Mobile phase— Prepare a filtered and degassed mixture of methanol and water (70:30). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve accurately weighed quantities of USP Methylparaben RS and USP Propylparaben RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having known concentrations of about 1.2 and 0.12 mg per mL, respectively. Pipet 5 mL of this solution into a 50-mL volumetric flask, add by pipet 30 mL of methanol, dilute with water to volume, and mix.

Test preparation— Pipet 1 mL of Injection into a 10-mL volumetric flask, add by pipet 7 mL of methanol, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation as directed for Procedure: the capacity factor, k¢, is 0.52 for methylparaben and 1.05 for propylparaben, with a minimum separation factor () of about 2.0.

Procedure— Separately inject equal volumes (about 12 µL) of the Standard preparation and the Test preparation into the chromatograph by means of a suitable microsyringe or sampling valve, adjusting the specimen size and other operating parameters such that the peak obtained with the Standard preparation is about 0.7 full scale. Record the chromatograms, and measure the height of the peaks, at identical retention times, obtained with the Test preparation and the Standard preparation, and calculate the concentration in mg per mL, in the portion of methylparaben or propylparaben taken by the formula: in which C is the concentration, in mg per mL, of USP Methylparaben RS or USP Propylparaben RS in the Standard preparation; and HU and HS are the peak heights obtained from the Test preparation and the Standard preparation, respectively.