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COMPARISON OF A CHEMICAL AND ENZYMATIC EXTRACTION OF ARSENIC FROM RICE AND AN ASSESSMENT OF THE ARSENIC ABSORPTION FROM CONTAMINATED WATER BY COOKED RICE
Citation:
ACKERMAN, A., P. A. CREED, A N. Parks, M. FRICKE, C A. Schwegel, J T. Creed, D. T. Heitkemper, AND N. Vela. COMPARISON OF A CHEMICAL AND ENZYMATIC EXTRACTION OF ARSENIC FROM RICE AND AN ASSESSMENT OF THE ARSENIC ABSORPTION FROM CONTAMINATED WATER BY COOKED RICE. ENVIRONMENTAL SCIENCE AND TECHNOLOGY. John Wiley & Sons, Ltd., Indianapolis, IN, 39(14):5241-5246, (2005).
Impact/Purpose:
The goal is to develop an extraction protocol that mimics the human digestive tract and then to use it to assess the bioavailable fraction of arsenic from complex dietary mixtures such as a daily composite -- to move current methods toward a better human physiologically-based exposure estimate method which approximates the "true" bioavailability of arsenic within an environmental or dietary matrix.
Description:
Rice represents a unique set of arsenic exposure assessment challenges in that it contains relatively high concentrations of arsenic and it absorbs about 100% of its dry weight in water during cooking. The actual arsenic exposure from rice consumption becomes difficult to calculate without knowing the arsenic concentration in rice before cooking, the arsenic concentration of the water, the percentage of arsenic absorbed by rice from water during the cooking process, and the bioavailability of the arsenic in cooked rice. While the first two unknowns are relatively straightforward to determine, the last two are unknown quantities and require further study.
Arsenic extraction efficiencies ranged from 83.7 to 103.5% for cooked rice samples using a trifluoroacectic acid (TFA) extraction procedure. Rice samples cooked in water containing no arsenic and in water containing a known concentration of arsenic were both analyzed. Chromatographic recoveries, measuring the percent of arsenic injected on the column that was determined chromatographically, ranged from 98.7 to 116.2%. Absorption of arsenic by rice from the total volume of water used in cooking was between 88.8 and 104.5% for two different contaminated drinking water samples. A detailed statistical analysis found replicate standard deviations for individual matrices to be between 2.0 and 4.0% for extraction efficiency, chromatographic recovery, and overall recovery. However, matrix standard deviation ranged between 4.6 and 6.2%, indicating that the rice matrix contributed the largest percentage of the variation. A comparison of TFA extraction to an enzymatic extraction produced overall recoveries of 96.6 and 93.3%, respectively. However, the presence of an unidentified arsenic species and an Ar40Cl35 peak in the chromatograms of the enzymatic extracts necessitated a significantly longer analysis time for those samples.
Per capita daily rice consumption rates in various countries range between 25 and 410 g, on an uncooked, dry-weight basis. The levels of inorganic of arsenic that would be found in these amounts of rice after cooking were calculated and compared to the U.S. drinking water maximum contaminant level (MCL) for inorganic arsenic. Finally, the inorganic arsenic exposure from rice was calculated based on mean consumption rates from a number of different countries. These ezposures were compared to the new drinking water MCL of 10 ppb using a 2L/day consumption rate.