pH2.8phosphate buffer— Dissolve 11.1g of monobasic sodium phosphate in 1000mLof water,adjust with phosphoric acid to a pHof 2.8±0.05,filter,and degas.

Diluent— Prepare a mixture of acetonitrile and pH2.8phosphate buffer(2:1).

Mobile phase— Prepare a mixture of pH2.8phosphate bufferand methanol (65:35).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).[NOTE—Minimize the time the Reference Standard,the Ophthalmic Solution,the standard stock solution,the Standard preparation,and the Assay preparation are exposed to direct light.]

Standard preparation— Transfer about 34mg of USP Timolol Maleate RS,accurately weighed,to a 25-mLvolumetric flask,dissolve in and dilute with water to volume,and mix.Transfer 5.0mLof this stock solution to a 50-mLvolumetric flask,add 15mLof Diluent,dilute with water to volume,and mix.

Assay preparation— Transfer an accurately measured volume of Ophthalmic Solution,equivalent to about 5mg of timolol,to a 50-mLvolumetric flask,add 15mLof Diluent,dilute with water to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 295-nm detector and a 4.6-mm ×15-cm column that contains 5-µm packing L1.The column temperature is maintained at 40,and the flow rate is about 1.2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor is not more than 2.0,the column efficiency is not less than 3600theoretical plates,and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the peak area responses for the major peaks.Calculate the quantity,in mg,of timolol (C13H24N4O3S)in each mLof Ophthalmic Solution taken by the formula: in which 316.43and 432.49are the molecular weights of timolol and timolol maleate,respectively,Cis the concentration,in mg per mL,of USP Timolol Maleate RSin the Standard preparation,Vis the volume,in mL,of Ophthalmic Solution taken,and rUand rSare the peak area responses of the timolol peaks obtained from the Assay preparationand the Standard preparation,respectively.