Evaluation of tetracationic salts as gas-phase ion-pairing agents for the detection of trivalent anions in positive mode electrospray ionization mass spectrometry

In previous studies, new electrospray ionization mass spectrometry (ESI-MS) approaches were developed for the highly sensitive detection of singly and doubly charged anions in positive mode ESI-MS by using specially synthesized dicationic and tricationic ion-pairing agents, respectively. By detecting the positively charged ion complex in the positive mode, limits of detection (LODs) for the anions can be lowered by several magnitudes. In this work, we used eighteen newly synthesized tetracationic ion-pairing agents, constructed with different geometries, linkages and cation moieties, for the detection of eighteen triply charged anions of different structural motifs. The LODs for these anions were from ten to several thousand times lower in the positive selective ion monitoring (SIM) mode than in the negative mode. These tetracationic agents also were shown to be useful for the detection of -1 and -2 anions. In addition, the LODs for -3 anions can be further lowered by monitoring the product fragments of the ion-pairing complexes in the single reaction monitoring (SRM) mode. Copyright (C) 2010 John Wiley & Sons, Ltd.