Education and Technology

* Plate Theory and Extensions

The plate theory is developed and equations for the retention volume of a solute, the capacity ratio of a solute, the separation ratio of two solutes and the conditions for chromatographic separation derived. The different volumes that make up the dead volume are discussed and experimental methods for measuring the different dead volume components given. The Gaussian form of the elution equation is derived and methods of measuring the retention volume of closely eluting peaks discussed. The concept of column efficiency is introduced and a method for measuring column efficiency described. The points of inflection of a peak are defined together with effective plate number and the resolving power of a column. The concept of the summation of variances is introduced and used to calculate the maximum volume of sample that can be placed on a column. The technique of vacancy chromatography is considered and an equation for the peak capacity of a column developed. Finally the temperature changes that take place on the passage of a solute through a theoretical plate are examined in detail both theoretically and experimentally.


*The Mechanism of Chromatographic Retention

Solutes are retained in a chromatographic column because the solute molecules interact more strongly with the molecules of the stationary phase than those of the mobile phase. The different types of molecular interaction are described which include dispersive interactions, polar interactions (which include both dipole-dipole interactions and dipole-induced dipole interactions) and ionic interactions. Molecular interactions with mixed phases are also discussed and it is shown that interaction occurs as though each component of the phase is a separate phase and its contribution is proportional to its concentration in the mixed phase. Retention by surface adsorption is also discussed and the theory of monolayer and bi-layer adsorption developed. The sorption and displacement adsorption processes are considered including retention by exclusion. The preparation of silica gel is described and the preparation of silica gel mixtures having different exclusion properties outlined. Chiral interactions and chiral phases are also discussed.

*The Thermodynamics of Chromatography

The basic thermodynamic equations pertinent to chromatography are introduced and the method of distribution analysis using the standard energy of distribution discussed. Thermodynamic analysis is demonstrated by analyzing the energy difference between the dispersive interactions of the methyl and methylene groups with an alkane stationary phase. Using the distribution data for the substituted methanes the energies involved in the dispersive interactions of carbon, hydrogen chlorine and bromine with an alkane stationary phase are also examined. Other types of molecular interaction are considered and the thermodynamic explanation of complex formation also examined. Thermodynamic argument is also used to identify the optimum operating conditions for chiral separations and the effect of solvent composition enantiomeric separations.

*Dispersion in Chromatography Columns

The principal of the summation of variances is first discussed and the alternative axis of the chromatogram introduced, namely curves relating sample concentration to either time, volume flow of mobile phase through the column or distance moved along the column. The Random Walk model for obtaining an expression for the variance of a dispersion process is explained and used to derive expressions for multipath dispersion, longitudinal diffusion and dispersion due to the resistance to mass transfer in the two phases. The effect of the compressibility of the mobile phase in a gas chromatography column on the variance equation is theoretically examined and the Van Deemmter equation discussed. The alternative dispersion equations developed by Giddings, Huber, Knox, Horvath for packed columns and by Golay for capillary columns are described and discussed.. Strong experimental evidence is given to indicate that the Van Deemter equation best explains the variance in a packed column over practical range of operating variable used in both gas and liquid chromatography.

*Extra Column Dispersion

Dispersion of an eluted solute in the chromatographic apparatus other than the column can be extremely important and in the worst case seriously impair the performance of the column. This book examines quantitatively and qualitatively the dispersion that can take place in sample valves, connecting conduits, unions, frits and in the sensing volume of the detector. The dispersion that takes place in the column is first considered and from that the limiting value of the extra column dispersion can be determined and an expression giving this limiting dispersion is included. Low dispersion tubing is discussed and the special case of low dispersion serpentine tubing considered in detail. The design of low dispersion gradient elution apparatus is also described and the special case of microbore columns examined.

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