A Method for Determination of Ion Distribution within Electrosprayed Droplets

A simplified method for determination of ion distribution within electrospray ionization droplets has been developed. The method is based on an electrospray ionization mass spectrometry equilibrium partitioning model recently developed by Enke (Enke, C. G. Anal Chem. 1997, 69, 4885-4893). This "simple" method required only two samples to be analyzed at each solution composition compared to the method previously reported. Furthermore, as the same m/z value is monitored in both experiments, possible effects of the mass-dependent ion transmission in the quadrupole were eliminated. Tetraalkylammonium compounds with variable hydrophobicity were used as model compounds and the effect of methanol-water composition in the electrosprayed solution was studied. It was found, as expected, that the signal optimized at a high content of methanol in the electrosprayed solution. The distribution coefficient for analyte ions between the droplet surface and bulk solution maximize, however, at a lower content of methanol in the electrosprayed solution.