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n X=1� Xi n i-1 Determine the standard deviation (s) for each analyte as follows: S = (S2)1/2 Determine the MDL for each analyte as follows: MDL = tcn-1 a = .99>(S) where t(n-1 _ is the one-sided t-statistic appropriate for the nu0er' oit samples used to determine (s) , at the 99 percent level . ORGANIC-FREE For volatiles, all references to water in the methods REAGENT WATER: refer to water in which an interferant is not observed at the method detection limit of the compounds of interest. Organic-free reagent water can be generated by passing tap water through a carbon filter bed containing about 1 pound of activated carbon. A water purification system may be used to generate organic-free deionized water. Organic-free reagent water may also be prepared by boiling water for 15 minutes and, subsequently, while maintaining the temperature at 90°C, bubbling a contaminant-free inert gas through the water for 1 hour. For semivolatiles and nonvolatiles, all references to water in the methods refer to water in which an interferant is not observed at the method detection limit of the compounds of interest. Organic-free reagent water can be generated by passing tap water through a carbon filter bed containing about 1 pound of activated carbon. A water purification system may be used to generate organic-free deionized water. PRECISION: The agreement among a set of replicate measurements without assumption of knowledge of the true value. Precision is estimated by means of duplicate/replicate analyses. These samples should contain concentrations of analyte above the MDL, and may involve the use of matrix spikes. The most commonly used estimates of precision are the relative standard deviation (RSD) or the coefficient of variation (CV) , RSD = CV = 100 S/x, ONE 27 Revision 1 July 1992