Internal standard solution— Prepare a solution in methylene chloride containing 1 mg of n-docosane in each mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Lindane RS in methylene chloride to obtain a solution having a known concentration of about 2 mg per mL. Transfer 5.0 mL of this solution to a graduated centrifuge tube, add 5.0 mL of Internal standard solution, mix, and evaporate with the aid of gentle heat and a current of dry air to 3 mL. [note—Avoid evaporating to dryness. If the mixture is inadvertently evaporated to dryness, discard it, and begin another Standard preparation.]

Solid support— Use 60- to 100-mesh magnesium silicate that has been previously heated at 300 for 2 hours.

Mobile phase— Mix 18 mL of anhydrous ethyl ether with 280 mL of chromatographic solvent hexane.

Assay preparation— Place a pledget of cotton on a removable porous plate at the base of a 25-mm × 200-mm chromatographic tube that is fitted with a polytef stopcock. Add 50 mL of Mobile phase and 10 g of Solid support, and stir the mixture to expel air bubbles. Add 1.5 g of anhydrous sodium sulfate to the column, and elute until the surface of the liquid is about 4 cm above the Solid support, discarding the eluate. Transfer an accurately weighed portion of Cream, corresponding to about 10 mg of lindane, to a 150-mL beaker, and add 10 g of Solid support. Mix with a spatula, adding chromatographic solvent hexane as necessary to produce a homogeneous mixture, and continue stirring until a free-flowing powder is produced. Transfer this mixture to the chromatographic column with the aid of three 5-mL portions of Mobile phase, and elute the column with 225 mL of the Mobile phase at a flow rate of 2 mL to 3 mL per minute, collecting the eluate in a 250-mL beaker. Remove the chromatographic column, add 5.0 mL of Internal standard solution to the eluate, and evaporate with the aid of gentle heat and a current of dry air to about 5 mL. Transfer this solution to a graduated centrifuge tube with the aid of 1 mL of methylene chloride, and evaporate with the aid of gentle heat and a current of dry air to about 3 mL. [See Note under Standard preparation.]

Chromatographic system (see Chromatography 621)— The gas chromatograph is equipped with a flame-ionization detector and contains a 1.8-m × 2-mm glass column packed with 3% liquid phase G3 on support S1A. Maintain the column temperature at 195, and maintain the injection port and the detector temperatures at 250. Use dry nitrogen as the carrier gas at a flow rate of about 40 mL per minute. Chromatograph six to ten replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 3.0%; the tailing factor is not more than 2.0; and the resolution factor between lindane and n-docosane is not less than 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 percentage of C6H6Cl6 in the Cream taken by the formula: in which C is the concentration, in mg per mL, of USP Lindane RS in the solution prepared, prior to the addition of Internal standard solution, for the Standard preparation; W is the weight, in mg, of Cream taken; and RU and RS are the ratios of the peak responses of lindane to those of n-docosane from the Assay preparation and the Standard preparation, respectively.