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 methodologies. The separation and detection of the most common, yet chemically diverse, arsenic species is one such challenge. In the present work, separation chemistry, system, and method features are discussed that allow the chromatographic retention of both anionic and cationic arsenic species using mobile phases that are compatible 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 environment, in toxic and nontoxic forms. It occurs in inorganic and organic compounds; in trivalent and pentavalent states; and as anions, cations, zwitterions, and neutral species. In general, methylated and other organo-arsenicals are less toxic than inorganic arsenic, and pentavalent arsenic is considerably less toxic than the trivalent 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 consumption. The inorganic forms of arsenic—arsenite and arsenate—are the usual forms found in drinking water, and the U.S. EPA has set a maximum contamination level (MCL) for total arsenic in drinking 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 contamination and biological processes. One important form, arsenobetaine, is considered 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 regulations on the individual arsenic species, only on the total arsenic found.

Table 1 - Chemical structures of five common arsenic species

The five most common arsenic species include arsenite (AsIII), arsenate (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 as As, at m/z 75. Speciation is commonly provided by chromatographic separation using multiple separation columns in order to achieve retention of all five species. The first four species listed above are anions, and the arsenobetaine can be retained as a cation at low pH. Alternately, anion exchange separations are shown in which the arsenobetaine is very close to the column void volume, making it susceptible to matrix interference.4,5 To date, there is no single chromatographic separation that provides sufficient retention and separation of both anionic and cationic arsenic species using ESI-MS-compatible mobile phases.

This article discusses the use of dual-selectivity ion chromatography coupled with electrospray ionization mass spectrometry (IC-MS or IC-MS-MS) to provide retention of all five species and structural information using ESI-MS detection. The use of MS-MS detection is needed for complex matrices, especially in the determination of arsenite. The ICS-3000 ion chromatograph6 (Dionex Corp., Sunnyvale, CA) was used with carefully selected ion chromatography methodologies to solve a tricky combination of separation and detection requirements posed by this analyte mixture.

Table 2 - Separation and detection parameters used in this method for five common arsenic species, sodium, and chloride

An anion exchange separation with electrolytic suppression and a mixed-mode anion/cation exchange separation without suppression were combined in one analytical method to provide retention of a model group of five arsenic species that includes anions and a betaine. Within the analyte set, the MMA, DMA, and AsB are best detected by positive-polarity ESI-MS, and the arsenite and arsenate are best detected using negative-polarity ESI-MS. In addition, because the DMA crosses the membrane in the suppressor, it needs to be separated using anion exchange without suppression. Table 2 summarizes the requirements of this analyte set. The overall method illustrates the ability to separate and detect a suite of analytes with diverse acid/base properties from important matrix ions.

Experimental

Instrumentation

Figure 1 - Schematic of the dual-selectivity IC-MS hardware configuration.

Figure 1 shows the flow diagram of the dual DS-IC-MS or DS-IC-MS-MS system with shared autosampler, chromatography module, and mass spectrometer. The ion chromatograph used in this work was the ICS-3000, which included a dual pump module with two analytical pumps (DP1 and DP2); an eluent generator (EG); conductivity detector (CD); autosampler (AS), including diverter valve; and a chromatography compartment (DC), including two high-pressure valves and two injection valves. The single-quadrupole mass spectrometer was the MSQ™ Plus (Dionex and Thermo Electron, Santa Clara, CA), and the triple-quadrupole mass spectrometer was the API 2000 (Applied Biosystems/MDS SCIEX, Foster City, CA). Chromeleon® 6.8 software (Dionex) was used for all instrument control, data collection, and data reduction with the ICS-3000/MSQ Plus system. DCMS Link software version 1.1 (Dionex) was used for instrument control, data collection, and data reduction with the ICS-3000/API 2000 system.

The anion exchange separation used an IonPac AS18 analytical column (250 × 2 mm i.d., Dionex) with AG18 guard column (50 × 2 mm i.d., Dionex). The mixed-mode separation was accomplished using an IonPac AC15 (50 × 2 mm i.d.) with an IonPac CS5A (250 × 2 mm i.d.) column. The suppressor was the ASRS® MS (Dionex) with external water at 50 mA current. The eluent for the AS18 anion exchange column was a KOH gradient, 6–52 mM in 15 min, 0.3 mL/min. The eluent for the mixed-mode separation was 80 mM formic acid, 0.37 mL/min.

Detection conditions for each analyte were optimized to provide the lowest detection limits within the capability of the single-quadrupole mass spectrometer. These were arsenite, SIM 107, –ESI, 50 V; arsenate, SIM 141, –ESI, 30 V; MMA, SIM 141, +ESI, 60 V; DMA, SIM 139, +ESI, 80 V; and arsenobetaine, SIM 179, +ESI, 70 V. ESI needle voltage was 3 kV. Two injections were made sequentially into two column sets, 13 min apart, and the detection data from the conductivity and ESI-MS detectors were collected in one data file.