Interference from multiple cations in MALDI-MS spectra of copolymers

by Kaufman, J. M.; Jaber, A. J.; Stump, M. J.; Simonsick, W. J.; Wilkins, C. L.

A matrix-assisted laser desorption/ionization Fourier transform mass spectral (MALDI-FTMS) investigation of copolymers of methyl methacrylate (MMA) and butyl methacrylate (BMA) is reported here. A key to the analysis of copolymers by mass spectrometry is the accurate assignment of a unique composition to each ion signal observed in a spectrum. In the present case, the spectra reveal numerous ions with nominal mass separations of 16 Da. This results from the presence of oligomers differing in composition by three extra MMA units and two lesser BMA units. Lithium-sodium and sodium-potassium adduct pairs produce ions that also differ by a nominal mass of 16 Da. Therefore, ambiguous assignments of compositions are possible, especially when the identities of the cations are unknown. When cesium ions are added to the sample, the adducts produced allow high resolution Fourier transform mass spectra to be used for assignment of unambiguous compositions to ions. Spectra with overlap from multiple cations were compared with the cesium adduct spectra using a mathematical analysis of relative ion abundances to determine the fraction of ionization that each cation contributed. When two cations were simultaneously added to a sample, the resulting fraction of ionization by one of the cations was related to the ratio of the two added salts. This relationship is well described by an empirical equation that allows the prediction of the ionization fraction for a given ratio and vice versa.

International Journal of Mass Spectrometry
Start Page
1873-2798; 1387-3806