Anion Detection in the Positive Ion Mode Electrospray Ionization Mass Spectrometry by Use of Cationic Ion Pairing Agents

Book excerpt: Anion detection and quantitation has long been of great interest to many scientific areas of research in academic and industrial settings. Their importance extends to areas such as water and environmental analysis, biological system analysis, and also the pharmaceutical and food industries. A variety of sensitive detection methods have been developed for anions. Many of these methods have significant advantages, but also drawbacks. In my dissertation I have described a new method for the detection of anions using electrospray ionization mass spectrometry (ESI-MS) positive ion mode, and cationic ion pairing reagents. Using this method, anions are paired with a plethora of different, di-, tri-, and tetra-cationic ion pairing reagents, thus forming positively charged complexes, which can then be detected in the positive ion mode ESI-MS. The behavior and selectivity of the ion pairing reagents is investigated for the different classes of analytes. During this study, large classes of small organic and inorganic anions were analyzed. In addition, three unique classes of moderate size molecules were investigated. Two of which were biological ones: nucleotides and phospholipids. These are very important molecules, which are responsible for the proper functioning of all biological entities. Nucleotides can exist as monomers or constituents of oligomers, thus they can have more than one negative charge. A total of 28 nucleotide and nucleotide based compounds were paired and analyzed with many different cationic ion pairing reagents. Phospholipids are unique molecules due to their amphipathic character. Phospholipids of different polar groups and different sizes were analyzed in the positive ion mode ESI-MS by using newly synthesized ion pairing reagents. Also, analytical separations of phospholipids were developed using both reverse phase high performance liquid chromatography (HPLC) and hydrophilic interaction liquid chromatography (HILIC) The last class of analytes studied were metal ions. They are already positively charged and therefore they can be detected directly in the positive ion mode ESI-MS, however most of them have a small mass and several oxidation states which usually makes them fall in the low mass to charge region of the mass spectra where the background noise is significant. In this study metals of different oxidation states were complexed with commercially available chelating agents forming negatively charged complexes. Ion pairing agents were added to these compounds forming overall ternary positively charged complexes. Analyses for all types of analytes were performed in both single ion monitoring (SIM) and single reaction monitoring (SRM) modes. Limits of detection (LODs) were easily achieved at the parts per billion (ppb) to parts per trillion (ppt) levels, and significant improvements were noticed in the positive versus negative ion mode ESI-MS. This novel method developed herein shows immense potential for sensitive trace studies of many molecules, high throughput and great simplicity. Lastly a mechanism study by ESI-MS was explored to further understand the low limits of detection achieved by this method.