Identification—
Transfer a portion of powdered Tablets, equivalent to about 10 mg of carbidopa, to a 100-mL volumetric flask containing about 50 mL of 0.05 N hydrochloric acid. Agitate for 20 minutes, add methanol to volume, mix, and filter or centrifuge. Separately prepare 2 Standard solutions containing 0.1 mg per mL of
USP Carbidopa RS and
USP Levodopa RS, respectively, in a solvent prepared by mixing equal volumes of 0.05 N hydrochloric acid and methanol. Apply 20 µL of the test solution and 20 µL of each Standard solution at separate points to a thin-layer chromatographic plate (see
Chromatography 
621
) coated with a 0.25-mm layer of chromatographic silica gel. Develop the chromatogram using a solvent system consisting of a mixture of acetone, chloroform,
n-butanol, glacial acetic acid, and water (60:40:40:40:35) until the solvent front has moved about 15 cm. Air-dry, spray uniformly with about 0.5 mL of ninhydrin reagent (prepared by dissolving 0.3 g of ninhydrin in 100 mL of
n-butanol acidified with 3 mL of glacial acetic acid), and heat at 105
for about 10 minutes: the solution under test exhibits two spots (reddish brown for levodopa and yellow-orange for carbidopa) having
RF values that correspond to those exhibited by the Standard solutions.
Dissolution 711—
Medium:
0.1 N hydrochloric acid; 750 mL.
Apparatus 1:
50 rpm.
Time:
30 minutes.
Procedure—
Determine the amounts of carbidopa and levodopa in solution in filtered portions of the solution under test, in comparison with a Standard solution having known concentrations of
USP Carbidopa RS and
USP Levodopa RS in the same medium, as directed for
Procedure in the
Assay.
Tolerances—
Not less than 80% (Q) of the labeled amounts of carbidopa (C10H14N2O4) and levodopa (C9H11NO4) is dissolved in 30 minutes.
Assay—
Sodium 1-decanesulfonate solution—
Dissolve 0.24 g of sodium 1-decanesulfonate in 1 L of water.
Mobile phase—
Mix 11.04 g of monobasic sodium phosphate and 950 mL of water in a beaker. Add 1.3 mL of Sodium 1-decanesulfonate solution, and adjust with phosphoric acid to a pH of 2.8. Transfer to a 1-L volumetric flask, dilute with water to volume, and pass through a membrane filter.
Standard preparation—
Transfer about 50 mg of
USP Levodopa RS, accurately weighed, to a 100-mL volumetric flask. Add an accurately weighed quantity of
USP Carbidopa RS, which is in a ratio with
USP Levodopa RS that corresponds with the ratio of carbidopa to levodopa in the Tablets. Add 10 mL of 0.1 N phosphoric acid. Warm gently to dissolve the standards. Dilute with water to volume, and mix.
Assay preparation—
Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 50 mg of levodopa, to a 100-mL volumetric flask, add 10 mL of 0.1 N phosphoric acid, dilute with water to volume, and mix.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate, about 2 mL per minute, is adjusted until the retention times for levodopa and carbidopa are about 4 minutes and 11 minutes, respectively. Chromatograph five replicate injections of the
Standard preparation, and record the peak responses as directed for
Procedure: the relative standard deviation is not more than 2.0%; and the resolution factor between levodopa and carbidopa is not less than 6.
Procedure—
Separately inject equal volumes (about 20 µL) of the
Standard preparation and the
Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of carbidopa (C
10H
14N
2O
4) in the portion of Tablets taken by the formula:
(100C)(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Carbidopa RS in the
Standard preparation; and
rU and
rS are the responses of the carbidopa peak obtained from the
Assay preparation and the
Standard preparation, respectively. Calculate the quantity, in mg, of levodopa (C
9H
11NO
4) by the same formula, reading the terms to refer to levodopa instead of carbidopa.
