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A Comparative Study of Different Chromatographic Techniques for Determination of Toxic Impurities of Some Commonly Used Anesthetics

2,6-Dimethylaniline (DMA) and o-toluidine (TOL) are considered as toxic impurities and carcinogenic metabolites as well as hydrolytic products upon degradation process for lidocaine (LD) and prilocaine (PR), respectively. The presence of such impurities above the permutable limits can adversely affect health and cause severe complications. Three different separation techniques, namely, high-performance liquid chromatography (HPLC), capillary zone electrophoresis (CZE), and thin-layer chromatography (TLC), were applied for the selective determination of the toxic compounds DMA and TOL and their intact drugs, LD and PR, respectively. The proposed HPLC method was carried out on Inertsil ODS 3 column (50 mm × 4.6 mm, 5 μm), utilizing a green mobile phase consisting of ethanol and 34 mM disodium hydrogen phosphate (20:80, v/v; pH = 6 adjusted using o-phosphoric acid). The flow rate was 1 mL min−1 with UV detection at 220 nm. The CZE separation was performed using 50 mM monobasic sodium phosphate (pH = 2.5), and voltage programming started with 22 kV for the first 6.7 min down to 18 kV from 6.9 min till 10 min. The suggested TLC method was achieved on TLC plates pre-coated with silica gel 60 F245 using a developing solvent containing toluene—chloroform—ethanol—triethylamine (5:5:1:0.1, by volume). These methods were validated with respect to linearity range, accuracy, precision, limit of detection, limit of quantification and robustness and were successfully applied for the determination of the two drugs in pure form and in pharmaceutical dosage form. A comparative study was held between the three chromatographic methods to evaluate the selectivity of the separation and the sensitivity, accuracy, and precision of the quantification.