Mobile phase— Prepare a mixture of water,methanol,and glacial acetic acid (60:40:1).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Diluent— Prepare a mixture of methanol,water,and glacial acetic acid (70:30:4).

Reference solutions— Prepare a solution in Diluentcontaining about 0.05mg of 4-hydroxybenzoic acid and 0.025mg of 4-hydroxyisophthalic acid per mL,and a second solution in Diluentcontaining about 0.05mg of 4-hydroxybenzoic acid and 0.01mg of phenol per mL.

Standard solution— Dissolve accurately weighed quantities of 4-hydroxybenzoic acid,4-hydroxyisophthalic acid,phenol,and the Salicylic Acid under test in Diluentto obtain a solution having known concentrations of about 0.05mg per mL,0.025mg per mL,0.01mg per mL,and 50mg per mL,respectively.

Test solution— Prepare a solution of Salicylic Acid in Diluentcontaining 50mg per mL.Sonicate until completely dissolved.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 270-nm detector and a 4.6-mm ×10-cm column containing 5-µm packing L1.The flow rate is about 0.5mLper minute.Chromatograph the Reference solutions,record the chromatograms,and note the retention times of the peaks.Chromatograph the Standard solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.35,0.45,0.5,and 1.0for 4-hydroxybenzoic acid,4-hydroxyisophthalic acid,phenol,and salicylic acid,respectively,and the peaks are all baseline-resolved from each other.Changes in the composition of the Mobile phasemade to optimize resolution may change the elution order of impurities.Use the chromatograms obtained from the Reference solutionsto determine the elution order of the specified impurities.

Procedure— Separately inject equal volumes (about 2µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the areas for the major peaks.Calculate the percentage of each relevant related compound taken by the formula: in which Ciis the concentration,in mg per mL,of the relevant related compound in the Standard solution;and riand rSiare the peak responses for the relevant related compound obtained from the Test solutionand the Standard solution,respectively:not more than 0.1%of 4-hydroxybenzoic acid,0.05%of 4-hydroxyisophthalic acid,and 0.02%of phenol is found.Calculate the percentage of any other impurity,other than the solvent peak,observed in the chromatogram of the Test solutionby the same formula,except to use the concentration of 4-hydroxyisophthalic acid in the Standard solutionas Ci;the response of the peak of 4-hydroxyisophthalic acid in the chromatogram obtained from the Standard solutionas rSi;and the response of any other impurity as ri:not more than 0.05%of any other impurity is found;and the sum of all of the related compounds and other impurities found is not more than 0.2%.