Proton-transfer reactions of mass-selected multiply charged ions

A triple‐quadrupole spectrometer has been used to study proton‐transfer reactions of multiply charged ions generated by electrospray ionization. Doubly and triply charged ions generated from the peptides Arg‐Lys‐Glu‐Val‐Tyr and Met‐Lys‐bradykinin, respectively, were found to undergo proton‐transfer reactions with ammonia molecules contained in the RF‐only quadrupole collision‐gas cell of the spectrometer. With horse‐heart myoglobin in the source, ions having charges of 20+, 19+, 16+ and 14+ were selected in turn by the first quadrupole and their proton‐transfer reactions with ammonia investigated. For each ion, numerous product ions were detected having charges (n‐1)+, (n‐2)+, (n‐3)+ … where n was the charge on the reacting parent ion. The possibility of using the experimental technique to measure approximately the proton affinities of multiply charged ions is discussed. Also, a procedure is outlined for identifying the charge states of product ions resulting from collision‐induced dissociation of multiply charged ions.