Mobile phase— Prepare a filtered and degassed mixture of 7mM ammonium acetate and acetonitrile (1:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Docusate Sodium RS in alcohol, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 1.0 mg per mL.

System suitability solution— Transfer about 15 mg of methylparaben, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with water to volume, and mix. Transfer 0.1 mL of this solution to a suitable container, add 10 mL of Standard preparation, and mix.

Assay preparation— Transfer 10 Tablets to a 1 L volumetric flask, add 200 mL of alcohol and 300 mL of water, and shake by mechanical means for not less than 90 minutes to completely disintegrate the Tablets. Dilute with water to volume, mix, and filter, discarding the first 3 mL of the filtrate.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm × 10-cm column that contains packing L1. The flow rate is about 1 mL per minute. The column temperature is maintained at 40. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.74 for methylparaben and 1.0 for docusate, and the resolution, R, between methylparaben and docusate is not less than 2.0. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 2.5; and the relative standard deviation for replicate injections is not more than 1.8%.

Procedure— Separately inject equal volumes (about 40 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C20H37NaO7S in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Docusate Sodium RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.