A novel approach to converting alkylated arsenic to arsenic acid for accurate ICP-OES determination of total arsenic in candidate speciation standards

Abstract

A new approach was developed for quantitatively converting alkylated arsenic to arsenic acid (AsV). Conversion of the organic As species to AsV was required to prevent bias in the ICP-OES measurements because the species in the calibration standard was AsV. The approach was used in the development of a new series of organic As reference materials, SRMs 3030, 3031, 3033 and 3034, for monomethylarsonic acid, dimethylarsinic acid, arsenobetaine and arsenocholine, respectively, at the National Institute of Standards and Technology. Trimethylarsine oxide (TMAO) was used as a model alkylated arsenic compound to assess the effectiveness of the conversion due to its structural stability and difficulty to convert to AsV. Hydrochloric acid (HCl) was the only reagent capable of quantitative conversion of the alkylated arsenic compound to AsV using microwave assistance. The minimum concentration of HCl required for quantitative conversion under the experimental conditions was 4.4 mol L−1, which was 3 mL of concentrated HCl in 8 mL of solution. This is the first report of HCl affecting the quantitative conversion of alkylated arsenic to AsV. After converting to AsV, the mass fraction of arsenic in the alkylated arsenate compounds was quantified using ICP-OES with an expanded uncertainty ≤ 2.4 %. These ICP-OES results were in good agreement with INAA, a technique that is not susceptible to interferences from elemental speciation.