Mobile solvent— Prepare a mixture of 2 volumes of methanol, 2 volumes of acetonitrile, and 6 volumes of water.

Standard preparation— Dissolve a suitable quantity of USP Hydrocortisone RS, accurately weighed, in alcohol and dichloromethane (75:25) to obtain a solution having a known concentration of about 0.1 mg per mL. Dilute this solution with alcohol to a concentration of 10 µg per mL.

Assay preparation— Weigh accurately an aliquot of Gel, equivalent to about 10 mg of hydrocortisone, and dilute with alcohol and dichloromethane (75:25) to obtain a solution having a concentration of about 0.1 mg per mL. Dilute this solution with alcohol to a concentration of 10 µg per mL.

Procedure— Introduce equal volumes (between 5 µL and 15 µL) of the Assay preparation and the Standard preparation into a high-pressure liquid chromatograph (see Chromatography 621) operated at room temperature, by means of a suitable microsyringe or sampling valve, adjusting the specimen size and other operating parameters such that the peak obtained from the Standard preparation is about 0.6 full-scale. Typically, the apparatus is fitted with a 4-mm × 30-cm column that contains packing L1 and is equipped with an UV detector capable of monitoring absorption at 254 nm, and a suitable recorder, and is capable of operating at a column pressure of up to 6000 psi. In a suitable chromatogram, the coefficient of variation for five replicate injections of the Standard preparation is not more than 3.0%. Determine the ratios of the peak heights, at equivalent retention times, obtained from the Assay preparation and the Standard preparation, and calculate the quantity, in mg, of hydrocortisone (C21H30O5) in the portion of Gel taken by the formula: in which C is the concentration, in µg per mL, of USP Hydrocortisone RS in the Standard preparation; and HU and HS are the peak heights of the Assay preparation and the Standard preparation, respectively.