Studies in MALDI -TOF -MS as a tool for the characterization of synthetic polymers
The method of preparation and methods of analysis of a narrow distribution polystyrene of about 7000 mass units (u) used in an interlaboratory comparison of matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) of synthetic polymers is described. Size exclusion chromatography (SEC) was used to measure the polystyrene sample variability. Fourier transform infrared spectroscopy (FTIR) and MALDI-TOF-MS were used to analyze end groups on the polymer. The polystyrene was analyzed by MALDI-TOF-MS and classical methods of polymer characterization. The number (Mn) and mass (M w) average of the molecular mass distribution (MMD) determined by the classical methods (light scattering and NMR) were compared with those obtained by MALDI-TOF-MS. Agreement between classical methods to obtain the moments of the MMD and the MALDI is found to be good overall. However, all the experimental values obtained by MALDI fell below the classical values. The statistical analysis of the data from the interlaboratory comparison conducted by NIST, which includes data from 23 different laboratories, is discussed. Analysis of variance (ANOVA) is used to examine the influences of the independent parameters (laboratory, matrix, instrument manufacturer, and instrument mode) on the data. The parameters, laboratory and instrument manufacturer, were determined to have an influence on the MMD, where matrix was found not to have a significant influence. The MMD that is obtained in polymer characterization may be biased by preferential desorption/ionization of low mass polymer molecules, preferential ion attachment to larger polymers, or degradation and fragmentation due to the desorption process. In this paper the effect of the laser energy and the effect of the matrix on the MMD of four narrow synthetic polymers of varying thermal stabilities are studied. The four polymers considered were polystyrene (PS), polyethylene glycol (PEG), polymethyl methacrylate (PMMA), and polytetrahydrofuran (PTHF). Statistical analyses of the MMDs revealed that all of the moments of the four polymers were lower when run in 2,5-dihydroxybenzoic acid (DHB) than in all-trans retinoic acid (RA). PS did not appear to fragment as laser power increased, but PEG and PTHF clearly fragmented with increased laser power.