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.

20%Phosphoric acid— Transfer 23.5mLof 85%phosphoric acid into a 100-mLvolumetric flask containing water,and dilute with water to volume.

Mobile phase— Prepare a filtered and degassed mixture of water,acetonitrile,20%Phosphoric acid,and triethylamine by using the following procedure.Add 20mLof 20%Phosphoric acidto 450mLof water,to this solution add 5mLof triethylamine,and adjust with 1Nsodium hydroxide to a pHof 3.0.Add 500mLof acetonitrile,and dilute with water to 1000mL.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Methotrimeprazine RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.1mg per mL.

System suitability preparation— Dissolve or add suitable quantities of 1%benzyl alcohol in Mobile phaseand USP Methotrimeprazine RSin Mobile phaseto obtain a solution containing about 2.0and 0.1mg per mL,respectively.

Assay preparation— Transfer an accurately measured amount of Injection,equivalent to about 20mg of methotrimeprazine,accurately weighed,to a 200-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm ×25-cm column that contains packing L7.The flow rate is about 1mLper minute.Chromatograph the System suitability preparation,and record the peak responses as directed for Procedure:the resolution,R,between benzyl alcohol and methotrimeprazine is not less than 4.0;and the tailing factor is not more than 1.2.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation for replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 20µL)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 methotrimeprazine (C19H24N2OS)in the portion of Injection taken by the formula: in which Cis the concentration,in mg per mL,of USP Methotrimeprazine RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.