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Low Detection Limit Methods for the Determination of VOCs in Air and Water as Applied in the USGS National Water-Quality Assessment (NAWQA) ProgramJames F. Pankow, D. L. Rose, W. Luo, D. A. Bender, L. M. IsabelleDepartment of Environmental Science and Engineering Oregon Graduate Institute P.O. Box 91000, Portland, OR 97291-1000, USA Phone (503) 690-1080, Telecopier (503) 690-1273, pankow@ese.ogi.edu The U.S. Geological Survey's National Water-Quality Assessment (NAWQA)program seeks a comprehensive understanding of the levels of VOCs and other contaminants in the natural waters of the Nation. For VOCs, this requiresan understanding of how these compounds move between the atmosphere and natural waters. This requires the application of appropriately sensitive analytical methods for VOCs in both air and water. Methods specific to the needs of NAWQAhave been developed. For VOCs in air, adsorption/thermal desorption (ATD) with multi-sorbent sampling cartridges was developed. The method allows the determination of 87 analytes, including numerous halogenated alkanes, halogenated alkenes, ethers, alcohols, nitriles, esters, ketones, aromatics, a disulfide, and a furan. The volatilities of the compounds range from that of dichlorofluoromethane (CFC12) to that of 1,2,3-trichlorobenzene. The compounds are determined in air samples that range in volume from 1.5 to 5 liters. Most of the method detection level (MDL) values are in the range 0.02 to 0.06 ppbV. The method has been tested extensively in a long term sampling program that has been conducted in New Jersey from April 1997 to May 1999. Breakthrough, reproducibility, analyte stability, and recovery were all studied, and problems were minimal. A detailed description of the method is available in Analytical Chemistry, vol. 70, pages 5213-5221, Pankow et al. (1998). For VOCs in water, a purge and trap capillary-column gas chromatography/mass spectrometry method was developed. The target VOCs include most of the compounds determined by the air method. Two of the air and water analytes, methyl-tert-butyl ether (MTBE), and tert-amyl methyl ether (TAME), are especially noteworthy because of their use in reformulated gasoline and the current national interest in the fact that their high water solubilities translate into high tendencies to enter the hydrologic cycle. The water method is unique relative to other water methods in that all VOCs positively identified by mass spectrometry are reported, regardless of the concentrations measured. For a 25 mL sample, long-term MDL values determined by conventional means for 85 VOCs were found to range from 0.013 to 2.5 micrograms per liter. Concentration values below the MDL values are noted as being "estimated" to indicate the existence of uncertainty in quantitation at these low levels. A detailed description of the method is available in U.S. Geological Survey Open-File Report 97-829 by Connor et al. (1998). Thousands of water samples, including ground-water, surface-water, and precipitation samples have been analyzed since 1996 as part of the NAWQA program. |
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URL: http://sd.water.usgs.gov/nawqa/pubs/abstracts/pankow/low.det.methods.html
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