A: Infrared Absorption 197M.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

C: It meets the requirements for Chloride 191.

Mobile phase— Prepare a filtered and degassed mixture of tert-butyl methyl ether, chromatographic n-heptane, acetonitrile, and water (63:35:2:0.2). Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability solution— Transfer about 18 mg of USP Dorzolamide Hydrochloride RS and 2 mg of USP Dorzolamide Hydrochloride Related Compound A RS, each accurately weighed, to a 15-mL centrifuge tube, dissolve in 4 mL of 0.5 N ammonium hydroxide, add 4 mL of ethyl acetate, and mix. Separate the ethyl acetate layer, and transfer to a 15-mL centrifuge tube. Add 4 mL of ethyl acetate to the aqueous layer, mix, separate the ethyl acetate layer, and combine it with the first extract. Evaporate the combined organic layers to dryness on a water bath maintained at 50 under a stream of nitrogen. Dissolve the residue in 3 mL of acetonitrile, add 3 drops of (S)-()--methylbenzyl isocyanate [note—Discard the reagent if it is colored.], and allow to react for 5 minutes on a water bath maintained at 50. Evaporate the mixture to dryness on a water bath maintained at 50 under a stream of nitrogen. Dissolve the residue in 10 mL of a mixture of tert-butyl methyl ether, glacial acetic acid, and acetonitrile (87:10:3).

Test solution— Transfer about 20 mg of Dorzolamide Hydrochloride, accurately weighed, to a 15-mL centrifuge tube, and proceed as directed for System suitability solution beginning with “dissolve in 4 mL”.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L3. The flow rate is about 2 mL per minute. Chromatograph the System suitability solution, and record the peak areas as directed for Procedure: the relative retention times are about 1.0 for dorzolamide and 1.5 for dorzolamide hydrochloride related compound A; the resolution, R, between dorzolamide and dorzolamide hydrochloride related compound A is not less than 4.0; the column efficiency for the dorzolamide hydrochloride peak is not less than 4000 theoretical plates; the tailing factor is not more than 1.4.

Procedure— Separately inject equal volumes (about 10 µL) of the System suitability solution and the Test solution into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the percentage of dorzolamide hydrochloride related compound A in the portion of Dorzolamide Hydrochloride taken by the formula: in which rI is the peak area of dorzolamide hydrochloride related compound A obtained from the Test solution; and rS is the peak area of dorzolamide hydrochloride obtained from the Test solution: not more than 0.5% is found.

Phosphate buffer, Solution A, Solution B, Mobile phase, and Chromatographic system— Proceed as directed in the Assay.

Test solution— Use the Assay preparation.

Procedure— Inject a volume (about 10 µL) of the Test solution into the chromatograph, record the chromatogram, and measure all of the peak areas. Calculate the percentage of each impurity in the portion of Dorzolamide Hydrochloride taken by the formula: in which ri is the peak area of each individual impurity obtained from the Test solution; and rs is the sum of all the peak areas obtained from the Test solution: not more than 0.1% of any individual impurity is found; and not more than 0.5% of total impurities is found.

Phosphate buffer— Dissolve 3.7 g of monobasic potassium phosphate in 1000 mL of water.

Solution A— Prepare a filtered and degassed mixture of Phosphate buffer and acetonitrile (94:6.5).

Solution B— Use acetonitrile.

Mobile phase— Use variable mixtures of Solution A and Solution B as directed for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve suitable quantities of USP Dorzolamide Hydrochloride RS in Solution A to obtain a solution having a known concentration of about 0.6 mg per mL.

Assay preparation— Transfer about 60 mg of Dorzolamide Hydrochloride, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with Solution A to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. The column temperature is maintained at 35. The chromatograph is programmed as follows. Chromatograph the Standard preparation, and record the peak areas as directed for Procedure: the column efficiency is not less than 6500 theoretical plates; the tailing factor is not less than 0.6 and not more than 1.2; and the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the quantity, in mg, of C10H16N2O4S3·HCl in the portion of Dorzolamide Hydrochloride taken by the formula: in which C is the concentration, in mg per mL, of USP Dorzolamide Hydrochloride RS in the Standard preparation; and rU and rS are the peak areas obtained from the Assay preparation and the Standard preparation, respectively.