Specific quantitative gas-liquid chromatographic analysis of methyldopa and some foreign related amino acids in raw material and commercial tablets.

A relatively simple gas-liquid chromatographic (GLC) procedure has been designed for the rapid detection and accurate quantitation of some theoretically possible foreign related amino acid contaminants in alpha-methyldopa raw material and commercial tablets. After trimethylsilylation of the drug or drug mixture with N,O-bis(trimethylsilyl)acetamide in acetonitrile at ambient temperature for 90 min, the derivatives are eluted from a methylsilicone column isothermally at 170 degrees. Quantitation of the components is effected by simple computation relative to dibenzyl succinate as the internal standard. The results obtained by applying the GLC procedure to the analysis of a number of multicomponent synthetic mixtures are in good agreement with the theoretical values. The percentage of label claim values obtained by the GLC method for commercial tablets are compared to those measured colorimetrically by the official U.S.P. procedure. No foreign related amino acid impurities were detected in any of the three commercial dosage forms examined.