Medium: water;900mL.

Apparatus 2: 75rpm.

Time: 45minutes.

Mobile phase— Mix 510mLof methanol,previously filtered through a membrane filter of 0.5µm or finer porosity,and 490mLof glacial acetic acid solution (1in 50),similarly filtered,and degas.

Standard preparation— Transfer about 90mg of USP Aspirin RSand 90Jmg of USP Carisoprodol RS,both accurately weighed,to a 250-mLvolumetric flask,Jbeing the ratio of the labeled amount,in mg,of carisoprodol to that of aspirin.Add 5mLof acetonitrile,previously filtered through a membrane filter of 0.5µm or finer porosity,swirl to dissolve,dilute with water to volume,and mix.

Resolution solution— Dissolve salicylic acid in Standard preparationto obtain a solution containing about 0.36mg of salicylic acid per mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a refractive index detector and a 3.9-mm ×30-cm column that contains packing L1.Maintain the detector and the column at 30±1.The flow rate is about 2mLper minute.Chromatograph the Resolution solutionand the Standard preparation,and record the peak responses as directed for Procedure:the resolution,R,between the aspirin and salicylic acid peaks,and between the carisoprodol and salicylic acid peaks,is not less than 1.5,and the relative standard deviation for replicate injections of the Standard preparationis not more than 2.0%.

Procedure— Separately inject equal volumes (about 300µL)of the Standard preparationand the solution under test,previously filtered,into the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.4for aspirin and 1.0for carisoprodol.Calculate the quantity,in mg,of aspirin (C9H8O4)dissolved by the formula: in which Cis the concentration,in µg per mL,of USP Aspirin RSin the Standard preparation,and rUand rSare the peak responses obtained for aspirin from the solution under test and the Standard preparation,respectively.Calculate the quantity,in mg,of carisoprodol (C12H24N2O4)dissolved by the same formula,except to read “USP Carisoprodol RS”where “USP Aspirin RS”is specified,and “carisoprodol”where “aspirin”is specified.

Tolerances— Not less than 75%(Q)of the labeled amounts of C9H8O4and C12H24N2O4are dissolved in 45minutes.

Mobile phase— Mix 5mLof glacial acetic acid and 500mLof water,and filter through a membrane filter of 0.5µm or finer porosity.Add 360mLof the filtrate to 640mLof methanol,similarly filtered,mix,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Solvent mixture— Prepare a mixture of water,acetonitrile,and glacial acetic acid (59:40:1).

Aspirinand carisoprodol standard preparation— Transfer about 80mg of USP Aspirin RSand 80Jmg of USP Carisoprodol RS,both accurately weighed,to a 25-mLvolumetric flask,Jbeing the ratio of the labeled amount,in mg,of carisoprodol to that of aspirin.Add about 15mLof Solvent mixture,swirl for 5minutes,and sonicate for 25to 30seconds.Dilute with Solvent mixtureto volume,and mix.

Salicylic acid standard preparation— Dissolve an accurately weighed quantity of USP Salicylic Acid RSin Solvent mixtureto obtain a solution having a known concentration of about 16µg per mL.

Resolution solution— Dissolve salicylic acid in Aspirin and carisoprodol standard preparationto obtain a solution containing about 0.5mg of salicylic acid per mL.

Assay preparation— Weigh and finely powder not less than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 325mg of aspirin,to a 100-mLvolumetric flask.Add about 50mLof Solvent mixture,swirl for 5minutes,sonicate for 25to 30seconds,shake by mechanical means for 30minutes,dilute with Solvent mixtureto volume,and mix.Filter a portion of this solution through a membrane filter of 0.5µm or finer porosity,and use the filtrate as the Assay preparation.[NOTE—Use within 8hours.]

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a refractive index detector,a 313-nm detector,and a 4.6-mm ×25-cm column that contains packing L7.Maintain the refractive index detector and the column at 30±1.The flow rate is about 1mLper minute.Chromatograph the Aspirin and carisoprodol standard preparationand the Resolution solution,and record the peak responses as directed under Procedure,using the refractive index detector:the resolution,R,between the solvent and aspirin peaks in the chromatogram of the Resolution solutionis not less than 1.2,the resolution,R,between the aspirin and salicylic acid peaks is not less than 1.5,and the relative standard deviation for replicate injections of the Aspirin and carisoprodol standard preparationis not more than 2.0%.Chromatograph the Salicylic acid standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation for replicate injections is not more than 5.0%.

Procedure— Separately inject equal volumes (about 50µL)of the Aspirin and carisoprodol standard preparation,the Salicylic acid standard preparation,and the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks,using the refractive index detector for the Aspirin and carisoprodol standard preparation,the 313-nm detector for the Salicylic acid standard preparation,and both detectors for the Assay preparation.The relative retention times are about 0.6for aspirin,0.7for salicylic acid,and 1.0for carisoprodol.Calculate the quantity,in mg,of aspirin (C9H8O4)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Aspirin RSin the Aspirin and carisoprodol standard preparation,and rUand rSare the peak responses,with the use of the refractive index detector,obtained for aspirin from the Assay preparationand the Aspirin and carisoprodol standard preparation,respectively.Calculate the quantity,in mg,of carisoprodol (C12H24N2O4)in the portion of Tablets taken by the same formula,except to read “USP Carisoprodol RS”where “USP Aspirin RS”is specified,and “carisoprodol”where “aspirin”is specified.Calculate the percentage of free salicylic acid in the Tablets taken by the formula: in which Cis the concentration,in µg per mL,of USP Salicylic Acid RSin the Salicylic acid standard preparation,ais the quantity,in mg,of aspirin in the portion of Tablets taken,based on the labeled amount,and rUand rSare the peak responses,with the use of the 313-nm detector,obtained for salicylic acid from the Assay preparationand the Salicylic acid standard preparation,respectively:not more than 3.0%is found.