Dynamics of Capillary Electrochromatography: Experimental Study of Flow and Transport in Particulate Beds (original) (raw)
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The Journal of Physical Chemistry B, 2001
The electrical conductivity of capillary electrochromatography (CEC) columns packed with macroporous particles has been investigated. Columns were prepared with commercially available octadecylsilane-coated 7 µm diameter particles (Nucleosil) having nominal pore diameters of 100, 300, 500, 1000, and 4000 Å, and operated under typical CEC conditions. The conductivity of the 100 Å column was in agreement with that predicted from theory for nonporous spheres, indicating that intraparticle current was negligible. Columns packed with the wide-pore media (1000 Å and 4000 Å), in contrast, yielded conductivity values over 2-fold greater than the 100 Å. The electroosmotic contribution to current flow in these columns was deemed insignificant on the basis of theoretical modeling and the experimental data. It was therefore concluded that the increased column conductivity of the wide-pore packed columns was the result of intraparticle current transport. These results further suggest that wide-pore packings are more permeable to fluid flow and thus can provide maximum gains in efficiency due to electroosmotic perfusion when electrical double layer thickness is small relative to the median pore diameter of the packing.
Journal of Chromatography A, 2002
The use of wide-pore stationary phases in capillary electrochromatography has shown exceptional increases in separation efficiency in conjunction with high electroosmotic flow. These effects are due to the perfusive flow mechanism which is primarily controlled by the ionic strength of the mobile phase. Good correlation between calculated values of electrochemical double-layer thickness and efficiency data have also been obtained. Reduced plate height values of ,0.5 have been observed with pore sizes of 4000 A. In addition, electroosmotic flow mobility twice that of 3 mm Spherisorb ODS-1 has been obtained.
Capillary electrochromatography: theories on electroosmotic flow in porous media
Journal of Chromatography A, 1997
In view of the present interest in capillary electrochromatography (CEC), theories dealing with electroosmotic flow (EOF) in porous media are reviewed with particular regard to the use of packed capillaries in CEC. Two of the models found in the pertinent literature are applicable to CEC and give simple analytical solutions. The first of the two models is based on yon Smoluchowski's work as adapted and extended by Overbeek. It deals with EOF through packed capillaries under conditions of low electric field ~,;trength where the EOF varies linearly with the field strength because there is no polarization of the double layer. Overbeek's model originally developed for porous media of infinite dimensions was modified in an attempt to account for the wall effect that assumes importance in the packed capillary columns used in CEC. The second model proposed by Dukhin and his coworkers predicts EOF of at least an order of magnitude higher than that expected by classical theories. This "electroosmosis of the second kind" is believed to occur in columns packed with conductive particles like ion exchangers at high electric field strengths when the double layer is polarized and the EOF becomes a non-linear function of the applied voltage. Conditions necessary for electroosmosis of the second kind are likely to arise upon the further development of CEC when further enhancement of the speed of analysis is brought about at electric field strength higher than that employed at present.
Analytical Chemistry, 2000
A model to estimate the extent of intraparticle, or perfusive, electroosmotic flow (EOF) in CEC capillaries packed with macroporous particles has been developed. Nucleosil packings (d p ) 7 µm) having nominal pore sizes of 500, 1000, and 4000 Å were studied. Intraparticle pores ranging from 50 to 10 000 Å in diameter were partitioned into 995 intervals of 10 Å. Using pore size distribution data for the sorbents obtained by mercury intrusion porosimetry, fractions of the total column void volume contributed by pores in the range of interest were determined. The average channel diameter of the interstitial space was estimated from the d p of the packing; its fraction of total column volume was determined from the interstitial porosity. Estimations of relative EOF velocity in the intraparticle and interstitial channels were made by treating the channels as parallel cylindrical capillary tubes. Relative EOF values were combined with the volume fraction data and used as weighting factors in calculating an effective particle diameter (d p,eff ) for each set of conditions (i.e., packing type, ionic strength of eluent). Values of d p,eff generated by the model correctly predict the trends observed in the experimental data. At the lowest ionic strength, plate height correlated inversely with the pore size of packing (h 4000 Å < h 1000 Å < h 500 Å ). Rate curves for each column tended toward lower plate heights with increasing eluent ionic strength before converging at some limiting point. The point of convergence was reached at moderate ionic strengths for the larger pore media (1000 and 4000 Å) and higher ionic strength for the 500 Å.
ELECTROPHORESIS, 2003
The relevance and magnitude of an electroosmotic perfusion mechanism in electrochromatography is analyzed. To systemize our studies we first considered the transport of an electroneutral and nonadsorbing tracer. Based on the refractive index matching in a microfluidic setup containing fixed spherical porous particles, we conducted a quantitative analysis in real time of the spatio-temporal distribution of fluorescent tracer molecules during their uptake by (and a release from) single particles using confocal laser scanning microscopy. Even under conditions of a significant electrical double layer overlap the intraparticle electroosmotic flow produces due to its unidirectional nature and in striking contrast to the symmetric (spherical) distributions typical for purely diffusive transport strongly asymmetric concentration profiles inside spherical particles as the locally charged pore liquid begins to respond to the externally applied electrical field. The profiles retain an axisymmetric nature, i.e., rotational symmetry with respect to the field direction. Results of our measurements could be successfully interpreted and further analyzed by a compact mathematical model. Intraparticle Peclet numbers of up to 150 have been realized and found to significantly enhance the mass transport on particle scale towards the convection-dominated regime when compared to a conventional (diffusion-limited) kinetics.
Perfusive electroosmotic transport in packed capillary electrochromatography: Mechanism and utility
Journal of Microcolumn Separations, 1997
In this study, silica-based packing materials with pore sizes herein . referred to primarily as channel diameters, d ranging from 60 to 4000 A were ch utilized to elucidate the role of pore size in CEC. Separations were conducted using a commercially available CE system, the inlet and outlet buffer reservoir compartments having been modified to allow constant application of ;150 psi of helium head pressure. This application of pressure was sufficient to prevent nucleation of gas bubbles, a common practical hindrance in CEC. On-column UV detection was performed following the outlet frit of the packed section of the capillary. Electrokinetic sample introduction was employed. The experimental results of these studies indicate that the use of wide-pore media in CEC may provide for notable increases in efficiency. When wide-channel, corpuscular silica packing materials are employed in CEC, the microbeads which comprise the larger particle begin to dictate the ''true'' or effective diameter of the packing particles, and analytes experience two distinct regions of pore space through which eluent Ž . flows: the intraparticle region inside the particles of packing material and the Ž . interparticle region between adjacent particles of packing material . The percent of total pore volume contributed by through-pores is likely to be relatively small in the packing materials used, suggesting that particles with a greater degree of perfusive character could provide even further increases in both efficiency and loading capacity. These larger diameter packing materials are easier to work with, less expensive, and offer increased probability of success in the manufacture of packed capillaries for CEC.
Journal of Microcolumn Separations, 1997
The influence of mobile phase composition variation, organic solvent type, and the concentration of buffer salts on the magnitude of the electroosmotic Ž . flow EOF velocity, retention, and selectivity in capillary electrochromatography Ž . CEC has been investigated systematically. The observed change in EOF is explained in terms of change of solvent and stationary phase properties. These findings provide guidelines for the practitioner to select optimal conditions for CEC separations. On the other hand, it is demonstrated that stationary phase properties also have a profound effect on EOF velocity, solute retention, and selectivity of separation. It is demonstrated that the column packed bed of silica-based reversed-phase particles is the main contributor to EOF in CEC. Variation of stationary phases in CEC can be used in a similar way as in HPLC to improve the selectivity of separation of neutral substances. This also applies to the separation of weakly basic substances like triazines. ᮊ 1997 John Wiley & Sons, Inc. J Micro Sep 9: 399᎐408, 1997 ( ) ( )
Theory of capillary electrochromatography
Journal of Chromatography A, 2001
The present state of the theory of capillary electrochromatography (CEC) is reviewed. Emphasis is placed on electroosmosis and the electrical double layer, and the generally good understanding of the factors affecting the electroosmotic flow in CEC columns. The relation of CEC to other electrically driven separations are described, along with band broadening, and the influence of column temperature in CEC. The theoretical potential of CEC is assessed from the standpoint of current and future column technology, and likely future application areas are described.
Electrophoresis, 2006
Different models have been described in the literature to evaluate the total porosity of CEC columns: gravimetric, flow as well as conductivity-based methods. In this study, these models have been compared for two kinds of CEC columns: two mixed-mode silica particle stationary phases and different monolithic columns (acrylate or polystyrene divinylbenzene-based). The total porosities measured from the conductivity-based methods were lower than the total column porosities obtained by gravimetric or flow methods for all the investigated columns while the wide distribution of observed values shows that conductivity-based methods discriminate columns more efficiently with very different properties. We propose a conductivity-based method taking into account the actual length proposed by Horvath, to evaluate what we call an ”actual electrokinetic” porosity (AEP). This parameter, based on electrokinetic theory only, affords the most consistent evaluation of porosity under experimental CEC conditions for the packed- and acrylate-based monolithic columns. To illustrate the potential of AEP and actual EOF for the estimation of the performances of a CEC system (stationary and mobile phases) we studied the influence of the mobile-phase composition on these parameters for CEC separations with an ammonium embedded packed stationary phase. The AEP and the actual electroosmotic mobility should allow a better understanding of the perfusive EOF and stationary-phase wettability. For neutral compounds (substituted phenols), AEP evaluation allowed us to predict the mobile-phase conditions able to enhance the efficiency while both AEP and actual EOF had to be considered in the case of peptide analysis.
Journal of Chromatography A, 2000
In capillary electrochromatography (CEC) the propulsion of the mobile phase is effected by electroosmosis. The velocity of the electroosmotic flow is dependent on surface properties of the stationary phase and on bulk properties of the mobile phase. Therefore, in CEC the optimization of the mobile phase composition must take more factors into account than in pressure-driven LC. In this paper, the impact of the electrolyte concentration in the mobile phase and of the volume fraction of the organic mobile phase constituent on the velocity of the electroosmotic flow and on the chromatographic efficiency is investigated for CEC with capillaries packed with octadecylsilica gel. Bias of the data by an open section of the capillary has been excluded by employing completely packed capillaries and detection in a packed section. Acetonitrile as organic constituent of the mobile phase is compared to other possible organic modifiers (polar organic solvents) concerning influence on velocity of the electroosmotic flow and retention of solutes.