Electrospray Charge-State Determination with Low Resolution Mass Analyzers

Abstract

The addition of metal salts to proteins produces metal adduct ions, as well as multiply protonated species, in electrospray mass spectra. These 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⁺, La³⁺ 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⁺ concentrations produce mass spectra with overlapping charge state series. The adduction of Ag⁺, Cd²⁺ and Zn²⁺ results in smaller separations, with no interference from adjacent charge states. However, higher concentrations of these cations are required to form reasonably intense adduct ion peaks. For La³⁺ and Tb³⁺ cations, significant adduction is only observed at very high metal salt concentrations. The small m/z separations for the zinc adducts cause problems with resolution, resulting in charge state assignment errors. Unlike their singly charged silver and mercury-containing counterparts, lead, cadmium and zinc adduct precursors do not readily form metal adduct fragment ions during collisional activation experiments, making them less suited to accurate charge state assignment for fragments.