(ek' wi lin).
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C18H20O2 268.35

Estra-1,3,5(10),7-tetraen-17-one, 3-hydroxy-.
3-Hydroxyestra-1,3,5(10),7-tetraen-17-one [474-86-2].
» Equilin contains not less than 97.0 percent and not more than 103.0 percent of C18H20O2, calculated on the dried basis.
Packaging and storage— Preserve in tight, light-resistant containers.
USP Reference standards 11
USP Equilin RS Click to View Structure
Clarity of solution— Add 100 mg to 100 mL of 1 N sodium hydroxide in a 125-mL conical flask, heat on a steam bath until solution is complete, then cool, and transfer to a 100-mL color-comparison tube: the solution is clear.
Solution: 50 µg per mL.
Medium: alcohol.
Specific rotation 781S: between +300 and +316.
Test solution: 20 mg per mL, in dioxane.
Loss on drying 731 Dry it in vacuum at 105 for 1 hour: it loses not more than 0.5% of its weight.
Residue on ignition 281: not more than 0.5%.
Mobile phase— Prepare a suitable and degassed solution containing 35 volumes of acetonitrile and 65 volumes of water.
Internal standard solution— Dissolve phenol in acetonitrile to obtain a solution having a concentration of about 35 µg per mL.
Standard preparation— Dissolve a suitable quantity of USP Equilin RS, accurately weighed, in Internal standard solution to obtain a solution having a known concentration of about 0.2 mg per mL.
Assay preparation— Transfer about 10 mg of Equilin, accurately weighed, to a 50-mL volumetric flask, dissolve in and dilute with Internal standard solution to volume, and mix.
Chromatographic system (see Chromatography 621)—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 2 mL per 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 preparation is about 0.7 full-scale.
Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into 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 14 and 3 minutes, respectively. Calculate the quantity, in mg, of C18H20O2 in the portion of Equilin taken by the formula:
50C(RU / RS)
in which C is the concentration, in mg per mL, of USP Equilin RS in the Standard preparation, and RU and RS are the ratios of the peak responses of the equilin peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.
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Topic/Question Contact Expert Committee
Monograph Domenick Vicchio, Ph.D.
Senior Scientific Liaison
(SM42010) Monographs - Small Molecules 4
Reference Standards RS Technical Services
USP35–NF30 Page 3062