Dissociation characteristics of alpha,omega-dihydride poly(dimethylsiloxane) ammonium adducts generated by electrospray ionization
Résumé
Dissociation behavior of ammonium adducts of alpha,omega-dihydride poly(dimethylsiloxane) (H-PDMS) produced by electrospray ionization was studied in collisional activation conditions. In contrast to MS/MS spectra of synthetic polymers usually displaying fragment series on the whole m/z range, ammonium adducts of H-PDMS were shown to always generate the same product ions regardless of the size of the precursor ion. Based on accurate mass measurement and ab initio calculation, mechanisms could be proposed to account for this MS/MS pattern. A primary reaction consisting of release of ammonia and hydrogen from activated oligomer adducts would be rapidly followed by elimination of a three dimethylsiloxane-containing cyclic species to generate the three main product ions at m/z 207, 281 and 355. Further dissociation of these three species would explain additional signals in MS/MS spectra. Although a similar behavior was previously observed for PDMS bearing methyl terminations (CH(3)-PDMS), strong variations in product ion relative abundance were measured as the size of oligomeric precursor increased in the case of H-PDMS. Perfect similarity of MS/MS spectra is shown to occur once hydride end-groups are no longer involved in primary dissociation reactions. Measuring the degree of similarity between MS/MS spectra of consecutive oligomers within PDMS distribution indicates a much higher reactivity of hydride terminations towards the adducted ammonium cation, as compared to methyl end-groups. (C) 2011 Elsevier B.V. All rights reserved.