Solution A— Transfer 0.55 g of sodium 1-heptanesulfonate to a 1000-mL volumetric flask, dissolve in and dilute with water to volume, and mix.

Mobile phase— Transfer 10 g of monobasic potassium phosphate to a 1000-mL beaker, dissolve in 300 mL of Solution A, add 250 mL of methanol, followed by another 300 mL of Solution A, and mix. Adjust the mixture with phosphoric acid to a pH of 2.2. Transfer the whole mixture to a 1000-mL volumetric flask, dilute with Solution A to volume, and mix. Filter and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of methionine in water containing 1.25 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Aminocaproic Acid RS in water to obtain a Stock solution having a known concentration of 12.5 mg per mL. Transfer 5.0 mL of the Stock solution to a 100-mL volumetric flask, add 2.0 mL of the Internal standard solution, dilute with water to volume, and mix.

Assay preparation— Transfer an accurately weighed quantity of 1.25 g of Aminocaproic Acid to a 100-mL volumetric flask, dissolve in and dilute with water to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, add 2.0 mL of Internal standard solution, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm × 15-cm column that contains packing L1 and is maintained at 30. The flow rate is about 0.7 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.76 for aminocaproic acid and 1.0 for methionine; the resolution, R, between aminocaproic acid and methionine is not less than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, and allow the Assay preparation to elute for not less than two times the retention time of aminocaproic acid. Record the chromatograms, and measure all the peak responses. Calculate the quantity, in g, of C6H13NO2 in the portion of Aminocaproic Acid taken by the formula: in which C is the concentration, in mg per mL, of USP Aminocaproic Acid RS in the Standard preparation; and RU and RS are the ratios of the aminocaproic acid peak response to the internal standard peak response obtained from the Assay preparation and the Standard preparation, respectively.