Identification of Monomethylmonothioarsonic Acid as the Major Thioarsenical Generated During Extraction of Arsenic Species from Rice

Arsenic speciation analysis for foodstuffs contaminated with arsenic is a requirement for meaningful risk assessment, since the toxicity of different arsenic species differ by orders of magnitude. The most challenging part of speciation analysis is the sample preparation aiming at the quantitative extraction of the different arsenic species from the food matrix.  One of the problems encountered during extraction is species degradation or transformation.  Acid extraction is commonly used to analyse arsenic species in rice.  During the extraction process, spiked monomethylarsonic acid (MMA) is often transformed into different compounds. A similar phenomenon is observed in the arsenic speciation analysis of seafood.

In order to study the transformation of MMA during extraction from food matrix, a group of Japanese researchers spiked MMA to rice and corn starch. Extraction of arsenic species was performed by using 0.15 mol/L nitric acid at 100°C for 2h. The species generated during extraction was identified by its retention time of liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS). For further determination the sample extract solution was purified and concentrated by liquid chromatography (LC) column fractionation.

The extract fraction containing the generated species was analysed using LC-TOF-MS against a control extract from a sample not containing MMA. A peak obtained in the differential analysis at a retention time identical with the generated product was further evaluated. The mass number of the major component of this peak was 154.9157, indicating a  proton-desorbed substance, CH₅AsO₂S.

Figure: Thioarsenates

The estimated substance monomethylmonothioarsonic acid (MMMTA) was synthesized by bubbling MMA solution with hydrogen sulphide (H₂S). The comparison of this substance with the extracted product gave identical data with respect to precise mass number and isotopic intensity ratio. When the extract solution was dried using a boiling water bath and then re-dissolved in water, CH₅AsO₂S was readily transformed into MMA.

The similar experiments were performed using extract solution spiked with DMA and resulted in the generation of another compound with an RT of approximately 11 min.

The results of this study confirmed that thioarsenicals are occasionally generated during the extraction of arsenic species for subsequent analysis depending on slight differences in heating and extraction conditions as well as the presence of sulphur.

The authors concluded, that it is very difficult to select extraction conditions avoiding species transformation.

The original publication

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