Mass discrimination in the analysis of polydisperse polymers by MALDI time-of-flight mass spectrometry .1. Sample preparation and desorption/ionization issues

A study of the experimental effects on the matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometric analysis of polydisperse polymers is presented, based on a time-lag focusing TOF configuration. Polydisperse polymers were simulated with the preparation of multicomponent blends of polystyrene of narrow polydispersity. Spectral collection was based on one sample preparation procedure. It is shown that many experimental parameters give rise to mass discrimination in the signal generated from a mass spectral analysis of polydisperse polymers. Several issues related to sample preparation and desorption/ionization are addressed in this work, rand the instrumental considerations such as ion focusing, transmission, and detection are presented in the following article of this issue. Ht is demonstrated that sample preparation and desorption/ionization can introduce serious mass biasing that appears to be due to the characteristics of the (MALDI) process. Experimental procedures are discussed that allow for the determination of this mass discrimination. Sample preparation guidelines are suggested for the minimization of mass discrimination. It is stressed that,with our current understanding of the MALDI process, great care must be exercised in interpreting average molecular weights and molecular weight distribution functions deduced from the direct MALDI analysis of polydisperse polymers.