EFFECT OF SOLUTION IONIC-STRENGTH ON ANALYTE CHARGE-STATE DISTRIBUTIONS IN POSITIVE AND NEGATIVE-ION ELECTROSPRAY MASS-SPECTROMETRY

The variability of ESMS charge state distributions was investigated for a number of analytes when the ionic strength of their solutions was altered over a wide range. Solution ionic strength was adjusted by adding varying amounts of electrolytes (NH4OAc, CsCl, or (n-C4H9)(4)N+Cl-) to protein solutions of myoglobin and lysozyme. Measured solution pH values were found to be nearly constant over the entire range of CsCl and (n-C4H9)(4)N+Cl- addition, and a slight increase in pH was observed upon NH4OAc addition. Analyte peak intensities in positive ion ESMS reproducibly decreased as the solution ionic strength increased. However, analyte charge state distributions remained quite constant despite 4 orders of magnitude variation in electrolyte concentration. In negative ion ESMS of phenolphthalein diphosphate (free acid form) and eosin Y, analyte charge state distributions also exhibited only minor variations while signal intensity decreased dramatically with increasing solution ionic strength upon addition of CsCl. These observations in positive and negative ion ESMS provide evidence that ''spectator'' electrolytes present in solution have no significant effect upon the degree of multiple charging. The factors which determine the distribution of multiply charged analyte ions in the gas phase are thus independent of certain others which influence the efficiency of production of gasphase analyte ions. In the context of theoretical descriptions of the electrospray process, the rate constants for desorption of multiply charged ions into the gas phase described in ion evaporation theory are likely to be invariant with spectator electrolyte concentration. Alternatively, the presence of added ion pairs in the ultimate droplet produced according to single ion in a droplet theory does not alter the level of charge attachment/charge separation of analyte molecules.