Mass spectrometry of nitroxide-terminated poly(4-vinylpyridine): A case of unwanted reactive MALDI

Reactive MALDI was evidenced to occur when using acidic matrices to assist ionization of poly(4-vinylpyridine) (P4VP) prepared by nitroxide mediated polymerization (NMP). The high accessibility of nitrogen atoms in each 4VP units would allow strong hydrogen bonding with acidic matrix molecules, which was shown to further weaken the fragile C-ON linkage between the last monomeric unit and the nitroxide end-group. As a result, release of the nitroxide upon laser irradiation of the MALDI sample would lead to radical chains capable of reacting with the matrix molecule in interaction with the last 4VP unit, yielding a new polymeric by-product where one matrix molecule is incorporated in the chain termination. The mechanism proposed for this reaction was supported by accurate mass measurements, MS/MS data as well as electron paramagnetic resonance (EPR) experiments. Interestingly, such matrix/polymer covalent adducts were no longer observed when using MALDI source operated at high vacuum. (C) 2016 Elsevier B.V. All rights reserved.