A: Infrared Absorption á197Kñ.

B: Ultraviolet Absorption á197Uñ—

Solution: 50µg per mL.

Medium: alcohol.

Test solution: 20mg per mL,in dioxane.

Mobile phase— Prepare a suitable and degassed solution containing 35volumes of acetonitrile and 65volumes of water.

Internal standard solution— Dissolve phenol in acetonitrile to obtain a solution having a concentration of about 35µg per mL.

Standard preparation— Dissolve a suitable quantity of USP Equilin RS,accurately weighed,in Internal standard solutionto obtain a solution having a known concentration of about 0.2mg per mL.

Assay preparation— Transfer about 10mg of Equilin,accurately weighed,to a 50-mLvolumetric flask,dissolve in and dilute with Internal standard solutionto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph five replicate injections of the Standard preparation,and record the peak responses as directed under Procedure:the relative standard deviation is not more than 2.0%,and the resolution factor between equilin and phenol is not less than 5.Adjust the operating parameters such that the peak obtained from the Standard preparationis about 0.7full-scale.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph by means of a suitable microsyringe or sampling valve,record the chromatograms,and measure the responses for the major peaks:the retention times for equilin and phenol are about 14and 3minutes,respectively.Calculate the quantity,in mg,of C18H20O2in the portion of Equilin taken by the formula: in which Cis the concentration,in mg per mL,of USP Equilin RSin the Standard preparation,and RUand RSare the ratios of the peak responses of the equilin peak to the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.