Mass Discrimination Effects in an Ion Detector and Other Causes for Shifts in Polymer Mass Distributions Measured by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

Abstract

Recent attempts to employ matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization have claimed successful matches with more traditional methods such as size exclusion chromatography. In this paper, we describe how the resulting molecular mass distribution is not only extremely sensitive to ionization conditions such as laser power but also sensitive to the mass- and velocity-dependent detector system. For a broad polymer distribution (polydispersity > 2), employing similar ionization conditions and varying only the kinetic energy with which an ion impinges onto a detector surface, the mass corresponding to the most probable peak can be significantly shifted. Differences in shape of the measured distribution and the shift in the most probable peak are explicable by existing theories for secondary electron emission. We propose a criterion for selecting the ion kinetic energy in order to minimize spectrum distortion.