Modeling and analysis of the electrokinetic mass transport and adsorption mechanisms of a charged adsorbate in capillary electrochromatography systems employing charged nonporous adsorbent particles

Mass-transfer systems based on electrokinetic phenomena (i.e., capillary el ectrochromatography (CEC)) have shown practical potential for becoming powe rful separation methods for the biotechnology and pharmaceutical industries . A dynamic mathematical model, consisting of the momentum balance and the Poisson equations, as well as the unsteady-state continuity expressions for the cation and anion of the background electrolyte and of a positively cha rged analyte (adsorbate), is constructed and solved to determine quantitati vely the electroosmotic velocity, the electrostatic potential, the concentr ation profiles of the charged species in the double layer and in the electr oneutral core region of the fluid in the interstitial channels for bulk flo w in the packed chromatographic column, and the axial current density profi les as the adsorbate adsorbs onto the negatively charged fixed sites on the surface of the nonporous particles packed in the chromatographic column. T he frontal analysis mode of operation is simulated in this work. The result s obtained from model simulations provide significant physical insight into and understanding of the development and propagation of the dynamic profil e of the concentration of the adsorbate (analyte) and indicate that sharp, highly resolved adsorption fronts and large amounts of adsorbate in the ads orbed phase for a given column length can be obtained under the following c onditions: (i) The ratio, gamma (2,0), of the electroosmotic velocity of th e mobile liquid phase at the column entrance after the adsorption front has passed the column entrance to the electrophoretic velocity of the anion is very close to -1. The structure of the equations of the model and model si mulations indicate that a stable adsorption front cannot develop when gamma (2,0) is less than -1 unless the value of the mobility of the cation is le ss than the value of the mobility of the analyte, which may be a rare occur rence in practical CEC systems. (ii) The ratio of the mobility of the catio n to the mobility of the analyte is less than two orders of magnitude. This effect becomes more significant as the value of the equilibrium adsorption constant, K-A,K-3,K- of the analyte increases. (iii) The concentration of the analyte relative to the concentration of the cation is increased (feed solutions with less dilute concentrations of the analyte are employed). The refore, to obtain good performance for CEC systems operated in the frontal analysis mode (well-resolved adsorption fronts and high adsorbate amounts i n the adsorbed phase), one can choose an electrolyte whose cation has a mob ility that is not more than one or two orders of magnitude greater than the mobility of the analyte and whose anion has a mobility such that the value of gamma (2,0) is close to -1; one can then bring the value of gamma (2,0) closer to -1 by decreasing the particle diameter, d(p), and/or making the value of the surface charge density, delta (0), of the particles more negat ive (in effect, making the value of the zeta potential, zeta (p), at the su rface of the particles more negative at time t = 0) to change the value of the velocity, < upsilon (x)>/(x=0), of the electroosmotic flow (EOF) at the column entrance (< upsilon (x)>/(x=0) is determined after the adsorption f ront has passed the column entrance). This approach could provide condition s in the column that avoid overloading of the adsorbate. One can obtain faster breakthrough times at the sacrifice of resolution and utilization of the adsorptive capacity of the packed bed if one employs a cation whose mobility is very large relative to the mobility of the analyte and/or an anion that provides a value of gamma (2,0) significantly greater than -1. If it is possible, one can increase the concentration of the anal yte in the feed stream to avoid sacrificing resolution and adsorptive capac ity of the packed bed and still decrease the time at which breakthrough occ urs. Also, the dynamic behavior of the axial current density, i(x), profile s indicates that the magnitude of i, and/or the change in the value of i, a cross the adsorption front could serve as a measurement for the rate of pro pagation of the adsorption front through the column. Furthermore, the effec t of the decreased magnitude of the velocity of the EOF in the region of th e column where the analyte is present in the adsorbed phase could act to de crease the effect of tailing when CEC systems are operated in the pulse inj ection mode (analytical electrochromatography) because the higher velocity of the fluid upstream of the migrating adsorption zone may compress the tai l of the peak. (C) 2001 Academic Press.