Anita M. Katti

Preparative liquid chromatography

SummaryHistorically, liquid chromatography has been a preparative technique. Its applications have been limited, however, until a decade ago. The needs of modern chemical, pharmaceutical and biochemical industries have motivated this period of phenomenal growth which is being witnessed now.Novel packing materials, new packing technologies, and advancements in instrumentation and process technologies have appeared in rapid succession. Instruments using columns with diameters ranging from a few inches to a few feet, can be packed with efficiencies comparable or better than analytical columns having the same packing material. This permits the development of new applications covering a wide variety of problems.The empirical approach, followed until recently for the development of new applications, is being improved by insights derived from a better understanding of the theory of large concentration chromatography. With increased computer power and a greater comprehension of the theoretical aspects, a fundamental approach to design and optimization of the operating parameters is being developed. Investigation of the components of the cost of industrial production is also in its early stages.Historical trends, theoretical treatments, column technologies, operating modes and guidelines for optimization will be discussed and reviewed.

Prediction of band profiles in displacement chromatography by numerical integration of a semi-ideal model

Abstract A numerical solution for the semi-ideal model of chromatography, incorporating dispersion in the numerical integration scheme, has been developed for the prediction of intermediate and isotachic peak profiles of a multi-component mixture in displacement chromatography. The model was used here to study the effect of various operating parameters on the band shapes, production and yield for a binary mixture. The effect of the displacer concentration and the column length on the yield and the production was investigated. Optimal concentrations can be found that maximize the yield or the production. Very low displacer concentrations lead to peak shapes identical with those in overloaded elution chromatography and extremely large displacer concentrations result in the lack of a separation. The band profiles obtained prior to the formation of the isotachic train are in agreement with profiles taken from the literature.

Effect of sample viscosity in high-performance size-exclusion chromatography and its control

Abstract When the sample viscosity greatly exceeds that of the mobile phase, flow instabilities occur, which lead to non-uniform flow in the radial direction. This “fingering” effect is usually greater with large-diameter than with micro-bore columns and may have worse effects in size-exclusion chromatography (SEC) than in any other chromatographic mode, as retention is shorter and dilution less important with SEC than with modes where retention is significant. This study suggests ways to reduce this viscosity effect. For example, this effect can be eliminated by controlling the concentration of a suitable mobile-phase additive which allows one to equate the viscosities of the eluent and the sample. Another possibility is to follow the sample with a plug of eluent, 0.3–0.5 column volumes wide, having a slightly higher viscosity than the sample. No spurious peaks are observed, as the plug acts as a wall prohibiting “fingering”.

Comparison of various isotherm models for predicting competitive adsorption data

Abstract Competitive adsorption isotherms were determined for 2-phenylethanol and 3-phenylethanol and 3-phenylpropanol on ODS-silica with methanol—water as the mobile phase. The experimental data were fitted to the Langmuir competitive isotherm, the second-order ideal adsorbed solution isotherm of LeVan and Vermeulen, the seven-parameter quadratic isotherm (ratio of two second-degree polynomials) and the competitive Fowler isotherm. The best results were obtained byn adjusting the five parameters of the competitive Fowler isotherm by a non-linear regression. Unfortunately, this isotherm, gives the mobile phase concentration as a function of the surface coverage and the equation cannot be inverted in closed form.

Quantitative comparison between the experimental band profiles of binary mixtures in overloaded elution chromatography and their profiles predicted by the semi-ideal model

Abstract The experimental elution profiles of the components of a binary mixture were determined under conditions where the column is strongly overloaded and the two bands interfere markedly. These profiles were derived from the analysis of approximately 100 fractions collected during the elution of large samples of synthetic mixtures of two similar compounds having different compositions. The system investigated was the separation of 2-phenylethanol and 3-phenylpropanol on Vydac ODS silica eluted isocratically with methanol—water (50:50). The results demonstrate the displacement and the “tag-along” effects previously predicted on the basis of theoretical investigations of the elution behavior of binary mixtures in non-linear chromatography. The displacement of the first band by the second takes place essentially when the concentration of the second eluted component is the larger. The tag-along effect of the second component is observed when the major component of the mixture is the first eluted. Comparison between the experimental profiles and the profiles predicted by the semi-ideal model was performed using the competitive Langmuir isotherm model. For numerical calculations, the parameters of these isotherms were determined by measurement of the single-component isotherms of each of the two compounds by frontal analysis. The results of the comparison show that the competitive Langmuir isotherm gives only a fair approximation of the multi-component adsorption behavior of the two compounds studied. Competition between them for adsorption seems to be stronger than predicted by the competitive Langmuir model.

Consolidation of the Packing Material in Chromatographic Columns under Dynamic Axial Compression. II. Consolidation and Breakage of Several Packing Materials

Abstract The consolidation of packing materials under dynamic axial compression is studied for one irregular and two spherical shaped particles. When the total axial compression stress is raised from 1 to 80 kg/cm2, a long column of initial length 25 cm shrinks by nearly 25%, while a short column of initial length 4 cm decreases by nearly 35%. This result illustrates the lack of homogeneity of the stress distribution during the compression. While the extent of consolidation is nearly the same with all three materials, its kinetics is different. The spherical materials consolidate rapidly and smoothly. The material made of irregular-shaped particles consolidates more slowly, in several decreasing steps. Finally, almost no breakage is observed with the spherical materials, while for the irregularshaped material fragmentation and chipping takes place to a significant extent under an axial compression stress of 80 kg/cm2.

Comparison of the performances of overloaded elution and displacement chromatography for a given column

Abstract Using the semi-ideal model, chromatograms were calculated for the separation of a binary mixture on a given column by displacement and elution chromatography. The optimum conditions for the maximum production rate with or without a recovery yield constraint were determined using the simplex algorithm. Mixtures of relative retention 1.20 and 1.70 and relative composition 1:3 and 3:1 were considered. The maximum production rates achieved in both modes of chromatography are close. In all instances studied the ratio of the production rates of the two modes was between 0.5 and 2. Without a yield constraint, for the cases studied the recovery yield achieved in overloaded elution is much higher than that achieved in displacement chromatography. However, the concentration of the fraction collected in the displacement mode is one to two orders of magnitude higher than that of the fraction collected in the elution mode. The choice between the modes of operation will depend largely on the comparative economics of displacer recovery and fraction concentrations.

Nonlinear chromatography Recent theoretical and experimental results.

The theory of nonlinear chromatography has been advanced by the incorporation of recent results obtained by the theory of partial differential equations. The system of equations of the ideal model has been solved analytically in the case of a single component for which the equilibrium isotherm between the mobile and the stationary phases is given by a Langmuir equation. A series of computer programs has been written which permits the calculation of numerical solutions of the semi-ideal model. The properties of the solutions obtained are described and discussed for a one-component system (profile of high concentration bands of a pure compound eluted by a pure solvent), several two-component systems (elution of a pure compound band by a binary mobile phase, separation of a binary mixture eluted by a pure mobile phase), and three-component systems (separation of a binary mixture eluted by a binary solvent, displacement and separation of a binary mixture). Experimental results are reported which validate the conclusions derived from the numerical integration of the model. The conclusions of the work apply to all high-performance chromatographic procedures, i.e., to those where the kinetics of mass transfer are fast enough for the mobile and stationary phases always to be near equilibrium. More specifically, the contribution from the kinetics of the retention mechanism to the mass transfer resistance must itself be negligible. This clearly excludes affinity chromatography.

Prediction of single and binary profiles in overloaded elution chromatography using various semi-ideal models

Abstract Experimental band profiles of 2-phenylethanol and 3-phenylpropanol were recorded for pure components in different amounts and for binary mixtures at different compositions in reversed-phase chromatography. The injection function was also measured. These experimental profiles are compared with those calculated using different finite difference methods ( i.e. , the Craig and Rouchon models). The results show that it is important to take into account the true injection profile. The different calculation procedures in most instances give profiles which are in close agreement with the experimental data. The Craig model gives profiles which are generally sharper than those given by the Rouchon model. Differences between the experimental and calculated profiles are ascribed to mathematical properties of the method implemented.