Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1 - Investigations of alkali metal chloride and sodium salt cluster ions

Salt cluster ions of alkali metal chlorides ACl (A = Li+, Na+, K+, Rb+ and Cs+) and sodium salts NaB (B = I-, HCOO-, CH3COO-, NO2-, and NO3-), formed by electrospray ionization, were studied systematically by mass spectrometr y. The influences on the total positive ion and negative ion currents of va riation of solvent, solution concentration, desolvation temperature, soluti on flow-rate, capillary voltage and cone voltage were investigated. Only co ne voltage was found to influence dramatically the distribution of salt clu ster ions in the mass spectra observed. Under conditions of normal cone vol tage of similar to 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of similar to 10 V, the distribution of cluster ions detected is c haracterized by a relatively low average mass/charge ratio due to the prese nce of multiply charged cluster ions; in addition, there is a marked reduct ion in duster ions having a magic number of molecules. Product ion mass spe ctra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and cl osest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixe d clusters of NaCl with HCl and NaOH, respectively. The effects of ionic ra dius on 20 distributions of cluster ions for 10 salts were investigated; ho wever, the fine structure of these effects is not readily discerned. Copyri ght (C) 2001 John Wiley & Sons, Ltd.