Internal standard solution— Weigh about 100 mg of procaine, and dissolve in 20 mL of methylene chloride.

Standard preparation— Weigh accurately about 10 mg of USP Methadone Hydrochloride RS, transfer to a 60-mL separator, add 1 mL of water and 2 mL of 0.5 N sodium hydroxide, and proceed as directed for Assay preparation, beginning with “extract with three 10-mL portions of chromatographic grade methylene chloride.”

Assay preparation— Transfer 1.0 mL of Injection, equivalent to 10 mg of methadone hydrochloride, to a 60-mL separator, add 2 mL of 0.5 N sodium hydroxide, and extract with three 10-mL portions of chromatographic grade methylene chloride, combining the extracts in a vessel containing about 3 g of anhydrous sodium sulfate. Transfer 2.0 mL of Internal standard solution to the vessel containing the extracts, insert the stopper, and mix. Decant about 15 mL of the methylene chloride solution to a test tube, and evaporate to a volume of 2 to 3 mL, using vacuum or a stream of nitrogen.

Procedure— Use a suitable gas chromatograph equipped with a flame-ionization detector and a glass column 1.2-m long and 4-mm in diameter, packed with 3% phase G2 on 100- to 200-mesh support S1A. Maintain the column temperature at 170, the injection port at 225, and the detector at 240. Use dry helium as the carrier gas, at a flow rate of about 55 mL per minute. In a suitable chromatogram, six replicate injections of the Standard preparation show a coefficient of variation of not more than 1% in the ratios of the peak areas of methadone to the peak area of procaine, and the resolution factor is not less than 5. Inject, separately, suitable volumes of the Assay preparation, containing about 5 µg of methadone, and of the Standard preparation. Calculate the quantity, in mg, of methadone hydrochloride (C21H27NO·HCl) in each mL of the Injection taken by the formula: in which W is the weight, in mg, of USP Methadone Hydrochloride RS in the Standard preparation; and RU and RS are the ratios of the peak areas of methadone to the peak area of procaine in the Assay preparation and the Standard preparation, respectively.