Test solution: 40 mg per mL, in 15 N formic acid.

Diluent— Prepare a mixture of water and methanol (9:1).

Mobile phase— Prepare a filtered and degassed solution by dissolving 5.6 g of monobasic potassium phosphate in 820 mL of water in a 1-liter volumetric flask, adjusting with phosphoric acid to a pH of 4.3, diluting with methanol to volume, and mixing. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Dissolve an accurately weighed quantity of USP Aspartame Related Compound A RS in Diluent, and dilute quantitatively with Diluent to obtain a solution having a known concentration of about 75 µg per mL.

Test solution— Transfer 50 mg of Aspartame, accurately weighed, to a 10-mL volumetric flask, dissolve in and dilute with Diluent to volume, and mix. [note—Avoid heat and excessive holding times.]

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and 4.6-mm × 25-cm column containing packing L1. The flow rate is about 2 mL per minute. The column temperature is maintained at 40. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the tailing factor is not more than 2.0, and the relative standard deviation for replicate injections is not more than 4.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the percentage of aspartame related compound A in the portion of Aspartame taken by the formula: in which C is the concentration, in µg per mL, of 5-benzyl-3,6-dioxo-2-piperazineacetic acid in the Standard solution; W is the weight, in mg, of Aspartame; and rU and rS are the peak responses of aspartame related compound A obtained from the Test solution and the Standard solution, respectively: not more than 1.5% is found.

Diluent , Mobile phase, Test solution, and Chromatographic system—Proceed as directed under Limit of 5-benzyl-3, 6-dioxo-2-piperazineacetic acid.

Diluted test solution— Pipet 2.0 mL of the Test solution into a 100-mL volumetric flask, dilute to volume with Diluent, and mix.

Procedure— Inject equal volumes (about 20 µL) of the Test solution and the Diluted test solution into the chromatograph, record the chromatograms, and measure the peak responses. [note—Continue the elution of the Test solution for twice the retention time of the aspartame peak.] The sum of the responses of all the peaks in the chromatogram of the Test solution, excluding the 5-benzyl-3,6-dioxo-2-piperazineacetic acid and aspartame peak responses, is not greater than the aspartame peak response obtained from the Diluted test solution, corresponding to not more than 2.0% of chromatographic impurities.

0.1 N Perchloric acid— Use perchloric acid, tenth-normal (in glacial acetic acid) as specified under Volumetric Solutions in the section Reagents, Indicators, and Solutions, but in the standardization, titrate to a green endpoint.

Procedure— Transfer about 300 mg of Aspartame, accurately weighed, to a 150-mL beaker, dissolve in 1.5 mL of anhydrous formic acid, and add 60 mL of glacial acetic acid. Add crystal violet TS, and immediately titrate with 0.1 N Perchloric acid to a green endpoint. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N perchloric acid is equivalent to 29.43 mg of C14H18N2O5. [note—A blank titration exceeding 0.1 mL may be due to excessive water content, and may cause loss of visual endpoint sensitivity.]