NOTE—Throughout the following procedures, protect test or assay specimens, the Reference Standard, and solutions containing them, by conducting the procedures without delay, under subdued light, or using low-actinic glassware.

Medium: 0.01 N hydrochloric acid; 1000 mL.

Apparatus 2: 100 rpm.

Time: 60 minutes.

Procedure— Determine the amount of C21H26N2OS2 dissolved by employing UV absorption at the wavelength of maximum absorbance at about 261 nm on filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Mesoridazine Besylate RS in the same Medium. [NOTE—A volume of methanol not exceeding 1% of the final total volume may be used to prepare the Standard solution.]

Tolerances— Not less than 80% (Q) of the labeled amount of C21H26N2OS2 is dissolved in 60 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile, water, and triethylamine (850:150:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Mesoridazine Besylate RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.35 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 50 mg of mesoridazine, to a 200-mL volumetric flask. Add about 150 mL of methanol, shake by mechanical means for about 15 minutes, dilute with methanol to volume, and mix. Sonicate for 30 minutes, and allow dispersed material to settle. Filter through a 0.25-µm disk, discarding the first 20 mL of the filtrate.

System suitability preparation— Dissolve a suitable quantity of thioridazine hydrochloride in a portion of the Standard preparation, and mix to obtain a solution containing 0.025 mg of thioridazine hydrochloride per mL.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 265-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 2.5 mL per minute. Chromatograph the Standard preparation and the System suitability preparation, and record the peak areas as directed for Procedure: the resolution, R, between mesoridazine besylate and thioridazine hydrochloride is not less than 1.0; the column efficiency determined from the analyte peak is not less than 750 theoretical plates; and the relative standard deviation for replicate injections of the Standard preparation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the quantity, in mg, of mesoridazine (C21H26N2OS2) in the portion of Tablets taken by the formula: in which 386.59 and 544.75 are the molecular weights of mesoridazine and mesoridazine besylate, respectively; C is the concentration, in mg per mL, of USP Mesoridazine Besylate RS in the Standard preparation; and rU and rS are the peak areas obtained from the Assay preparation and the Standard preparation, respectively.