WALL ADSORPTION IN CAPILLARY ELECTROPHORESIS EXPERIMENTAL-STUDY AND COMPUTER-SIMULATION

A semi-quantitative model based on non-linear equilibrium chromatograp hy coupled with diffusion-driven sample sorption at the wall was devel oped to account for and predict potential binding of an analyte to the wall in It was then used for computer simulation of sample concentrat ion profiles corresponding tal conditions (sorption kinetics, capillar y length, wall capacity and initial sample concentration). The binding phenomena were also studied experimentally by means of analysis of th e sample peak shape (including peak height and area). Contrary to expe ctations, it was found that the interaction of small monovalent cation s with the charged capillary wall does not lead to strong adsorption, as the sample mass is not lost during experiment and the peak shape re mains close to that which one could expect in the absence of interacti on. For polycations (e.g., poly-L-histidine) at any pH above 3, sample adsorption is evident by a lack of return of the baseline to zero, af ter peak passage, with progressively higher levels at progressively in creasing buffer pH values. Upon several runs with a polycation, the su rface charge on the wan changes from negative to positive, as evidence d by reversal of electroosmotic flow. However, it was discovered that even under these last conditions, the sample-wall interaction was rath er strong. The influence of NaOH washing and the addition of different substances (urea, Tween-20, sodium chloride) on adsorption was studie d. The comparison between simulated results and experimental data is d iscussed.