MASS DISCRIMINATION ITS THE ANALYSIS OF POLYDISPERSE POLYMERS BY MALDI TIME-OF-FLIGHT MASS-SPECTROMETRY .2. INSTRUMENTAL ISSUES

Mass discrimination is generally observed in the analysis of polydispe rse polymers by matrix-assisted laser desorption/ionization (MALDI) ti me-of-flight (TOF) mass spectrometry. Using multicomponent blends of p olystyrene of narrow polydispersity, instrumental effects on the mass analysis of polydisperse polymers were investigated in a time-lag focu sing linear TOF mass spectrometer. It is shown that the lensing action provided by the source electrodes in TOF gives rise to a mass-depende nt distribution of ions in the plane of the microchannel plate detecto r. Therefore, the size of the detector aperture must be sufficient to ensure acceptance of the full ion packet. This distribution is also af fected by the choice of push-out pulse delay. For the mass range inves tigated, ion detection with the use of a multichannel plate detector r esults from a combination of primary ion to electron conversion, as we ll as secondary ion to electron conversion. Both processes are shown t o give rise to different influences on the measured M-n values of the polystyrene blends. Detector saturation also adds to the observed mass discrimination, due mainly to the saturation caused by matrix species , The issue regarding preservation of the measured polymer distributio n function in data presentation is also discussed.