Inter-laboratory validation of an inexpensive streamlined method to measure inorganic arsenic in rice grain

Authors: CHANEY, RUFUS - FORMER ARS EMPLOYEE; GREEN, CARRIE; Lehotay, Steven

Submitted to: Analytical and Bioanalytical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2018
Publication Date: 5/4/2018
Citation: Chaney, R.L., Green, C.E., Lehotay, S.J. 2018. Inter-laboratory validation of an inexpensive streamlined method to measure inorganic arsenic in rice grain. Analytical and Bioanalytical Chemistry. https://doi.org/10.1007/s00216-018-1075-x.
DOI: https://doi.org/10.1007/s00216-018-1075-x

Interpretive Summary: International regulations pertaining to arsenic in rice have just recently been changed to set maximum limits only of inorganic forms of arsenic at 200 ng/g in white rice and 350 ng/g in brown rice. Thus, analytical methods need only determine the inorganic species, not total arsenic including organoarsenic species, for regulatory, trade, and many research purposes. This change means that previous full speciation methods can be streamlined to lower costs and increase sample throughput, which better meets the increased demands for analysis by rice growers, food industry, and monitoring labs worldwide. In this study, a new streamlined method of analysis for inorganic arsenic was developed and validated in an inter-laboratory study involving 14 labs in 5 countries. The new method was demonstrated in this gold standard form of method validation to achieve acceptable analytical performance by international standards. Labs may use the method to save time, costs, and labor in high throughput monitoring of inorganic arsenic in rice for many purposes.

Technical Abstract: With the establishment by CODEX of a 200 ng/g limit of inorganic arsenic (iAs) in polished rice grain, more analyses of iAs will be necessary to ensure compliance in regulatory and trade applications, to assess quality control in commercial rice production, and to conduct research involving iAs in rice crops. Although analytical methods using high-performance liquid chromatography - inductively-coupled plasma - mass spectrometry (HPLC-ICP/MS) have been demonstrated for full speciation of As, this expensive and time-consuming approach is excessive when regulations are based only on iAs. We report a streamlined sample preparation and analysis of iAs in powdered rice based on heated extraction with 0.28 M HNO3 followed by hydride generation (HG) under control of acidity and other simple conditions. Analysis of iAs is then conducted using flow-injection HG and inexpensive ICP - atomic emission spectroscopy (AES) or other detection means. A key innovation compared with previous methods was to increase the acidity of the reagent solution with 4 M HCl [prior to reduction of As(V) to As(III)], which minimized interferences from dimethylarsinic acid. An inter-laboratory method validation was conducted among 12 laboratories worldwide in the analysis of 6 shared blind duplicates and a NIST Standard Reference Material involving different types of rice and iAs levels. Also, 4 laboratories used the standard HPLC-ICP-MS method to analyze the samples. The results between the methods were not significantly different, and the Horwitz ratio averaged 0.52 for the new method, which meets official method validation criteria. Thus, the simpler, more versatile, and less expensive method may be used by laboratories for several purposes to accurately determine iAs in rice grain.