A general, positive ion mode ESI-MS approach for the analysis of singly charged inorganic and organic anions using a dicationic reagent

Anion analysis continues to be of great importance to many scientific and technical fields. We propose here a general and sensitive method of detecting singly charged anions by ESI-MS and LC-ESI-MS as positive ions. This method utilizes a dicationic reagent to form a complex with the anion that retains an overall positive charge for analysis by MS. Nitrate, thiocyanate, perchlorate, perfluorooctanoic acid (PFOA), halogenated acetic acids, and various other inorganic and organic anions and are investigated. The use of tandem mass spectrometry to enhance the detection limits of some of the anions is demonstrated. Chaotropic anions provided the lowest detection limits, with PFOA detected at the hundreds of femtograms level. Indeed, this single approach provides the lowest reported detection limits for a variety of anions, especially PFOA, nitrate, monochloroacetic acid, dichloroacetic acid, and bromochloroacetic acid, among others. The integrated areas and signal-to-noise ratios for five ions during a chromatographic run in both the positive and negative ion modes are compared. The ability of this method to detect differences in related ions is shown for four arsenic species. Finally, a tap water sample is analyzed for the anions in this study using the dicationic reagent method.