New arsenic compound found in rice: dimethyl-arsonyl-dimethyl-arsinic acid

Rice is known to uptake more arsenic from the soil than other cereals due to the special growing conditions resulting from paddy field cultivation. Especially inorganic arsenic is considered to be a health hazard and therefore rice is routinely monitored throughout the world for compliance with regulatory limits of inorganic arsenic. However, rice grains are known to contain also relatively high arsenic concentration in the form of organic arsenic species, with dimethylarsinic acid (DMA) and methylarsonic acids (MMA) being the main components. Other organic arsenic compounds have been found in rice when grown on contaminated soil or using contaminated manure as fertilizer.

During routine surveys by the US FDA performed between 2013 and 2018, that included arsenic speciation of rice and rice products available on the US market (imports and domestic) an unexpected arsenic compound (uAs) was noted. The unidentified compound was found at relatively high concentration (> 100 µg/kg) in less than 1% of the samples investigated but also at trace levels in a larger number of samples.

A group of researchers from the University of Graz and US FDA now aimed at the identification of the unknown arsenic compound found in some rice samples. As a tool for speciation analysis, the group used HPLC coupled to ICP-MS and simultaneously to ESI-MS/MS. Two rice grain samples were used for the analysis. Both were brown rice samples from Texas known to contain the unknown arsenic compound at high levels.

Three different methods for arsenic extraction were applied. Finely ground rice grain was extracted either by utilizing 0.28 M HNO3 heated at 95°C for 90 min, with 20 mM TFA in a water-bath for 60 min at 95°C, or with 1% /v/v) ammonia solution for 25 h at room temperature under constant shaking. The extracts were centrifuged, and the supernatants stored at 4°C until final analysis.  Using anion-exchange chromatography for separation, the unknown compound eluted before DMA when using a basic pH and after DMA at acidic pH (< 5.5). The retention time of the unknown species could not be assigned to any known compound, indicating that the compound had not yet been identified in any samples investigated for arsenic speciation.

Several experiments were performed in order to gain information about the unknown species (uAs). For these experiments, isolation of the compound by fraction collection was utilized. During these experiments, it became clear that the compound is not completely acid stable, but stable under neutral or slightly alkaline conditions. The compound (uAs) was shown to be unaffected by hydrogen peroxide indicating a pentavalent arsenical, not containing sulphur. High resolution ESI-MS gave a signal at 258.9291 Da indicating an elemental composition of C₄H₁₃As₂O₃⁺.

Comparing the different extraction procedures, the extraction with NH₃-solution increased the overall yield of uAs by more than 30 %. Analysis of the alkaline extract showed some thiolated compounds, including thiolated forms of uAs. Treatment of the extract with H₂O₂ resulted in the complete loss of thiolated compounds and the concomitant increase of uAS and DMA. Thiolation of the As compounds in the extract using H₂S resulted in the near complete loss of uAs and DMA and the concomitant formation of DMMTA, DMDTA and mono-, di- and tri-thiolated forms of uAs.

Figure 1: Structure of dimethylarsonyl- dimethylarsinic acid

To assign the correct structure for the uAS with the elemental composition C₄H₁₂As₂O₃, MS/MS experiments in positive mode were done. The fragments found are consistent with the structure given in figure 1. Other potential structures could be excluded.

Using all information gained, the unknown arsenic species uAs found in rice grain samples was identified as dimethylarsonyl-dimethylarsinic acid. It is present in some rice samples both as oxide and as mono- and di-thiolated compound. The thiolated forms are highly sensitive to the extraction conditions, especially the application of heat and the use of hydrogen peroxide.

The authors concluded that the compound could have been overlooked at low concentrations in other biological materials because of its behaviour during extraction and chromatographic separation. Also, no statement about the compound's toxicity could be given.

The original study

Thermo Scientific - Exactive High-Performance LC/MS

Related studies (newest first)

Britta Planer-Friedrich, Carolin F. Kerl, Andrea E. Colina Blanco, Stephan Clemens, Dimethylated Thioarsenates: A Potentially Dangerous Blind Spot in Current Worldwide Regulatory Limits for Arsenic in Rice, J. Agric. Food Chem., 70/31 (2022) 9610–9618. DOI: 10.1021/acs.jafc.2c02425

A.E. Colina Blanco, C.F. Kerl, B. Planer-Friedrich, Detection of Thioarsenates in Rice Grains and Rice Products. J. Agricult. Food Chem., 69/7 (2021) 2287–2294. DOI: 10.1021/acs.jafc.0c06853

K.M. Kubachka, N.V. Shockey, T.A. Hanley, S.D. Conklin, D.T. Heitkemper, Elemental Analysis Manual: Arsenic Speciation in Rice and Rice Products Using High Performance Liquid Chromatography - Inductively Coupled Plasma - Mass Spectrometric Determination. (2012) EVISA Link Database

T. Arao, A. Kawasaki, K. Baba, S. Matsumoto, Effects of arsenic compound amendment on arsenic speciation in rice grain. Environ. Sci. Technol., 45/4 (2011) 1291–1297. DOI: 10.1021/es1033316

H.R. Hansen, A. Raab, A.H. Price, G. Duan, Y. Zhu, G.J. Norton, J. Feldmann, A.A. Meharg, Identification of tetramethylarsonium in rice grains with elevated arsenic content. J. Environ. Monitor., 13/1 (2011) 32–34. DOI: 10.1039/c0em00460j

D.T. Heitkemper, K.M. Kubachka, P.R. Halpin, M.N. Allen, N.V. Shockey, Survey of total arsenic and arsenic speciation in US-produced rice as a reference point for evaluating change and future trends. Food Addit. Contam. Part B, 2/2 (2009) 112–120. DOI: 10.1080/02652030903148298

C.-W. Liu, C.-C. Lin, C.-S. Jang, G.-R. Sheu, Lo Tsui. Arsenic accumulation by rice grown in soil treated with roxarsone. J. Plant Nutr. Soil Sci., 172/4 (2009) 550–556. DOI: 10.1002/jpln.200800206

K. Baba, T. Arao, Y. Maejima, E. Watanabe, H. Eun, M. Ishizaka, Arsenic speciation in rice and soil containing related compounds of chemical warfare agents. Anal. Chem., 80/15 (2008) 5768–5775. DOI: 10.1021/ac8002984

Related EVISA Resources

Brief summary: LC-ICP-MS - The most often used hyphenated system for speciation analysis

Brief summary: Speciation and Toxicity
Brief summary: Standard methods for arsenic speciation analysis
Link database: Toxicity of organic arsenic species
Link database: Toxicity of inorganic Arsenic
Link database: Human exposure from arsenic in the diet
Link database: Analytical Methods for Arsenic Speciation Analysis
Link Page: All about food science

last time modified: September 18, 2024