Monitoring radicals in water

29 September 2010

A sensitive probe to monitor hydroxyl radicals in water has been developed by a team of Swiss and US scientists.

Hydroxyl radicals are high-energy oxidants that are important in the biology of ageing and radiation damage, as well as in environmental chemistry. In natural water systems, the radicals are produced photochemically from nutrients such as nitrates or nitrites and pollutants such as dissolved organic matter. And accurately measuring hydroxyl radical concentrations can help understand the fate of pollutants.

Current hydroxyl radical probes, such as benzoic acid, often have limited sensitivity, require long irradiation times or high concentrations of the probe, which can affect the sample. Now Kristopher McNeill at the Swiss Federal Institute of Technology (ETH) in Zurich and colleagues at the University of Minnesota in Minneapolis have discovered that terephthalate is a more sensitive probe for hydroxyl radicals in aquatic environments.

Terephthalate picks up an OH radical to form fluorescent hydroxyterephthalate

Terephthalate is used as a probe molecule in biology and radiation research, but McNeill is the first to use it in aquatic chemistry. It reacts with hydroxyl radicals to produce hydroxyterephthalate - a fluorescent molecule that is easily detected. 'The system can detect about 10 times less hydroxyl radical than the comparable method that uses benzoic acid as a probe. This may be important in marine systems or other systems in which hydroxyl radical concentrations are low,' comments Matthew Tarr, an expert in environmental chemistry at the University of New Orleans in the US.

However, McNeill and colleagues did find some limitations in using this probe as hydroxyterephthalate degrades on exposure to UV radiation. 'Consequently, the system cannot readily be used to study systems exposed to natural sunlight or full spectrum simulated sunlight,' adds Tarr.

'One of the unsolved mysteries of aquatic photochemistry is what all of the sources of hydroxyl radicals are and what their relative importances are,' says McNeill. The team now plans to use the probe to study different components of aquatic systems, particularly organic matter, to assess each one's ability to generate hydroxyl radicals.

Rachel Cooper


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