TIME-LAG FOCUSING MALDI TIME-OF-FLIGHT MASS-SPECTROMETRY FOR POLYMER CHARACTERIZATION OLIGOMER RESOLUTION, MASS ACCURACY, AND AVERAGE WEIGHT INFORMATION

We report a polymer characterization study by matrix-assisted laser de sorption/ionization (MALDI) on a linear time-of-flight instrument equi pped with pulsed ion extraction for time-lag focusing. It is demonstra ted that time-lag focusing MALDI provides improved mass resolution and mass accuracy over continuous extraction instruments. Oligomer resolu tion is extended to a much higher mass range than that observed even b y continuous extraction reflectron systems. This allows new opportunit ies to study the chemical composition and determine the molecular weig hts of individual components in a mixture of higher molecular weight p olymers. It is shown that oligomer resolution can be obtained for poly (ethylene glycol) (repeat unit mass of 44) of mass up to 25 000 u and poly(styrene) (repeat unit mass of 104) up to 55 000 u. Mass measureme nt accuracy of 80 ppm or better is demonstrated, and the relevance to end-group analysis is shown for two derivatives of poly(ethylene glyco l) used as slow-release drugs. The analysis of the molecular weight di stribution was investigated at several extraction pulse potentials to determine if there was an effect on the relative peak area. We found t hat the values of the number-average molecular weight (Mn) and the wei ght-average molecular weight (Mw) do not change significantly for a po ly(styrene) blend with oligomer masses between 2000 and 15 000 u and a polydispersity of 1.155. The values are within the 1.6% standard devi ation observed for repeat analyses at the same extraction pulse.