A highly efficient ion-pair reversed-phase chromatography for arsenic species characterization and determination with combined ion trap MS and ICP-MS detection

ALTAMIRANO, JORGELINA C.; WUILLOUD, RODOLFO G.; HEITKEMPER, DOUGLAS

Nashville, Tennesse

Conferencia; 52nd ASMS (AMERICAN SOCIETY OF MASS SPECTROMETRY) CONFERENCE ON MASS SPECTROMETRY AND ALLIED TOPICS; 2004

AMERICAN SOCIETY OF MASS SPECTROMETRY

Arsenic is a widely distributed element in environmental systems and occurs in a variety of chemical forms which differ in toxicity, bioavailability and biotransformation. The analytical challenge of As speciation is based on the nature of As species differing in oxidation state, charge, molecule size and functional organic groups. Consequently, many chromatographic principles have been applied such as, anion exchange, cation exchange, reversed-phase and size-exclusion. However, incomplete resolution of several As species has been observed and the samples could only be totally characterized by running combinations of these mechanisms. In this work, an ion-pair  reversed-phase chromatographic method suitable for on-line coupling to ion trap MS and ICP-MS detectors is described.A perfluorinated carboxylic acid, tridecafluoroheptanoic acid (TDFHA), was employed as the ion-pairing agent. Chromatographic separation of As species was achieved using a Luna C-18(2) reversed-phase column (3 x 150 mm) with a mobile phase consisting of TDFHA : acetonitrile at 0.3 ml min-1.  An injection volume of 10 ?ÝL was used. The HPLC system was coupled on-line with ICP-MS or ion trap MS for element-specific detection and qualitative analysis, respectively.Several methods involving ion pair-reversed phase HPLC for the analysis of As species have been developed. However, they have generally been coupled to elemental detectors such as ICP-MS, GF-AAS, HG-AAS and HG-AFS. These techniques provide excellent sensitivity and detection limits, simplicity and short analysis times. However, the techniques have a fundamental limitation, originating from the fact that identification of metal compounds is based on matching peak retention times of samples and standards. As a consequence, errors can result from using this approach, especially in cases of co-eluting metal species. Electrospray mass spectrometry (ESI-MS) has successfully been used for structural characterization of numerous compounds of biological and environmental interest. The combination of the information obtained from HPLC-ICP-MS with that collected from an HPLC-ESI-MS constitutes a powerful combination of tools for the speciation of arsenic.  The challenge in this field is to find a volatile ion pairing reagent, compatible with both ICP-MS and ESI-MS that allows the separation of the major arsenical species in a single chromatographic run. With this in mind, TDFHA was chosen as a suitable ion pairing reagent.In preliminary studies, twelve arsenical species were baseline separated using a mobile phase of 4 mM TDFHA : acetonitrile; pH 2.5. Among the studied species were: monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), tetramethylarsonium ion (TMA), arsenobetaine (AsB), arsenochline (AsC), as well as phenyarsonic acids used as animal feed additives.  These include p-arsanilic acid (p-ASA), o-arsanilic aicd (o-ASA), 4-nitrophenylarsonic acid (4-NPAA), 3-nitro-4-hydroxyphenylarsonic acid (3-NHPAA), and p-ureidophenylarsonic acid (p-UPAA). In this work, the effect of different chromatographic variables including, TDFHA concentration, pH, temperature, and organic solvent were examined to obtain complete chromatographic resolution of the As species. Finally, the method was applied for the analysis of As species in certified reference materials of biological origin.