The University of Bristol, School of Chemistry

MASS SPECTROMETRY RESOURCE

Time-of-Flight (TOF) Mass Analysis


Theory and History:

    Time-of-flight mass spectrometry (TOF-MS) is probably the simplest method of mass measurement to conceptualise, although there are hidden complexities when it comes to higher resolution applications. The first commercial TOF instrument was marketed by the Bendix corporation in the late 1950's. Their design was based on the Wiley & MacLaren instrument that was published in 1955 [1]. TOF-MS has really come into its own in recent years as being an essential instrument for biological analysis applications - this is especially the case with the coupling of TOF-MS to MALDI and ESI ionisation methods and the development of high-resolution and hybrid instruments (for example Q-TOF and TOF-TOF configurations). The inherent characteristics of TOF-MS are extreme sensitivity (all ions are detected), almost unlimited mass range and speed of analysis (modern instruments can obtain full spectra in seconds). This makes TOF-MS one of the most desirable methods of mass analysis.

Fig. 1: A Schematic of a Time-of-Flight mass spectrometer operating in Reflectron Mode

Time of Flight Schematic figure

    The general set up of TOF is shown in Fig. 1. The ions are introduced either directly from the source of the instrument or from a previous analyser (in the case of Q-TOF) as a pulse. This results in all the ions receiving the same initial kinetic energy. As they then pass along the field free drift zone, they are separated by their masses, lighter ions travel faster. This enables the instrument to record all ions as they arrive at the detector and so accounts for the techniques high sensitivity. The equation governing TOF separation is:

TOF equation
m/z is mass-to-charge ratio of the ion
E is the extraction pulse potential
s is the length of flight tube over which E is applied
d is the length of field free drift zone
t is the measured time-of-flight of the ion

Theoretically then, all the ions are given the same initial kinetic energy by the extraction pulse and then drift along the field free drift zone where they will be separated so that all ions of the same m/z arrive at the detector at the same time. In practice, the pulse is not felt by all ions to the same intensity and so a kinetic energy distribution for each discrete m/z exists. This lowers the resolution by creating a time-of-flight distribution for each m/z [2]. This is relatively easily corrected for by the application of a reflectron at the end of the drift zone [3]. This consists of a series of electric fields which repulse the ions back along the flight tube - usually at a slightly displaced angle (see figure) - resulting in a refocusing of ions with the same m/z value on the reflectron detector.

References:

[1]   W.C. Wiley & I.H. MacLaren; The Review of Scientific Instruments, 26; 1955; p1150.
[2]   R.J. Cotter; Analytical Chemistry, 64; 1992, p1027A.
[3]   Mamyrin et al.; Soviet Physics - JETP, 37; 1973, p45 and International Journal of Mass Spectrometry and Ion Processes, 131; 1994, p1.
[4]   A tutorial review of TOF-MS was published in 1995: M. Guilhaus; Journal of Mass Spectrometry, 30; 1995, p1519.

©2004 Paul Gates, University of Bristol
Last updated January 24th 2004