Time-Lag Focusing MALDI Time-of-Flight Mass Spectrometry for Polymer Characterization: Oligomer Resolution, Mass Accuracy, and Average Weight Information

Abstract

We report a polymer characterization study by matrix-assisted laser desorption/ionization (MALDI) on a linear time-of-flight instrument equipped with pulsed ion extraction for time-lag focusing. It is demonstrated that time-lag focusing MALDI provides improved mass resolution and mass accuracy over continuous extraction instruments. Oligomer resolution is extended to a much higher mass range than that observed even by continuous extraction reflectron systems. This allows new opportunities to study the chemical composition and determine the molecular weights of individual components in a mixture of higher molecular weight polymers. 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 measurement accuracy of 80 ppm or better is demonstrated, and the relevance to end-group analysis is shown for two derivatives of poly(ethylene glycol) used as slow-release drugs. The analysis of the molecular weight distribution was investigated at several extraction pulse potentials to determine if there was an effect on the relative peak area. We found that the values of the number-average molecular weight (M_n) and the weight-average molecular weight (M_w) do not change significantly for a poly(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 deviation observed for repeat analyses at the same extraction pulse.