(Official January 1, 2007)

Internal standard solution— Transfer 250 mg, accurately weighed, of n-dodecane to a 25-mL volumetric flask, dilute with chloroform to volume, and mix.

Standard preparation— Transfer about 96 mg of USP Phenol RS, accurately weighed, to a 10-mL volumetric flask. Add about 224 mg of USP Camphor RS, accurately weighed, to the flask. Dilute with chloroform to volume, and mix. Combine 5.0 mL of this solution with 5.0 mL of Internal standard solution in a 50-mL volumetric flask, dilute with chloroform to volume, and mix.

Assay preparation— Transfer about 1 g of Topical Gel, accurately weighed, to a 50-mL flask. Add 5.0 mL of Internal standard solution, dilute with chloroform to volume, and mix.

Chromatographic system (see Chromatography 621)— The gas chromatograph is equipped with a flame-ionization detector (200) and a 2-mm × 1.8-m glass column packed with 100- to 120-mesh S1A, coated with 15% G44. The carrier gas is helium. Adjust the column temperature to about 140 so that the relative retention times are 0.3 for phenol, 0.8 for camphor, and 1.0 for the internal standard. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between the camphor peak and the internal standard peak is not less than 2.0 and between the phenol peak and the camphor peak is not less than 5.0; and the relative standard deviation of the peak response ratio of the camphor peak and phenol peak to the internal standard peak for five consecutive injections of the Standard preparation is not more than 2.0%.

Procedure— Inject 1 to 2 µL of the Assay preparation and the Standard preparation into the gas chromatograph, record the chromatograms, and determine the peak response ratios. Calculate the percentage of camphor (w/w) and the percentage of phenol (w/w) in the sample according to the formula: in which WR is the weight, in mg, of the appropriate USP Reference Standard in the Standard preparation; W is the weight of Topical Gel, in mg, taken to prepare the Assay preparation; and rU and rS are the response ratios of the corresponding analyte peaks in the Assay preparation and the Standard preparation, respectively.