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6.4 Matrix Spikes and Matrix Spike Duplicates <br /> An MS is a second, extra aliquot of an environmental sample to which known <br /> concentrations of analytes have been added. The MS is taken through the entire <br /> preparation and analytical procedure and the recovery of the analytes is calculated. <br /> Results are expressed as percent recovery. The MS is used to evaluate the effect of the <br /> sample matrix on the accuracy of the analysis. <br /> An MSD is a third, extra aliquot of an environmental sample that is also spiked with the <br /> same, known concentrations of analytes used for the MS. The two spiked aliquots are <br /> processed separately, and the results are compared to determine the effects of the matrix <br /> on the precision and accuracy of the analysis. Results are expressed as RPD and percent <br /> recovery. <br /> 6.5 Laboratory Quality Control Samples <br /> Laboratory QC samples will be introduced into the measurement process to evaluate <br /> laboratory performance and sample measurement for all matrices, as defined by the <br /> governing analytical methods. Control samples will be prepared from environmental <br /> samples or be generated from standard materials in the laboratory. Laboratory QC <br /> samples include method blanks, LCSs, and laboratory duplicates, as appropriate for the <br /> method. <br /> Appendix C 15 <br />