Formation and Reactivity of Multiply Charged Ions Studied by ESI/MS

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Mechanisms for the formation of positive and negative ions from electrosprayd charged liquid droplets were investigated. With increase of the polarity of the solvents used, the intensities of multiply-charged ions were found to increase. Solvents such as methanol and isopropanol are suitable for the observation of the highly charged ions. The reason why the polar solvents give the stronger intensities of higher charged ions is that the solvation energies of ions become larger with more polar solvents and this makes it easier to separate the positive and negative ions in the electrospray Taylor cone. This results in the formation of more charged liquid droplets and the highly charged ions can be produced by the evaporation of the solvent molecules from the charged droplets. It was also found that the addition of dimethyl sulfoxide increases the evaporation efficiencies of the multiply charged metal ions from the chraged liquid droplets.For the observation of the positive (protonated) and negative (deprotonated) ions, acids and alkalines, respectively, are generally added into the solution in order to promote the protonation and deprotonation from the sample molucules. We found that the intensities of positive and negative ions of amino acids were increased with the addtion of ammonia and acetic acid, respectively. This result cannot be exlained by the conventional idea of the solution chemistry. This new experimental fact can be explicable if one assumes that NH_4^+ and CH_3COO^- act as the protonation and deprotonation reagents, respectively, in the charged liquid droplets.It is known that the analysis of iodine is very complicated and the detection limit is quite low. We found that iodine can be detected as I^- and I_3^- with very high sensitivity by using the electrospray ionization. It is likely that iodine is reduced to I^- in the liquid-metal interface.