(Official January 1, 2007)

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

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

Assay preparation— Transfer an accurately weighed quantity of Gel, equivalent to 2 g of dimethyl sulfoxide, to a 100-mL volumetric flask. Add 1.0 mL of dimethylformamide and about 75 mL of methanol. Dilute with methanol to volume, and mix.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and a 4-mm × 1.8-m column packed with 15% liquid phase G39 on support S1A. The column is maintained at about 160 and the injection port and detector block at about 200. Helium is used as the carrier gas, flowing at a rate of about 30 mL per 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 0.4 for dimethylformamide and 1.0 for 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 2 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms for not less than 40 minutes, and measure the responses for the dimethylformamide and dimethyl sulfoxide peaks. Calculate the percentage (w/w) of C2H6OS in the portion of Gel taken by the formula: in which C is the concentration, in mg per mL, of USP Dimethyl Sulfoxide RS in the Standard preparation; W is the weight, in g, of Gel taken to prepare the Assay preparation; and RU and RS are the ratios of the dimethyl sulfoxide peak response to the dimethylformamide peak response in the chromatograms obtained from the Assay preparation and the Standard preparation, respectively.