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

Abstract

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.

The evaluation and comparison of trigonal and linear tricationic ion-pairing reagents for the detection of anions in positive mode ESI-MS

“…Additionally, LTC 1 and TTC 1, which are the phosphonium containing reagents, (Figure 1), account for 26 (of 34) of the lowest LODs. The exceptional overall performance of the TPP reagents for this set of divalent anions is in agreement with previous studies [45].…”

“…In the case of these analytes, the halogen is lost from the anion and remains paired with the trication (or a portion of it). This was seen in our previous study on the linear trications [45]. Figure 4a illustrates a proposed fragmentation pattern for dibromomaleate using TTC 1.…”

“…The tricationic reagents used in this study were chosen to represent the best performing trigonal and linear trications used in previous studies [44,45]. These four tricationic reagents offer a variety of functional groups as well as differences in rigidity.…”

“…The second group of tricationic reagents is linear and more flexible [46]. The limit of detection (LOD) for many of the divalent anions tested was lower for the linear trications than the trigonal cations [45]. In the present study, the best two trigonal and two linear tricationic reagents from these previous studies are used to determine detection sensitivity for a wide variety of divalent anions.…”

“…Most recently, tricationic reagents were paired with divalent anions, which again could be detected as a singly charged complex [44,45]. The first group of tricationic reagents used as pairing agents were classified as trigonal trications [44].…”