(Official January 1, 2007)

Sodium chloride solution— Dissolve 250 g of sodium chloride in water to make 1000 mL.

Internal standard solution— Dissolve suitable quantities of anethole in hexanes to obtain a solution having a concentration of about 2 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Menthol RS in Internal standard solution to obtain a solution having a known concentration of about 0.20L mg per mL, L being the labeled quantity, in mg, of menthol in each Lozenge.

Assay preparation— Transfer 20 Lozenges to a 1-liter screw-capped conical flask. [NOTE—Use caps with inert white rubber liners.] Add 200 mL of water, 260 mL of Sodium chloride solution, and 100.0 mL of Internal standard solution, and shake by mechanical means for 30 minutes. Allow the phases to separate, and transfer a portion of the hexanes phase to a suitable container.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector, a split injection system with a split ratio of about 10:1, and a 0.53-mm × 30-m fused silica column coated with a 1-µm layer of Gl6 stationary phase. The column is maintained isothermally at about 125, and the injection port and the detector block are maintained at about 250. The carrier gas is helium at a flow rate of about 10 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.5 for menthol and 1.0 for anethole, the resolution, R, between the menthol and the anethole peaks is not less than 15, the tailing factor for the menthol and anethole peaks is not more than 2.0, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard preparation and the Assay preparation into the gas chromatograph, and measure the responses for the menthol and anethole peaks. Calculate the quantity, in mg, of C10H20O in each Lozenge taken by the formula: in which C is the concentration, in mg per mL, of USP Menthol RS in the Standard preparation, and RU and RS are the ratios of the menthol peak to the anethole peak obtained from the Assay preparation and the Standard preparation, respectively.