Determination of related substances of levodopa including the R-enantiomer by CE

A CE assay for the simultaneous determination of the impurities of levodopa listed in the European Pharmacopoeia including the (R)-enantiomer was developed and validated. The analysis was performed in a fused-silica capillary employing sulfated beta-cyclodextrin as chiral selector at an applied voltage of 20 kV and a temperature of 18 degrees C. The optimized background electrolyte consisted of 0.1 M sodium phosphate buffer, pH 2.0, containing 6 mg/mL sulfated beta-cyclodextrin. L-phenylalanine was used as the internal standard. The assay was validated in the range of 0.1-1.0% for the impurities at a concentration of levodopa of 2 mg/mL. The effect of different batches of sulfated beta-cyclodextrin was investigated using levodopa and L-tyrosine as critical pair. The method was applied to determine the purity of several samples of levodopa including the chemical reference substance of the European Pharmacopoeia.