Mechanisms of ESI-MS Selectivity and Sensitivity Enhancements When Detecting Anions in the Positive Mode Using Cationic Pairing Agents

Abstract

Recently, we have shown that dilute multivalent cationic reagents can be paired with analyte anions in ESI-MS, thereby allowing them to be detected in the positive mode at very low limits of detection. However, there can be differences in the efficiency of this technique depending on the nature of the cationic pairing agent and the anion being analyzed. In this study, three dicationic ion-pairing agents and four singly charged anionic species were examined in a series of experiments to elucidate the mechanism of action that allows for such sensitive detection and the profound differences in the selectivity of this ion-pairing method. The binding constants for the dication/anion complexes were determined by NMR and ESI-MS. The results indicated that the binding of these species is greatly enhanced as they move from the solution phase to the gas phase. Furthermore, surface tension measurements for the complexes were performed. This test revealed that, as the dication pairs with the anion, it creates a surface-active species within the ESI droplet. This is determined to be one of the major factors that leads to the overall sensitivity enhancement. This has led to a better understanding of how this ion-pairing technique produces unprecedented limits of detection for anions and why there are selectivity differences in pairing agents of different structures.