Use of Dual-Selectivity IC-ESI-MS for the Separation and Detection of Anionic and Cationic Arsenic Species

There are times when an analytical challenge demands a solution that reaches beyond standard methodolo-gies. The separation and detection of the most common, yet chemically diverse, arsenic species is one such challenge. In the present work, sepa-ration chemistry, system, and method features are discussed that allow the chromatographic retention of both anionic and cationic arsenic species using mobile phases that are compat-ible with electrospray ionization-mass spectrometry (ESI-MS) detection. The requirement is that all of the species be well retained in the separation in order to minimize void volume interferences and be detected with the structural information, as provided by ESI-MS. Arsenic is ubiquitous in the environ-ment, in toxic and nontoxic forms. It occurs in inorganic and organic com-pounds; in trivalent and pentavalent states; and as anions, cations, zwit-terions, and neutral species. In gen-eral, methylated and other organo-arsenicals are less toxic than inorganic arsenic, and pentavalent arsenic is considerably less toxic than the triva-lent state. The speciation of arsenic is important to assess the risk to human health, and since some forms are not considered toxic, the suitability of certain As-containing foods for human consump-tion. The in organic forms of arsenic—arsenite and arsenate—are the usual forms found in drinking water, and the U.S. EPA has set a maximum con-tamination level (MCL) for total arsenic in drink-ing water, which took effect in January 2006, at 10 µg/L.1 Some foods, such as fish and seaweed, can contain organic forms of arsenic resulting from con-tamination and biological processes. One important form, arsenobetaine, is con-sidered stable, metabolically inert, and relatively nontoxic, but is a common form of arsenic in some foods such as seafood.2,3 Currently, there are no regu-lations on the individual arsenic spe-cies, only on the total arsenic found. The five most common arsenic spe-cies include arsenite (AsIII), arse-nate (AsV), monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV), and arsenobetaine (AsB) (see Table 1). ICP-MS is the most popular detection scheme for arsenic species, but it detects all species a