Molecular weight distributions on an external injection matrix-assisted laser desorption/ionization Fourier transform mass spectrometer are subject to time-of-flight distortions as different ion velocities are probed with varied delay times between ionization and trapping (i.e., the trapping time). This phenomenon is used to advantage to reject low-mass matrix ions which would otherwise saturate the trapped ion cell; however, for accurate determination of molecular weight distributions of complex samples like polymeric systems, several mass spectra must be acquired at a series of different trapping times to compensate for this distortion. The spectra acquired should be superimposed (not summed) on the same m/z axis to yield the correct molecular weight distribution as summation of these spectra merely causes further distortions and can cause loss of signal/noise. Distortions due to TOF effects are probed with a calibration compound, poly(ethylene glycol) of peak mass (Mp) ~ 1000 Da, as well as the more difficult to ionize polystyrene, which was obtained as a chromatographic molecular weight standard (MW 950). This polystyrene reference material was determined to have a ~20% error in Mp.

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Journal Anal. Chem.
O'Connor, P.B, Duursma, M.C, van Rooij, G.J, Heeren, R.M.A, & Boon, J.J. (1997). Correction of time-of-flight shifted polymeric molecular weight distributions in matrix-assisted laser desorption/ionization Fourier transform mass spectrometry. Anal. Chem., 69, 2751–2755. doi:10.1021/ac961040h