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reported as individual compounds on chromatograms. The chromato- <br /> graph is operated in backflush mode to prevent contamination of <br /> the analytical column with high concentrations of interfering <br /> compounds . The instrument is calibrated by in3ecting known <br /> amounts of vapor standards. Compound retention time and response <br /> data are stored in the instrument integrator and subsequently <br /> used to make identifications and to quantify unknowns in samples . <br /> Blanks are run to ensure that the system is free of contamina- <br /> tion. As necessary, the instrument is re-calibrated by infecting <br /> standards and by running ambient air blanks approximately every <br /> two hours during sampling This re-calibration ensures that the <br /> system is operating consistently and that parameter changes <br /> caused by temperature changes during the day are accounted for . <br /> These data, along with multiple standard runs ensure system <br /> reproducibility. <br /> 5 <br />