[Research on the separation of caffeine and its nine analogues by micellar electrokinetic capillary chromatography]

The separation of caffeine and its nine analogues by micellar electrokinetic capillary chromatography (MECC) using sodium dodecyl sulphate (SDS) as the micellar phase was investigated. The effects of pH and concentration of phosphate buffer solution, SDS micelle concentration, methanol volume fraction, applied voltage and temperature on the separation were studied. It was found that the migration of these compounds was affected by these factors, especially by pH of the solution. The order of elution, as well as the migration time and separation efficiency of these compounds changes with the acidity of the solution, which suggests the disassociation of the hydrogen on the secondary amino group in these compounds. The optimized separation conditions consisted of a running buffer of 20 mmol/L sodium phosphate buffer, at pH 11.0, containing 20 mmol/L SDS, with an applied voltage of 25 kV and a temperature of 25 degrees C. All of the compounds were resolved within 9 min with the detection limits of 0.70 mg/L and the linearity in the range of 1.40 mg/L-45.5 mg/L with the correlation coefficients of 0.998-0.999.