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4 - <br /> be expressed as the relative percent difference (RPD). A quantitative definition of the <br /> RPU is &M-o b:aow: <br /> RPD = xz - x, x 100 <br /> (X1 f x2)/2 <br /> where: x, = first duplicate point value <br /> xZ second duplicate point value <br /> The sampling frequency for precision measurements will be at a minimum of 1 in 20 or <br /> - I per set. <br /> 4.2.2 ACCURACY <br /> Acc!)racy is an assessment of the closeness of the measured value to the true. value. <br /> Accuracy of chemical tr.st residis is assessea by spiking samples with known standards <br /> and establishing the average recovery. In general, for organics, two types of recoveries <br /> are measured: matrix spike recoveries and surrogate spike recoveries. For a matrix <br /> spike, known amounts of standard compounds identical to the compounds presort in <br /> the sample of interest are added to the sample. For a surrogate spike, the standards <br /> are chemically similar but not identical to the compounds in the fraction being .:;:. . <br /> analyzed. The purpose of the surrogate s►)ike is to provide quality control on every <br /> sample Ly constantly monitoring for unusual matrix effects and gross sample processing <br /> errors. For inorganics, only matrix spikes are measured in general. A quantitative <br /> definition of average recovery is given below. <br /> For spiked samples, the recovery (R) can be defined as a mea:;ure of accuracy: <br /> 1 <br /> R (c� - co)/(cs x vs x 10-3) <br /> where: <br /> CO = measured concentration analyzed in the sample without the addition of a <br /> spike (mg/kg) <br /> cs == concentration of standard added to the sample (mg/kg) <br /> v, = amount of standarc; added to the sample (mg/kg) <br /> c, = measured concentration analyzed in the sample after adding the spike <br /> (mg/kg) <br /> or, on a percentage basis.- <br /> %R <br /> asis:%R = (c, - C.)100/(C.'x v5 x 10.3) <br /> �+ SF031o051W1'ti00i.5! 4-2 <br />