Determination of 2,6-dimethylaniline and o-toluidine impurities in preparations for local anaesthesia by the HPLC method with amperometric detection.

2,6-Dimethylaniline (2,6-DMA) and o-toluidine (o-TLD) together with their decomposition products are potential technological impurities of Pharmaceuticals used for local anaesthesia, in which lidocaine and prilocaine appear as active substances. Pharmacopoeial analytical methods for the determination of these impurities are little sensitive (from about 1 microg ml(-1)) and accurate and provide results which are difficult to interpret. Taking the above into account, a sensitive and specific amperometric method has been developed, which enables, after separation with the use of HPLC, an accurate determination of the content of 2,6-DMA and o-TLD in various pharmaceutical preparations. The determinations were performed at a glassy carbon electrode at a potential of +0.85 V. The limit of detection for both 2,6-DMA and o-TLD was 0.8 ng ml(-1). On the other hand, the limit of quantitation, considering a signal to noise ratio, was 1.5 ng ml(-1). The developed method allows to determine low concentrations of the impurities in question, which are hardly 1/120000 of the main substance. Preparation and determination of samples is carried out in a relatively short time, thus the method can be applied to routine investigations. Statistical evaluation of the obtained results shows that the accuracy and precision of the elaborated HPLC-ED method is good.