Arsenic speciation by gradient anion exchange narrow bore ion chromatography and high resolution inductively coupled plasma mass spectrometry detection

Based on gradient anion exchange chromatography (AEC), a new strategy in As-speciation was evaluated. A narrow bore chromatographic system with lower flow rates (≤300 μL) well suitable for the low flow requirements of higher efficiency nebulizers was splitless coupled to a high resolution sector field ICP MS. The AEC system takes full advantage of the detector sensitivity allowing more diluted samples (50–100 times) to be injected, delivering substantially less sample matrix to the column and a lower eluent load to the plasma. The unique plasma compatibility of the NH4NO3-eluent salt used in this study enabled high linear salt ramps in gradient applications, highly reproducible retention times (±1%) and detection limits in the low ng/L range. The separation conditions were applied on two different polymeric anion-exchangers: a low capacity, weakly hydrophobic material (AS11, Dionex) and a more frequently used higher capacity, higher hydrophobic material (AS7, Dionex). On both columns, As-species (As(III/V), MMA, DMA, AsB) and Cl− were separated in less than nine minutes and co-elution was circumvented by adapting the separation pH to the optimal column selectivity. The key-advantage of the NH4NO3-eluent is that it can adopt any separation pH without compromising the eluent strength which is not possible with all other eluents used so far. The influences of chloride and methanol were investigated and found not to affect the chromatographic performance. Column deposits caused strong reversible As(v) adsorption which reduced As(v) to As(III). A corresponding phosphate excess in the injected sample eliminated the adsorption and prevented artefacts in As(v)/As(III) ratios. The method applied to ground water samples provided robust separations and is compatible with any sample preservation procedure.