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Selenium sensing sans calibration Everything is good in moderation, the saying goes, and that holds true for selenium. An essential dietary nutrient, selenium is required in trace amounts, ~50 µg/day, mostly for the production of selenoproteins. Ingest too little selenium, and you could end up with a condition such as Keshan disease or Kashin– Beck disease, forms of cardiomyopathy and osteoarthritis, respectively. At high concentrations, >800 µg/day, selenium is toxic. But somewhere in the middle, ~200–300 µg/day, it may reduce the risk of some forms of cancer. Traditional methods for quantifying selenium, such as hydride-generation atomic absorption spectrometry, use bulky instrumentation that requires technical expertise and sensitive signal calibration; this precludes deployment in the field. In the June 15 issue of Analytical Chemistry (pp 4558–4563), David T. Pierce of the University of North Dakota Grand Forks and graduate student Sandra G. Hazelton report a new ultratrace method to quantify selenium in aqueous samples that is relatively inexpensive to implement; is potentially portable; and, most importantly, requires no signal calibration. Hazelton and Pierce’s electrochemical method relies on quantitative deposition and coulometric stripping of Se(IV) onto a novel, gold flow-through electrode. The working electrode is a pair of 1-mm-diam gold wires, one wrapped helically around the length of the other, that is placed within a piece of Nafion tubing. The electrode–tubing assembly is inserted into the counter electrode, a piece of stainless steel tubing, which is then placed in a flow circuit so that injected volumes of the test solution can flow through the electrode. The curved channel created by wrapping the gold electrodes maximizes the capture of selenium, says Pierce. “We force the Se(IV) in solution to have as much contact with the electrode as possible. The gold wire is serpentine in shape, like a caduceus, so that it pro-
vides a very efficient deposition of selenium.” Using spike recovery qualitycontrol methods, the authors determined capture rates of >95%. Once the Se(IV) has been deposited on the electrode by reduction to Se(0), the system is flushed to remove impurities that might interfere with the measurement process. Then, the selenium is reoxidized back to Se(IV) for quantitation via chronopotentiometry. This stripping step relies on Faraday’s law, and that absolute relationship eliminates the need for calibration, Pierce explains. “Because we can count on Faraday’s law, it’s just a matter of relating the amount of electricity to numbers of moles of selenium.” By contrast, with techniques such as hydride-generation atomic absorption spectrometry, absorption measurements must be related back to selenium concentration via a standard curve—a slow and potentially error-prone step, Pierce says. “This is not a new type of analysis, but it is one of the rare ways where you can measure an analyte amount without having to do calibration.” The flow cell was able to detect 8–800 ng of Se(IV) in 0.5–20 mL, or anywhere from the parts-per-million range down to just below the parts-perbillion range; that performance is on a par with that of hydride-generation atomic absorption spectrometry. That level of sensitivity makes the cell appropriate for environmental monitoring, Pierce says, because, even though most river water contains 10 ppm; however, contrary to previous reports, cadmium did not interfere. Dissolved organic material, which is found in river water (a typical test sample), also confounded the device. However, the researchers eliminat-
Outlet
CE WE Potentiostat
RE Inlet
The flow-through cell at the heart of the calibration-free selenium sensor. Critical parts are the helical gold working electrode (WE), the porous stainless steel counter electrode (CE), and the Ag/AgCl quasireference electrode (RE).
ed this problem by treating such samples with alkaline hydrogen peroxide before conducting the analysis. According to Pierce, the flow-cell electrode currently is implemented as a benchtop device for aqueous samples. But Pierce lives and works in America’s agricultural heartland; he says he would like to develop a smaller, portable instrument for solid samples that could be deployed at grain elevators. Farmers could then bin their wheat into lowand high-selenium batches, the more selenium-rich of which could be sold at a higher price because of its potential health benefits. “We are currently working with chemical engineers on that, as well as with the [U.S. Department of Agriculture] and a local coop,” he says. a — Jeffrey M. Perkel J U LY 1 , 2 0 0 7 / A N A LY T I C A L C H E M I S T R Y
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