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-" a 12 / ORGANIC ANALYTICAL METHODS - GC increased. Detection limits to be used for groundwater samples are indicated in Table 1. Where doubt exists over the identification of a peak on the chromatograph, confirmatory techniques such as mass spectroscopy should be used. 8.3 The method detection limit (MDL) is defined as the minimum concen- tration of a substance that can be measured and reported with 99% confidence that the value is above zero. The MDL concentrations listed in Table 1 were obtained using reagent water. Similar results were achieved using represen- tative wastewaters. The MDL actually achieved in a given analysis will vary depending on instrument sensitivity and matrix effects. 8.4 In a single laboratory, using reagent water and wastewaters spiked at or near background levels, the average recoveries presented in Table 2 were obtained. The standard deviation of the measurement in percent recovery is also included in Table 2. 9.0 References 1. Bellar, T.A., and J.J. Lichtenberg. 1974. J. Amer. Water Works Assoc. 66(12):739-744. 2. Bellar, T.A., and J.J. Lichtenberg. 1979. Semi -automated headspace analysis of drinking waters and industrial waters for purgeable vola- tile organic compounds. In: Van Hall (ed.), Measurement of organic pollutants in water and wastewater. ASTM STP 686, pp. 108-129. 3. Dowty, B.J., S.R. Antoine, and J.L. Laseter. 1979. Quantitative and qualitative analysis of purgeable organics by high resolution gas chromatography and flame ionization detection. In: Van Hall (ed.), Measurement of Organic Pollutants in Water and Wastewater. ASTM STP 686, pp. 24-35. 4. Development and application of test procedures for specific organic toxic substances in wastewaters. Category 11 - Purgeables and Category 12 - Acrolein, Acrylonitrile, and Dichlorodifluoromethane. Report for EPA Contract 68-03-2635 (in preparation).