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

The dynamic mathematical model of Grimes and Liapis [J. Colloid Interf. Sci . 234 (2001) 223] for capillary electrochromatography (CEC) systems operate d under frontal chromatography conditions is extended to accommodate condit ions in CEC systems where a positively charged analyte is introduced into a packed capillary column by a pulse injection (analytical mode of operation ) in order to determine quantitatively the electroosmotic velocity, electro static potential and concentration profiles of the charged species in the d ouble layer and in the electroneutral core region of the fluid in the inter stitial channels for bulk flow in the packed chromatographic column as the adsorbate adsorbs onto the negatively charged fixed sites on the surface of the non-porous particles packed in the chromatographic column. Furthermore , certain key parameters are identified for both the frontal and analytical operational modes that characterize the performance of CEC systems. The re sults obtained from model simulations for CEC systems employing the analyti cal mode of operation indicate that: (a) for a given mobile liquid phase, t he charged particles should have the smallest diameter, d(p), possible that still provides conditions for a plug-flow electroosmotic velocity field in the interstitial channels for bulk flow and a large negative surface charg e density, delta (0), in order to prevent overloading conditions; (b) sharp , highly resolute adsorption zones can be obtained when the value of the pa rameter gamma (2,min), which represents the ratio of the electroosmotic vel ocity of the mobile liquid phase under unretained conditions to the electro phoretic velocity of the anions (0 > gamma (2,min)> -1), is very close to n egative one, but the rate at which the solute band propagates through the c olumn is slow; furthermore, as the solute band propagates across larger axi al lengths, the desorption zone becomes more dispersed relative to the adso rption zone especially when the value of the parameter gamma (2,max), which represents the ratio of the electroosmotic velocity of the mobile liquid p hase under retained conditions to the electrophoretic velocity of the anion s (0>gamma (2,max)>-1), is significantly greater than gamma (2,min); (c) wh en the value of the equilibrium adsorption constant, K-A,K-3, is low, very sharp, highly resolved adsorption and desorption zones of the solute band c an be obtained as well as fast rates of propagation of the solute band thro ugh the column; (d) sharp adsorption zones and fast propagation of the solu te band can be obtained if the value of the mobility, nu (3) of the analyte is high and the value of the ratio nu (1)/nu (3), where nu (1) represents the mobility of the cation, is low; however, if the magnitude of the mobili ty, nu (3), of the analyte is small, dispersed desorption zones are obtaine d with slower rates of propagation of the solute band through the column; ( e) good separation of analyte molecules having similar mobilities and diffe rent adsorption affinities can be obtained in short operational times with a very small column length, L, and the resolution can be increased by provi ding values of gamma (2,min) and gamma (2,max) that are very close to negat ive one; and (f) the change in the magnitude of the axial current density, i(x), across the solute band could serve as a measurement for the rate of p ropagation of the solute band. (C) 2001 Elsevier Science B.V. All rights re served.