NMR spectroscopy has been used to study the kinetics of atmospheric pollutants in the gas phase for the first time. The method provides an empirical correlation between the atmospheric lifetimes of hydrofluorocarbons and hydrofluoroethers and their relative reaction rates with excess photolysed chlorine at ambient temperatures. The researchers say that the method is simple and uses only the common tools and techniques of an industrial chemical laboratory.

Humans are pumping pollutants into the atmosphere at a rate never before seen. The current focus on global climate change, damage to the ozone layer, and other environmental concerns means it is critical to understand how volatile products behave in the atmosphere. For instance, new refrigerants that replaced the chlorofluorocarbons (CFCs), such as hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs) as well as the byproducts of industrial chemistry are of particular interest.

Now, Alexander Marchione, Paul Fagan, Eric Till, Robert Waterland, and Concetta LaMarca of DuPont Central Research and Development, in Wilmington, Delaware, USA, have turned to NMR to help them reveal the details of the degradation of many atmospheric pollutants in the troposphere, the lowest portion of the Earth's atmosphere.

The tropospheric chemistry of such pollutants hinges on free-radical attack by the hydroxyl (OH) radical. And, according to Marchione and colleagues determining the reaction rates of a particular compound with hydroxyl radicals can allow researchers to estimate the atmospheric lifetime of the pollutant. However, hydroxyls are not the only radicals. Chlorine, or chloro, radicals take part in minor atmospheric degradation pathways, which can also affect overall pollutant lifetimes. As such, it is important to consider chloro radical reactions in any overarching analysis of atmospheric pollutants.

The chloro radical operates through similar radical mechanisms to the hydroxyl and there are various methods for determining its reaction rates. Direct methods, such as laser-induced fluorescence, simple resonance fluorescence with flash photolysis, fast flow discharge, and free-radical titration are commonly used. Indirect methods, use Fourier-transform infrared spectroscopy (FTIR) or gas chromatography (GC) to monitor the rate of product formation.

All these techniques provide valid results but require specialized analytical equipment or reaction apparatus. The researchers explain that with heightened attention and regulatory pressure on the introduction of new volatile commercial compounds, there is a new urgency to finding a quick and inexpensive way to learn of the potential environmental fate and atmospheric lifetime of such compounds. Such studies might reveal possible long-term problems with a persistent species earlier in the cycle of synthesis, production, and commercialization, say the researchers.

One might ask whether the potent greenhouse gas nitrogen trifluoride, used in flat-screen television manufacture, that piqued the media's interest recently, would have been approved for such use if its kinetics had been known more widely in advance.

Marchione and colleagues have now taken an analytical approach that could provide an estimate of atmospheric lifetimes for any new compound. To be viable, such a technique would have to operate without the need for sophisticated analytical instrumentation. They suggest that, ideally, such a technique would rely solely on the equipment and analytical techniques common to an industrial synthetic chemistry laboratory instead.

"The validation of this new method would be achieved by fulfilling two criteria: demonstrating an acceptably accurate empirical correlation between observed reaction rates derived from it and accepted atmospheric lifetimes of known species, and yielding such results on a rapid experimental time scale," the researchers say.

The researchers report details of a rapid analytical method based on chlorine photolysis under ultraviolet light with gas phase ¹⁹F NMR spectroscopy, a highly sensitive NMR technique. By carrying out the chlorine photolysis within a valved NMR tube rather than in a separate apparatus prior to the measurements they avoided the need for a complex preparative method. The degradation chemistry is effected directly in the NMR tube, they say and the high sensitivity of ¹⁹F NMR in the gas phase entails the use of only very small samples.

"The use of NMR spectroscopy enables the facile positive identification of the product species, yielding further kinetic and mechanistic insight," the researchers add.

Related links:

• Anal Chem, 2008, in press
• Dupont R&D

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.