SPECIES-SELECTIVITY EFFECTS IN THE PRODUCTION OF ELECTROSPRAY IONS

Positive- and negative-ion production from electrosprayed charged liqu id droplets has been studied. For acidified solutions of arginine, Arg -Arg-Arg, Lys-Lys, and (CH3)4NBr in methanol + water (80/20, v/v), the relative intensities of CH3OH2+ ions were found to decrease faster th an those of other sample ions as the distance (d) between the capillar y tip and the interface plate was increased. The radial distribution o f ion intensities in the spray cone was also examined. While the CH3OH 2+ ions and multiply charged ions had maximum intensities in the centr al region of the spray cone, NH4+ and singly charged oligopeptide ions were found to be distributed in its peripheral region. The observed s pecies-selectivity effects in the production of electrospray ions were interpreted in terms of the volatility of ions in the charged liquid droplets. The negative-ion intensities of the oligopeptide Glu-Glu wer e enhanced greatly when ammonia was added to the solution. An optimum ammonia concentration which gives the strongest negative-ion signals w as approximately 10(-4) M.