Solvent— Use dimethyl sulfoxide.

Internal standard solution— Dissolve about 500mg of 4-tert-butylphenol,accurately weighed,in acetone in a 100-mLvolumetric flask,add acetone to volume,and mix.

Standard preparation— Dissolve accurately weighed quantities of USP3-tert-Butyl-4-hydroxyanisole RSand USP2-tert-Butyl-4-hydroxyanisole RSin Internal standard solutionto obtain a solution having known concentrations of about 9mg per mLand about 1mg per mL,respectively.

Assay preparation— Dissolve about 100mg of Butylated Hydroxyanisole,accurately weighed,in Internal standard solutionin a 10-mLvolumetric flask,dilute with Internal standard solutionto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The gas chromatograph is equipped with a flame-ionization detector,and contains a 2-mm ×1.8-m stainless-steel column packed with 10%liquid phase G26on support S1A.The carrier gas is helium.The column is maintained isothermally at a temperature between 175and 185.The injection port temperature is maintained at 200,and the detector temperature is maintained at about 250.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the resolution,R,between the 3-tert-butyl-4-hydroxyanisole isomer and the 2-tert-butyl-4-hydroxyanisole isomer is not less than 1.3;the tailing factor is not more than 2.0;and the relative standard deviation is not more than 2.0%determined from the 3-tert-butyl-4-hydroxyanisole isomer and not more than 6.0%determined from the 2-tert-butyl-4-hydroxyanisole isomer.

Procedure— Separately inject equal volumes (about 5µL)of the Standard preparationand the Assay preparationinto the gas chromatograph,record the chromatograms,and measure the peak areas for each isomer and the internal standard.Calculate the quantity,in mg,of each isomer in the portion of Butylated Hydroxyanisole taken by the formula: in which Cis the concentration,in mg per mL,of the appropriate USP Reference Standard;and RUand RSare the peak area ratios of the corresponding isomer to the internal standard obtained from the Assay preparationand the Standard preparation,respectively.Calculate the weight,in mg,of C11H16O2in the portion of Butylated Hydroxyanisole taken by adding the quantities of the two isomers.