A: Shake a volume of Oral Concentrate,equivalent to about 5mg of methadone hydrochloride,with 5mLof sodium carbonate TS,and extract with 5mLof chloroform:the extract so obtained responds to the Thin-layer Chromatographic Identification Test á201ñ,a solvent mixture of alcohol,glacial acetic acid,and water (5:3:2)being used for development and iodoplatinate TSbeing used to visualize the spots.

B: It responds to the tests for Chloride á191ñ.

Mobile phase— Prepare a suitable mixture of 0.033Mmonobasic potassium phosphate and acetonitrile (60:40),adjust with phosphoric acid to a pHof 4.0,filter,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

Assay preparation— Transfer an accurately measured volume of Oral Concentrate,equivalent to about 50mg of methadone hydrochloride,to a 50-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.Transfer 10.0mLof this solution to a 25-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—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 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the column efficiency determined from the analyte peak is not less than 1500theoretical plates,the tailing factor for the analyte peak is not more than 2.0,and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of methadone hydrochloride (C21H27NO·HCl)in each mLof the Oral Concentrate taken by the formula: in which Cis the concentration,in mg per mL,of USP Methadone Hydrochloride RSin the Standard preparation;Vis the volume,in mL,of Oral Concentrate taken;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.