The calibration equation is derived from the relationship of kinetic energy to mass and velocity. Rearranging Equation 1 to solve for velocity, v, gives the relationship in Equation 2. Velocity is distance divided by time or d / t. Substituting d / t into Equation 2 and solving for t produces Equation 3.
KE = 0.5mv2 [Eq. 1]
v = [2KE/m]1/2 [Eq. 2]
t = m1/2 * d/[2KE]1/2 [Eq. 3]
Distance and KE are assumed to be constant and can be replaced with A. The relationship between t and m1/2 is linear and an intercept is added to produce a simple equation for a straight line (Equation 4).
t = Am1/2 + B [Eq. 4]
Figure 1. Plot of abundance versus mass-to-charge ratio.
Solving the calibration equation requires two standards. The equation is solved for the two unkowns A and B using the known m/z ratios and their flight times. The current technology permits mass measurement accuracy of 10 parts-per-million (ppm) or better for peptides in the range of 500-5000 m/z [1]. Mass measurement accuracy of 100 ppm or better can be obtained for proteins in the range of 5000-30000 m/z [2].
Time-of-Flight Mass Spectrometry - Instruments and Applications in Biological Research by Robert J. Cotter [3] is an excellent resource for learning the basics of TOF mass spectrometry.
[1] Edmondson, R.D., Russell, D.H., J. Mass Spectrom., 1996, 7, 995-1001.
[2] Watkins, L.K., Bondarenko, P.V., Cockrill, S.L., Song, S., Barbacci, D.C., Russell, D.H., Macfarlane, R.M, J. Chrom. A., 1999.
[3] Cotter, R.J., Time-of-Flight Mass Spectrometry - Instruments and Applications in Biological Research; American Chemical Society: Washington, DC, 1997.