A: Dilute a volume of Injection with a mixture of chloroform and methanol (1:1),if necessary,to obtain a solution containing about 5mg of methyldopate hydrochloride per mL.Apply separately 10µLof this solution and 10µLof a Standard solution of USP Methyldopate Hydrochloride RSin a solvent mixture of chloroform and methanol (1:1)containing 5mg per mLto a suitable thin-layer chromatographic plate (see Chromatography á621ñ)coated with a 0.25-mm layer of chromatographic silica gel.Allow the spots to dry,and develop the chromatogram in a saturated chamber with a solvent system consisting of a mixture of butyl alcohol,water,and formic acid (7:2:1),until the solvent front has moved about three-fourths of the length of the plate.Remove the plate from the developing chamber,mark the solvent front,and allow the solvent to evaporate.Locate the spots on the plate by lightly spraying with Folin-Ciocalteu phenol TSfollowed by spraying with sodium carbonate solution (1in 10):the RFvalue of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

B: It responds to the tests for Chloride á191ñ,with the exception that the 6Nammonium hydroxide is omitted.

Buffer solution— To 214g of monobasic potassium phosphate add 700mLof water,and stir.Cautiously add 75mLof sodium hydroxide solution (1in 2),and stir until solution is complete.Adjust with sodium hydroxide solution (1in 2)to a pHof 8.0,and dilute with water to 1000.0mL.

Water-saturated tributyl phosphate— Shake 800mLof tributyl phosphate with 100mLof water,and discard the lower,aqueous phase.Filter the upper phase.

Standard preparation— Transfer about 25mg of USP Methyldopate Hydrochloride RS,accurately weighed,to a 25-mLvolumetric flask,add water to volume,and mix.Transfer 5mLof this solution to a 100-mLvolumetric flask,add 0.1Nsulfuric acid to volume,and mix.Use a freshly prepared solution.The Standard preparationcontains about 50µg per mL.

Assay preparation— Transfer to a 50-mLvolumetric flask an accurately measured volume of Injection,equivalent to about 50mg of methyldopate hydrochloride,add water to volume,and mix.Transfer a 5.0-mLaliquot of the solution to a 60-mLseparator,add 15mLof Buffer solutionand 10mLof Water-saturated tributyl phosphate,and shake for about 1minute.Allow the phases to separate,and transfer the lower,aqueous phase to a second 60-mLseparator.To this separator add a second 10-mLportion of Water-saturated tributyl phosphate,shake for about 1minute,allow the phases to separate,discard the lower,aqueous phase,and add the upper tributyl phosphate phase to the phase retained in the first separator.Rinse the second separator with about 2mLof Water-saturated tributyl phosphate,and add the rinsing to the first separator.Extract the phase contained in the first separator with two 25-mLportions of 0.1Nsulfuric acid.Collect the acid extracts in a 100-mLvolumetric flask,add 0.1Nsulfuric acid to volume,and mix.Filter,if necessary,to obtain a clear solution.

Procedure— Concomitantly determine the absorbances of the Assay preparationand the Standard preparationin 1-cm cells at the wavelength of maximum absorbance at about 283nm,with a suitable spectrophotometer,using 0.1Nsulfuric acid as the blank.Calculate the quantity,in mg,of methyldopate hydrochloride (C12H17NO4·HCl)in each mLof the Injection taken by the formula: in which Cis the concentration,in µg per mL,of USP Methyldopate Hydrochloride RSin the Standard preparation;Vis the volume,in mL,of Injection taken;and AUand ASare the absorbances of the Assay preparationand the Standard preparation,respectively.