Thin Layer Chromatography Theory

The interactions between solute and the pha.ses are exactly the same as those present in LC separations, namely, dispersive, polar and ionic interactions. At one extreme, the plate coating might be silica gel, which would offer predominately polar and induced polar interactions with the solute and, con.sequently, the separation order would follow that of the solute polarity. To confine the polar selectivity to the stationai y phase, the mobile phase might be -hexane which would offer only dispersive interactions to the solute. The separation of aromatic hydrocarbons by induced polar selectivity could be achieved, for example, with such a system. 

On the other hand, if the stationary phase consists of bonded silica containing hydrocarbon chains, then the stationary phase interactions with the solute would be 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

TLC Plate Layer of Stationary Phase Layer of Mobile Phase 

Distance Determind by the Concntration of Component (A) Distance Determind by the Concntration of Component (B) Distance Determind by Migration of Solvent Front (A) 

---

Most of the work, however, tend to give the impression of being somewhat esoteric, and of uncertain value in the practice of thin layer chromatography. A more detailed discussion of TLC theory can be found elsewhere. (4)... 

Data from Kowalska, T., Kaczmarski, K., and Prus, W., Theory and mechanism of thin-layer chromatography, in Handbook of Thin-Layer Chromatography, 3rd ed., revised and expanded, Chromatographic Science Series, 89, Sherma, J. and Fried, B., Eds., Marcel Dekker, New York, 2003, chap. 2. 

The identification of separated componnds is primarily based on their mobility in a snitable solvent, which is described by the Rp valne of each compormd. Kowalska et al. have nicely discnssed in greater detail the theory of planar chromatography and separation efficiency parameters in Chapter 2 of the third edition of the Handbook of Thin-Layer Chromatography, published in 2003. 

Rowe, K., Bowlin, D., Zou, M., Davis, J.M. (1995). Application of 2-D statistical theory of overlap to three separation types 2-D thin-layer chromatography, 2-D gas chromatography, and liquid chromatography/capillary electrophoresis. Anal. Chem. 67, 2994. 

We have purposely narrowed the scope of all multidimensional chromatography to those techniques that incorporate separations in the liquid phase and to those in which the use of the comprehensive mode prevails but is not exclusive. This text neither incorporates elements of multidimensional thin-layer chromatography, multidimensional separations in gel media such as those commonly employed for the separation of complex mixtures of proteins, nor the techniques that utilize multidimensional gas chromatography. Some of the same principles apply, particularly in the theory section, but our emphasis is strictly on separations carried out in the liquid phase and by columns, rather than in the gas phase or in planar configurations. 

From this expression (Kubelka Munk function) it follows that, within the range of validity of the theory, q,/ depends only on the ratio of the absorption coefficient to the scattering coefficient, and not on their individual values. The equation has been most useful where reflectance measurements are used to obtain information about absorption and scattering (e.g., in textile dyeing, thin layer chromatography, and IR spectroscopy). 

Because of the similarities in the theory and practice of these two procedures, they will be considered together. Both are examples of partition chromatography. In paper chromatography, the cellulose support is extensively hydrated, so distribution of the solutes occurs between the immobilized water (stationary phase) and the mobile developing solvent. The initial stationary liquid phase in thin-layer chromatography (TLC) is the solvent used to prepare the thin layer of adsorbent. However, as developing solvent molecules move through the stationary phase, polar solvent molecules may bind to the immobilized support and become the stationary phase. 

DFT = density functional theory HMQC = heteronuclear multiple quantum coherence SOMO = singly occupied molecular orbital PES = photoelectron spectroscopy Cl = configuration interaction LUMO = lowest unoccupied molecular orbital HOMO = highest occupied molecular orbital TLC = thin layer chromatography. 

The history, theory, and applications of paper and thin-layer techniques are described in several books. " The first report on the subject of thin-layer chromatography was by Izmailov and Shraiber, who applied the technique to the separation of pharmaceutical tinctures. 

Belenky, B.G. Nesterov, V.V. Smirnov, M.M. Theory of thin-layer chromatography. I. Differential equation of thin-... 

Kowalska, T. Theory and Mechanism of Thin-Layer Chromatography. In Handbook of Thin Layer Chroma-... 

Gazes, J. Scott, R.P.W. Thin Layer Chromatography. In Chromatography Theory Marcel Dekker, Inc. New York, 2002 443-454. 

Initial training of analysts new to the QC laboratory typically involves reading the General Chapters for tests such as thin-layer chromatography, loss on drying, and water testing. These usually contain more detailed information on the theory associated with the test. 

C. Poole, The Essence of Chromatography, Elsevier, Amsterdam, 2003. (A 927-page book that provides a comprehensive survey of the current practice of chromatography. It includes an in-depth and well-referenced treatment of chromatographic theory and quantitative principles. It covers HPLC, GC, thin-layer chromatography, supercritical fluid chromatography, and capillary electrophoresis (CE).)... 

The theory of bed efficiency in thin-layer chromatography (TLC) can be developed from the same basic premises which apply to separations in columns. The dependence of bed efficiency on separation conditions might therefore be expected to be quite similar for separations on columns and "plates. Actually there are some important differences. In the following discussion we shall follow the treatment of Stewart (25,26). 

Equation (6.13) provides values that are about 25% smaller that those calculated by Eq. (6.12). Some typical results from theory or determined by experiment are summarized in Table 6.4. Results from theory are probably too high and represent an upper limit. Experiment indicates a zone capacity of about 12-14 for a single development with capillary flow. This rises to about 30 - 40 for forced flow. Automated multiple development with capillary flow provides a similar zone capacity to forced flow. Two-dimensional thin-layer chromatography employing different retention... 

---