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6 / ORGANIC ANALYTICAL METHODS - GC and hold until all compounds have eluted. For a higher boiling range of compounds, operate at 50' C isothermal for 2 min, then program at 3''min to 110' C and hold until all compounds have eluted. Column 1 provides outstanding separations for a wide variety of aromatic hydrocarbons. Column 1 should be used as the primary analytical column because of its unique ability to resolve para, meta, and ortho aromatic isomers. Column 2: The carrier gas is helium at a flow rate of 30 ml/min. The temperature program sequence is as fellows: 40' C isothermal for 2 min, then 2'/min to 100' C and hold until all compounds have eluted. Col- umn 2, an extremely high polarity column, has been used for a number of years to resolve aromatic hydrocarbons from alkanes in complex samples. However, since the resolution between some of the aromatics is not as efficient as with Column 1, Column 2 should be used as a confirmatory column. 7.2 Calibration. Assemble necessary gas chromatographic apparatus and establish operating parameters equivalent to those indicated in Section 7.1. 7.2.1 By injecting secondary standards, adjust the sensitivity of the analytical system for each compound being analyzed so as to detect quantities of less than or equal to 1 µg for waste samples. Detection limits to be used for groundwater analysis are given in Table 1. Calibrate the chromatographic system using either the external standard technique (Section 7.2.2) or the internal standard technique (Section 7.2.3). 7.2.2 External standard calibration procedure 7.2.2.1 Prepare calibration standards at a minimum of three concentration levels for each parameter by carefully adding 20.0 µl of one or more secondary dilution standards to 100, 500, or 1,000 ml of reagent water or the matrix under study. A 25-µ1 syringe should be used for this operation. One of the external standards should be at a concentration near, but above, the method detection limit and the other concentrations should correspond to the expected range of concentrations found in real samples or should define the working range of the detector. These aqueous standards must be prepared fresh daily. 7.2.2.2 Analyze each calibration standard according to the procedure being used (direct aqueous injection, headspace, or purge -and -trap) and tabulate peak height or area responses against the concentration in the standard. The results can be used to prepare a calibration curve for each compound. Alternatively, if the ratio of response to concentration (calibration factor) is a constant over the working range (less than 10% relative standard deviation), linearity through the origin can be assumed and the