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G Guiochon

Publications and source records attributed to G Guiochon.

At least 55 records · Page 3Linked to original sources
PubMed2000

Modeling of separations by closed-loop steady-state recycling chromatography of a racemic pharmaceutical intermediate.

Closed-loop steady-state recycling (SSR) is a cyclic, one-column process that is similar to simulated moving bed (SMB) chromatography in several respects. Both processes are cyclic. In both SMB and SSR, fractions are collected from the leading and trailing portions of the circulating chromatographic profile, and fresh sample is injected into the interior of the profile. However, SMB is a continuous process whereas SSR is a discontinuous, repetitive process. This paper presents a model for the closed-loop SSR process and its experimental validation in a case of practical importance. For this last purpose, we used the closed-loop SSR separation of the enantiomers of a racemic pharmaceutical intermediate. The experimental determination of the competitive adsorption equilibria was performed by frontal analysis in a system composed of a chiral HPLC column as the stationary phase and pure acetonitrile as the mobile phase. All the adsorption data were well correlated by the Langmuir model. The Langmuir model was used to calculate overloaded band profiles corresponding to the separation of racemic mixtures at both analytical and preparative scales. Theoretical band profiles were calculated using the equilibrium-dispersive model. With proper corrections for the contributions of the sources of extra-column band broadening, the model properly predicts the experimental band profiles obtained in the closed-loop SSR setup and demonstrates that a cyclic steady-state develops after the completion of a finite number of cycles. The results also show that the extra-column effects must be accounted for in order to model the closed-loop SSR process accurately.

Chromatography, High Pressure Liquid↗
PubMed2000

On-column visualization of sample migration in liquid chromatography.

A quantitative on-column visualization technique for evaluating solute migration in liquid chromatography columns was described. This technique employed a matched refraction index phase system in high-pressure glass columns. In this case, the mobile phase was carbon tetrachloride and the stationary phase was a C18 silica. Because the refractive indexes of the two phases were the same, the column bed, otherwise opaque, was transparent to the eye. Zones of colored solutes could be injected (iodine for instance) and their migration studied along the column. Photographing the sample band during its migration allowed chromatographic information to be extracted using digital technology. As an example of applications for this technique, we show the sample entry through various inlet fittings. The results demonstrate that chromatographers should pay more attention to the selection of inlet frits. Importantly, the frit porosity should be matched to the particle size of the packing.

Calibration↗
PubMed2000

Kinetic study of the mass transfer of bovine serum albumin in anion-exchange chromatography.

A kinetic study was made on the mass transfer phenomena of bovine serum albumin (BSA) in two different anion-exchange columns (Resource-Q and TSK-GEL-DEAE-5PW). The analysis of the concentration dependence of the lumped mass transfer rate coefficient (km,L) provided the information about the kinetics of the several mass transfer processes in the columns and the anion exchangers, i.e., the axial dispersion, the fluid-to-particle mass transfer, the intraparticle diffusion, and the adsorption/desorption. In the Resource-Q column, the intraparticle diffusion had a dominant contribution to the band broadening compared with those of the other processes. The surface diffusion coefficient (Ds) of BSA showed a positive concentration dependence, by which the linear dependence of km,L on the BSA concentration seemed to be interpreted. On the other hand, in the TSK-GEL-DEAE-5PW column, the contribution of the adsorption/desorption was also important and almost same as that due to the intraparticle diffusion. There are some differences between the intrinsic properties of the mass transfer kinetics inside the two anion exchangers. It was likely that the positive concentration dependence of Ds was explained by the heterogeneous surface model.

Chromatography, Ion Exchange↗
PubMed2000

Apparent and true enantioselectivity in enantioseparations.

The separation factor of two compounds in chromatography is the ratio of their equilibrium constants or retention factors. This parameter is universally employed to investigate their resolution and to optimize the experimental conditions of their analysis. In enantioseparations, the situation is more complex because there is a mixed retention mechanism. The retention factor is the sum of two contributions, one enantioselective, the other nonselective. Although both contribute to retention, the latter being identical for the two enantiomers and does not contribute to their separation. We show how these two contributions can be measured and how it becomes necessary to distinguish between the apparent, alpha(app), and the true, alpha(true), separation factors. The existence of nonselective sites is responsible for alpha(app) being less than alpha(true). Depending on the difference between these two factors, the more effective approach to improve a separation is either to increase the enantioselectivity or to reduce the nonselective interactions. Practical applications to separations of different beta-blockers on cellobiohydrolase are discussed. The apparent enantioselectivity of alprenolol is larger and increases faster with increasing pH than that of the more hydrophobic propranolol, in spite of the importance of hydrophobic interactions in the enantioselective mechanism. These two unexpected properties are discussed and explained.

Adrenergic beta-Antagonists↗
PubMed2000

Influence of the modification conditions of alkyl bonded ligands on the characteristics of reversed-phase liquid chromatography.

The parameters characterizing retention and the mass transfer mechanisms in reversed-phase liquid chromatography were derived from data acquired for a series of compounds on a silica gel bonded to alkyl ligands of various lengths and ligand densities. The adsorption equilibrium constant, the absolute value of the isosteric heat of adsorption, and the activation energy of surface diffusion increase while the surface diffusion coefficient decreases with increasing length of the alkyl chain and density of the C18 ligand. These results are consistent with increasing restriction of the mobility of an increasingly adsorbable molecule. There is a critical level of carbon content above which the four parameters just listed no longer change with increasing alkyl ligand density. This level is likely to depend on the sizes of the alkyl ligand and the sample molecule. Even at high densities of C18 chains, there may be a limitation in the contact area of the sample molecule and the alkyl ligand. The retention behavior on the stationary phases studied is explained by assuming that the sample molecules penetrate into the layer of alkyl ligands. Enthalpy-entropy compensation was demonstrated for surface diffusion. Its mechanism is probably similar on the different stationary phases studied, irrespective of the length and density of the bonded ligand.

Chromatography, Liquid↗
PubMed1999

Visualization of sample introduction in liquid chromatographic columns. Contribution of a flow distributor on the sample band shape.

The contributions to the radial distribution of the sample concentration across the column inlet and to the axial band dispersion resulting from a column header containing a distributor were evaluated using a band-visualization process entailing matching the refractive indices of the stationary and mobile phases in a glass column. This study illustrates graphically how a distributor fitted to the column can increase the axial dispersion of the sample band compared to an inlet containing only a frit. The distributor did not provide a uniform sample distribution across the column. In fact, for 17-mm inner diameter columns and high-porosity frits, the distribution was no better than with the frit having no distributor. However, when low-porosity frits were employed, improved peak shapes were obtained with a distributor. In addition, we observed that the inlet header configuration influenced dramatically the flow stream established along the column. The radial distribution of the efficiency of the columns was nearly homogeneous for those having only a frit but not for those having also a distributor. For the latter, the efficiency decreased from the column axis to its wall.

Chemical Phenomena↗
PubMed1999

Visualization of viscous fingering in high-performance liquid chromatographic columns. Influence of the header design.

Using an on-column visualization technique, band profiles of solutes migrating along an HPLC column were studied. The study showed that, under conditions where viscous fingering is prevalent, the design of the inlet header has little influence on the outcome of the viscous fingers. Two types of headers were studied. The first contained a small diameter inlet frit, which localized the majority of the sample in or near the central region of the column. The second header contained a wide frit and produced a more uniform radial distribution of the sample. In both cases, the extent of viscous fingering was essentially the same.

Chemical Phenomena↗
PubMed1999

Influence of column radial heterogeneity on peak fronting in linear chromatography.

Using numerical calculations of elution peak profiles, an explanation of the fronting behavior of elution peaks in linear chromatography was found in certain radial distributions of the mobile phase flow velocity and local bed efficiency. Fronting peaks are observed only if the flow velocity is higher in the wall region than in the center part of the column and the local efficiency is lower near the wall than in the center. By contrast, tailing or symmetrical peaks are observed if only the flow velocity or the local efficiency are radially heterogeneous. The degree of peak fronting increases with increasing amplitude of the radial distributions. The influence of the radial heterogeneity of the flow velocity on the degree of peak fronting is more severe for high than for low efficiency columns. An equation is suggested to correlate peak fronting behavior for columns of different efficiencies and a procedure proposed for the estimation of the radial distributions of the flow velocity and the local efficiency by analyzing some characteristics of asymmetric peaks.

Chromatography↗
PubMed1999

Visualization of sample introduction in liquid chromatography columns. The effect of the frit diameter.

A previously developed on-column detection technique using 35 mm SLR cameras [J. Chromatogr. A 826 (1998) 1] was employed to visualize colored sample bands as they elute through frits of differing diameter. Head fittings containing a 4.0 mm frit and a 15.9 mm frit were mounted in a 17 mm I.D. glass column packed with C18 silica with an average particle size of 21 microns. A carbon tetrachloride mobile phase of matching refractive index to that of the silica provides clarity along the column diameter during band migration. The photographs of the migrating sample zones were scanned and analyzed with appropriate imaging software. The smallest diameter frit induced severely parabolic sample distributions at the column inlet compared to the larger frit. Local axial dispersion coefficient values, expressed as local reduced plate height, were calculated as well as local zone velocities at the column inlet. The results demonstrate clearly the need to match the diameter of the inlet frit to the I.D. of the column so as to avoid the initial onset of zone broadening due to the frit.

Chromatography, Liquid↗
PubMed1999

Repeatability and reproducibility of retention data and band profiles on reversed-phase liquid chromatography columns. III. Results obtained with Kromasil C18 columns.

The reproducibility of the retention data and the band profiles was investigated with Kromasil C18 columns (silica-based monomeric type reversed-phase packing material). High precision data were obtained and statistically compared among five columns from the same batch (column-to-column reproducibility) and six columns, one from each of six different batches (batch-to-batch reproducibility). These data were acquired under five different sets of chromatographic conditions, for a group of 30 neutral, acidic and basic compounds selected as probes following an experimental protocol previously described. Data characterizing the retention time, the retention factor, the separation factor, the column efficiency and the peak asymmetry for the different probe compounds are reported. Factors describing the silica surface interaction with the selected probe compounds, such as the hydrophobic interaction selectivity, the steric selectivity, and the separation factors of basic compounds at different pH values were also determined. The influence of the underlying silica on these data and correlations between the chromatographic and physico-chemical properties of the different batches are discussed.

Chromatography, Liquid↗
PubMed1999

Kinetic study of the mass transfer of S-Tröger's base in the system cellulose triacetate and ethanol.

A recent study of the mass transfer kinetics of (-)- or S-Tröger's base (TB) between ethanol and microcrystalline cellulose triacetate (CTA) allows an analysis of the concentration dependence of the mass transfer rate coefficient (k(m)). S-TB elutes before R-TB. The retention time of the both compounds decreases with increasing temperature. In this study, experimental data measured between 30 and 50 degrees C were analyzed to provide information on the kinetics of several mass transfer processes which take place in the chromatographic column, i.e., axial and intraparticle dispersion, the fluid-to-particle mass transfer, and the kinetics of adsorption/desorption at the actual adsorption sites. Intraparticle diffusion has the dominant contribution to band broadening at high flow-rates. Both intraparticle diffusivity and the surface diffusion coefficient exhibit a small concentration dependence. The positive dependence of k(m) on the concentration of S-TB seems to result from the properties of the adsorption/desorption kinetics and can be interpreted by considering the phase equilibrium properties. A quantitative analysis of the activation energy of the mass transfer kinetics of S-TB in the CTA column was also attempted.

Cellulose↗
PubMed1998

Effect of the column length on the characteristics of the packed bed and the column efficiency in a dynamic axial compression column.

The axial homogeneity of preparative-scale chromatography columns was studied by measuring the overall properties of similar columns differing only by their lengths. The properties investigated were the packing density, the external porosity, the permeability and the column efficiency. Two different materials were used, one made of large, irregular silica particles compressed under either high or a low degree of stress; the other of small, spherical and rigid particles. The columns made with the spherical particles were more homogeneous, had a higher external porosity and specific permeability, and a higher efficiency than those made with the irregular ones. For this latter material, the columns prepared with a low level of stress were better than those compressed under high stress. The latter ones experienced extensive particle breakage. These results indicate that the long columns are heterogeneous in their axial direction.

Chromatography, Liquid↗
PubMed1998

Comparing the optimum performance of the different modes of preparative liquid chromatography.

A comparative study of the optimization of the different modes of the preparative separation of binary mixtures by liquid chromatography is presented. Band profiles were calculated by means of the equilibrium-dispersive model of chromatography in the cases of isocratic elution, gradient elution, and displacement chromatography. The objective function to be maximized was the product of the production rate and the recovery yield. The production rate was calculated using the same definition of the cycle time in all cases. This common definition accounts for column regeneration after each run in each mode of the separation. The calculations reveal that the number of experimental parameters to be adjusted to achieve optimum separations is relatively small. The major parameters are the loading factor and the number of theoretical plates, besides the displacer concentration in displacement chromatography, or the gradient steepness in gradient elution. The relative advantages of the different modes of preparative chromatography are discussed.

Chromatography, Liquid↗
PubMed1998

Extension of a Jovanovic-Freundlich isotherm model to multicomponent adsorption on heterogeneous surfaces.

A recently proposed Jovanovic-Freundlich isotherm model for single component adsorption without lateral interactions on heterogeneous surfaces is extended to account for lateral interactions and for competitive adsorption. The model is tested using previously reported single component and competitive adsorption data of 2-phenylethanol and 3-phenylpropanol on ODS-silica with methanol-water as the mobile phase. A comparison is made regarding the ability of the Jovanovic-Freundlich and Langmuir-Freundlich models to predict competitive equilibria using the single component identified parameters. Fair predictions of the competitive data were obtained when using heterogeneous-surface models which do not take into account the possible interactions of phenylalcohols in the adsorbed phase via hydrogen bonding. Markedly improved predictions were obtained with models which account simultaneously for the two main sources of adsorbed phase nonideal behavior, i.e. adsorbate-adsorbate interactions and heterogeneity of the adsorbent surface.

Adsorption↗
PubMed1998

Optimization of the recovery yield and of the production rate in overloaded gradient-elution reversed-phase chromatography.

To investigate the effects of various parameters on the production rate and on the recovery yield in overloaded reversed-phase gradient-elution chromatography, band profiles of binary mixtures of phenol and o-cresol were calculated using the experimental parameters of the distribution isotherms determined by binary frontal analysis. The effects of the feed volume and of the steepness of a continuous gradient (gradient time) of methanol in aqueous-organic mobile phases on the separation were studied. If the sample feed in a solvent with weak elution strength is used, combined effects of on-column enrichment, frontal chromatography and sharpening of the later eluted bands may enhance the production rate and the recovery yield. Steep continuous gradients offer better results than isocratic elution if the feed is dissolved in water and injected into the mobile phase with a higher elution strength, because larger volumes of dilute feed can be separated with high recovery yields.

Chromatography, Liquid↗
PubMed1997

Consolidation of particle beds and packing of chromatographic columns.

A variety of complementary evidence reviewed here demonstrates that the packing of chromatographic columns is heterogeneous. It is denser and less well organized close to the column wall than in the center of the column. The mechanism of particle consolidation under stress explains the origin of this phenomenon and provides a fundamental justification to what is known in column chromatography as "the wall effect".

Chromatography, Liquid↗