‘Wrong-way-round’ Electrospray Ionization of Amino Acids

‘Wrong-way-round’ electrospray ionization, in the present context, refers to observation of intense M + H+ ions electrosprayed from strongly basic solutions and of M − H− ions from strongly acidic solutions. Most previous investigations of this phenomenon have been directed at variations in charge-state distributions for polyfunctional peptides and proteins as a function of bulk solution pH. The present work extends that of Hiraoka et al. (J. Mass Spectrom. Soc. Japan 43, 127 (1995)) on the pH dependence of absolute mass spectral intensities of ions in electrospray mass spectra of amino acids. This choice of test analyte permits investigation of both positive- and negative-ion mass spectra without potential complications from changes in secondary and tertiary structures as the pH is varied. The intensities of M + H+ and M − H− ions, over the pH range 3 to 11, varied by factors of 3–5 despite calculated variations of several orders of magnitude in equilibrium concentrations in the bulk solution. The same behaviour was observed for derivatized amino acids such as amides and methyl esters. Measurements of pH of collected spray, and of the total current carried by the charged droplets, confirmed that these observations can not be accounted for in terms of wholesale pH switching from acidic to basic or vice versa by electrochemical reactions at the electrospray needle. Precursors of the ‘wrong-way-round’ ions were sought by conventional precursor-ion scanning experiments but with minimal declustering conditions in the atmospheric-pressure ion-source interface. In the case of added electrolytes, such as ammonia and acetic acid, which are both volatile and capable of Brønsted acid–base behaviour, the observations were consistent with earlier interpretations involving e.g. M + NH4+ precursors for M + H+ ions. Such explanations were not applicable to similar observations made for solutions with added tetramethylammonium hydroxide or hydrochloric acid. © 1997 Crown Copyright, Canada