EVALUATION OF MATRIX-ASSISTED LASER-DESORPTION IONIZATION MASS-SPECTROMETRY FOR POLYMER CHARACTERIZATION

A protocol for the preparation of polymeric samples for time-of-flight matrix-assisted laser desorption ionization mass spectrometry (TOF-MA LDI-MS) analysis was developed. Dithranol was identified as a good mat rix for polystyrene (PS), and the addition of silver for cationization of molecules was determined to be necessary. Based on this preparativ e method, low molecular weight samples of other polymers [polyisoprene , polybutadiene, poly(ethylene oxide), poly(methyl methacrylate), and polydimethylsiloxane] were analyzed with molecular weights up to 49 ku . The effects of laser intensity were determined to influence the mole cular weight distribution of intact oligomers, most significantly for low molecular weight polymers. Linear and reflectron modes of analysis were evaluated; better signal intensity and resolution were obtained in the reflectron mode. The TOF-MALDI-MS measurements are compared wit h time-of-flight secondary ion mass spectrometry (TOF-SIMS) and gel pe rmeation chromatography (GPC) for same polymers. The M(n) values calcu lated by TOF-MALDI-MS consistently are higher than values calculated b y TOF-SIMS for all classes of polymers with molecular weights up to 8 ku. The molecular weights of the PS calculated. from TOF-MALDI-MS are in good agreement with GPC (+/-10%). The composition of the terminal g roup on a polymer chain may affect the ion yields. The ion yields of i ntact oligomers were evaluated as a function of end group composition for both TOF-MALDI-MS and TOF-SIMS. The slight disparity of results be tween TOF-SIMS and TOF-MALDI-MS for the perfluoroalkyl-terminated PS s uggests that the oligomers are desorbed preferentially from the surfac e in the TOF-SIMS analysis, rather than having an increased ionization probability.