Effects of liquid phase composition on salt cluster formation in positive ion mode electrospray mass spectrometry: implications for clustering mechanism in electrospray.

Potassium bromate salt clusters, [KBrO3]nKx(x+), formed by electrospray ionization were studied as a function of solution properties. Clusters with up to 4 positive charges were observed. Their abundance, charge state and distribution were shown to vary with the organic solvent in solution. The effects of 7 solvents, including methanol, ethanol, isopropanol, acetonitrile, acetone, pyridine, and 1,4-dioxane, were thoroughly investigated. Solvents with a low dielectric constant and a high viscosity seem to favor clustering in solution but do not systematically allow high charge state ion formation. On the other hand, cluster charge reduction during desolvation was not correlated with solvent cation affinity over the range of solvents examined. However, ion distribution in mass spectra could be rationalized as a combination of these two competing phenomena. Charge state increases with the cluster size but may be reduced during ion desolvation when high cation affinity solvent molecules are actually involved in the ion solvation shell. This assumption could be envisaged in either Iribarne or Dole mechanisms of ion release in the gas phase. However, intensity profiles of multiply charged clusters could only be understood in terms of the ion evaporation mechanism.