Prediction of the band profiles of the mixtures of the 1-indanol enantiomers from data acquired with the single racemic mixture

Investigation of the adsorption behavior of glycine peptides on 12% cross-linked agarose gel media

The highly cross-linked 12% agarose gel Superose 12 10/300 GL causes retardation of glycine peptides when mobile phases containing varying concentrations of acetonitrile in water are used. An investigation has been made into the retention mechanism behind this retardation using the glycine dipeptide (GG) and tripeptide (GGG) as models. The dependence of retention times of analytical-size peaks under different experimental conditions was interpreted such that the adsorption most probably was caused by the formation of hydrogen bonds but that electrostatic interactions cannot be ruled out. Thereafter, a nonlinear adsorption study was undertaken at different acetonitrile content in the eluent, using the elution by characteristic points (ECPs) method on strongly overloaded GG and GGG peaks. With a new evaluation tool, the adsorption energy distribution (AED) could be calculated prior to the model selection. These calculations revealed that when the acetonitrile content in the eluent was varied from 0% to 20% the interactions turned from (i) being homogenous (GG) or mildly heterogeneous (GGG), (ii) via a more or less stronger degree of heterogeneity around one site to (iii) finally a typical bimodal energy interaction comprising of two sites (GG at 20% and GGG at 10% and 20%). The Langmuir, Tóth and bi-Langmuir models described these interesting adsorption trends excellently. Thus, the retardation observed for these glycine peptides is interpreted as being of mixed-mode character composed of electrostatic bonds and hydrogen bonds.

Investigation of the concentration dependence of mass transfer coefficients in reversed-phase liquid chromatography

In order to investigate the concentration dependence of mass transfer coefficients in RPLC, experimental breakthrough curves obtained by staircase frontal analysis (FA) were fitted to the simplified models such as multiplate (MP) model, equilibrium dispersive (ED) model, and transport model, and the sophisticated models such as lumped pore diffusion (POR) model and general rate (GR) model. The MP model was used to obtain the initial guesses of the parameters of the ED and the transport models. Then the best values were obtained by minimizing the differences between theoretical and experimental values with a nonlinear fitting procedure. The values of the parameters of the POR and the GR models can be calculated by using the expressions derived from the plate height equations, which was further validated by using the fitting method. It was found that the mass transfer coefficients would depend on the solute concentration. This can be ascribed to the surface diffusivity, which correlates with the concentration and is lumped into the mass transfer coefficients for both simplified and sophisticated models.

Adsorption of the enantiomers of 3-chloro-1-phenyl-propanol on silica-bonded chiral quinidine carbamate

The interactions of 3-chloro-1-phenyl-propanol with a quinidine carbamate-bonded chiral stationary phase under NPLC conditions were studied by measuring the adsorption isotherm data of its enantiomers by frontal analysis, modeling these data with a suitable isotherm model, and comparing the experimental overloaded elution band profiles with those calculated with this isotherm and the equilibrium dispersive model of liquid chromatography. The affinity energy distribution was calculated from the adsorption isotherm data. The results show that the surface of the adsorbent is heterogeneous and exhibits a bimodal adsorption energy distribution. This fact is interpreted in terms of the presence of two different types of adsorption sites on the stationary phase, nonselective and enantioselective sites. Albeit the bi-Langmuir isotherm model successfully accounts for the single-component data corresponding to both enantiomers, the competitive bi-Langmuir isotherm model does not allow an accurate prediction of the overloaded band profiles of the racemic mixture. Thermodynamic data are drawn for explanation. Some aspects of the retention mechanism are discussed in the light of the data obtained.

Adsorption of the enantiomers of 2,2,2-trifluoro-1-(9-anthryl)-ethanol on silica-bonded chiral quinidine-carbamate

The adsorption isotherms of the enantiomers of 2,2,2-trifluoro-1-(9-anthryl)-ethanol from a toluene-acetonitrile solution onto a Chiris Chiral AX:QD1 column were measured using the pulse method. The isotherm data were modeled with a bi-Langmuir isotherm model, indicating the presence of two different types of adsorption sites on this stationary phase, nonselective and enantioselective sites. The latter are homogeneous but interact with both enantiomers, albeit with different energies. The thermodynamic characteristics of these two types of sites were characterized by their adsorption constants and saturation capacities and by the influence of the temperature on these different parameters.

Experimental determination of single solute and competitive adsorption isotherms

In order to design and to optimise preparative liquid chromatography, the knowledge of the underlying thermodynamic functions, i.e. the adsorption isotherms, is of large importance. Usually these functions can not be predicted and various techniques have been suggested to determine them experimentally. In this paper, several important methods to measure adsorption equilibrium data are discussed and evaluated. The main focus is set on dynamic methods analysing concentration profiles that could be detected at the outlet of fixed-beds packed with the stationary phase of interest. The theoretical background of the different methods is explained using classical equilibrium theory and the equilibrium dispersion model. Each method is illustrated based on experimental data collected in our laboratory. Based on these personal experiences recommendations are given regarding the potential and the applicability of the methods discussed.

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.

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.

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.