Internal standard solution— Transfer 1.0mLof dimethylformamide to a 100-mLvolumetric flask.Add about 75mLof methanol,and shake for about 1minute.Dilute with methanol to volume,and mix.

Standard preparation— Transfer about 2g of USP Dimethyl Sulfoxide RS,accurately weighed,to a 100-mLvolumetric flask,add 1.0mLof dimethylformamide and 75mLof methanol,and shake for about 1minute.Dilute with methanol to volume,and mix.This solution contains about 20mg of USP Dimethyl Sulfoxide RSper mL.

Assay preparation— Transfer an accurately weighed quantity of Gel,equivalent to 2g of dimethyl sulfoxide,to a 100-mLvolumetric flask.Add 1.0mLof dimethylformamide and about 75mLof methanol.Dilute with methanol 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 15%liquid phase G39on support S1A.The column is maintained at about 160and the injection port and detector block at about 200.Helium is used as the carrier gas,flowing at a rate of about 30mLper minute.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 0.4for dimethylformamide and 1.0for dimethyl sulfoxide;the tailing factor is not more than 2.0;the resolution,R,between the dimethylformamide peak and the dimethyl sulfoxide peak is not less than 4;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 40minutes,and measure the responses for the dimethylformamide and dimethyl sulfoxide peaks.Calculate the percentage (w/w)of C2H6OSin the portion of Gel taken by the formula: in which Cis the concentration,in mg per mL,of USP Dimethyl Sulfoxide RSin the Standard preparation;Wis the weight,in g,of Gel 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.