Quantitative speciation of arsenic compounds when using organic solvent gradients in HPLC-ICPMS

Abstract

Quantitative analysis with HPLC-ICPMS under gradient elution conditions with increasing content of organic solvents is severely restricted because the elemental response can vary significantly with the amount of carbon reaching the plasma. The present work offers a solution to this challenge to elemental species quantification by introducing a volatile organic solvent directly into the spray chamber whereby the sample introduction system is saturated with organic vapours and no change in sensitivity is observed during a methanol gradient separation. This gradient compensation method was tested with a newly developed chromatographic separation of 12 arsenic species comprising arsenite, arsenate, methylarsonate, dimethylarsinate, four oxo-arsenosugars and four thio-arsenosugars within a run of 25 min using a Hamilton PRP-X100 column applying a methanol gradient up to 50% (v/v). Under these conditions 10 of the 12 species could be separated, whereas arsenite and oxo-arsenosugar glycerol co-elute near the front. Species independent quantification was performed against dimethylarsinate as calibrant and the results were compared with isocratic quantifications. Without compensation arsenic species eluting with more methanol in the organic phase were overestimated by factors of 8 to 10, whereas with gradient compensation the results for all arsenic species were comparable with results obtained from isocratic separations. The method was applied to the determination of arsenosugars in the BCR 710 oyster tissue candidate reference material. Results from these experiments underline the applicability of the proposed gradient compensation method for species independent quantification under reversed phase conditions. The method enables accurate quantification without post column modification.