(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 Solution, accurately weighed, to a 50-mL volumetric 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, maintained at a temperature of 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 camphor and the internal standard is not less than 2.0, and between phenol and camphor is not less than 5.0; and the relative standard deviation of the peak response ratio of camphor and phenol to the internal standard for five consecutive injections of the Standard preparation is not more than 2.0%.

Procedure— Separately 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 portion of Topical Solution taken by 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 Solution, 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.