Biomedical subjects

Georges Guiochon

Publications and source records attributed to Georges Guiochon.

124 records · Page 7Linked to original sources

PubMed2002

Extrathermodynamic relationships in reversed-phase liquid chromatography.

Enthalpy-entropy compensation (EEC) and linear free energy relationships (LFER) are extrathermodynamic correlations frequently used to discuss the mechanistic similarities of chemical equilibria and reaction kinetics. Although empirical, they are widely applied, proving the substantial effectiveness of fundamental studies based on them. Many attempts have been made to interpret theoretically the necessary conditions (or preconditions) of EEC and LFER. LFER is known to rest on the existence of EEC. However, the intimate correlations between EEC on one hand and LFER and the temperature dependence of LFER on the other hand were insufficiently discussed from the viewpoint of molecular structure contributions. We present a simple LFER model relating the slope and intercept of LFER to the compensation temperatures, themselves derived from EEC analyses, and to several parameters characterizing the molecular contributions to the changes in enthalpy and entropy associated with the passage from one phase of the chromatographic system to the other. A theoretical explanation is supplied for the intimate correlation between the two types of extrathermodynamic relationships, EEC and LFER. We demonstrate also that the characteristics of EEC and LFER depend on the structural parameters. This new model allows a proper interpretation of the temperature dependence of LFER. It should permit further progress of fundamental studies of chemical reaction mechanisms based on extrathermodynamic relationships.

Journal Article↗

PubMed2002

Determination of the porosities of monolithic columns by inverse size-exclusion chromatography.

Retention data of polystyrene samples of narrow molecular size distribution and known average molecular mass were measured on several monolithic columns (Chromolith Performance, Merck) and one conventional packed column (Luna C18, Phenomenex) by size-exclusion chromatography. These data were used to determine the external, the internal, and the total porosities of these columns. These data provided also information on the pore-size distribution of the adsorbent medium. The external and the total porosities of these columns are much higher than those of conventional packed columns. The results illustrate the profound changes brought by monolithic columns to the balance of the hydrodynamic and the mass transfer kinetic properties of chromatographic columns. Classical methods of comparison between column performance must be re-evaluated.

Chromatography, Gel↗

PubMed2002

Preparative liquid chromatography.

The status of the theory and the main methods of implementation of preparative liquid chromatography are reviewed. On the theory front, the focus has recently shifted. The theory of non-linear, non-ideal chromatography has given rise to numerous models whose advantages, disadvantages and ranges of application are now well understood. Interest now resides in investigating the equilibrium thermodynamics of complex new systems, in the study of the kinetics of mass transfers in conventional chromatographic systems, and in the application of the various models of chromatography to optimize the experimental conditions. Progress in computer technology allows the use of sophisticated models, provided their parameters can be measured. This allows the detailed investigation of separations for which the mass transfer kinetics is slow such as chiral separations, the purification of basic compounds, and the extraction of recombinant proteins. On the applied front, in addition to numerous incremental improvements in reliability and economic performance, a few essential new features should be noted, i.e. the availability of instruments for simulated moving bed separations at the scale needed for preparative chiral separations, the use of expanded beds for the extraction of recombinant proteins from fermentation broths, and the attention given to improvements in the performance of packed beds. A survey of the literature dealing with practical applications and recent meetings shows that preparative chromatography is becoming a well established separation and purification method in the pharmaceutical industry.

Chromatography, Liquid↗

PubMed2002

Energetic heterogeneity of the surface of a molecularly imprinted polymer studied by high-performance liquid chromatography.

The influence of thermal annealing on the surface homogeneity of a polymer imprinted against L-phenylalanine anilide (LPA) was examined using high-performance liquid chromatography (HPLC) for the measurement of the adsorption isotherms. The isotherms obtained for LPA and for its enantiomer, D-phenylalanine anilide (DPA) were fitted to the Freundlich (F) equation which accounts for the energetic heterogeneity of the surface with a separate parameter. Changes in the adsorptive properties of the polymer produced by thermal annealing were deduced by comparing the heterogeneity parameters given by the nonlinear regression. These changes were also illustrated by deriving the isosteric heats of adsorption as functions of the amounts adsorbed and by calculating the associated affinity distributions. This latter technique involves an application of the affinity spectrum (AS) combined with the F adsorption model. The plausibility and accuracy of the combination is discussed. It is shown that the derivation of the amplitudes of the affinity distributions from the F parameters is inaccurate, making difficult the proper estimate of the changes in the total population of adsorption sites. In contrast, the AS method gives correct estimates of the parameter that characterizes the slope of the affinity distributions. The results derived from the three sets of results (F model parameters, isosteric heats of adsorption, AS + F method) show consistently that annealing reduces the energetic heterogeneity of the polymer surface for both LPA and DPA. In practice, however, the improvement of the polymer performance in HPLC is relatively limited.

Chromatography, High Pressure Liquid↗

PubMed2002

Application of the general rate model and the generalized Maxwell-Stefan equation to the study of the mass transfer kinetics of a pair of enantiomers.

The general rate model of chromatography can be coupled with the generalized Maxwell-Stefan equation that describes the surface diffusion flux. The resulting model is useful to describe the behavior of two enantiomers during their separation on chiral phases, cases in which the mass transfer kinetics is known to be sluggish. A case in point is the modeling of the elution profiles of the racemic mixture of the two enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate coated on silica, a popular chiral stationary phase. The competitive equilibrium isotherm behavior of the two enantiomers on the chiral stationary phase was described using the competitive Tóth isotherm model. An excellent agreement between the experimental and the calculated profiles was observed in the whole range of experimental conditions investigated, at low and high column loadings.

Kinetics↗

PubMed2002

New model of surface diffusion in reversed-phase liquid chromatography.

A new model of surface diffusion in reversed-phase liquid chromatography (RPLC) was derived by assuming a correlation between surface and molecular diffusion. Analysis of surface diffusion data under different conditions of sample compounds, mobile and stationary phases, and temperature in RPLC systems validates this assumption and shows that surface diffusion should be regarded as a molecular diffusion restricted by the adsorptive interactions between the adsorbate molecule and the stationary phase surface. A surface-restricted molecular diffusion model was proposed as a first approximation for the mechanism of surface diffusion. The model is formulated according to the absolute rate theory. The activation energy of surface diffusion (Es) was quantitatively interpreted assuming that Es consists of the contributions of two processes, a hole-making and a jumping one. The former contribution is nearly equal to the activation energy of molecular diffusion and is correlated with the evaporative energy of the mobile phase solvent. The latter contribution is a fraction of the isosteric heat of adsorption. An appropriate explanation based on this new model of surface diffusion is provided for two contradictory results related to the relationship between retention equilibrium and surface diffusion in RPLC and to the surface diffusion coefficient for weakly retained sample compounds.

Chromatography, Liquid↗

PubMed2002

Repeatability and reproducibility of retention data and band profiles on six batches of monolithic columns.

Chromatographic data were acquired for eight different mixtures, under five different sets of experimental conditions, for a total of 30 neutral, acidic and basic test compounds, on a series of six Chromolith Performance columns from Merck. These columns are made of a C18 chemically bonded silica monolith. Each column belonged to a different production batch, so the data reported here characterize their batch-to-batch reproducibility. The parameters studied in this work were the retention times, the retention and separation factors, the hydrophobic and the steric selectivities, the column efficiencies, and the tailing factors for all 30 compounds.

Chromatography, Liquid↗

PubMed2002

Adsorption equilibria of butyl- and amylbenzene on monolithic silica-based columns.

The adsorption isotherms of butyl- and amylbenzene on silica monolithic columns were measured by frontal analysis. The external, internal and total porosities of these columns were determined by inverse size-exclusion chromatography. The adsorption isotherms are concave upward in the entire concentration range investigated. They were fitted to the anti-Langmuir model, an unusual model in liquid-solid and liquid-liquid phase equilibria. Band profiles under overloaded conditions were recorded. They were in good agreement with the profiles calculated using th,e lumped pore diffusion model of chromatography and these adsorption isotherms.

Adsorption↗

PubMed2002

Study of the adsorption equilibria of the enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate using a microbore column.

Using competitive frontal analysis, the binary adsorption isotherms of the enantiomers of 1-phenyl-l-propanol were measured on a microbore column packed with a chiral stationary phase based on cellulose tribenzoate. These measurements were carried out using only the racemic mixture. The experimental data were fitted to four different isotherm models: Langmuir, BiLangmuir, Langmuir-Freundlich and Tóth. The BiLangmuir and the Langmuir-Freundlich models accounted best for the competitive adsorption data. An excellent agreement between the experimental and the calculated overloaded band profiles for various samples of racemic mixture was obtained when the equilibrium dispersive model of chromatography was used together with the BiLangmuir competitive isotherm. The isotherm parameters measured under competitive conditions were used to calculate the overloaded band profiles of large samples of the pure S- and R-enantiomers, too. A satisfactory agreement between the experimental and calculated band profiles was observed when using in the computation the corresponding single component BiLangmuir isotherm derived from the binary isotherm previously determined. Thus oniy data derived from the racemic mixture are required for computer optimization of the preparative chromatography separation of the enantiomers.

Adsorption↗

PubMed2002

Effect of the homogeneity of the column set on the performance of a simulated moving bed unit. II. Experimental study.

Previous studies of the simulated moving bed (SMB) process assume identical characteristics of all the columns incorporated in a given unit. Due to the practical impossibility to manufacture identical columns, numerical applications of the theory to modeling and optimization use for each of the needed column parameter the average value for the entire column set. In this study, the effects of these simplifications on the actual productivity of the SMB process are evaluated by making exact calculations, i.e., by taking the differences in the porosity values into account. We apply a revised set of separation conditions previously introduced and derived from the equilibrium theory. Earlier theoretical results are compared to experimental results obtained in the study of the enantiomeric separation of Tröger's base on Chiralpak AD. Due to the nonLangmuirian character of the adsorption isotherms of these two compounds on the packing material used, the separation area cannot be determined analytically. As an alternative, a reliable numerical algorithm was used to scan a wider region and to define the separation area. The form of this area depends on the applied porosity values. A UV detector and a laser polarimeter located at one node of the SMB monitor on-line the internal concentration profiles. Excellent agreement between the calculated and the experimentally determined concentration profiles was obtained under nonlinear conditions. The influence of column-to-column variations on the performance of the SMB process was found to be more significant under nonlinear than under linear conditions.

Adsorption↗

PubMed2002

Behavior of packing materials in axially compressed chromatographic columns.

The behavior of a packing material (Luna C18 from Phenomenex, Torrance, CA, USA) was studied during the consolidation of a column bed under axial compression stress. The kinetics of this consolidation, the permeability and efficiency of the columns obtained, and the reproducibility of these column properties were measured under different conditions. The consolidation process and the column properties are considerably affected by the friction between the packing material in the bed and the column wall. Clear evidence of this wall effect was demonstrated. The apparent permeability of columns consolidated under the same axial stress increases with increasing column length. The apparent modulus of elasticity of the beds increases with increasing column length. The shear resistance between the packed bed and the column wall was measured for columns of different lengths. It increases rapidly with increasing bed length. The column efficiency for thiourea (unretained) and phenyloctane (retention factor, k' approximately 1) was much poorer after recompression than after the first compression. It depended little on the compression stress. The effect of the column length was small.

Chromatography, Liquid↗

PubMedDate not supplied

Modeling of preparative reversed-phase HPLC of insulin.

The adsorption isotherms of three recombinant proteins, human insulin, porcine insulin, and Lispro, were measured by frontal analysis on a YMC-ODS C18 column with an aqueous solution at 31% acetonitrile (0.1% TFA) as the mobile phase. The retention behavior of insulin, its related molecular structure, its conformation, and its aggregation in this phase system are discussed. The experimental isotherm data were fitted to the Langmuir, the Langmuir-Freundlich, and the Toth models. The results allow for a quantitative comparison of the saturation capacities, the equilibrium constants, and the exponents that represent the heterogeneity of the stationary phase obtained for the different insulin variants studied. The Toth model provided the best fit of the experimental data. The overloaded band profiles were calculated using the lumped pore diffusion and the equilibrium-dispersive model of chromatography. An excellent agreement between calculated and experimental profiles was demonstrated.

Adsorption↗

PubMedDate not supplied

True and apparent temperature dependence of protein adsorption equilibrium in reversed-phase HPLC.

The adsorption behavior of bovine insulin on a C(8)-bonded silica stationary phase was investigated at different column pressures and temperatures in isocratic reversed-phase HPLC. Changes in the molar volume of insulin (deltaV(m)) upon adsorption were derived from the pressure dependence of the isothermal retention factor (k'). The values of deltaV(m) were found to be practically independent of the temperature between 25 and 50 degrees C at -96 mL/mol and to increase with increasing temperature, up to -108 mL/mol reached at 50 degrees C. This trend was confirmed by two separate series of measurements of the thermal dependence of ln(k'). In the first series the average column pressure was kept constant. The second series involved measurements of ln(k') under constant mobile-phase flow rate, the average column pressure varying with the temperature. In both cases, a parabolic shape relationship was observed between ln(k') and the temperature, but the values obtained for ln k' were higher in the first than in the second case. The relative difference in ln(k'), caused by the change in pressure drop induced by the temperature, is equivalent to a systematic error in the estimate of the Gibbs free energy of 12%. Thus, a substantial error is made in the estimates of the enthalpy and entropy of adsorption when neglecting the pressure effects associated with the change in the molar volume of insulin. This work proves that the average column pressure must be kept constant during thermodynamic measurements of protein adsorption constants, especially in RPLC and HIC. Our results show also that there is a critical temperature, T(c) approximately equals 53 degrees C, at which ln(k') is maximum and the insulin adsorption process changes from an exothermic to an endothermic one. This temperature determines also the transition point in the molecular mechanism of insulin adsorption that involves successive unfolding of the protein chain.

Adsorption↗

PubMedDate not supplied

Equivalent models of indanol isomers adsorption on cellulose tribenzoate.

The adsorption isotherm data of (R)- and (S)-1-indanol and of their racemic mixture on cellulose tribenzoate were measured by frontal analysis. The experimental data for each enantiomers were fitted to the single-component bilangmuir isotherm model. The competitive experimental data were fitted to the ideal adsorption solution model (IAS), the real adsorption solution model (RAS), and the bilangmuir thermodynamically consistent model (BTC). The mass transfer kinetic parameters were estimated from systematic comparisons between the experimental single-component band profiles and profiles calculated using the general rate model (GR) of chromatography coupled with the generalized Maxwell-Stefan equation (GMS). The validation of the isotherm model and of the mass transfer kinetic model was made by comparing the experimental band profiles obtained for solutions of the two enantiomers and those calculated with the competitive GR-GMS model. The excellent agreement observed proves that a combination of the BTC isotherm model and the GMS kinetic model, using the best values of the BTC and GMS parameters estimated from single component experiments, allows an excellent prediction of the binary isotherm and the binary mass transfer kinetics.

Adsorption↗

PubMedDate not supplied

Prediction of band profiles of mixtures of bradykinin and kallidin from data acquired by competitive frontal analysis.

The competitive adsorption isotherms of two closely related peptides, bradykinin and kallidin, were measured by frontal analysis on a Zorbax SB-C18 microbore column. An aqueous soluton at 20% acetonitrile (0.1% TFA) was used as the mobile phase. The competitive isotherm data were fitted to four different models: Langmuir, Bilangmuir, Langmuir-Freundlich, and Toth. These data fitted best to a Bilangmuir isotherm model. The influence of the pressure on the retention factors of the two peptides was found to be small and was not investigated in detail. The band profiles of large samples of the single components and of their mixtures were recorded. The overloaded profiles calculated using either the equilibrium-dispersive or POR model are in excellent agreement with the experimental profiles in all cases. Our results confirm that the competitive isotherm data derived from mixtures may suffice for a reasonably accurate prediction of the band profiles of all mixtures of the two components, provided their composition is close to 1/1.

Algorithms↗

PubMedDate not supplied

Mass transfer kinetics in the chromatography of insulin variants under nonlinear conditions.

Experimental data regarding the thermodynamics and kinetics of adsorption of lispro, an insulin variant, onto a YMC ODS-A column, from an aqueous solution of acetonitrile (31%) and TFA are reinterpreted, using a more complex model of the mass transfer kinetics. The adsorption behavior follows the Toth isotherm model, suggesting either a strongly heterogeneous surface or, rather, that when insulin molecules adsorb they contact the surface along different areas of the molecule. The lumped pore diffusion (POR) model of chromatography accounts well for the band profiles. The internal mass transfer resistances are higher than expected, which suggests that intraparticle diffusion is slower. Furthermore, the pore diffusion coefficient increases with decreasing sample size. That surface diffusion accounts for the mass transfer kinetics inside particles explains these results. Assuming that the gradient of the surface concentration is the driving force of surface diffusion, it is possible to account very well for the band profiles of samples of widely different sizes, using a single value of the surface diffusivity.

Acetonitriles↗