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<br />K - known value of the spike in the sample. <br />8.3 All qualfty control measures described in the ap~r~priate analytic,l <br />methods shall be followed. <br />8.4 The use of internal calt'~ation Guanti[atian methcds shall be <br />employed for a metallic contaminant if: (5) Recovary of the contaminant from the <br />1312 extract is not at least SC;. and the concentration does not exceed the <br />appropriate regulatory level, and (2) The concentration cf the contaminant <br />measured in the extract is within 20~ of the appropriate regulatory level. <br />8.4.1. The method of standard additions shall be employed as the <br />internal calibration quan*_itation method far each metallic contaminant. <br />8.4.2 The method of standard additions requires preparing <br />calibration standards in the sample matrix rather than reagent water or <br />blank solution. It requires taking four identical aliquots of the <br />solution and adding known amounts of standard to three of these aliquots. <br />The forth aliquot is the unknown. Preferably, the first addition should <br />be prepared sa that the resulting concentration is approximately 50:C of <br />the expected concentration of the sample. The second and third additions <br />should be prepared so that the concentrations are approximately 100 and <br />150y of the expected concentration of the sample. All four aliquots are <br />maintained at the same final volume by adding reagent water or a blank <br />solution, and may need dflution adjustment to maintain the signals in the <br />linear range of the instrument technique. All four aliquots are analyzed. <br />8.4.3 • Prepare a plot, ar subject da*_a to linear regression, o= <br />instrument signals or external-calibration-derived concentrations as the <br />dependant variable (y-axis) versus concentrations of the additions of_ .-. <br />standards as the independent variable {x-axis). Solve for the intercept <br />of the abscissa (the independent variable, x-axis) which is the concentra- <br />tion in the unknown. <br />8.4.4 Alternately, subtract the instrumental signal or external- <br />calibratien-derived concentration of the unknown (unspiked) sample from <br />the instrumental signals or external-calibration-derived concentrations of <br />the standard additions. Plat or subject to linear regression of the <br />corrected instrument signals or external-calibration-derived concentra- <br />tions as the dependant variable versus the independent variable. Derive <br />concentrations for the unknowns using the internal calibration curve as if <br />it were an external calibration curve. <br />8.5 Samples must undergo 1312 extraction within the following time •_ <br />periods: <br />]312 - 19 Revision 0 <br />November ]992 <br />