
Plate +15 kV
Detector
Flight
Tube
Entrance
0 V