Mobile phase— Prepare a filtered and degassed mixture of 0.03 M monobasic potassium phosphate and acetonitrile (60:40). Adjust with phosphoric acid to a pH of 3.2. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Methadone Hydrochloride RS in Mobile phase to obtain a solution having a known concentration of about 0.4 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets for Oral Suspension. Transfer an accurately weighed portion of the powder, equivalent to about 10 mg of methadone hydrochloride, to a 25-mL volumetric flask. Add 10 mL of Mobile phase, and sonicate briefly. Shake by mechanical means for 15 minutes, dilute with Mobile phase to volume, mix, and filter.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L11. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 2.0, and the relative standard deviation 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 responses for the major peaks. Calculate the quantity, in mg, of methadone hydrochloride (C21H27NO·HCl) in the portion of Tablets for Oral Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Methadone Hydrochloride RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.