A: The chromatogram of the Assay preparation,obtained as directed in the Assay,exhibits a major peak for dimethyl sulfoxide,the retention time of which corresponds to that exhibited in the chromatogram of the Standard preparation,obtained as directed in the Assay.

B: To 2mLof Topical Solution in a test tube add 5mLof cold hydriodic acid.Filter the mixture rapidly,and collect the precipitate.Dry the precipitate in vacuum:a deep violet,crystalline solid is obtained,and it is soluble in chloroform,yielding a red solution.

C: To 0.1g of solid sodium hydroxide in a test tube add 10mLof Topical Solution,and heat on a steam bath until the sodium hydroxide has dissolved.Add 10mLof chloroform,and shake.Filter the chloroform layer through a filter funnel containing about 10g of anhydrous sodium sulfate.Heat the filtrate on a steam bath to remove the chloroform.Determine the IRspectrum of the liquid residue as directed under Spectrophotometric Identification Tests á197Fñ.The spectrum so obtained corresponds to that of a spectrum of USP Dimethyl Sulfoxide RS,similarly prepared.

Internal standard solution— Transfer 3.0mLof dimethylformamide to a 250-mLvolumetric flask.Add about 200mLof acetone,and shake for about 1minute.Dilute with acetone to volume,and mix.

Standard preparation— Transfer about 2g of USP Dimethyl Sulfoxide RS,accurately weighed,to a 250-mLvolumetric flask,add 3.0mLof dimethylformamide and 200mLof acetone,and sonicate for about 1minute.Dilute with acetone to volume,and mix to obtain a solution having a known concentration of about 8mg of USP Dimethyl Sulfoxide RSper mL.

Assay preparation— Transfer an accurately measured volume of Topical Solution,equivalent to about 2g of dimethyl sulfoxide,to a 250-mLvolumetric flask,add 3.0mLof dimethylformamide and about 200mLof acetone,and sonicate for about 1minute.Dilute with acetone to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and a 4-mm ×1.8-m column packed with 10%liquid phase G16on support S1A.The column is maintained at about 170,the injection port at about 200,and the detector block at about 300.The carrier gas is helium,flowing at a rate of about 20mLper minute.Chromatograph two portions of the acetone used to prepare the Standard preparation,the Assay preparation,and the Internal standard solution,and record the peak responses as directed under Procedure.Discard the first chromatogram,and examine the second to confirm that there are no peaks present that would interfere with the measurement of the dimethylformamide peaks and the dimethyl sulfoxide peaks in subsequent chromatograms.Chromatograph the Internal standard solution,and record the peak responses as directed for Procedure:examine the chromatogram to confirm that there are no peaks present that would interfere with the measurement of the dimethylformamide peaks and the dimethyl sulfoxide peaks in subsequent chromatograms.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.2for acetone,0.5for dimethylformamide,and 1.0for dimethyl sulfoxide;the resolution,R,between the dimethylformamide peak and the dimethyl sulfoxide peak is not less than 2;the tailing factors for the dimethylformamide peak and the dimethyl sulfoxide peak are not more than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— [NOTE—Use peak heights where peak responses are indicated.]Separately inject equal volumes (about 2µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms for not less than 20minutes,and measure the responses for the dimethylformamide and dimethyl sulfoxide peaks.Calculate the percentage (v/v)of C2H6OSin the portion of Topical Solution taken by the formula: in which 1.096is the specific gravity of dimethyl sulfoxide,Cis the concentration,in mg per mL,of USP Dimethyl Sulfoxide RSin the Standard preparation;Vis the volume,in mL,of Topical Solution taken to prepare the Assay preparation;and RUand RSare the ratios of the dimethyl sulfoxide peak response to the dimethylformamide peak response in the chromatograms obtained from the Assay preparationand the Standard preparation,respectively.