ANALYSIS OF THE ACCURACY OF DETERMINING AVERAGE MOLECULAR-WEIGHTS OF NARROW POLYDISPERSITY POLYMERS BY MATRIX-ASSISTED LASER-DESORPTION IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY

Matrix-assisted laser desorption ionization time-of-flight mass spectr ometry (MALDI-TOFMS) can be used to determine number-and weight-averag e molecular weights of narrow polydispersity polymers. In this work, s everal possible sources of error in determining molecular weights of p olymers with narrow polydispersity by MALDI-TOFMS are rigorously exami ned. These include the change in polymer distribution function, broade ning or narrowing of the overall distribution, and the truncation of s elected oligomer peaks within a distribution (i.e., the oligomer peaks at the high-and low-mass tails expected to be observed are not detect ed). These variations could be brought about by a limited detection se nsitivity, background interference, and/or mass discrimination of olig omer analysis in MALDI-TOFMS. For narrow polydispersity polystyrenes, it is shown that by using an appropriate MALDI matrix and sample prepa ration protocol and a sensitive ion detection instrument, no systemati c errors from these possible variations were detected within the exper imental precision (0.5% relative standard deviation) of the MALDI meth od. It is concluded that MALDI mass spectrometry can provide accurate molecular weight and molecular weight distribution information for nar row polydispersity polymers, at least for polystyrenes examined in thi s work. The implications of this finding for polymer analysis are disc ussed. (C) 1998 American Society for Mass Spectrometry.