The analysis of pharmaceutical compounds using electrochromatography (original) (raw)
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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.
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 ( ) ( )
Chromatographia, 2014
The aim of this work was to find a relationship between electroosmotic flow (EOF) velocity of the mobile phase in pressurized planar electrochromatography (PPEC) and physicochemical properties like zeta potential, dielectric constant, and viscosity of the mobile phase as well as its composition. The study included different types of organic modifiers (acetonitrile, methanol, ethanol, acetone, formamide, N-methylformamide and N,N-dimethylformamide) in the full concentration range. In all experiments, chromatographic glass plates HPTLC RP-18 W from Merck (Darmstadt) were used as a stationary phase. During the study we found that there is no linear correlation between EOF velocity of the mobile phase and single variables such as zeta potential or dielectric constant or viscosity. However, there is quite strong linear correlation between EOF velocity of the mobile phase and variable obtained by multiplying zeta potential of the stationary phase-mobile phase interface, by dielectric cons...
Journal of Liquid Chromatography, 1992
This study examines typical organic solvents which may be used for analyte dissolution and as reference solvent for determination of electroosmotic velocity. Five solvents were investigated: acetone, acetonitrile, methanol, I-propanol, and tetrahydrofuran at SOS concentrations of 40,60, SO, 100, and 120 mM. A precise method for measuring e1ectroosmotic velocity is presented which minimizes run to run variations and yields electroosmotic velocities that are reproducible. Tetrahydrofuran exhibits the greatest electroosmotic velocity at 40 mM 80S, while I-propanol has the lowest electroosmotic velocity at 40 mM SOS. Acetone, acetonitrile, and methanol are determined to have electroosmotic velocities that are approximately the same at 40 mM 80S. At 60, SO, 100, and 120 mM SOS the migration velocity of each of the five solvents decreased linearly under our operating conditions and are approximately the same at each of these SOS concentrations.
ELECTROPHORESIS, 2003
Peak dispersion effects in nonaqueous capillary electrophoretic separations of aromatic anionic analytes were investigated in a propanolic background electrolyte solution. Poly(glycidylmethacrylate-co-N-vinylpyrrolidone) coating was applied to the capillary to suppress the electroosmotic flow and to improve the repeatability of the migration times. Electrical field strengths up to 2000 Vcm 21 were applied in separations and the separation efficiencies were compared with theoretical values calculated on the basis of plate height theory. The contributions to the total plate height were calculated for injection plug length, diffusion, Joule heating, electromigration dispersion, analyte adsorption to the capillary wall, and detector slit aperture length. Analyte diffusion coefficients were measured by Taylor dispersion method, while distribution constants were measured chromatographically. Agreement between the calculated and empirical results was fairly good even though some approximations were required. In most cases the longitudinal diffusion contribution governed the total plate height, while the contribution of Joule heating was insignificant even at exceptionally high field strengths used. The relatively long detection slit aperture was found to influence the separation efficiency strongly, while the other dispersion sources that were investigated were of minor importance, except for adsorption in the case of one analyte. With all analytes, the dispersive effect of longitudinal diffusion was reduced as the field strength was increased, leading to enhanced migration velocities and faster separations.
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.
Chemical Engineering & Technology, 2004
The chromatographic performance with respect to the flow behaviour and dispersion in fixed beds of nonporous and macroporous particles (having mean intraparticle pore diameters of 41 nm, 105 nm, and 232 nm) has been studied in capillary HPLC and electrochromatography. The existence of substantial electroosmotic intraparticle pore flow (perfusive electroosmosis) in columns packed with the macroporous particles was found to reduce stagnant mobile mass transfer resistance and decrease the global flow inhomogeneity over the column cross-section, leading to a significant improvement in column efficiency compared to capillary HPLC. The effect of electroosmotic perfusion on axial dispersion was shown to depend sensitively on the mobile phase ionic strength and mean intraparticle pore diameter, thus, on an electrical double layer interaction within the particles. Complementary and consistent results were observed for the average electroosmotic flow through packed capillaries. It was found to depend on particle porosity and distinct contributions to the electrical double layer behaviour within and between particles. Based on these data an optimum chromatographic performance in view of speed and efficiency can be achieved by straightforward adjustment of the electrolyte concentration and characteristic intraparticle pore size.
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.
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.