Overloaded Band Profiles Research Articles

Determination of the single component and competitive adsorption isotherms of the 1-indanol enantiomers by the inverse method

The inverse method of isotherm determination consists in calculating the numerical values of the coefficients of an isotherm model that give a set of chromatographic profiles in best possible agreement with the set of experimental profiles available. This method was applied to determine the adsorption isotherms of the 1-indanol enantiomers on a cellulose tribenzoate chiral stationary phase. Both single-component and competitive isotherms were determined by using no more than one or two overloaded band profiles. The isotherms determined from the overloaded band profiles agreed extremely well with the isotherms determined by frontal analysis. Several isotherm models were used and tested. The best-fit isotherm was selected by means of statistical evaluation of the results. The results show that the adsorption is best characterized with a model describing heterogeneous adsorption with bimodal adsorption energy distribution.

Study of the competitive isotherm model and the mass transfer kinetics for a BET binary system

The competitive adsorption behavior of the binary mixture of phenetole (ethoxy-benzene) and propyl benzoate in a reversed-phase system was investigated. The adsorption equilibrium data of the single-component systems were acquired by frontal analysis. The same data for binary mixtures were acquired by the perturbation method. For both compounds, the single-component isotherm data fit best to the multilayer BET model. The experimental overloaded band profiles are in excellent agreement with the profiles calculated with either the general rate model or the modified transport-dispersive models. The competitive adsorption data were modeled using the ideal adsorbed solution (IAS) theory. The numerical values of the coefficients were derived by fitting the retention times of the perturbation pulses to those calculated using the IAS theory compiled with the coherence conditions. Finally, the elution profiles of binary mixtures were recorded. They compared very well with those calculated. As a characteristic feature of this case, an unusual retainment effect of the chromatographic band of the more retained component by the less retained one was observed. The combination of the General Rate Model and the adsorption isotherm model allowed an accurate prediction of the band profiles.

Repeatability and reproducibility of high concentration data in reversed-phase liquid chromatography: I. Overloaded band profiles on Kromasil-C 18

Single-component adsorption-isotherm data were acquired by frontal analysis (FA) for six low-molecular-mass compounds (phenol, aniline, caffeine, theophylline, ethylbenzene and propranolol) on one Kromasil-C 18 column, using water–methanol solutions (between 70:30 and 20:80, v/v) as the mobile phase. Propranolol data were also acquired using an acetate buffer (0.2 M) instead of water. The data were modeled for best agreement between calculated and experimental overloaded band profiles. The adsorption energy distribution was also derived and used for the selection of the best isotherm model. Widely different isotherm models were found to model best the data obtained for these compounds, convex upward (i.e. Langmuirian), convex downward (i.e. anti-Langmuirian), and S-shaped isotherms. Using the same sample size for all columns (loading factor, Lf≈10%), overloaded band profiles were recorded on four different columns packed with the same batch of Kromasil-C 18 and five other columns packed with different batches of Kromasil-C 18. These experimental band profiles were compared to the profile calculated from the isotherm measured by FA on the first column. The repeatability as well as the column-to-column and the batch-to-batch reproducibilities of the band profiles are better than 4%.

Determination of single component isotherms and affinity energy distribution by chromatography

Adsorption isotherm data were acquired by frontal analysis (FA) and large sample-size band profiles were recorded for phenol and caffeine. For both compounds, the isotherm data fit well to the Langmuir, Toth, and Bi-Langmuir models of adsorption. The Langmuir model must be dismissed because it does not predict accurately the overloaded band profiles. However, profiles calculated using the unimodal Toth and the bimodal Bi-Langmuir models are indistinguishable. The expectation-maximization procedure was used to calculate directly the affinity energy distribution (AED) from the raw FA data points. For both compounds, the AED converges to a bimodal distribution at high numbers of iterations. This result, which shows the high sensitivity of the EM method, suggest that the Bi-Langmuir model makes better physical sense than the Toth model. This model also permits a detailed investigation of the properties of active sites, a feature often evoked in chromatography but so far rarely the topic of a quantitative investigation.

Numerical determination of the competitive isotherm of enantiomers

A numerical method was developed and used to determine adsorption isotherms in chromatography. The numerical parameters of an isotherm model were derived from the recorded band profiles of the racemic mixture of the 1-phenyl-1-propanol enantiomers, by means of a nonlinear least-squares method. We used the equilibrium-dispersive model of chromatography with several isotherm models. The numerical constants of the isotherm models were tuned so that the calculated and the measured band profiles match as much as possible. We show that this numerical inverse method can be applied even without the knowledge of the individual band profile of the pure enantiomers. The isotherms determined from the—usually unresolved—overloaded band profiles matched extremely well the isotherms determined by frontal analysis. Several isotherm models were used and tested—such as Langmuir, biLangmuir, Tóth, Langmuir–Freundlich. The best-fit isotherm was selected by means of statistical evaluation of the results.

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-1-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 only data derived from the racemic mixture are required for computer optimization of the preparative chromatography separation of the enantiomers.

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 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.

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.

Prediction of a protein band profile in preparative reversed‐phase gradient elution chromatography

The overloaded band profiles of lysozyme in reversed-phase preparative chromatography were recorded on a C18 chemically bonded silica column, with acetonitrile/water as the mobile phase. These experiments were carried out under isocratic conditions at 31.6, 31.9, and 32.2% acetonitrile (ACN) for loading factors up to 43% of the column saturation capacity and under linear-solvent-strength gradient-elution with gradient slopes of 0.5 and 1% ACN/min, for loading factors up to 11.3%. The adsorption isotherms of lysozyme were measured for the same solvent compositions and found to be accurately accounted for by a bi-Langmuir isotherm model.With the use of a Craig model implementation of the equilibrium-dispersive model of chromatography, the band profiles of lysozyme were calculated. An excellent agreement was observed between these calculated profiles and the experimental profiles recorded at loading factors below 5%. By contrast, band profiles calculated using a Langmuir isotherm failed to describe the experimental bands. At column loadings exceeding 8%, a slight but systematic deviation takes place between calculated and experimental profiles. It is most probably explained by the considerable concentration effect of the gradient, making the band experience phase equilibrium in a concentration range that exceeds largely the one where the isotherm data have been measured.

Precision and accuracy of the gas—solid adsorption isotherms derived by the elution-by-characteristic-points method

The influence of the fluctuations of the experimental parameters of the reproducibility of the overloaded band profiles used to determine the isotherms by the elution-by-characteristic-points (ECP) method was investigated using two new tools. A parameter measuring the difference between two similar curves, such as overloaded elution profiles or adsorption isotherms, was defined. The band profiles measured were compared with those calculated from the derived isotherm, using models from the theory of non-linear chromatography. The agreement between both profiles was defined as a distance lower than twice the average distance between two profiles measured on the same day on the same column. The degree of validity of the various assumptions made in the ECP method was assessed.

Experimental study of the elution profiles of high concentration bands of binary mixtures eluted by a binary eluent containing a strongly retained additive

When the mobile phase of a chromatographic system contains a strong solvent or an additive that has an adsorption strength larger than that of the components of the analyzed mixture, the band profiles depend much on the additive concentration as well as on the sample size. The phenomena can be predicted with the semiideal model of chromatography, using a three-component set of equations and ternary competitive isotherms. At low concentrations of the modifier, the overloaded band profiles are similar to those predicted by a competitive binary Langmuir isotherm. At higher concentrations, the direct of band asymmetry is reversed and the band profiles are very much like those obtained with anti-Langmuir isotherms. With a binary mixture, retainment and pull back effects, which are the reverse of the effects found with a competitive binary Langmuir isotherm when the additive is weakly adsorbed, are observed. The exact band shapes depend on the strong solvent concentration, on the sample size and the relative concentration of the two components, on the column saturation capacity, and most importantly, on the ratios of the origin slopes of the equilibrium isotherms of the modifier and components in the pure weak solvent. In this paper, the author describes experimentalmore » results that duplicate the elution profiles of the overloaded bands of a binary mixture predicted by a previous theoretical paper. This work deals essentially with the influences of the solvent concentration and the sample size and composition on the band profiles. Their experimental results validate the models of nonlinear chromatography that they have been using.« less