Desorption behavior and distributions of fluorinated polymers in MALDI and electrospray ionization mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) have been used to desorb fluorinated polymers for mass spectrometric analyses. Compounds with high fluorine content are insoluble or only sparingly soluble in conventional solvent systems, making them difficult candidates for mass spectrometry studies. Among 11 tested MALDI matrixes, 2,5-dihydroxybenzoic acid produced the highest ion yield for a model fluorinated analyte at a matrix/analyte molar ratio of 1000. Use of this matrix allowed determination of repeat units and the polymer distribution parameters: M-n, M-w, and D. In MALDI, solubility requirements for a fluorinated polymer may be incompatible with those of the matrix, potentially leading to inhomogeneous crystallization which may influence the observed distribution of oligomers. In ESI, when precipitation was not an overriding factor, a minimal aqueous content in the employed solvent vastly improved fluorinated polymer signal intensities compared to purely organic solvent systems. Dilution with small amounts of higher polarity solvents could promote the desorption of longer chain fluorocarbons, presumably due to augmented solvophobicity. However, a very high aqueous content (high polarity) may disfavor the desorption of longer chain fluorocarbons, especially at higher polymer concentrations. This latter observation has been attributed to preferential intermolecular aggregation of longer fluorocarbon chains. The presence of fluorinated groups offers the advantage of inductive stabilization of anionic charge sites for improved signals in negative ion MALDI and ESI, while low molecular weight halogenated solvents used for dissolution of fluorinated polymers can suppress the tendency toward discharge in negative ion ESI.