Structure, anion, and solvent effects on cation response in ESI-MS

The abundance of an ion in an electrospray ionization mass spectrum is dependent on many factors beyond just solution concentration. Even in cases where the ions are permanently charged and do not rely on protonation or other chemical processes to acquire the necessary charge, factors such as cation structure, molecular weight, solvent, and the identity of the anion can perturb results. Screening of a variety of combinations of cation, anion and solvent provided insight into some of the more important factors. Rigid cations and high conductivity anions tended to provide the highest responses, while acetonitrile was the most accurate solvent for reflecting solution composition. Functional groups that had affinity for the solvent tended to depress response. These observations will provide predictive power when accounting for ions that for reasons of high reactivity can not be isolated.