Fig. 5.

A comparison of high-pH solvent systems used. (A) Base peak chromatograms show that choice of buffer additives, rather than pH alone, generate increased ionization with negative ESI. Piperidine solvents generate nearly an order of magnitude higher precursor ion signal than ammonium formate solvents. (B) Beyond increasing the number of precursors selected for MS/MS, piperidine solvents shift the charge state distributions of precursor anions to be more highly charged (more negative) than ammonium formate solvents. Notably, DMSO in the 5 mm piperidine solvent system expanded the distribution of charge states to more highly charged precursors, rather than collapsing it to lower charges as reported in positive electrospray.