NEW MATRICES AND ACCELERATING VOLTAGE EFFECTS IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION OF SYNTHETIC-POLYMERS

Most matrices used in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) have been selected for their sound performance in the detection of proteins. We evaluated a series of new matrices for the analysis of synthetic polymers and compared their performance to that of conventional ones. Among substituted polycyclic aromatic hydrocarbons 1,4-dihydroxy-2-naphthoic acid, 9-anthracenecarboxylic acid (9-ACA), and its mixtures with 5-methoxysalicylic acid ('super' 9-ACA) offered better signal quality for analysis of synthetic polymers. To test the performance of these new matrices, authentic standards of poly(ethylene glycol), poly(propylene glycol), Jeffamine, polybutadiene, poly(methyl methacrylate), poly(dimethyl siloxane), and some of their mixtures were analyzed. By generating mainly [M + Na](+) ions, 9-ACA showed superior performance to most conventional matrices and provided higher effective mass resolution. The 'super' 9-ACA composite matrix was found to be more suitable for larger polymer molecules (>8000 Da). Broader acceptance of MALDI-MS for synthetic polymer analysis has been delayed by systematic differences observed between MALDI-MS and gel permeation chromatography (GPC) in terms of oligomer size distributions (R. S. Lehrle and D. S. Sarson, Rapid Commun. Mass Spectrom., Vol. 9, p. 91 (1995). We found that part of the discrepancy can be explained by the profound dependence of mass distributions on the accelerating voltage of the Lime-of-flight mass spectrometer. With decreasing accelerating voltage both the ion intensity and the mass resolution decreased, and the high-mass end of the mass spectrum became truncated. For example, when the accelerating voltage was changed from 30 kV to 18 kV the center of the peak area distribution for poly(methylmethacrylate) (PMMA) 6000 shifted to lower values by about 210 Da. At the same time, the skewness of the distribution increased tenfold indicating serious discrimination phenomena. These detrimental effects of lower accelerating voltage were particularly strong for relatively large synthetic polymer molecules.