Comparing the fragmentation chemistry of gas-phase adducts of poly(dimethylsiloxane) oligomers with metal and organic ions

Abstract

Gas-phase ions of poly(dimethylsiloxane) oligomers were formed by electrospray ionization either by protonating them in solution with formic acid or by generating adducts of the oligomers with the metal ions Li+, Na+, K+, and Ag+ as well as with the organic cations NH4+, CH3CH2NH3+, and protonated glycine, aspartic acid, and 1,2-diphenylethylamine. The collision-induced fragmentation of the oligomeric ions was strongly dependent on the nature of the charging species. Ag+ adducts dissociated in a manner previously observed in secondary ion mass spectrometry experiments generating a series of linear and cyclic fragment ions, while Li+ adducts fragmented to form two ions: an adduct of the metal ion with the oligomer end-group and one with the remaining oligomer. Na+ and K+ adducts simply dissociate to form the bare metal ion. The organic species, to varying extents, transfer the proton to the oligomer to form a protonated poly(siloxane) ion. These protonated oligomers then dissociate at very low laboratory-frame collision energy along the siloxane backbone by loss of a silanol. These backbone fragments can then lose a methyl group to form a second series of fragment ions. Suggestions for probable mechanistic pathways for these processes are presented.