CORRECTION OF TIME-OF-FLIGHT SHIFTED POLYMERIC MOLECULAR-WEIGHT DISTRIBUTIONS IN MATRIX-ASSISTED LASER DESORPTION IONIZATION FOURIER-TRANSFORM MASS-SPECTROMETRY/

Molecular weight distributions on an external injection matrix-assiste d 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 summ ed) on the same mit axis to yield the correct molecular weight distrib ution 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 mas s (M-p) similar to 1000 Da, as well as the more difficult to ionize po lystyrene, which was obtained as a chromatographic molecular weight st andard (MW 950), This polystyrene reference material was determined to have a similar to 20% error in M-p.