A:Infrared Absorption 197M.

B:Ultraviolet Absorption 197U—

C: 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.

Test solution— Transfer about 500 mg of Levodopa, accurately weighed, to a 25-mL volumetric flask, add 10 mL of 1 N hydrochloric acid to dissolve the solid, then add 5 g of methenamine, swirl the contents to dissolve the methenamine, add 1 N hydrochloric acid to volume, and mix. Allow to stand in the dark at 25 for 3 hours, and measure the rotation.

Diluent, Mobile phase, System suitability solution, and Chromatographic system— Proceed as directed in the Assay.

Standard solution— Use the Standard preparation, prepared as directed in the Assay.

Test solution— Use the Assay preparation, prepared as directed in the Assay.

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 peak areas. Calculate the percentage of each impurity in the portion of Levodopa taken by the formula: in which C is the concentration, in mg per mL, of USP Levodopa RS in the Standard solution; F is the relative response factor of each impurity according the table below; W is the weight, in mg, of Levodopa, on the dried basis, used to prepare the Test solution; ri is the peak area for any impurity in the Test solution; and rS is the peak area for levodopa in the Standard solution: the impurities meet the requirements given in the table below.

(Official January 1, 2007)

Diluent— Prepare a mixture of trifluoroacetic acid and water (1 in 1000).

Mobile phase— Prepare a filtered and degassed mixture of Diluent and tetrahydrofuran (97:3). Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability solution— Dissolve suitable quantities of USP Levodopa RS, USP 3-Methoxytyrosine RS, and USP L-Tyrosine RS in Diluent to obtain a solution containing about 10 µg of each per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Levodopa RS in Diluent, and dilute quantitatively, and stepwise if necessary, with Diluent to obtain a solution having a known concentration of about 0.4 mg per mL.

Assay preparation— Transfer about 40 mg of Levodopa, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with Diluent to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains double-endcapped packing L1. The flow rate is about 1.0 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 1.0 for levodopa, 1.3 for L-tyrosine, and 1.6 for 3-methoxytyrosine; the resolution, R, between levodopa and L-tyrosine is not less than 3.0; the tailing factor is not more than 2.0 for levodopa; and the relative standard deviation determined from levodopa 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, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C9H11NO4 in the portion of Levodopa taken by the formula: in which C is the concentration, in mg per mL, of USP Levodopa RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.