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L <br />5.2.4 Data Accuracy <br />assessed b analyzing a sample with a Certified <br />Data accuracy �s y y g p <br />Reference Mixture (CRM) and comparing the measured concentration to <br />the known standard concentration. The CRM is obtained by the labora- <br />tory from U.S. National Bureau of Standards. The concentrations of <br />these VOCs in the CRM are identified in Tables 9a and 9b. The mean <br />accuracy of the data ranged from -11 to 2 percent (see Table 9a). A <br />low-level (5 ppb) CRM was also analyzed (see Table 9b). Analytical <br />reports for the certified reference mixtures are included in <br />Appendix I. <br />5.2.5 Air Blanks <br />Additional quality control checks were performed by running the <br />instrument blank or zero air (VOC-free) blank for each day of analy- <br />sis. The zero air blank analyzed on March 4, 1988 contained 0.6 ppb <br />of methylene chloride, a concentration below its laboratory detection <br />limit of 1 ppb. The zero air blank analyzed on March 6, 1988 con- <br />tained 0.6 ppb of methylene chloride and 0.3 ppb of tetrachloro- <br />ethylene. The laboratory detection limit for tetrachloroethylene is <br />0.1 ppb. No VOCs were detected in the other zero air samples. <br />Analytical results for the zero air blanks are presented in Appen- <br />dix I. <br />11 <br />11 <br />r <br />L <br />PJ9 9390304.00D 5-11 <br />1 <br />Rev. 0 09/20/88 <br />