Guoming Zhong

Study of the thermodynamics and mass transfer kinetics of two enantiomers on a polymeric imprinted stationary phase

The adsorption isotherms of d- and l-phenylalanine anilide (PA) on an l-phenylalanine anilide imprinted stationary phase have been determined using staircase frontal analysis. An aqueous buffer–organic solvent mixture has been used as mobile phase. The measurements were done at temperatures of 40, 50, 60 and 70°C for sample concentrations ranging between 5·10−4 to 1 g/l. It was found that the adsorption data fit well to both the Freundlich and the Bi-Langmuir isotherm models. Examination of the best values of the numerical coefficients of the Bi-Langmuir model shows that the site class representing the binding sites with the highest binding energy exhibits a very low saturation capacity for the non-imprinted enantiomer, indicating a high selectivity for the imprinted l-enantiomer. The low energy site class also shows some selectivity for the l-enantiomer. Mass transfer rate coefficients were obtained for each single breakthrough curve by using the transport model of chromatography. It was found that the mass transfer coefficient of l-PA increases very rapidly with the sample concentration while there is only a slight increase for the other enantiomer.

Peak tailing and mass transfer kinetics in linear chromatography

The origin of peak tailing under linear conditions, at very low analyte concentrations, is investigated using the equations of the transport-dispersive model of chromatography. It is shown that a general explanation can be obtained by assuming the existence of two different types of adsorption sites having different equilibrium isotherms and different rates of mass transfer kinetics. Even if the experimental conditions are such that both mechanisms operate linearly, tailing can be observed if the mass transfer kinetics is much slower on one type of sites than on the other. The most pronounced and typical peak tailings occur when the slow type of adsorption sites give a smaller contribution to the retention than the fast type and if the rate constant of mass transfer for the slow sites is between 20 and 2000 times smaller than that of the fast sites. Axial dispersion (caused by molecular and eddy diffusion) dampens the effects. Therefore, peak tailing of kinetic origin will be more important on highly efficient columns than on mediocre ones. An important case in point is that of enantiomeric separations. Chiral phases are designed as two-types-of-sites phases since it is impossible to eliminate the general non-selective interactions between analytes and bonded groups. The chiral selective sites involve strong interactions between the stationary phase and at least one of the enantiomers, conditions which slow down mass transfers.

Analytical solution for the linear ideal model of simulated moving bed chromatography

An analytical solution for linear ideal simulated moving bed (SMB) chromatography is derived. This solution is a set of algebraic equations which allows the calculation of the concentration profiles along the columns and of the concentration histories at the raffinate and extract ports. Using this solution, the operations of SMB under steady-state conditions and during the transition from start-up to steady state are examined. An asymptotic property is found for the steady state. A comparison between SMB and true moving bed (TMB) illustrates their similarities and differences.

Peak tailing and slow mass transfer kinetics in nonlinear chromatography

Peak tailing has two main origins in chromatography: (i) heterogenous mass transfer kinetics, (ii) heterogenous thermodynamics with overloading of a nonlinear isotherm. The individual effects of ea ...

Application of the Shock Layer Theory to the Determination of the Mass Transfer Rate Coefficient and Its Concentration Dependence for Proteins on Anion Exchange Columns

The extension of the shock layer theory to systems having a slow mass transfer kinetics and a concentration‐dependent rate coefficient is discussed. Experiments were carried out with bovine serum albumin on two anion exchangers, TSK‐GEL‐DEAE‐5PW and Resource‐Q. The adsorption isotherm data, determined by single‐step frontal analysis, could be fitted to simplified bi‐Langmuir equations with very small residuals. A lumped kinetic model (solid film linear driving force model, with rate coefficient kf) was used to account for the mass transfer kinetics. The profile of each breakthrough curve (BC) was fitted to the curve calculated with this transport model and the rate coefficient kf obtained by identification. A linear dependence of kf on the average concentration of the step of the BC was found. The shock layer thicknesses (SLT) calculated for different relative concentrations agreed very well with the experimental results. This justifies the use of the SLT for the direct determination of rate coefficients.

Influence of the concentration dependence of the mass transfer properties on chromatographic band profiles: II. Accuracy of the determination of isotherm data by frontal analysis

Using known relationships between the apparent axial dispersion coefficient and the concentration, the process of isotherm determination by frontal analysis is modeled by calculating numerical solutions of the equilibrium-dispersive model and processing these simulated experimental data using classical methods of frontal analysis. The results show that significant systematic errors can take place when the apparent axial dispersion coefficient depends strongly on the concentration in the range investigated.

Study of the mass transfer kinetics of BSA on a TSK-GEL DEAE-5PW anion exchanger in a wide concentration range

The adsorption isotherms of bovine serum albumin (BSA) on TOSOHAAS TSK‐GEL DEAE‐5PW were determined in a pH = 7.5 buffer for a 1.0 mL/min flow rate and at pH = 6.0 for 1.0, 2.0, and 3.0 mL/min flow rates. The isotherm data were derived from the breakthrough curves obtained by step series frontal analysis (FA) method. The sample concentrations ranged from ca. 0.2 to 3 mg/mL. From the Scatchard plots, it was found that the bi‐Langmuir isotherm model fits the adsorption isotherm data better than the single Langmuir isotherm model at both pH = 7.5 and 6.0, for flow rates up to 2.0 mL/min. The rate of the mass transfer kinetics was determined for each breakthrough curve by using a lumped kinetic model. An average rate coefficient of mass transfer kinetics was derived. This rate coefficient depends linearly on the average sample concentration.

Simulated moving bed chromatography. Effects of axial dispersion and mass transfer under linear conditions

In a previous publication, we have given the analytical solution of Simulated Moving Bed (SMB) chromatography in the linear, ideal case for a binary mixture. This solution predicts the concentration profiles of the two components along the columns and the concentration histories at the raffinate and the extract ports. It shows the asymptotic nature of SMB operation. In this report, the influence of the non-ideal effects arising from the axial dispersion and the finite rate of the mass transfer kinetics are investigated. Results obtained by the numerical simulation of the non-ideal model are compared with those given by the analytical solution of the ideal model. The consequences of a progressive increase of the dispersion of the solution are illustrated. The consequences of the non-ideal effects on the design of the industrial SMB process are also discussed.

Validity of Darcy’s law at low flow-rates in liquid chromatography

Abstract The applicability of Darcy’s law at extremely low flow velocities has been questioned in the scientific literature. It was investigated using a simple chromatographic system. Ethylene glycol (viscosity, 19.9 cP at 20°C) was pumped through 10 to 25 cm long chromatographic columns packed with small porous spherical particles (size, 5 to 10 μm). The dependence of the linear velocity of this liquid on the inlet pressure was found to be linear in the range of Reynolds numbers (based on particle diameter) between Re=1·10−6 and 1·10−4. These results contradict earlier reports by Fand et al. and more recent findings by Kececioglu and Jiang that suggest a limited applicability of Darcy’s law to the range Re=0.3 to 0.7.

Profiles of large-size system peaks and vacancy bands in liquid chromatography I. Analytical solution of the ideal model

Often in chromatography, a compound which participates in the retention mechanism is added to the mobile phase. This permits control of the retention factors of the analytes and allows their indirect detection. Injection of large volumes of solutions having a composition different from that of the mobile phase causes perturbation of the equilibrium between the two phases of the chromatographic system, and may result in large positive or negative system peaks when the additive signal is monitored. If the equilibrium behavior is accounted for by competitive Langmuir isotherms, the positive bands, which contain an excess of the additive, are Langmuirian-shaped as in the classical elution case, while the large vacancy bands have an anti-Langmuirian profile. An explicit analytical solution is derived within the framework of the ideal model for the profiles of large-size pulses and vacancies. This solution permits the easy calculation of the retention times of the different parts of the bands, i.e., of its shocks and diffuse boundaries. This solution provides also a more profound explanation of the formation of the large system peaks than available so far and of their distinctive properties as compared to those of large-size bands in classical elution.