Determination of 5-aminosalicylic acid related impurities by micellar electrokinetic chromatography with an ion-pair reagent

A micellar electrokinetic chromatographic (MEKC) method was developed for the quantification of mesalazine or 5-aminosalicylic acid (5-ASA) and its major impurities 3-aminosalicylic acid, salicylic acid, sulfanilic acid and 4-aminophenol. The optimisation of the experimental conditions was carried out considering some important requirements: resolution, reproducibility, detection limits of 0.1% (m/m) or less, short total analysis time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds; the addition of tetrabutylammonium bromide (TBAB) to the SDS micellar system resulted in the complete separation of all the compounds. The effects on the separation by several parameters such as TBAB concentration, SDS concentration, background electrolyte pH and concentration, were evaluated. Using a fused-silica capillary (8.5 cm effective length) complete analysis was obtained in a very short time. Under the optimised final conditions [120 mM 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid buffer, pH 10.20, 65 mM SDS in the presence of 55 mM TBAB and 5% methanol] the method was validated for specificity, precision, linearity, limits of detection and quantitation, and robustness: the 5-ASA related impurities can be quantified at least at the 0.1% (m/m) level.