Simpler less expensive method for analysis of inorganic as (iAs) in rice

Authors: Chaney, Rufus; Green, Carrie; Lehotay, Steven; Bukowski, Michael

Submitted to: The Rice Foundation
Publication Type: Other
Publication Acceptance Date: 10/12/2016
Publication Date: 10/12/2016
Citation: Chaney, R.L., Green, C.E., Lehotay, S.J., Bukowski, M.R. 2016. Simpler less expensive method for analysis of inorganic as (iAs) in rice. Final Project Report to USA Rice Foundation. 30 pp.

Interpretive Summary: Inorganic arsenic in rice grain comprises a potential risk to consumers. The U.S. Food and Drug Administration (U.S.-FDA) established a limit on inorganic As in infant cereal rice products, and the International CODEX program has established a limit on inorganic arsenic in all milled rice products. Thus a method to measure inorganic arsenic in rice was needed to verify compliance with these regulations. Unfortunately, the existing methods for analysis of inorganic arsenic in rice used chromatographic separation and then analysis of all chemical species of arsenic in the sample using inductively coupled plasma atomic emission spectrometry, a good but very expensive method (about $200 per sample). Such high cost limits analysis of inorganic arsenic in samples from experiments to breed lower arsenic rice or to test soil amendments to reduce inorganic arsenic in rice. With funding from the U.S.A. Rice Foundation a simple method to measure only inorganic arsenic using selective hydride generation of arsine was developed and tested in an Inter-Laboratory Evaluation. Inorganic arsenic in powdered rice products is extracted in 0.28 molar nitric acid with heating at 95°C for 90 min., a standard method of U.S.-FDA. This new inorganic arsenic analysis method uses hydride generation of arsine only from inorganic arsenic in the sample. The arsenic analysis can be achieved using much less expensive equipment and lower cost staff costing <<$50 per sample. The method was shown to be reliable among 14 labs in 5 countries. Additional testing showed that the extraction of inorganic arsenic from powdered rice could be accomplished in 30 min rather than needing the full 90 min required for the U.S.-FDA method with full speciation. The new method will achieve significant savings for the rice industry and allow researchers to more quickly identify methods to produce rice with lower inorganic arsenic by improved genotypes or production practices.

Technical Abstract: New limits on iAs in rice products require that samples be analyzed for iAs to assure compliance. Initially reported methods used measurement of all species of As present in rice and other foods, which requires very expensive staff and equipment, and a high cost per sample for rice iAs analysis. Industry needed a reliable less expensive method to measure the needed iAs, not full As speciation, in order to comply with market limits. We developed and then conducted an Inter-Laboratory Evaluation of a simple Hydride Generation (HG) method to measure iAs directly rather than as part of As speciation. The U.S.-FDA method to extract iAs from powdered rice [90 min. at 95°C in 0.28 M HNO3] was analyzed by pre-reduction of arsenate to arsenite in a stronger HCl solution so that the HG method was capable of minimizing measurement of dimethylarsinic acid (DMA) present in rice extracts. This method can use much less expensive analytical instruments, and be conducted by normal laboratory staff much more rapidly than the U.S.-FDA full As speciation method. Other laboratories obtained correct iAs results for unknown rice samples (both brown and milled, high medium and low iAs levels) using the HG method. Attempts to use simple shaking of rice powder with several extraction solutions were partially successful in that neutral pH H2O2 solutions could release the rice iAs, but filtering the extracts took much longer (~3 hr) and defeated any benefit compared to HotBlock extractions. Additional testing of the time required for the HotBlock extraction of iAs showed that full extraction with small variance could be achieved in 15-30 min. rather than the 90 min. in the U.S.-FDA method. Although methods for analysis of truckload samples before delivery appears unnecessary because nearly all U.S. rice complies with CODEX iAs limits, it may be possible to dehull, mill, grind, extract and analyze iAs in such samples using the shorter HotBlock extraction and the HG-ICP-AES methods if trucks continue to wait long periods before delivering their loads.