The cationization of synthetic polymers in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Investigations of the salt-to-analyte ratio.

Abstract

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique that when used to analyze synthetic polymer analytes often requires the addition of a metal cationization agent (herein termed the "salt"). The choice of both the matrix and the cationization agent needs to be taken into account when considering the polymer under study; different polymers have shown different affinities toward different cationization agents, and their selectivity can change as the matrix changes. Salt-to-analyte ratio (S/A) plots are used in this work to investigate the effect of the quantity of cationization agent employed in the analysis of a poly (methylmethacrylate) (PMMA) analyte with different MALDI matrices. The point at which analyte signal stops increasing with the added cationization agent is termed the "cation saturation point," and it was found to occur around a S/A of 1. When the analyte signal after this point remains constant, it is termed an "ideal case." The "non-ideal case" occurs when the analyte signal decreases after the cation saturation point. The amount of matrix present (measured as the matrix-to-analyte molar ratio, M/A) and the use of different counterions for the salt are also found to affect the intensity of the analyte signal. In non-ideal cases, changes in the counterion or an increase in the M/A are found to increase the analyte signal, often converting an initially observed non-ideal case into an ideal case. Several experiments attempting to uncover the reason for observation of the non-ideal S/A behavior are also described.