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Method 102 Revision 1 <br />the concentration of a secondary working standard. <br />7.3.1.2 Any secondary standards prepared from the reference <br />standard must show the same response factor as the <br />original reference standard. Intercomparisons are <br />made on a monthly basis. <br />7.3.1.3 A working standard, prepared by diluting an NBS <br />0 reference material of about 10 ppm to about 10 ppb, <br />is generally used for daily calibrations. <br />7.3.1.4 The stability of working standards must be such that <br />there is less than a 10% change in thirty days. <br />7.3.1.5 There shall be at least one working standard whose <br />concentration lies within the interval of 5 to 20 ppb. <br />7.3.1.6 A second working standard of a higher concentration <br />shall be prepared for use in two point calibrations. <br />7.3.1.7 A quality assurance audit of the standards is prepared <br />annually. <br />7.3.2 Calibrations are performed on a daily schedule. <br />7.3.2.1 The daily calibration consists of at least two calibra- <br />tion points bracketing the anticipated sample con- <br />centrations. <br />7.3.2.2 The calibration is repeated if either the slope or the <br />response at the limit of detection (LOD) of the fitted <br />line changes by more than 5%. If the calibration fails <br />on both runs, an NBS 0.25 ppm reference standard is <br />used to validate the calibration. <br />7.3.2.3 If the lamp voltage is adjusted, allow time for the <br />lamp to stabilize and repeat the calibration. <br />7.3.2.4 A record is kept of the lamp voltage settings and all <br />preventative maintenance procedures i.e. lamp replace- <br />ments, cleaning of lamp windows. <br />7.3.2.5 Blank samples are run daily between calibrations and <br />sample analyses as necessary. <br />7.3.2.6 A single point span calibration may be substituted for <br />the two point calibration procedure for a maximum of <br />four consecutive days provided the response factor <br />does not change by more than 10% during the time <br />interval.. <br />Linearity is a fac-tol- that is checked periodically. <br />7.3.3.1 A gas chromatographic linearity check is performed <br />annually with standards of at least 4 different con - <br />6 <br />