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

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

Assay preparation— Transfer 1.0mLof Injection,equivalent to 10mg of methadone hydrochloride,to a 60-mLseparator,add 2mLof 0.5Nsodium hydroxide,and extract with three 10-mLportions of chromatographic grade methylene chloride,combining the extracts in a vessel containing about 3g of anhydrous sodium sulfate.Transfer 2.0mLof Internal standard solutionto the vessel containing the extracts,insert the stopper,and mix.Decant about 15mLof the methylene chloride solution to a test tube,and evaporate to a volume of 2to 3mL,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 G2on 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 55mLper minute.In a suitable chromatogram,six replicate injections of the Standard preparationshow 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 5µg of methadone,and of the Standard preparation.Calculate the quantity,in mg,of methadone hydrochloride (C21H27NO·HCl)in each mLof the Injection taken by the formula: in which Wis the weight,in mg,of USP Methadone Hydrochloride RSin the Standard preparation;and RUand RSare the ratios of the peak areas of methadone to the peak area of procaine in the Assay preparationand the Standard preparation,respectively.