FORMATION OF CHARGED CLUSTERS DURING ELECTROSPRAY IONIZATION OF ORGANIC SOLUTE SPECIES

Electrospray dispersion of solutions into a bath gas can produce solute ions with many charges per molecule. In contrast to such multiple charging, it can also result in 'fractional charging' by which cluster ions are formed with more than one solute molecule per charge. Examples of such fractional charging or clustering are presented and discussed. In the case of arginine solutions, which are reported in some detail, ion masses corresponding to clusters containing as many as 24 solute molecules and up to four charges were identified. The number and size of these cluster ions increased markedly with increase in the initial solute concentration. When co-solutes consisting of acids, salts or bases were present, at up to millimolar concentrations in the initial solution, clustering was often strongly suppressed, especially by strong acids and their salts. This clustering phenomenon not only provides insight into the chemistry of solute precipitation in solution, but also offers a means of producing new kinds of clusters for experimental studies.