A: Infrared Absorption á197Kñ.

B: Ultraviolet Absorption á197Uñ—

Solution: 5µg per mL.

Medium: alcohol.

Mobile phase— Mix 300mLof methanol and 10mLof glacial acetic acid with 690mLof water.Allow the mixture to cool,and filter,if necessary,through a suitable microporous membrane filter.Degas the solution.

Internal standard solution— Dissolve salicylic acid in methanol,by sonicating,to obtain a solution having a concentration of about 7mg per mL.

Standard preparation— Dissolve,by sonicating,an accurately weighed quantity of USP Aminobenzoic Acid RSin methanol,quantitatively dilute with methanol to obtain a solution having a known concentration of about 0.42mg per mL,and mix.Pipet 5mLof this solution and 5mLof the Internal standard solutioninto a 50-mLvolumetric flask,dilute with methanol to volume,and mix.Pass through 0.6-µm filter paper.Throughout the preparation,protect against actinic light.

Assay preparation— Transfer an accurately weighed quantity of Gel,equivalent to about 4.2mg of aminobenzoic acid,to a 100-mLvolumetric flask,and add 10.0mLof Internal standard preparationand about 50mLof methanol.Shake or sonicate,as necessary,dilute with methanol to volume,and mix.Filter,if necessary,through filter paper (Whatman No.41or equivalent).Pass through 0.6-µm filter paper.Throughout this preparation,protect against actinic light.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×30-cm column that contains packing L11.The flow rate is about 1.0mLper minute.Chromatograph replicate 15-µLinjections of Standard preparationuntil the response ratio variability is within 1.0%of average.The resolution factor is not less than 3.0between aminobenzoic acid and salicylic acid.

Procedure— Separately inject equal volumes (about 15µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The retention time of salicylic acid is about 3.0relative to that of aminobenzoic acid as 1.0.Calculate the quantity,in mg,of aminobenzoic acid (C7H7NO2)in the portion of Gel taken by the formula: in which Cis the concentration,in mg per mL,of USP Aminobenzoic Acid RSin the Standard preparation;and RUand RSare the ratios of the peak responses of the aminobenzoic acid peak to the salicylic acid peak obtained from the Assay preparationand the Standard preparation,respectively.