Internal standard solution— Dissolve n-octadecane in methylene chloride to obtain a solution having a concentration of about 0.5 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Lindane RS in Internal standard solution to obtain a solution having a known concentration of about 2 mg per mL.

System suitability solution— Prepare solutions of -benzene hexachlorides (BHC) at 1000 µg per mL of methanol, -BHC at 1000 µg per mL of acetone, and -BHC at 1000 µg per mL of methanol. Transfer 100 µL each of -BHC, -BHC and -BHC solutions to a 4-mL conical vial, and evaporate under a stream of nitrogen to dryness. Add to the vial a 100-µL aliquot of the Standard preparation. Insert the stopper, and shake vigorously to dissolve the residue.

Assay preparation— Transfer about 10 mg of Lindane, accurately weighed, to a 5-mL volumetric flask. Dissolve in and dilute with methylene chloride to volume.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and a 0.32-mm×30-m fused-silica column coated with a 1-µm phase G46. The chromatograph is programmed as follows. The initial column temperature is maintained at 120 for 1 minute. Then, the temperature is increased at a rate of 20 per minute to 150, and then ramped at a rate of 10 per minute to 280 and maintained at that temperature for 4 minutes. The injection port and detector temperatures are maintained at 300. The injection split ratio is 50:1. Chromatograph the Standard preparation and the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times for n-octadecane and lindane are about 0.85 and 1.0, respectively. [note—Typical retention times for -BHC, -BHC, -BHC, -BHC, and n-octadecane are 15.7, 17.8, 16.5, 18.8, and 13.9 minutes, respectively.] The resolution, R, between n-octadecane and -BHC is not less than 21, between lindane (-BHC) and -BHC is not less than 9, between -BHC and lindane is not less than 14, and between -BHC and -BHC is not less than 8; the tailing factors for n-octadecane and lindane are less than 1.5 and 1.2, respectively; and the relative standard deviation of the ratios of peak area responses of lindane to n-octadecane for replicate injections of Standard preparation is not more than 1.5%.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of -C6H6Cl6 in the portion of Lindane taken by the formula: in which C is the concentration, in mg per mL, of USP Lindane RS in the Standard preparation; and RU and RS are the ratios of the peak responses of lindane to n-octadecane, obtained from the Assay preparation and the Standard preparation, respectively.