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Georges Guiochon

Publications and source records attributed to Georges Guiochon.

At least 91 records · Page 5Linked to original sources
PubMed2004

Band splitting in overloaded isocratic elution chromatography III. Modeling of adsorbate-adsorbate interactions by a two-component extension of a BET kinetic isotherm model.

A new two-component competitive adsorption model was derived to account for the competitive adsorption data of mixtures of ethylbenzoate and 4-tert-butylphenol, on a C18-Kromasil column under RPLC conditions (mobile phase, methanol/water, 62/38, v/v). The derivation is based on kinetic arguments and is an extension to multicomponent systems of the single-component BET isotherm. The model assumes that the molecules of the first compound (A) can adsorb on layers made of molecules of either A or B, while molecules of B can only adsorb on layers made of molecules of A. This makes the competitive isotherm consistent with the single-component isotherms of ethylbenzoate and 4-tert-butylphenol, the multilayer BET and the monolayer Langmuir isotherm models, respectively. The competitive adsorption data were acquired by frontal analysis (FA) with equimolar mixtures of eight different concentrations. For the seven lowest concentrations, these data were derived from the retention times of the shocks of the two compounds and the concentration of the intermediate plateau of the less retained compound. At the highest concentration (25 g/l), the individual band profiles were measured by collecting and analyzing twenty fractions. The low concentration data (C < or = 10 g/l) are well accounted for by the two competitive isotherm models derived previously but these models fail to describe the experimental data of 4-tert-butylphenol at high concentrations. By contrast, the new model predicts very well the experimental adsorption data for mixtures of ethylbenzoate and 4-tert-butylphenol in the whole range of concentration studied. Our results suggest that the adsorption constant of 4-tert-butylphenol onto layers made of ethylbenzoate (b(B,A) = 0.0120 l/g) is intermediate between those of ethylbenzoate on layers made of 4-tert-butylphenol (b(A,B) = 0.0105 l/g) and of ethylbenzoate on itself (b(A,A) = 0.0145 l/g). This new model should give an improved description of the band splitting observed for 4-tert-butylphenol in the presence of ethylbenzoate.

Adsorption↗
PubMed2004

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography.

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.

Adsorption↗
PubMed2004

Physical origin of peak tailing on C18-bonded silica in reversed-phase liquid chromatography.

Single component isotherm data of caffeine and phenol were acquired on two different stationary phases for RPLC, using a methanol/water solution (25%, v/v, methanol) as the mobile phase. The columns were the non-endcapped Waters Resolve-C18, and the Waters XTerra MS C18. Both columns exhibit similar C18 -chain densities (2.45 and 2.50 micromol/m2) and differ essentially by the nature of the underivatized solid support (a conventional, highly polar silica made from water glass, hence containing metal impurities, versus a silica-methylsilane hybrid surface with a lower density of less acidic free silanols). Thirty-two adsorption data points were acquired by FA, for caffeine, between 10(-3) and 24 g/l, a dynamic range of 24,000. Twenty-eigth adsorption data points were acquired for phenol, from 0.025 to 75 g/l, a dynamic range of 3000. The expectation-maximization procedure was used to derive the affinity energy distribution (AED) from the raw FA data points, assuming a local Langmuir isotherm. For caffeine, the AEDs converge to a bimodal and a quadrimodal distribution on XTerra MS-C18 and Resolve-C18, respectively. The values of the saturation capacity (q(s,1) approximately equal to 0.80 mol/l and q(s,2) approximately equal to 0.10 mol/l) and the adsorption constant (b1 approximately equal to 3.11/mol and b2 approximately equal to 29.1 l/mol) measured on the two columns for the lowest two energy modes 1 and 2, are comparable. These data are consistent with those previously measured on an endcapped Kromasil-C18 in a 30/70 (v/v), methanol/water solution (q(s,1) = 0.9 mol/l and q(s,2) = 0.10 mol/l, b1 = 2.4 l/mol and b2 = 16.1 l/mol). The presence of two higher energy modes on the Waters Resolve-C18 column (q(s,3) approximately equal to 0.013 mol/l and q(s,4) approximately equal to 2.6 10(-4) mol/l, b3 approximately equal to 252 l/mol and b4 = 13,200 l/mol) and the strong peak tailing of caffeine are explained by the existence of adsorption sites buried inside the C18-bonded layer. It is demonstrated that strong interactions between caffeine and the water protected bare silica surface cannot explain these high-energy sites because the retention of caffeine on an underivatized Resolve silica column is almost zero. Possible hydrogen-bond interactions between caffeine and the non-protected isolated silanol groups remaining after synthesis amidst the C18-chain network cannot explain these high energy interactions because, then, the smaller phenol molecule should exhibit similarly strong interactions with these isolated silanols on the same Resolve-C18 column and, yet, the consequences of such interactions are not observed. These sites are more consistent with the heterogeneity of the local structure of the C18-bonded layer. Regarding the adsorption of phenol, no matter whether the column is endcapped or not, its molecular interactions with the bare silica were negligible. For both columns, the best adsorption isotherm was the Bilangmuir model (with q(s,1) approximately equal to 2 and q(s,2) approximately equal to 0.67 mol/l, b1 0.61 and b2 approximately equal to 10.3 l/mol). These parameters are consistent with those measured previously on an endcapped Kromasil-C18 column under the same conditions (q(s,1) = 1.5 and q(s,2) = 0.71 mol/l, b1 = 1.4 l/mol and b2 = 11.3 l/mol). As for caffeine, the high-energy sites are definitely located within the C18-bonded layer, not on the bare surface of the adsorbent.

Chromatography, Liquid↗
PubMed2004

Single-component shock layer analysis in elution chromatography.

One of the fundamental results of the theory of nonlinear chromatography is that a propagation velocity is associated with each concentration. This velocity is related to the slope of the isotherm at the corresponding concentration. It follows that if a continuous concentration gradient is injected into a column, the gradient profile will not propagate in a mere translation but will progressively change shape. In the most common case of a convex upward isotherm (e.g., Langmuir), a linear gradient will become curved upward, the high concentrations migrating faster than the low ones. However, high concentrations cannot pass low ones, so the concentrations pile up, a concentration shock forms, and its height increases. In practice, axial dispersion and the mass-transfer resistances combine and prevent the formation of a true shock. A shock layer, a region where the concentration gradient is very steep, is formed. This shock layer migrates at the same velocity as the ideal shock would. Many characteristics of concentration shocks and shock layers have been determined previously, but not the time that it takes for a continuous gradient to turn into a shock layer, the circumstances of the birth of the shock layer, and of its growth. Yet, this is important to know to understand certain aspects of gradient elution. We have derived simple equations relating the circumstances of the birth of shocks to the phase equilibrium isotherm and to the column characteristics. The results of experimental measurements made with a high-efficiency analytical column are in excellent agreement with these theoretical predictions.

Journal Article↗
PubMed2004

Repeatability and reproducibility of high-concentration data in reversed-phase liquid chromatography. III. Isotherm reproducibility on Kromasil C18.

Single component equilibrium isotherms of six compounds (aniline, caffeine, ethylbenzene, phenol, propranolol, and theophylline) were determined by the inverse method on 10 Kromasil-C18 columns, using water-methanol solutions as the mobile phase. This method offers an economic and fast isotherm determination by means of the overloaded band profiles of the compounds. Five out of the ten columns used in this test come from the same batch whilst the other five columns represent five additional batches. Statistical evaluation was used to assess the reproducibility of the isotherm parameters. We found that the column-to-column reproducibility of the isotherm parameters is of the same magnitude as the batch-to-batch reproducibility (with the exception of one outlier column). In most of the cases, the reproducibilities of the saturation capacities and that of the retention factors are excellent, they are typically between 1.2 and 3%, and very often below 2%. Within the limits of the experimental precision, these results agree with those obtained earlier, using a conventional method of isotherm determination.

Adsorption↗
PubMed2004

Improved model of multicomponent adsorption in reversed-phase liquid chromatography.

This work presents a modification to the real adsorbed solution model using a Flory-Huggins type of expression that was reported previously. This modification consists in replacing the Flory-Huggins activity coefficient by the spreading pressure dependent approach. This new model takes into account explicitly the adsorbate-adsorbate interactions taking place in the adsorbed phase. It provides an excellent prediction of the competitive, ternary adsorption equilibrium of benzyl alcohol, 2-phenylethanol and 2-methyl benzyl alcohol observed in a reversed-phase liquid chromatographic system using information merely derived from the single-component adsorption experimental data.

Adsorption↗
PubMed2004

Heterogeneous adsorption of 1-indanol on cellulose tribenzoate and adsorption energy distribution of the two enantiomers.

The distributions of the adsorption energies (AED) of two enantiomers, (R)-1- indanol and (S)-1-indanol, on a chiral stationary phase were measured and the results are discussed. The chiral phase used is made of cellulose tribenzoate coated on porous silica. The AEDs were determined using the expectation maximization method, a numerical method that uses directly the raw experimental isotherm data, inverts this set of data into an AED, and introduces no arbitrary information in the calculation. However, it uses the Langmuir equation as the local isotherm. The experimental data fit very well to the bi-Langmuir isotherm model for the more retained enantiomer. Our results show that the AEDs of these two enantiomers have no energy modes that would be identical (same mean energy, mode profile, and mode area), in contrast to numerous cases previously studied, e.g., that of the beta-blockers on a Cel7A column. This indicates a significantly different retention mechanism.

Adsorption↗
PubMed2004

Characterization of monolithic columns for HPLC.

Monolithic stationary phases and columns have rapidly become highly popular separation media for liquid chromatography, in spite of their recent discovery. However, their most important features have not yet been completely clarified. A complete understanding of their performance and of their intrinsic characteristics will require the systematic acquisition of many series of reliable experimental data and their consistent analysis from different points of view. Progress in their design and production requires now that the chromatographic behavior of monolithic columns be studied in close connection with their physico-chemical and structural properties. The main goal of this review is to summarize fundamental information on some physico-chemical and chromatographic characteristics of monolithic stationary phases and columns for RPLC. The material reviewed deals only with silica-based monolithic columns. First, structural information on the porosities and the size of the pores in monolithic columns is reported. Second, results of chromatographic studies that deal with the characterization of monolithic columns are summarized. Third, results of detailed studies made on the adsorption equilibrium and the surface heterogeneity of monolithic stationary phases are presented. Finally, results on the mass transfer kinetics in monolithic columns derived from the applications of the classical random-walk model and of the moment theory to a new model of the monolith are discussed.

Adsorption↗
PubMed2003

Reproducibility of low and high concentration data in reversed-phase liquid chromatography. II. Overloaded band profiles on Chromolith-C18.

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for six low molecular weight compounds (phenol, aniline, caffeine, o-toluidine, p-toluidine and propylbenzoate) on one Chromolith-C18 column (#30, Merck, Darmstadt, Germany), using different methanol:water solutions (composition between 60/40 and 15/85 v/v, depending on the solute) as the mobile phase. These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions were also derived and used for the selection of the best isotherm model. Widely different models were obtained for these six compounds, four being convex upward (i.e., Langmuirian) and two having at least one inflection point. Overloaded band profiles corresponding to two different sample sizes (a low and a high loading factor) were recorded on six monolithic columns (#30-35) belonging to the same manufactured lot. These experimental band profiles were compared to the profiles calculated from the isotherm measured by FA on the first column, using the equilibrium-dispersive (ED) model of chromatography. For four of the six columns (#30, #32, #33, and #35), the reproducibility was better than 5 and 2.5% for the low and the high concentration profiles, respectively. On the other two columns (#31 and #34), the bands showed significant and systematic retention time shifts for all six compounds (with nearly identical band shapes), the relative adsorption being between 6 and 15% stronger on column #31 or between 2 and 7% lower on column #34. These differences seem to be correlated with the differences in the total porosities of these columns, which differ by 3% from columns #31 to #34, the higher porosity column giving the stronger adsorption.

Adsorption↗
PubMed2003

Moment analysis of mass-transfer kinetics in C18-silica monolithic columns.

The moment analysis of elution peak profiles based on new moment equations provides information on the mass-transfer characteristics of C(18)-silica monolithic columns. The flow rate dependence of the HETP data was analyzed using the generalized van Deemter equation, after correction of these data by subtraction of the external mass-transfer contribution to band broadening. Kinetic parameters and diffusion coefficients related to the mass-transfer processes in monolithic columns were derived by taking advantage of the different flow velocity dependence of their contributions to band broadening. At high flow rates, axial dispersion and diffusive migration across the monolithic C(18)-silica skeleton contribute much to band broadening, suggesting that it remains important to reduce the influence of eddy diffusion and the mass-transfer resistance in the stationary phase to achieve fast separations and a high efficiency. Surface diffusion plays a predominant role for molecular migration in the monolithic stationary phase. Although the value of the surface diffusion coefficient (D(s)) depends on an estimate of the external mass-transfer coefficient, D(s) values of the order of 10(-7) cm(2) s(-1) were calculated for the first time for the C(18)-silica monolithic skeleton. The value of D(s) decreases with increasing retention of sample compounds. Analysis of a kind of time constant calculated from D(s) suggests that the "chromatographic corresponding particle size" is approximately 4 microm for the C(18)-silica monolithic stationary phase used in this study. The accuracy of the D(s) values determined was discussed.

Journal Article↗
PubMed2003

Modeling of the separation of two enantiomers using a microbore column.

A microbore column packed with Chiralcel OB (cellulose tribenzoate coated silica) was used for the measurement of the single and competitive equilibrium-isotherm data of the 1-indanol enantiomers by frontal analysis. The amount of sample needed for the isotherm data acquisition was about 20 times less than that required with a conventional column. The data obtained were fitted to different single and competitive isotherm models. Both the single and the competitive data sets fitted best to the same Bilangmuir (BL) isotherm model with small differences in the numerical values of the parameters. The best fitted Bilangmuir single and competitive isotherm models were used to predict the overloaded experimental profiles of both pure enantiomers, of the racemic mixture, and of different enantiomeric mixtures. All the calculated profiles were in excellent agreement with the experimental ones. This agreement confirms that in many chiral separations, the competitive isotherms can be derived from data acquired from the mere racemic mixture with a sufficient accuracy for a correct prediction of the band profiles of all kinds of enantiomer mixtures, making possible the computer-assisted optimization of the experimental conditions.

Chromatography↗
PubMed2003

Surface heterogeneity of six commercial brands of end-capped C18-bonded silica. RPLC separations.

Single component adsorption isotherms of phenol and caffeine were measured on six different commercial brands of end-capped C(18)-bonded silica columns (five monomeric bonded phases: Kromasil, Waters Symmetry, Phenomenex, Hypersil, and Chromolith from Merck; one polymeric bonded phase, Vydac) with the same methanol/water solution (30/70, v/v) as the mobile phase. Adsorption data were acquired by frontal analysis (FA) for all these columns in the same way. Depending on their solubility in the mobile phase, the concentrations used ranged between 1 and 100 g/L and between 0.35 and 35 g/L for phenol and caffeine, respectively. Twenty-two adsorption data points were recorded over these ranges. In each case, the best isotherm model accounting for all sets of adsorption data is the bi-Langmuir model, all columns behaving as heterogeneous adsorbents despite the endcapping. Depending on the column, the high-energy sites accounts for between 30 and 40% and between 4 and 7% of the total saturation capacity for phenol and caffeine, respectively. Except for the polymeric phase (Vydac), the ratio of the adsorption constants on the high- and low-energy sites is constant at around 10 for both phenol and caffeine, corresponding to an average adsorption energy difference of 5 kJ/mol between these two sites. The exact nature of the high-energy sites is illustrated by the following properties: (i) they have a very low selectivity for caffeine, with alpha(caffeine/phenol) close to 0.4 for the five monomeric columns, which suggests the complete derivatization of residual silanols; (ii) the high-energy sites account for a large fraction of the surface area of these packing materials (35% for phenol, 6% for caffeine); (iii) there is a small adsorption energy difference between high and low adsorption energy sites (5 kJ/mol); and (iv) the adsorption constants increase with increasing surface coverage of the monomeric columns. Thus, the high energy sites cannot be residual free silanols of the bare silica. More likely, they are related to the local heterogeneity of the C(18)-bonded-layer structure. Caffeine is more strongly retained on the low-energy sites than phenol (the product q(s,) (1)b(1) is larger for caffeine) but the contribution of the high-energy sites (q(s,) (2)b(2)) is markedly lower for caffeine than for phenol, despite the larger value of the adsorption constant, b(2). Because of a larger molecular size, caffeine cannot penetrate as deeply as phenol inside the bonded layer. This explains the paradox of a stronger retention for phenol than for caffeine on end-capped C(18)-bonded stationary phases.

Journal Article↗
PubMed2003

Overloaded gradient elution chromatography on heterogeneous adsorbents in reversed-phase liquid chromatography.

Overloaded band profiles of phenol were measured on a C18-Kromasil column in gradient elution conditions. The mobile phase used was a mixture of methanol and water. The volume fraction of methanol was allowed to vary between 0 and 0.5. A general adsorption model, which expresses the amount of phenol adsorbed q* as a function of both its concentration C and the composition phi of the organic modifier (methanol) in the mobile phase, was empirically derived from previous independent adsorption experiments based on frontal analysis (FA) and frontal analysis by the characteristic point (FACP). Accordingly, the general model was an extension of the simplest heterogeneous model, the Bilangmuir model, to non-isocratic conditions. The low-energy sites followed the classical linear solvent strength model (LSSM), but not the high-energy sites whose saturation capacity linearly decreased with phi. The general model was validated by comparing the experimental and simulated band profiles in gradient elution conditions, in linear and non-linear conditions, as well. The band profiles were calculated by means of the equilibrium-dispersive model of chromatography with a finite difference algorithm. A very good agreement was observed using steps gradient (delta phi) from 0 to 50% methanol and gradient times t(g) of 20, 25, 30, 40, 60, 80 and 100 min. The agreement was still excellent for steps gradient from 5 to 45% (t(g) = 25 min), 5 to 35% (t(g) = 50 min), 5 to 25% (t(g) = 50 min) and 5 to 15% (t(g) = 50 min). Significative differences appeared between experience and simulation when the slope of the gradient (delta phi/t(g)) became too strong beyond 3.3% methanol per minute. This threshold value probably mirrored the kinetic of arrangement of the G18-bonded chains when the methanol content increased in the mobile phase. It suggested that the chromatographic system was not in a full thermodynamic equilibrium state when very steep mobile phase gradients were applied.

Adsorption↗
PubMed2003

Comparison of the binary equilibrium isotherms of the 1-indanol enantiomers on three high-performance liquid chromatography columns of different sizes.

The competitive isotherm data for the enantiomers of 1-indanol were measured on three columns, a microbore column (15 cm x 0.1 cm), a conventional analytical column (15 cm x 0.46 cm), and a semi-preparative column (20 cm x 1.0 cm), packed with Chiralcel OB. The sets of isotherm data measured on each one of these three columns could be fitted well by a bi-Langmuir isotherm model. The experimental elution band profiles of mixtures of the 1-indanol isomers were recorded on the three columns. The isotherm model, combined with the equilibrium dispersive model of chromatography, gave calculated profiles that are in excellent agreement with the experimental profiles in all cases investigated. It was found that the value of the inner diameter of the column is an important parameter in the calculation of the isotherm parameters from the measured isotherm data. In order to use isotherm data obtained on one column to account for the phase equilibrium on another one, the inner diameters of these columns must be measured accurately. The diameters of the three columns were all slightly off their nominal value. Without correction, an important systematic error was made on the isotherm data obtained with the microbore column while only negligible errors were made on the data obtained with the other two columns. After due correction for this effect, the relative difference between the isotherm data for the microbore and the semi-preparative column is still, on the average, about 10%, a difference that might be explained by the limited precision of the measurement of the microbore column diameter. The relative difference between the isotherm data for the analytical and the semi-preparative columns was about 1%, a reasonable value, since the two columns came from different batches of the same packing material.

Chromatography, High Pressure Liquid↗
PubMed2003

Pressure-induced effects in the heterogeneous adsorption of insulin on chromatographic surfaces.

The effect of increasing the average column pressure (ACP) on the heterogeneous adsorption of insulin variants on a C18-bonded silica was studied in isocratic reversed-phase HPLC. Adsorption isotherm data of lispro and porcine insulin obtained for values of the ACP ranging from 57 to 237 bar were fitted to the Langmuir-Freundlich and the Tóth equation. The resulting isotherm parameters, including the equilibrium adsorption constant and the heterogeneity index, were next used for the calculation of distribution functions characterizing the energy of interactions between the adsorbed insulin molecules and the stationary phase. It was observed that increasing the pressure by 180 bar causes a broadening of the distribution functions and a shift of the position of their maximum toward lower interaction energies. These findings suggest that, under high pressures, the insulin molecules interact with the stationary phase in a more diversified way than under low pressures. Additionally, the most probable value of the energy of the insulin-surface interactions becomes lower when the ACP increases. The pressure-induced changes in the interaction of insulin variants with the hydrophobic surface are attributed to a possible conformational flexibility of the molecular structure of this protein.

Adsorption↗
PubMed2003

Comparison between adsorption isotherm determination techniques and overloaded band profiles on four batches of monolithic columns.

The adsorption isotherms of 4-tert.-butyl phenol were measured on four different monolithic columns, using three different techniques, classical frontal analysis (FA), the perturbation on a plateau method (PP) and the recently introduced numerical procedure known as the inverse numerical method (IN). This last approach requires only the recording of a few overloaded profiles and has the potential advantage of affording a dramatic decrease of the amounts of compounds, solvent, and time needed to determine accurate estimates of the coefficients of the isotherm. The reproducibility of the adsorption data measured on the four columns is discussed with reference to the specific techniques used for obtaining these data and to the most suitable equation used for modeling them. The data obtained for the different columns were highly consistent. The inverse numerical approach was confirmed to provide a powerful, accurate, and economic method for measuring single component adsorption data.

Adsorption↗
PubMed2003

A graphitized-carbon monolithic column.

The preparation of a novel carbon monolithic column for high performance liquid chromatography is described. A phenolic resin rod with embedded 10-microm silica beads was prepared by acid-catalyzed polymerization of a resorcinol/iron(III) complex and formaldehyde. This rod was carbonized and graphitized under inert atmosphere with a programmed temperature cycle from room temperature to 1250 degrees C. Subsequently, the silica beads along with iron catalysts were removed, leaving a porous carbon rod. Imaging of this monolithic rod by scanning and transmission electron microscopies revealed a highly interconnected bimodal porous structure. The porosity and pore size distribution of the mesopores were characterized by N2 absorption/desorption. Graphene sheets were found in the TEM images of the carbon rod, and the graphite index was characterized by Raman spectrum and X-ray diffraction. A monolithic column prepared with the aforementioned carbon rod was evaluated using a mixture of alkylbenzenes. It exhibited an excellent separation power and a low hydraulic resistance. The bundle-of-capillaries model was used to characterize the hydrodynamics of this monolith. Its permeability was found to agree well with the theoretical one.

Journal Article↗