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Enseco I Sample Description See the attached Sample Description Information. The samples were received under chain-of-custody. II Analysis Request The following analytical tests were requested. Lab ID Analysis Description 044892-1 thru 6 Benzene, Toluene, Ethyl Benzene and Xylenes Total Petroleum Hydrocarbons III Quality Control A. Project Specific QC. No project specific QC (i .e. , spikes and/or duplicates) was requested. B. Method Blank Results. A method blank is a laboratory-generated sample which assesses the degree to which laboratory operations and procedures cause false-positive analytical results for your samples. No target parameters were detected in the method blanks associated with your samples at the reporting limit levels noted on the data sheets in the Analytical Results section. C. Laborator Control Samples. An LCS is a well-characterized matrix blank water, sand or celite) which is spiked with certain target parameters and analyzed at approximately 10% of the sample load in order to establish method-specific control limits. The LCS results associated with your samples are on the attached Laboratory Control Sample Report. Accuracy is measured by Percent Recovery as in: % recovery = (measured concentration) x 100 actual concentration Precision is measured using duplicate tests by Relative Percent Difference (RPD) as in: RPD = (% recovery test 1 - % recovery test 2) x 100 % recovery test 1 + % recovery test 2)/2 Control limits for accuracy (percent recovery) are based on the average, historical percent recovery +/-3 standard deviation units. Control limits for precision (relative percent difference) range from 0 (identical duplicate LCS results) to the average, historical relative percent difference + 3 standard deviation units. These control limits are updated on a quarterly basis. In cases where there is not enough historical data, EPA limits or advisory limits are set, with the approval of the Quality Assurance department.