Biomedical subjects

G Guiochon

Publications and source records attributed to G Guiochon.

At least 37 records · Page 2Linked to original sources

PubMed2001

Description of adsorption equilibria in liquid chromatography systems with binary mobile phases.

Adsorption of some simple compounds from pure and mixed solvents and of some solvents from mixed binary solvent mixtures on columns used in normal-phase, aqueous-organic and non-aqueous liquid chromatography was investigated. The distribution of the compounds between the liquid and the stationary phase is affected by the composition of the solvent mixture. Although preferential sorption of stronger solvents can often be described by the Langmuir isotherm, significant deviations are observed in some systems. A model was suggested accounting for the deviations from Langmuir isotherm by association on already adsorbed molecules. In most systems studied in this work, a simple competitive Langmuir isotherm did not provide a good fit to the experimental distribution data from mixed solvents when competition between the solute and the strong solvent for the adsorption sites was considered. A competitive isotherm taking into account possible association of solute on its own already adsorbed molecules and on the adsorbed molecules of strong solvent improves the fit to the experimental distribution data and enables description of the distribution data in dependence on the concentration of the strong solvent in mixed solvents in normal-phase, aqueous-organic and non-aqueous reversed-phase systems. Even though we need more data to prove the validity of the present model, we observe a good qualitative agreement of the experimental isotherm coefficients with the relative strength of the interactions expected in various chromatographic systems employing mixed solvents.

Adsorption↗

PubMed2001

Influence of the solute hydrophobicity on the enantioselective adsorption of beta-blockers on a cellulase protein used as the chiral selector.

Adsorption isotherm data were acquired at different eluent pH values for the enantiomers of several beta-blockers on cellobiohydrolase I on silica gel. They fit well to the biLangmuir model, allowing the determination of the equilibrium constants and the monolayer capacities for chiral and nonselective adsorption. The adsorption of the S-enantiomers (eluted second) is exothermic at low pH, endothermic at high pH, and athermal in a narrow pH range depending on the beta-blocker. This transition pH range is lower for S-alprenolol than for the more hydrophobic S-propranolol, although their endothermic adsorption originates from hydrophobic interactions. This surprising observation is explained by the relative values of the isotherm coefficients. S-Alprenolol seems to have a more pronounced endothermic behavior than S-propranolol because the nonselective interactions of both compounds with the stationary phase are exothermic but their contribution to retention, relative to that of the endothermic chiral interactions, is less important for alprenolol. The order of increasing energy of the chiral interactions is the same as that of hydrophobicity, propranolol>alprenolol>metoprolol.

Adrenergic beta-Antagonists↗

PubMed2000

A kinetic study of mass transfer in reversed-phase liquid chromatography on a C18-silica gel

The characteristic features of mass-transfer kinetics in a reversed-phase (RP) column packed with a C18-silica were studied. The relevant information on phase equilibrium thermodynamics and mass-transfer kinetics was obtained by frontal analysis and the pulse method, respectively. The equilibrium isotherm was accounted for by the simple Langmuir model. The ratio of the axial dispersion coefficient to the mobile-phase flow velocity increased almost linearly with increasing solute concentration. Similarly, the mass-transfer rate coefficient (km) showed a linear dependence on the solute concentration. The positive concentration dependence of km resulted from that of the surface diffusion coefficient, which was interpreted with the chemical potential driving force model. The contribution of axial dispersion to band broadening was predominant in the RP column packed with the medium-size packing material used (particle diameter, 12 microns) whereas that of the kinetics of adsorption/desorption was negligibly small. The results of this study demonstrate how an analysis of the dependence of the mass-transfer kinetics on the flow velocity and the solute concentration allows a better understanding of this kinetics.

Journal Article↗

PubMed2000

Enantioseparation of 1-phenyl-1-propanol by simulated moving bed under linear and nonlinear conditions.

Approximately optimum operating conditions needed to separate 1-phenyl-1-propanol (PP) enantiomers by simulated moving bed (SMB) were determined using the "safety margin" approach in the linear case and the "triangle theory" in the nonlinear case. Previous results showed the adsorption isotherm data to fit well to the competitive Langmuir model. This allowed the use of the triangle theory approach that applies straightforwardly to Langmuir systems. Experimentally, the operating conditions under nonlinear isotherm behavior were determined for a feed solution with a total concentration of 5 g/l. The purity of the products exceeded 98% for the raffinate and 95% for the extract. Failing to reach complete purity while the experimental conditions were chosen inside the separation zone is explained by the nonideality of the system used, which violates one of the triangle theory assumptions. The computed overall daily production rate was 11.6 g of racemic PP processed per day per kg of stationary phase, a result that compares favorably with previous ones.

Adsorption↗

PubMed2000

Influence of the column radial heterogeneity on the determination of single-component isotherms by the elution by characteristic point method.

The influence of the column radial heterogeneity on the determination of equilibrium isotherm data by the elution by characteristic point (ECP) method is studied using nondimensional numerical calculations and taking into account typical radial distributions of the mobile phase flow velocity and the column efficiency across a column. Overloaded elution peaks were calculated with the equilibrium-dispersive model as a function of four dimensionless parameters, the number of theoretical plates at the center of the column, the Langmuir equilibrium constant, the retention and the loading factors. The influence of the mass transfer resistances and the radial heterogeneity of the column on the ECP data was analyzed by comparing the true isotherm and the one estimated from the diffuse profile of overloaded peaks. The results provide information on the accuracy of the ECP method. The error made increases with increasing degree of radial heterogeneity. This error can be corrected by using the results of the nondimensional numerical calculations, allowing a further extension of the applicability of the ECP method.

Chromatography↗

PubMed2000

Determination of the lumped mass transfer rate coefficient by frontal analysis.

The validity of measurements of the lumped mass transfer rate coefficient (km,L) is studied on the basis of experimental data acquired under Langmuir isotherm conditions, in reversed-phase liquid chromatography. Two different methods were used, the perturbation method and frontal analysis. Accurate values of km,L can be properly obtained by the perturbation method because, with this method, the chromatographic processes take place under locally linear isotherm conditions. Values of km,L can also be derived from the breakthrough curves obtained in frontal analysis. Because the contribution of axial dispersion to band broadening was larger than that of the mass transfer resistances, the apparent axial dispersion coefficient (Da) was first derived from the breakthrough curve by applying the equilibrium-dispersive model. Then, the value of km,L was calculated from Da. The values of km,L determined by the two methods were in close agreement in the range of nondimensional Langmuir equilibrium constants (r= 1/[1+KLC0]) between 0.32 and 0.85, irrespective of the mobile phase flow velocity. Thus, frontal analysis can be used for kinetic studies of the mass transfer in chromatographic columns.

Chromatography, Liquid↗

PubMed2000

Retention mechanism of beta-blockers on an immobilized cellulase. Relative importance of the hydrophobic and ionic contributions to their enantioselective and nonselective interactions.

The adsorption isotherms of the enantiomers of three beta-blockers, metoprolol, alprenolol, and propranolol, were measured on cellobiohydrolase I (CBH I) immobilized on silicagel, in the concentration range between 0.25 microM and 1.7 mM, at pH = 5.0, 5.5, and 6.0. In agreement with previous results, these data are accounted for by a two-sites physical model and fit closely to a Bilangmuir equation. The saturation capacities and the binding constants were determined for each enantiomer on the chiral and the nonchiral sites. The chiral sites are shown to be strongly ionic, in contrast to the nonchiral ones, which are mainly hydrophobic. However, the chiral binding of (S)-propranolol is endothermic, with a high adsorption entropy, in contrast to the chiral interactions of (R)-propranolol and to the nonchiral interactions, which are all exothermic. This indicates that hydrophobic interactions also play a role in the chiral binding. The dependence of the adsorption parameters on the hydrophobicity of the solute is discussed and interpreted in terms of the retention mechanism. The results are compared with the structure of the protein, recently elucidated by X-ray crystallography.

Adrenergic beta-Antagonists↗

PubMed2000

Physical evidence of two wall effects in liquid chromatography.

Using optical on-column visualization for the study of the migration of sample bands, the radial variations of the local migration rate were studied in the region near the column wall. Photographs of small sample bands migrating along the column at various radial locations were obtained. On-column chromatograms extracted from these photographs showed evidence of two wall effects. The first of these effects was present only within the immediate vicinity of the wall. It is a direct result of the inability of the packing material to form a close packed configuration against the rigid column wall surface. The second wall effect causes a systematic variation of the migration rate of the sample band in the region of the wall, this rate increasing from the wall to the central region of the column. The corresponding images portrayed the classical "wall effect" that chromatographers have long discussed. They also show that this effect extends further into the column than anticipated. As to what are the relative contributions to the results of our observations of the wall effect and of a frit effect discussed in previous publications, this could not be ascertained.

Artifacts↗

PubMed2000

Evaluation of the secondary consolidation of columns for liquid chromatography by ultrasonic irradiation.

The consolidation of packed analytical chromatography columns was carried out under ultrasonic irradiation. Columns were first packed using a conventional high pressure downward slurry method. Then, they were subjected to further bed consolidation in the presence of ultrasonic vibration. This process of further bed consolidation is referred to as secondary consolidation. Secondary consolidation was observed to occur more readily in solvents of low viscosity and at low flow-rates (low pressures). Column efficiency was not observed to be a factor affecting the process of secondary consolidation of the packed bed.

Chromatography, Liquid↗

PubMed2000

Study of the adsorption behavior of the enantiomers of 1-phenyl-1-propanol on a cellulose-based chiral stationary phase.

Using single-step frontal analysis, we measured single-component and competitive adsorption isotherm data for the two enantiomers of 1-phenyl-1-propanol (PP). These experimental data were fitted to several competitive bi-Langmuir models (with 8, 6, 5 and 4 parameters) and to the competitive Langmuir model. The latter model accounted well for the behavior of both PP enantiomers on Chiracel OB (cellulose tribenzoate coated on silica gel). The parameters obtained were used in numerical calculations to predict the band profiles of the two single components and of their mixtures under overloaded conditions. The equilibrium-dispersive model provides satisfactory results, with minor differences between the calculated and the experimental profiles. These differences became negligible when a more complex kinetic model was used, with a concentration-dependent rate coefficient.

Adsorption↗

PubMed2000

Adsorption equilibria and overloaded band profiles of basic drugs in a reversed-phase system.

Single-solute adsorption equilibrium isotherms of three basic drugs: buspirone, doxepin and diltiazem were determined by frontal analysis in a reversed-phase system composed of an octadecylsilica packing material and a buffered mobile phase containing acetonitrile. The adsorption data were fitted to the bi-Langmuir model. Within the framework of this model, the adsorption of the drugs is assumed to occur on two distinct kinds of sites with different average adsorption energies. The data are consistent with the assumption that the low energy sites account for the hydrophobic interactions between the solutes and the chemically bonded alkyl chains and the high energy sites account for the ion-exchange interactions between the residual active silanols and the protonated bases. Multisolute, overloaded band profiles were also measured for the three binaries and for mixtures of the three drugs. Theoretical band profiles were calculated using the equilibrium dispersive model and the ideal adsorbed solution theory model which uses the parameters determined from the correlation of the single-solute adsorption data. Good agreement was found between the experimental and calculated overloaded band profiles.

Adsorption↗

PubMed2000

Adsorption behavior and prediction of the band profiles of the enantiomers of 3-chloro-1-phenyl-1-propanol. Influence of the mass transfer kinetics.

The single-component and competitive adsorption isotherms of the enantiomers of 3-chloro-1-phenyl-1-propanol were measured by frontal analysis. The stationary phase was a cellulose tribenzoate coated on silica, the mobile phase an n-hexane-ethyl acetate (95:5) solution. The adsorption data measured fitted well to the Langmuir isotherm model. The band profiles of single components and of their mixtures were calculated using the equilibrium-dispersive model. These profiles were found to match quite satisfactorily the experimental band profiles. However, the agreement between calculated and experimental band profiles was significantly improved when a more complex model taking into account the mass transfer kinetics was used. The mass transfer rate coefficients, k(f), for both single components were determined by using the transport-dispersive model of chromatography. The coefficients obtained were used to predict the band profiles of mixtures of the two enantiomers to good agreement.

Adsorption↗

PubMed2000

High-concentration band profiles and system peaks for a ternary solute system

High-concentration HPLC band profiles of single solutes and the individual band profiles of the components of binary and ternary mixtures are reported for benzyl alcohol, 2-phenylethanol, and 2-methylbenzyl alcohol. These solutes were eluted from a C18 column by a binary mobile phase (MeOH:H2O = 1:1, v/v). High-concentration system peaks were obtained using mixtures of benzyl alcohol and 2-phenylethanol at different relative concentrations as the feed and 2-methylbenzyl alcohol as the strong mobile phase additive. Band profiles and system peak profiles were calculated using the equilibrium-dispersive model of chromatography. The adsorption equilibrium in the multicomponent system was characterized by the competitive Langmuir model. Excellent quantitative agreement was found between the experimental and the calculated profiles. This work confirms that extremely unusual system peak profiles can be obtained even when the adsorption behavior is quite simple. Under certain circumstances, the use of a properly chosen additive could markedly increase the separation between bands and hence the production rate, the recovery yield, and/or the purity of the fractions.

Journal Article↗

PubMed2000

Restricted diffusion model for surface diffusion in reversed-phase liquid chromatography

The analysis of experimental results in reversed-phase liquid chromatography (RPLC) allows further discussion of the restricted diffusion model of surface diffusion formulated on the basis of the absolute rate theory. Chromatographic data were acquired on different RPLC systems with two series of homologous compounds, several stationary phases having different alkyl ligand densities and ligands of various lengths, and methanol/water mobile phases of different compositions. The enthalpy-entropy compensation observed and the linear free energy relationships found for surface diffusion suggest that the surface diffusion mechanism remains probably the same in all RPLC conditions studied. Whereas the isosteric heat of adsorption approaches zero with decreasing retention, the activation energy of surface diffusion tends toward a finite limit and the surface diffusion coefficient tends toward a value near the corresponding molecular diffusivity. These results support the validity of the restricted diffusion model. The influence of different factors on the validity of this model (i.e., the activation energy and the frequency factor of surface diffusion, and the surface tortuosity) was also considered.

Journal Article↗

PubMed2000

Comparison of the repeatability of quantitative data measured in high-performance liquid chromatography with UV and atmospheric pressure chemical ionization mass spectrometric detection.

A detailed comparison of the repeatability of the retention times, the peak efficiencies and the peak areas of a dozen probe compounds achieved in HPLC, using either HPLC-UV or HPLC-MS for detection purpose, is reported. Three groups of conventional analytes, each one separated under a different set of experimental conditions, were selected for this study. Most of the compounds are basic, the other ones being neutral. The repeatabilities of the retention times do not exhibit any influence of the mode of detection. However, the repeatabilities of the peak areas and the column efficiencies are generally (although not always) better in HPLC-UV than that in HPLC-MS. On average, the precision for the UV peak area detection was 2.5% versus 6.8% for MS detection. Experimental results show that the response factor of the UV detector is more constant than that of the MS detector, probably because the HPLC flow-rate was sufficiently stable. The results obtained in the different tests are discussed.

Atmospheric Pressure↗

PubMed2000

Study on the accuracy of the elution by characteristic point method for the determination of single component isotherms.

The accuracy of the method of elution by characteristic point (ECP) used to determine single component isotherms was studied numerically. Overloaded elution peaks were calculated using the equilibrium-dispersive model of nonlinear chromatography while varying the four parameters, i.e., the number of theoretical plates, the dimensionless Langmuir equilibrium constant, the retention and the loading factor, involved in the fundamental equations of the problem. Single component isotherms were estimated by analyzing the diffuse profile of the elution peaks by the ECP method. Similar results were obtained with the transport-dispersive model. The comparison of these calculated isotherms with the initial Langmuir isotherms provided detailed information on the influence of the mass transfer resistances on the accuracy of the isotherms afforded by the ECP method. The systematic error made on the isotherms depends on the experimental conditions, described by the four parameters. It is expected that this error could be eliminated by correcting the influence of nonequilibrium in the column on the basis of results of the nondimensional calculations. The concentration range of the objective isotherm which can be determined by the ECP method can also be predicted by numerical calculations. The usefulness of the correction strategy and the prediction of the concentration range were experimentally demonstrated.

Chromatography↗

PubMed2000

Repeatability and reproducibility of retention data and band profiles on reversed-phase liquid chromatography columns. IV. Results obtained with Luna C18 (2) columns.

The reproducibility of retention data and band profile characteristics was investigated for a series of columns packed with Luna C18 (2), a silica-based, reversed-phase adsorbent. High precision data were obtained and statistically compared among five columns from the same batch (column-to-column reproducibility) and nine columns from as many 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. This work is part of a study on the precision of chromatographic analyses. Its purpose is to illustrate the contribution of the stationary phase reproducibility to this precision.

Chromatography, Liquid↗

PubMed2000

Visualization of solute migration in chromatographic columns quantitation of the concentration in a migrating zone.

The concentration distribution across a zone of iodine migrating along a column made of glass, packed with C18-bonded silica, and eluted with carbon tetrachloride was derived from a quantitative analysis of the photographs of the zone. The photographs were scanned and turned into digital images. The intensity distributions obtained from the measurement of the grayscale intensity were converted into concentration profiles using a calibration method. This procedure is illustrated and suitable corrections are introduced to account for the transverse variation of the optical path length, as a result of using a cylindrical detector cell (the column itself), and for the refraction of light due to the differences between the refraction indices of the glass wall and the liquids involved. An error analysis is also reported. It shows that the method can reliably produce results with a precision of a few percent, allowing on-column evaluation of column performance and the derivation of the radial distributions of the column efficiency, the migration velocity of the zone, and the sample distribution at the head of the column.

Calibration↗