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

Standard preparation— Dissolve an accurately weighed quantity of USP Lindane RSin methylene chloride to obtain a solution having a known concentration of about 2mg per mL.Transfer 5.0mLof this solution to a graduated centrifuge tube,add 5.0mLof Internal standard solution,mix,and evaporate with the aid of gentle heat and a current of dry air to 3mL.[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 300for 2hours.

Mobile phase— Mix 18mLof anhydrous ethyl ether with 280mLof 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 50mLof Mobile phaseand 10g of Solid support,and stir the mixture to expel air bubbles.Add 1.5g of anhydrous sodium sulfate to the column,and elute until the surface of the liquid is about 4cm above the Solid support,discarding the eluate.Transfer an accurately weighed portion of Cream,corresponding to about 10mg of lindane,to a 150-mLbeaker,and add 10g 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-mLportions of Mobile phase,and elute the column with 225mLof the Mobile phaseat a flow rate of 2mLto 3mLper minute,collecting the eluate in a 250-mLbeaker.Remove the chromatographic column,add 5.0mLof Internal standard solutionto the eluate,and evaporate with the aid of gentle heat and a current of dry air to about 5mL.Transfer this solution to a graduated centrifuge tube with the aid of 1mLof methylene chloride,and evaporate with the aid of gentle heat and a current of dry air to about 3mL.[See Noteunder Standard preparation.]

Chromatographic system(see Chromatography á621ñ)— 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 G3on 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 40mLper 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 1µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the percentage of C6H6Cl6in the Cream taken by the formula: in which Cis the concentration,in mg per mL,of USP Lindane RSin the solution prepared,prior to the addition of Internal standard solution,for the Standard preparation;Wis the weight,in mg,of Cream taken;and RUand RSare the ratios of the peak responses of lindane to those of n-docosane from the Assay preparationand the Standard preparation,respectively.