Medium: water;1000mL.

Apparatus 2: 50rpm.

Time: 60minutes.

Tolerances— Not less than 75%(Q)of the labeled amounts of butalbital (C11H16N2O3),caffeine (C8H10N4O2),and aspirin (C9H8O4)is dissolved in 60minutes.

NOTE—Use glassware in this test.

Solvent— Add 1mLof phosphoric acid to 1000mLof methanol,and mix.

Standard preparation— Prepare a solution of USP Salicylic Acid RSin Solventhaving a known concentration of about 0.0012mg per mL.Use this solution promptly.

Test preparation— Transfer an accurately weighed portion of the powder remaining from the preparation of the Assay preparationin the Assay,equivalent to about 65mg of aspirin,to a 200-mLflask,add 100.0mLof Solvent,and shake for about 1minute.Promptly filter a portion of this solution,discarding the first 15mLof the filtrate,and use the clear filtrate as the Test preparation.Use this solution within 20minutes after the addition of the Solvent.

Procedure— Immediately place the cell containing the solution under test in the cell compartment of a spectrophotofluorimeter,and allow to equilibrate for 2minutes.Concomitantly measure the intensities of the fluorescence of the Test preparationand the Standard preparationat 444nm,using an excitation wavelength of 305nm.Calculate the percentage of salicylic acid in the portion of Capsules taken by the formula: in which Cis the concentration,in mg per mL,of USP Salicylic Acid RSin the Standard preparation;ais the quantity,in mg,of aspirin in the portion of Capsules taken to prepare the Test preparation,based on the labeled amount;and IUand ISare the fluorescence intensity readings obtained from the Test preparationand the Standard preparation,respectively.[NOTE—If the intensity of the Test preparationgreatly exceeds that of theStandard preparation,immediately transfer 5.0mLof theTest preparationto a 50-mLvolumetric flask,dilute with Solventto volume,and mix.Immediately determine the intensity of this solution,and calculate the percentage of salicylic acid in the portion of Capsules taken by the formula: Not more than 2.5%is found.

0.01M Phosphate buffer— Dissolve 1.361g of monobasic potassium phosphate in 1000mLof water,and mix.

Mobile phase— Prepare a suitable filtered and degassed mixture of 0.01M Phosphate bufferand methanol (550:450),and adjust with phosphoric acid to a pHof 3.9.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

pH2.5Buffer— Prepare a mixture of 0.01M Phosphate bufferand methanol (550:450),and adjust with phosphoric acid to a pHof 2.5±0.05.

Aspirin standard stock solution— Dissolve an accurately weighed quantity of USP Aspirin RSin pH2.5Bufferto obtain a solution having a known concentration of about 1.6mg per mL,sonicating and shaking the solution,if necessary,to achieve complete dissolution.Use this solution within 24hours.

Standard preparation— Dissolve accurately weighed quantities of USP Butalbital RSandUSP Caffeine RSquantitatively in Aspirin standard stock solutionto obtain a solution having known concentrations of about 1.6Jmg of USP Butalbital RSand 1.6J¢mg of USP Caffeine RSper mL,Jand J¢being the ratios of the respective labeled amounts,in mg,of butalbital and caffeine to the labeled amount,in mg,of aspirin per Capsule,sonicating and shaking the solution,if necessary,to achieve complete dissolution.Use this solution within 24hours.

Salicylic acid solution— Prepare a solution of salicylic acid in pH2.5Buffercontaining about 0.1mg per mL.Pass this solution through a suitable filter of 0.5-µm or finer porosity.

Assay preparation— Remove,as completely as possible,the contents of not fewer than 20Capsules.Transfer an accurately weighed portion of the powder,equivalent to about 325mg of aspirin,to a 200-mLvolumetric flask,dilute with pH2.5Bufferto volume,sonicate for about 30minutes,and mix.Pass a portion of this solution through a suitable filter of 0.5-µm or finer porosity,and use the filtrate as the Assay preparation.Use this solution within 24hours.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a detector set at the wavelength of the isosbestic point of aspirin and salicylic acid at about 277nm and a 210-nm detector,and a 3.9-mm ×30-cm column that contains packing L1and is maintained at 35±1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.45for caffeine,0.6for aspirin,and 1.0for butalbital;the resolution,R,between the caffeine and aspirin peaks is not less than 2.0;the column efficiency determined from the butalbital peak is not less than 2000theoretical plates;and the relative standard deviations of the caffeine,aspirin,and butalbital responses for replicate injections are not more than 2.0%.Inject 10µLof the Salicylic acid solution,and record the peak response as directed for Procedure:the salicylic acid peak has the same retention time as that of the aspirin peak obtained in the chromatogram of the Standard preparation.[NOTE—If the retention time of the salicylic acid peak differs from that of the aspirin peak,adjust the pHof the Mobile phasewith 0.2Npotassium hydroxide or 1Mphosphoric acid so that the salicylic acid peak has the same retention time as that of the aspirin peak.The retention time of the salicylic acid peak decreases about 0.3minute for each 0.1pHincrease.The retention time of the aspirin peak is essentially unaffected by such pHadjustments.]

Procedure— [NOTE—Use peak areas where peak responses are indicated.]Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,using the 277-nm detector to record the caffeine and aspirin peak responses and the 210-nm detector to record the butalbital peak responses,and measure the peak responses for the major peaks.Calculate the quantities,in mg,of caffeine (C8H10N4O2),aspirin (C9H8O4),and butalbital (C11H16N2O3)in the portion of Capsules taken by the same formula: in which Cis the concentration,in mg per mL,of the appropriate USP Reference Standard in the Standard preparation;and rUand rSare the peak responses of the corresponding analyte obtained from the Assay preparation and the Standard preparation,respectively.Correct the amount of aspirin obtained for the amount of salicylic acid present by the formula: in which Ais the quantity,in mg,of aspirin in the portion of Capsules taken to prepare the Assay preparation;and Fis the percentage of salicylic acid obtained in the test for Limit of free salicylic acid.