A field kit could give rice producers a simple, quick, and cheap way to screen their product for arsenic (Anal. Chem. 2015, DOI: 10.1021/acs.analchem.5b02386). Rice absorbs arsenic from soil and irrigation water, where the element can occur both naturally and in residue from pesticides and fertilizers. Because inorganic arsenic is a carcinogen, the World Health Organization recently set maximum contaminant levels in white rice at 200 μg/kg. In June, the European Union adopted that limit, as well as one of 100 μg/kg for rice used in food for babies and children.
Given these regulations, rice producers would benefit from screening their rice for inorganic arsenic before bringing it to market. But in Southeast Asia, the world’s top rice-exporting region, and other rice-producing areas in the developing world, this isn’t easy. The gold standard of analysis is high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS), which requires a lab with expensive instrumentation, expert operators, and reliable power.
Jörg Feldmann of the University of Aberdeen and his colleagues wanted to find a simple device that would give rice producers a direct answer. So they adapted a commercial field kit called the Arsenator, which is designed to measure inorganic arsenic in drinking water by converting these species into arsine gas and reacting it with mercuric bromide, a color-changing reaction known as the Gutzeit reaction.
To adapt the kit for rice, the team needed to find a way to get the arsenic from the rice into solution in a reliable way. Then they could use the Arsenator kit to produce and quantify arsine gas and determine the levels in the rice.
Their method takes about an hour, and involves four steps. First, the researchers boil a 5-g sample of ground rice in nitric acid for about 30 minutes to extract inorganic arsenic in the form of arsenic acid. Once the solution cools, they transfer it to a flask, after which they can use the Arsenator kit: They add packets of sulfamic acid and sodium borohydride, which react with arsenic acid to produce arsine gas. A piece of filter paper atop the flask impregnated with mercuric bromide catches the gas, which reacts with the mercury compound to form a colorful complex, H2As–HgBr. The darker the paper becomes—ranging from light yellow to dark brown—the greater the concentration of inorganic arsenic in the rice.
To determine concentration, the color readout is compared visually with that of standards, or using a spectrophotometer if available. With a 5-g sample, the method has a quantification limit of 50 μg/kg inorganic arsenic, or half the EU limit for baby-food rice.
The researchers analyzed 30 rice samples with the field kit and with HPLC-ICP-MS. The field kit results were reproducible within 12%, and were within 7% of those from the HPLC. Moreover, the kit produced less than 10% false-positive and false-negative results for assessing the EU limit on baby-food rice, and none for that on white rice. For a field-based screening method, this exceeded the researchers’ expectations, Feldmann says.
Because it involves boiling a strong acid and produces arsine gas, the test must be done in a well ventilated room by trained personnel, and with proper waste disposal.
Brian P. Jackson, an environmental analytical chemist at Dartmouth College, calls the method an “innovative adaptation.” Given the recent regulations on arsenic in rice, he says, “it seems reasonable to suppose that there’ll be a demand for a field test kit that will allow producers and suppliers to quickly know whether the rice is below the regulatory limit.”