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7.5.3 A repeated multipoint calibration should not differ from the previous calibration by more than 10%. 7.5.4 Any region of concentration that deviates more than 5% from the least -square line is considered nonlinear. 7.5.5 Data is reported only for compounds whose concentra- tions lie in the linear range. 7.6 Limits of detection are established. 7.6.1 The limit of detection (LOD) is based on three standard deviations (SD) of runs near the LOD (within 10 SD of the LOD, Winefordner and Long, 1983). 7.6.2 The LOD should be determined at least on an annual basis. 7.6.3 If the instrument response changes by more than 15%, the instrument must be checked and the LOD redetermined. 7.6.4 The presence in a sample of a very large adjacent peak will often raise the LOD in the sample. 7.7 Analytical instruments have quality control procedures. 7.7.1 Column conditions are checked periodically and as needed. 7.7.1.1 All GC accessible parameters is logged when a column is first installed. These parameters are checked daily and recorded on integrator reports. 7.7.1.2 The efficiency and resolution of the column are checked every month. If the tests show more than a 10% change, the column is replaced. 7.7.1.3 If the headpressure required to maintain a specified flow through the column increases by more than 100%, the column is replaced. 7.7.1.4 If the drift of retention times of peaks results in peak misidentification, all instrument parameters are checked. 7.7.2 Replicate analyses are a quality control procedure. 7.7.2.1 A duplicate analysis is performed on at least one sample per day. 7.7.2.2 If the duplicate analysis (replicate) differs by more than 20%, and if the concentration of the sample is higher than 3X LOD, then an additional analysis is performed. 8