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_ maintenance. Daily Calibration Check Standards verify that the instrument is functioning well and is properly calibrated, Specific procedures and acceptance limits for calibration vary from method to method. Standard Operating Procedures for each method are kept in the analysts' Q.C. notebook. Standards for instrument calibration are obtained from a variety of sources. Elemental standards are purchased from commercial suppliers, dated on receipt and replaced according 7 to the methodology. A Standards log is kept, containing date of receipt, supplier, lot #, concentration, any dilutions made, and a unique two digit code number to identify the standard (Figure 12) . The documentation of the use of the standard is accompanied by the two digit code at every entry in the analyst's notebook. organics - Gas Chromatography" and G.C./M.S. EPA method 8000 from SW-846, Revision 1, December 1987, is a general introduction to the quality control requirements for gas chromatography. It is followed by more specific methods developed for specific organic compounds. Sequoia Analytical adheres to the duality control measures set out in EPA method 8000 as well as any additional measures required by specific EPA methods. Standard operating Procedures for all quality control measures are kept in the analysts' notebooks. Appropriate procedures for sample extraction, preparation, and clean-up are referenced in the individual methods and are followed accordingly. The Gas chromatography department calibrates each system with external standards. G.C./M.S. syst,:ms are calibrated with internal standards. Calibration curves are established for each analyte. A minimum of 5 concentrations of each analyte are prepared, one of which must be just above the method detection limit. The remaining concentrations define the linear quantitation range for that analyte. The percent relative standard deviation (%RSD) of the calibration factors (CF) calculated from those five concentrations must be <20% for the calibration curve to be considered linear. The calibration curves, the %R.SD's, and the CF's are documented in the analysts' Q.C. notebook and are verified with a Calibration Check Standard at least once daily. More sensitive detectors require more frequent verification. The response from the Calibration Check Standard must be within 15% of the established calibration curve. In addition the G.C./M.S. systems must be tuned every 12 hours with 4-Bromofluorobenzene (BFB) for volatiles analyses it