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Environment

New Method Detects Particularly Toxic Arsenic Compound

Water Pollution: Scientists measure chemical at levels one-million times greater than previously detected

by David Pittman
July 12, 2010

Researchers at the University of Alberta in Canada have developed a new analytical technique to detect a harshly toxic arsenic compound that was once difficult to measure (Environ. Sci. Technol., DOI: 10.1021/es100273b). With this new method, scientists may find that the chemical contaminates more groundwater sources than previously thought, the researchers say.

The toxicities of arsenic compounds vary by orders of magnitude, so simply detecting the element isn't enough to understand a contaminated site's impact on human health. Unfortunately, scientists have had trouble detecting one of the most toxic arsenic species, monomethylarsonous acid (MMAIII). This particularly dangerous compound often defies detection by mass spectroscopy, because the acid is too stable and does not ionize under normal conditions.

University of Alberta chemist X. Chris Le, who studies the detection of trace toxic elements in biological and environmental systems, and collaborators found a fix to the problem. To help the molecules ionize, the researchers added dimercaptosuccinic acid to their samples to add negative charge to MMAIII. Le and his colleagues could then use tandem mass spectroscopy to definitively characterize the toxic arsenic compound.

With this new technique, the researchers measured MMAIII levels near a plant in Wisconsin that produced arsenic-containing herbicides from 1957 to 1977. They tested 60 groundwater samples and found that MMAIII concentrations ranged from 3.9 mg/L to 274 mg/L. These concentrations are 1,000-times greater than the Environmental Protection Agency's arsenic drinking water guidelines and about one-million-times greater than the largest-reported MMAIII concentrations (Appl. Organomet. Chem. 1997, 11, 305). 

Le thinks that reducing chemicals from the herbicide plant may have seeped into the soil around the plant and, in an uncontrolled environment, produced this abundance of MMAIII. Similar reducing conditions may exist elsewhere, Le says, so scientists may find high MMAIII levels in places other than near herbicide plants.

This new detection method "opens a new window of environmental arsenic investigation," says Ronald S. Oremland of the U.S. Geological Survey in Menlo Park, California. "While the MMAIII concentrations detected at this particular site are probably at the extremely high end of what is possible, these data make the case for closer scrutiny of methylated arsenic chemicals in the environment."

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