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  • On the trail of arsenic

Photonics Spectra
Jun 2009
Rebecca C. Jernigan,

AMHERST, Mass. – In stories of murder by poisoning, both fictitious and real, arsenic has long been a key player. Just the mention of the word brings to mind dark deeds, secret plans and – the family deck?


CCA-treated wood, a type of pressure-treated lumber processed using chromium, copper and arsenic, could be leaching arsenic into the environment. Commonly available less than a decade ago, the wood is still in use in outdoor structures, including decks and docks. Unfortunately, separating arsenic from the iron oxides to which it binds makes quantitative measurement nearly impossible.

Scientists have been working to develop a more accurate test for arsenic in the environment, hoping to track its movement through soil and water to determine whether naturally occurring bacteria can convert the poison into a less-toxic form or whether more drastic cleanup steps are necessary to prevent contamination of groundwater and other resources.

Researchers at the University of Massachusetts Amherst, led by analytical chemist Julian F. Tyson, have created a technique that measures arsenic compounds in soil using a two-stage extraction procedure. To test the process, they added 20 mg kg-1 of arsenite, arsenate, monomethylarsonate and dimethylarsinate to a silt loam soil, then mixed a sample of the soil with dilute phosphoric acid.

After shaking the sample for 24 hours, they filtered it and injected a small amount into a liquid chromatograph, which was interfaced to a plasma source emission spectrometer with a multimode sample introduction system in hydride generation mode. The system separated out the arsenic compounds and measured each one individually. The entire process then was repeated using a soil sample mixed with a sodium hydroxide solution, and the resulting measurements for each species of arsenic were added together. These final numbers were found to be very accurate, as there were no transformations of the compounds resulting from the process.

The detection limits varied, depending on the species of arsenic, but they ranged from 10 to 25 μg kg-1. When each species was added individually, the scientists recovered 100 percent of the arsenic over a period of several months; when added together, recovery was 89 percent.

The method has not yet been tested in other scenarios, such as soils with a higher organic content, where difficulties may occur. The scientists plan to investigate the applications and limitations of the process in future research.

Although a more accurate measurement of arsenic in soil probably will not change the way we use CCA-treated wood – which has been on its way out for some time – there are other applications for the technology. Tyson said that one use is to measure and study the amount of arsenic being absorbed into rice plants in countries such as Bangladesh, which have high naturally occurring levels of the poisonous substance.

This knowledge may not have an immediate impact on the distribution of rice from these plants, but it could help determine what, if any, risks the contamination poses to humans.

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