SPECIES-SELECTIVITY EFFECTS IN THE PRODUCTION OF ELECTROSPRAY IONS

Positive- and negative-ion production from electrosprayed charged liquid 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 than those of other sample ions as the distance (d) between the capillary tip and the interface plate was increased. The radial distribution of ion intensities in the spray cone was also examined. While the CH3OH2+ ions and multiply charged ions had maximum intensities in the central region of the spray cone, NH4+ and singly charged oligopeptide ions were found to be distributed in its peripheral region. The observed species-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 were enhanced greatly when ammonia was added to the solution. An optimum ammonia concentration which gives the strongest negative-ion signals was approximately 10(-4) M.