Evaluation of Flexible Linear Tricationic Salts as Gas-Phase Ion-Pairing Reagents for the Detection of Divalent Anions in Positive Mode ESI-MS

Anion analysis is of great importance to many scientific areas of interest. Problems with negative mode ESI-MS prevent researchers from achieving sensitive detection for anions. Recently, we have shown that cationic reagents can be paired with anions, such that detection can be done in the positive mode, allowing for low limits of detections for anions using ESI-MS. In this analysis, we present the use of 16 newly synthesized flexible linear tricationic ion-paring reagents for the detection of 11 divalent anions. These reagents greatly differ in structure from previously reported trigonal tricationic ion-pairing agents, such that they are far more flexible. Here we present the structural features of these linear trications that make for good ion-pairing agents as well as show the advantage of using these more flexible ion-pairing reagents. In fact, the limit of detection for sulfate using the best linear trication was found to be 25 times lower than when the best rigid trication was used. Also, MS/MS experiments were performed on the trication-dianion complex to significantly reduce the detection limit for many dianions. Limits of detection in this analysis were as low as 50 fg.