Negative Ion Mode Electrospray Tandem Mass Spectrometry of Hydroxy-Terminated Polydimethylsiloxanes Formed upon in situ Methanolysis.

Ethoxy-, methoxy- and hydroxy-terminated polydimethylsiloxanes (PDMS) are formed as the result of the methanolysis of diethoxy-ended PDMS during its infusion in electrospray ionization. The negative ion mode permits only hydroxy-ended products to be detected, and isomeric interference is avoided in single stage and tandem mass spectrometry. The routes for the fragmentation of (ethyl, hydroxy)-, (methyl, hydroxy)- and (hydro, hydroxy)-ended PDMS upon collision activated dissociation (CAD) were explored in the negative ion mode using either formate or acetate anion adduction. Symmetrical (hydro, hydroxy)-ended PDMS decomposed to product ions carrying one of the hydroxy terminations through the abstraction of an acidic hydrogen and depolymerization (expulsion of cyclic neutral species) regardless of the adducted anion. Asymmetric (ethyl, hydroxy)-ended (resp. (methyl, hydroxy)-ended) PDMS yielded both ethoxy-ended (resp. methoxy-ended) fragment ions through the abstraction of the only acidic hydrogens and linear product ions carrying both terminations still interacted with the anion. The production of information-rich ethoxy-ended (resp. methoxy-ended) fragment ions was limited by formate but favored when acetate (higher proton affinity) was used in a CAD fingerprint complementary to the positive ion mode.