pH 9.0 Buffer— Dissolve 34.8 g of dibasic potassium phosphate in 900 mL of water, and adjust to a pH of 9.0, determined electrometrically, by the addition of 3 M hydrochloric acid or 1 M sodium hydroxide, as necessary, with mixing.

Internal standard solution— [note—Prepare fresh daily.] Transfer about 25 mg of homatropine hydrobromide to a 50-mL volumetric flask, dissolve in and dilute with water to volume, and mix.

Standard preparation— [note—Prepare fresh daily.] Dissolve an accurately weighed quantity of USP Atropine Sulfate RS in water, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 0.1 mg per mL. Pipet 10 mL of this solution into a separator, and proceed as directed for the Assay preparation, beginning with “Add 2.0 mL of Internal standard solution.”

Assay preparation— Transfer an accurately measured volume of Ophthalmic Solution, equivalent to about 10 mg of Atropine Sulfate, to a 100-mL volumetric flask, dilute with water to volume, and mix. Pipet 10 mL of this solution and treat as follows. Add 2.0 mL of Internal standard solution and 5.0 mL of pH 9.0 Buffer, and adjust the solution in the separator with 1 M sodium hydroxide to a pH of 9.0. Extract with two 10-mL portions of methylene chloride, filter the methylene chloride extracts through 1 g of anhydrous sodium sulfate supported by a small cotton plug in a funnel into a 50-mL beaker, and evaporate under a stream of nitrogen to near-dryness. Dissolve the residue in 2.0 mL of methylene chloride.

Chromatographic system (see Chromatography 621)— The gas chromatograph is equipped with a flame-ionization detector and a 2-mm × 1.8-m glass column packed with a 3% phase G3 on support S1AB. The carrier gas is nitrogen, flowing at a rate of 25 mL per minute. The column temperature is maintained at 225. The injection port and detector temperatures are maintained at 250. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, is not less than 4.0; the tailing factor is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 1 µL) of the Assay preparation and the Standard preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the quantity, in mg, of atropine sulfate [(C17H23NO3)2·H2SO4·H2O] in each mL of Ophthalmic Solution taken by the formula: in which 694.85 and 676.83 are the molecular weights of atropine sulfate monohydrate and anhydrous atropine sulfate, respectively; W is the weight, in mg, of USP Atropine Sulfate RS in the Standard preparation; V is the volume, in mL, of Ophthalmic Solution taken; and RU and RS are the peak area ratios of atropine sulfate to homatropine hydrobromide obtained from the Assay preparation and the Standard preparation, respectively.