Mobile phase Chromatography Theory

When the paper is impregnated with an organic liquid, the drying period is shortened to remove only the volatile mobile phase. The theory of unidimensional multiple development has been described [48], and the method has been used to analyze steroids [49] and amino acids [50] among other classes of compounds. Fractional chromatography is a special variation of multiple development in which a solvent of increasing polarity is employed for each subsequent development [51]. 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

The function (vm + Kvs) is termed the plate volume and so the flow through the column will be measured in plate volumes instead of milliliters. The plate volume is defined as that volume of mobile phase that can contain all the solute in the plate at the equilibrium concentration of the solute in the mobile phase. The meaning of plate volume must be understood, as it is an important concept and is extensively used in different aspects of chromatography theory. 

D. E. Martire, Unified Approach to the Theory of Chromatography Incompressible Binary Mobile Phase (Liquid Chromatography) in Theoretical Advancement in Chromatography and Related Separation Techniques (Ed. F. Dondi, G. Guiochon, IGuwer, Academic Publishers, Dordrecht, The Netherlands,(l993)261. 

Dispersion in Columns and Mobile Phase Conduits, the Dynamics of Chromatography, the Rate Theory and Experimental Support of the... 

The profile of the concentration of a solute in both the mobile and stationary phases is Gaussian in form and this will be shown to be true when dealing later with basic chromatography column theory. Thus, the flow of mobile phase will slightly displace the concentration profile of the solute in the mobile phase relative to that in the stationary phase the displacement depicted in figure 1 is grossly exaggerated to demonstrate this effect. It is seen that, as a result of this displacement, the concentration of solute in the mobile phase at the front of the peak exceeds the equilibrium concentration with respect to that in the stationary phase. It follows that there is a net transfer of solute from the mobile phase in the front part of the peak to the... 

The capacity ratio of a solute (k ) was introduced early in the development of chromatography theory and was defined as the ratio of the distribution coefficient of the solute to the phase ratio (a) of the column. In turn the phase ratio of the column was defined as the ratio of the volume of mobile phase in the column to the volume of stationary phase in the column. 

Liquid-liquid chromatography in its simplest form involves two solvents that are immiscible. However, many recently developed media consist of a liquid (the stationary phase) that is firmly bound to a solid supporting medium. As a result, it is possible to use a second solvent (the mobile phase) which under normal conditions would be miscible with the first solvent. The second solvent is permitted to move in one direction across the stationary phase to facilitate the separation process. The presence of a supporting medium introduces some problems in the system and, in theory, it should be completely inert and stable, showing no interaction with the solutes in the sample. However, this is not always the case and sometimes it affects the partitioning process, resulting in impaired separation. 

From both theory and experimental evidence, raising the temperature by 10°C decreases the retention time by about 20% in isocratic chromatography and decreases the backpressure by 10% to 20% because of a reduction in the viscosity of the mobile phase. This can help to overcome the instrument limitations associated with running shorter columns packed with smaller particles, i.e., the pressure limitations of current HPLC systems. However, since the majority of reversed-phase columns available are silica-based, operating at temperatures above... 

CEC is a miniaturized separation technique that combines capabilities of both interactive chromatography and CE. In Chapter 17, the theory of CEC and the factors affecting separation, such as the stationary phase and mobile phase, are discussed. The chapter focuses on the preparation of various types of columns used in CEC and describes the progress made in the development of open-tubular, particle-packed, and monolithic columns. The detection techniques in CEC, such as traditional UV detection and improvements made by coupling with more sensitive detectors like mass spectrometry (MS), are also described. Furthermore, some of the applications of CEC in the analysis of pharmaceuticals and biotechnology products are provided. 

Eluents used in reversed-phase chromatography with bonded nonpolar stationary phases are genei ly polar solvents or mixtures) of polar solvents, such as acetonitrile, with water. The properties of numerous neat solvents of interest, their sources, and their virtues in teversed-phase chromatography have been reviewed (128). Properties of pure solvents which may be of value as eluents are summiuized in Table. VII. The most significant properties are surface tension, dielectric constant, viscosity, and eluotropic value. Horvath e/ al. 107) adapted a theory of solvent effects to consider the role of the mobile phase in determinmg the absolute retention and the selectivity found in reversed-phase chromatography. 

In chromatography there are at least three equilibria analyte/mobile phase, analyte/stationary phase and mobile phase/stationary phase. The origin of the term theoretical plate in chromatography comes from the adaptation of an older plate theory for distillation described by Martin and Synge (Nobel Prize for Chemistry, 1952). This term which is universally used for historical reasons, has no physical significance. It may have been preferable to call it a Tswett ... 

A stochastic theory provides a simple model for chromatography.11 The term stochastic implies the presence of a random variable. The model supposes that, as a molecule travels through a column, it spends an average time Tm in the mobile phase between adsorption events. The time between desorption and the next adsorption is random, but the average time is Tm. The average time spent adsorbed to the stationary phase between one adsorption and one desorption is rs. While the molecule is adsorbed on the stationary phase, it does not move. When the molecule is in the mobile phase, it moves with the speed ux of the mobile phase. The probability that an adsorption or desorption occurs in a given time follows the Poisson distribution, which was described briefly in Problem 19-21. 

---