ELECTROSPRAY CHARGE-STATE DETERMINATION WITH LOW-RESOLUTION MASS ANALYZERS

The addition of metal salts to proteins produces metal adduct ions, as well as multiply protonated species, in electrospray mass spectra. Th ese adducts can be used for charge state determination of both intact and fragment ions using low resolution mass analyzers. In this study, the effectiveness of MeHg(+), Ag+, PhHg(+), Zn2+, Cd2+, Pb2+, La2+ and Tb3+ as adducts with myoglobin and ubiquitin, and their resistance to loss during collisional activation experiments, are investigated. Due to the large m/z separations, high MeHg(+), PhHg(+) and Pb2+ concentr ations produce mass spectra with overlapping charge state series. The adduction of Ag+, Cd2+ and Zn2+ results in smaller separations, with n o interference from adjacent charge states. However, higher concentrat ions of these cations are required to form reasonably intense adduct i on peaks. For La3+ and Tb3+ cations, significant adduction is only obs erved at very high metal salt concentrations. The small m/z separation s for the zinc adducts cause problems with resolution, resulting in ch arge state assignment errors. Unlike their singly charged silver and m ercury-containing counterparts, lead, cadmium and zinc adduct precurso rs do not readily form metal adduct fragment ions during collisional a ctivation experiments, making them less suited to accurate charge stat e assignment for fragments.