Evaluation of mass discrimination effects in the quantitative analysis of polydisperse polymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using uniform oligostyrenes

Mass discrimination effects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were quantitatively investig ated using equiweight and equimolar mixtures of uniform polystyrene (PS) ol igomers. Uniform PS oligomers were separated by preparative super-critical fluid chromatography (SFC) from commercial standard PS samples. The separat ed PS oligomers, with degrees of polymerization n = 2-25, have absolutely n o molecular weight distributions. Equiweight and equimolar mixtures of unif orm PS oligomers were accurately prepared by weighing by microbalance, and their spectra were recorded using a MALDI-TOF mass spectrometer. In the low er molecular weight region (less than about 10(3)) the oligomers with lower molecular weights give lower mass spectral, intensities, with no correlati on with laser power. In contrast higher laser powers yield a decrease of ma ss spectral intensities in the higher molecular weight region. These result s clearly show that mass discrimination effects occur at lower and higher m olecular weights depending on the laser power, and provide quantitative inf ormation about the discrimination. Using the data on equiweight and equimol ar mixtures of PS oligomers, it was possible to calibrate the MALDI-TOF mas s spectral data for an analysis of molecular weight distribution of a stand ard monodisperse PS sample with number-averaged molecular weight of 103, an d to compare it with the molecular weight distribution measured by analytic al SFC. The result from the calibrated MALDI-TOF mass spectrum, however, do es not agree perfectly with that from the SFC results, because undetectable peaks in MALDI-TOF mass spectra at lower and higher molecular weights coul d not be included in the calibration of peak intensities. Copyright (C) 200 1 John Wiley & Sons, Ltd.