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 polydisperse polymers by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry. Using multicomponent blends of polystyrene of narrow polydispersity, instrumental effects on the mass analysis of polydisperse polymers were investigated in a time-lag focusing linear TOF mass spectrometer. It is shown that the lensing action provided by the source electrodes in TOF gives rise to a mass-dependent distribution of ions in the plane of the microchannel plate detector. Therefore, the size of the detector aperture must be sufficient to ensure acceptance of the full ion packet. This distribution is also affected by the choice of push-out pulse delay. For the mass range investigated, ion detection with the use of a multichannel plate detector results from a combination of primary ion to electron conversion, as well as secondary ion to electron conversion. Both processes are shown to 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 distribution function in data presentation is also discussed.