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i� <br /> E. VII . Frequency of AnalUs of Method Blanks ,3 <br /> Sample blanks are analyzed for each method, and the blanks <br /> are analyzed with each batch, or every ten samples, for each method. <br /> The blanks, prepared from clean sample matrix using the same procedure <br /> samples, allow laboratory contamination to be detected. <br /> d for the , <br /> use P <br /> I VIII . Frequency of Analysis of Duplicate Samples t <br /> Duplicate samples are analyzed for each method, and the duplicate <br /> F samples are analyzed with each batch, or every ten samples , for each <br /> method. As these analyses are a measure of `analytical precision, <br /> precision data can also be obtained by analyzing :duplicate spike samples. <br /> This method for monitoring precision is recommended for those samples <br /> which do not usually contain reportable levels ofthe specified analyte. <br /> When metal analysis of drinking water exceeds MCL levels, the analysis <br /> should be repeated and/or checked by an alternative method. For organic <br /> analyses performed by GC, all positive results should be confirmed either <br /> by using a second, dissimilar column or by using GC/MS. When gross alpha <br /> analysis of drinking water exceeds 5 pCi/L the sample is verified by <br /> recounting unless the data comes from an overnight count. <br /> i <br /> IX. Frequency of Analysis of Spiked Samples <br /> Spiked samples are analyzed for each method; and' the spiked <br /> ach <br /> samples are analyzed with each batch, <br /> measu�e r every ofles , for <br /> laboratoryten paccuracy,esamples <br /> method. As these analyses are a ,� <br /> are spiked with all of the analytes to a concentration of five to ten <br /> times the reporting limit. For samples which usually contain a higher <br /> concentration of the analyte, the samples a're spiked with an amount of <br /> analyte similar to that of the sample. <br /> l <br /> 'j. <br /> X. Establishment of Acceptance Limits for Precision and Accurac <br /> Acceptance limits for precision and accuracy are based on analysis of <br /> duplicate samples and of matrix spike samples. In general , the <br /> adceptable control limits are defined as the mean value plus or minus <br /> 20 percent of the expected value. The percent recovery for spiked <br /> samples is calculated as follows: <br /> % R = 100% _ (Theoretical Value'!- Value Found) x 100% <br /> (TheoreticalValue <br /> Thep ercent difference in duplicate sampler is calculated by: <br /> Value(1 - Vaiue(2 ii x 100% <br /> % D = Va ue 1 + Value 2 /2 <br /> and sample standard deviation (when applicable) may be calculated by: <br /> 1/2 ,1 <br /> i = n (Xi <br /> - X)2 <br /> i 1 <br /> n - i <br /> Where: Xi = The ith sample observation <br /> X = The sample average <br /> n = The total numberl;of sample observations <br /> F. . <br /> i f a <br />