[Analysis of three antipyretic analgesic drugs by open-tubular capillary electrochromatography].

The advantages of capillary electrophoresis, such as small sample consumption, high separation efficiency, and multiple separation modes, have been known for decades. However, exploring unique capillary electrophoresis techniques for the analysis of fluid drugs in living bio-systems remains an important and urgent task. Owing to the similar structures and mass-to-charge ratios of antipyretic analgesic drugs, efficient baseline separation of these analytes by capillary zone electrophoresis method cannot be easily achieved. Micellar electrokinetic chromatography can improve the baseline separation of these drugs, but the substantial amounts of non-volatile surfactants (such as sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium deoxycholate and cetylammonium bromide) in running buffer solutions would pollute the ion source during mass spectrometric analysis. For this reason, it is difficult to analyze unknown drugs by capillary electrophoresis-electrospray ionization-mass spectrometry. To overcome these drawbacks, much attention has been paid to capillary electrochromatography (CEC) because combines the high separation efficiency of capillary electrophoresis with the high selectivity of high performance liquid chromatography (HPLC). Recent challenges encountered in open-tubular capillary electrochromatography (OT-CEC) expanding the range of suitable functional polymer monomers and improvement of the separation efficiency by tuning the characteristics of the polymer coatings without using any organic solvent additives. In this study, a protocol based on OT-CEC using a block co-polymer coating is proposed for the analysis of three test antipyretic analgesic drugs (4-aminoantipyrine, antipyrine and phenacetin), without adding organic solvents and surfactants in the running buffer solutions. First, an amphiphilic block co-poly(styrene-co-glycidyl methacrylate) (P(St-GMA)), was synthesized by reversible addition-fragmentation chain transfer polymerization under mild conditions. Then, P(St-GMA) was coated onto the capillary surface, and an OT-CEC analysis was performed. Next, the effect of some key factors, including the polymerization time for obtaining P(St-GMA) with different molecular weights, coating concentrations of the block copolymer, the species of the running buffer solutions, pH and concentrations of the running buffer solutions, and organic solvent additives, on the OT-CEC separation efficiency were investigated. Under the optimized conditions with 50.0 mmol/L NaAc-HAc as the running buffer solution at pH 5.7, the three test antipyretic analgesic drugs were base-line separated by the constructed OT-CEC system. Good linear relationships between peak area and concentration of the test analytes in the range of 8.0-2.5×103 μmol/L were obtained (R2 ≥ 0.995). The limits of detection (LODs) were 1.0-2.5 μmol/L. Furthermore, the reason for the OT-CEC separation efficiency was clarified based on the decreased electro-osmotic flow in the coated capillary compared with that in the uncoated capillary. Finally, the proposed OT-CEC assay without using any organic solvents and surfactants as additives was applied for analysis of the three test antipyretic analgesic drugs in rat serum samples. Importantly, it was found that despite peak tailing, the OT-CEC separation efficiency of the drugs was dramatically enhanced because the block co-polymer could self-assemble in the solution and form pseudo-micelles, which further increased the interactions between the P(St-GMA) and these drugs. Our results not only reveal the great potential of block co-polymer coatings in OT-CEC for the analysis of drugs in real biological samples, but also serve asa platform for the preparation of diverse block co-polymers to be used in OT-CEC analysis. We believe that in the near future, the peak tailing problem in OT-CEC analysis can be resolved by using the designed unique block co-polymers, which possess a greater number of functional sites, as coatings and by appropriately tuning the interactions between the analytes and the coatings.