Efficient structural characterization of synthetic polymers by activated electron photo-detachment dissociation
Résumé
Tandem mass spectrometry of poly(styrene sulfonate sodium salt) (PSS) and poly(methacrylic acid) (PMAA) was performed after activated electron photo-detachment dissociation (activated EPD). In this technique, doubly charged oligomers were first produced in negative mode electrospray ionization, then oxidized into radical anions upon electron photo-detachment using a 220 nm laser wavelength, and further activated by collision. In contrast to CID of negatively charged PSS oligomers, which does not provide informative data with regards to the end-groups, activated-EPD is shown here to promote radical-induced dissociation reactions thanks to the oxidation of a sulfonate group upon laser irradiation. Major product ions generated after backbone bond cleavages contained one or the other chain terminations and could be accounted for by four main mechanisms, depending on the location of the oxidized monomer near one or the other chain terminal moieties. As a result, combination of these two fragments allowed a straightforward mass characterization of each end-group. In case of PMAA ions, fragmentation of odd-electron species is first shown to proceed via a radical-induced decarboxylation, followed by reactions which involved backbone bond cleavages and gave rise to product ions containing one or the other oligomer termination, contrasting with conventional collision induced dissociation of negatively charged PMAA, which mainly consists of multiple dehydration steps. Activated EPD spectra displayed quite intense product ions throughout the whole m/z range, allowing unambiguous and straightforward determination of each end-group mass. Experiments performed using PMAA sodium salts allowed us to account for relative intensities of peaks measured within product ion series obtained from PMAA, which would reveal that ion stability is ensured by hydrogen bonds within pairs of MAA units.