Publications : 2011

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.

Ammonium adducts of trimethylsilyl-terminated poly(dimethylsiloxane) (CH(3)-PDMS) produced by electrospray ionization were submitted to collision induced dissociation and revealed a particular MS/MS behavior: the same three main product ions at m/z 221, 295, and 369 were always generated in very similar relative abundances regardless of the size of the precursor ion. Combining accurate mass measurements and ab initio calculation allowed very stable cyclic geometries to be obtained for these ionic species. Dissociation mechanisms were proposed to account for the three targeted ions to be readily generated in a two-step or a three-step reaction from any CH(3)-PDMS ammonium adducts. A second set of three product ions was also observed with low abundance at m/z 207, 281, and 355, which were shown in MS(3) experiments to be formed in secondary reactions. An alternative dissociation process was shown to consist of a concerted elimination of ammonia and methane and the need for a methyl of an end-group to be involved in the released methane molecule would account for this reaction to mainly proceed from the smallest precursor ions.