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8.2.1 Matrix spikes are to be added after filtration of the TCLP extract and before preservation. Matrix spikes should not be added prior to TCLP extraction of the sample. 8.2.2 In most cases, matrix spikes should be added at a concentration equivalent to the corresponding regulatory level . If the analyte concentration is less than one half the regulatory level , the spike concentration may be as low as one half of the analyte concentra- tion, but may not be not less than five times the method detection limit. In order to avoid differences in matrix effects, the matrix spikes must be added to the same nominal volume of TCLP extract as that which was analyzed for the unspiked sample. 8.2.3 The purpose of the matrix spike is to monitor the performance of the analytical methods used, and to determine whether matrix interferences exist. Use of other internal calibration methods , modification of the analytical methods , or use of alternate analytical methods may be needed to accurately measure the analyte concentration in the TCLP extract when the recovery of the matrix spike is below the expected analytical method performance. 8.2.4 Matrix spike recoveries are calculated by the following formula: %R (%Recovery) = 100 (XS - Xu)/K where: XS = measured value for the spiked sample, Xu = measured value for the unspiked sample, and K = known value of the spike in the sample. 8.3 All quality control measures described in the appropriate analytical methods shall be followed. 8.4 The use of internal calibration quantitation methods shall be employed for a metallic contaminant if: (1) Recovery of the contaminant from the TCLP extract is not at least 50% and the concentration does not exceed the regulatory level , and (2) The concentration of the contaminant measured in the extract is within 20% of the appropriate regulatory level . 8.4. 1. The method of standard additions shall be employed as the internal calibration quantitation method for each metallic contaminant. 8.4.2 The method of standard additions requires preparing calibration standards in the sample matrix rather than reagent water or blank solution. It requires taking four identical aliquots of the solution and adding known amounts of standard to three of these aliquots. The forth aliquot is the unknown. Preferably, the first addition should be prepared so that the resulting concentration is approximately 50% of the expected concentration of the sample. The second and third additions should be prepared so that the concentrations are approximately 100% and CD-ROM 1311- 20 Revision 0 July 1992