Nonlinear chromatography

trends in analytical chemistry,

[ 5) P. W. Araujo ( 1995) 2497.

vol. 15, no. 3, 1996

and R, G, Brereton,

163

Analyst,

120

[ 6 1R. G. Brereton, Multivariate Pattern Recognition in Chemometrics, Illustrated by Case Studies, Elsevier, Amsterdam, 1992. [ 7 1 B. Flury and H. Riedwyl, Multivariate Statistics, A Practical Approach, Chapman and Hall, London, 1988. [ r3] M. A. Sharaf, D. L. Illman and B. R. Kowalski, Chemometrics. Wiley, New York, 1986.

[9] H. Martens and T. Nm, Multivariate Wiley, Chichester, 1989.

Pedro W. Araujo and Richard G. Brereton are at the School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK. For bibliographical details see Part I of this article_

Nonlinear chromatography Fundamentals of Preparative and Nonlinear Chromatography, by G. Guiochon, S.G. Shirazi and A.M. Katti, Academic Press, Boston, MA, 1994, $95.00 (xv + 701 pages), ISBN 0- 12-305530-X This book treats the fundamental concepts which form the basis for the development of preparative separations by chromatography on the laboratory, pilot, and process scale. It aims to provide the reader with a better understanding of the methods needed to apply them to the solution of practical problems. The recent pressure by the regulatory agencies on pharmaceutical comto produce high-purity panies chemicals, to identity drug metabolites and to perform toxicological studies, has increased the need to extract and purify a wide variety of chemicals on the laboratory scale. In such cases, small amounts of material are needed and the application of a few simple rules and the knowledge of the basics of nonlinear chromatography will help. The contribution of the cost of solvent, stationary phases, chemicals and instrument amortization will be small in comparison to the cost of the time spent in developing the separation. Preparative chromatographic separations are also required in the production and sale of a final material. Their purpose is to optimize the experimental conditions for

Calibration,

maximum production rate, minimum solvent consumption and other production cost. The reader will find here a complete mathematical development of the models of chromatography and other physical laws which will help the chemical engineer in the development of these two aspects of preparative chromatography. In Chapter I, after an introduction relating the history of chromatography and especially of preparative chromatography as a separation process, the authors present a number of useful and necessary definitions such as linear and nonlinear, ideal and nonideal chromatography, separation, extraction and purification. Chapter II provides the fundamental theoretical basis of chromatography such as the mass balance equation and relationship between the concentrations in the stationary and the mobile phases. Depending on the assumptions, the various models (ideal, equilibrium-dispersive, kinetics ...) are discussed. After demonstrating that thermodynamics of phase equilibrium is of prime importance to the separation process, the equilibrium isotherms for single components are discussed in Chapter III. The models of adsorption isotherms (Langmuir, bi-langmuir, Fowler, Freundlich, S-shape and quadratic) are presented. Many examples of experimental isotherms are fitted to these models. The different methods used for determining

single-component isotherms are given in the last part of Chapter III and include the diagrams of the chromatograph designed for the determination of equilibrium isotherms. The last section explains how experimental data should be processed and fitted to an isotherm model. Chapter IV examines the additional complexity which results from the competition between the different components for interaction with the stationary phase, since chromatography often deals with multicomponent mixtures. The authors point out that in a finite-concentration solution, the amount of a component adsorbed at equilibrium is a function of the concentration of the component, as for single-component isotherms, but also of the concentrations of all the other components present in the solution which are absorbed by the stationary phase. The competitive character of adsorption is examined for each isotherm model. The determinations of competitive isotherms by frontal and pulse methods are presented. Chapter V completes the first part of the book by a study of the transport phenomena (diffusion, axial dispersion and mass transfer resistance, importance of the viscosity of the mobile phase). In the second part, the separation process is described according to the different models of chromatography. Chapter VI is devoted to linear chromatography, defined as a specific and limiting case of nonlinear chromatography because thermodynamics controls only the band positions whereas

VIII kinetics controls their profile. Although linear chromatography is not encountered in preparative chromatography, it permits investigation of the thermodynamics in the formation and migration of band profiles and in band separation. The reader will find a rigorous presentation of the different approaches of the plate models and of the general rate model of chromatography. After discussing the sources of band asymmetry and tailing in linear chromatography, it is explained why the equilibrium isotherms no longer behave linearly when a large amount of a concentrated solute mixture is injected: different concentrations will move with various velocities with the effect of deformation of band profiles during their migration. As mathematical aspects of the theory become much more complex, so simplified models are considered. Chapter VII and VIII discuss the solution in nonlinear chromatography with the assumption of the ideal model, i.e. with assumption of an infinite column efficiency, in the case of the elution of a single component band and two-component bands. Chapter IX deals also with ideal model, but in displacement chromatography. As chromatographic columns have a finite efficiency, the equilibrium-dispersive model is studied in Chapter X for the elution of single-component bands, in Chapter XI for isocratic separations of twocomponent bands and gradient elution and in Chapter XII for frontal analysis and displacement. Applications of displacement chromatography are presented for the separation of rare earths and other cations, organic compounds, peptides and proteins, as well as nucleic acid constituents. Chapter XIII deals with the important problem of perturbation of the equilibrium caused by the sample injection, when the mobile phase is a mixture of compounds, which generates a number of additive peaks named system peaks. Feed injection in preparative chromatography causes major perturbation of the additive concentrations which are coupled with the profile of the bands of feed components and this phenomenon may decrease the production rate considerably. After a theoretical

trends in anafyticai chemistry, vol. 75, no. 3, 7 996

study of system peaks in linear chromatography, high-concentration system peaks are examined for single-component and two-component samples. Chapters XIV and XV deal with kinetic models which allow the study of chromatography with slow kinetics of mass transfer. Again, frontal analysis and elution of single-component bands are discussed before frontal analysis, elution and displacement of multicomponent bands. The last Chapter will interest many readers as it explains thoroughly the optimization of the experimental conditions in preparative chromatography. After some definitions specific to the topic and some consideration about the economics of chromatographic separations, optimization is presented in detail, taking into account both theoretical considerations and the use of numeric solutions. Emphasis is given to optimization of the column operating and design parameters for maximum production rates and minimum production costs and also to a comparison between the performance of the different modes of chromatography. The authors state in the introductory chapter that the “book is written for those who want to understand the mechanisms through which band

profiles are generated and separated in preparative and process chromatography. It should also be useful to those who develop and optimize chromatographic separations”. I would add that this book is useful for all who want to have a general understanding of chromatography. There is a good balance between theory, modelling and experimental data and it contains a considerable amount of original work. All these chapters contain many calculated or computed chromatograms, and the authors report many experimental data for comparison. They make an admirable effort to clearly describe the fundamentals and the assumptions of the different models before writing the corresponding equations. This book will certainly serve as a reference for all chromatographers, even if they are not (yet) closely involved with nonlinear and preparative chromatography. MARIE-CLAIRE

HENNION

Professor M-C. Hennion is at the Laboratoire de Chimie Analytique of the Ecole Supkrieure de Physique et de Chimie Industrielles de Paris, 10 rue Vauquelin, 75231 Paris cedex 05, France.

TrAC/Internet column In order to inform analytical chemists about the Internet and the role it could play in their lives, Dr. Stephen Heller was invited to become a Contributing Editor of TrAC. The Internet Column has now become a feature of the journal. The column can also be found on the World Wide Web. Anyone interested in contributing to this column is invited to contact Stephen Heller at: [email protected] The Internet Column articles of TrAC can also be found on the Web. If you have a browser, to access the TrAC column on the Web simply point to: http:/www.elsevier.nl/locate/trac