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

Abstract

AbstractMass discrimination effects in matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) were quantitatively investigated using equiweight and equimolar mixtures of uniform polystyrene (PS) oligomers. Uniform PS oligomers were separated by preparative super‐critical fluid chromatography (SFC) from commercial standard PS samples. The separated PS oligomers, with degrees of polymerization n = 2–25, have absolutely no molecular weight distributions. Equiweight and equimolar mixtures of uniform PS oligomers were accurately prepared by weighing by microbalance, and their spectra were recorded using a MALDI‐TOF mass spectrometer. In the lower molecular weight region (less than about 103) the oligomers with lower molecular weights give lower mass spectral intensities, with no correlation with laser power. In contrast, higher laser powers yield a decrease of mass spectral intensities in the higher molecular weight region. These results clearly show that mass discrimination effects occur at lower and higher molecular weights depending on the laser power, and provide quantitative information about the discrimination. Using the data on equiweight and equimolar mixtures of PS oligomers, it was possible to calibrate the MALDI‐TOF mass spectral data for an analysis of molecular weight distribution of a standard monodisperse PS sample with number‐averaged molecular weight of 103, and to compare it with the molecular weight distribution measured by analytical SFC. The result from the calibrated MALDI‐TOF mass spectrum, however, does not agree perfectly with that from the SFC results, because undetectable peaks in MALDI‐TOF mass spectra at lower and higher molecular weights could not be included in the calibration of peak intensities. Copyright © 2001 John Wiley & Sons, Ltd.