Principles of Gas and Liquid Chromatography

A chromatographic column filled in three sections with ground sugar, chalk, and alumina. When a petroleum extract of spinach leaves is run onto the top of the column, ihe extract spreads down the column, but not uniformly bands of green chlorophylls stop near the top. yellow xanthophyll further down, and red carotene near the bottom. 

For chromatography to occur, a mobile phase and a stationary phase are needed. The mobile phase is a gas in gas chromatography and is a liquid in liquid chromatography. In GC, the stationary phase is almost always a column of liquid, gum, or elastomer but not a solid, while in LC the stationary phase is generally a column of a porous solid. Strictly, the names should be gas liquid chromatography (GLC) and liquid solid chromatography (LSC), as will be found in the early literature. The shortened names (GC, LC) came into use through widespread 

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All these types of solute-solvent associations are summed up in a rule of thumb learned by all chemists like dissolves like. The chemical processing industry depends on the ability to separate a useful chemical from a solvent by an extraction process. If a chemist wants to extract nonpolar chemicals, he or she would use a nonpolar sorption material. The opposite is equally true. In a mixture of polar and nonpolar chemicals, the two classes of compounds could be separated from each other. The analytical techniques of gas and liquid chromatography are based on this principle. In applying this principle to an enviromnental issue, however, the fact that the pollutants have a range of polarities makes the system problematic. Thus, carbon is the material of choice because its affinity is based on molecular size, not on polarity. 

Earlier methods used to determine mercury in biological tissue and fluids were mainly colorimetric, using dithizone as the com-plexing agent. However, during the past two to three decades, AAS methods - predominantly the cold vapor principle with atomic absorption or atomic fluorescence detection - have become widely used due to their simplicity, sensitivity, and relatively low price. Neutron activation analysis (NAA), either in the instrumental or radiochemical mode, is still frequently used where nuclear reactors are available. Inductively coupled plasma mass spectrometry (ICP-MS) has become a valuable tool in mercury speciation. Gas and liquid chromatography, coupled with various detectors have also gained much importance for separa-tion/detection of mercury compounds (Table 17.1). 

Chromatography is the most powerful tool in an analytical chemist s arsenal for separating and measuring components of a complex mixture. With mass spectromet-ric detection, we can identify the components as well. This chapter discusses principles of chromatography and mass spectrometry, and the following chapters take up gas and liquid chromatography. 

Chromatography is based on the same principle as adsorption. Chromatographic techniques can be used to measure a wide variety of thermodynamic, kinetic, and physico-chemical properties. Separation occurs because of differing velocities of travel of components in the mobile and stationary phases. Theoretical plate model can be applied to chromatography. Chromatographic methods can be classified as gas and liquid chromatography depending on the nature of the mobile phase. 

Qualitative and quantitative analysis of bioactive principles in Zingiberis rhizoma by means of high performance liquid chromatography and gas liquid chromatography on the evaluation of Zingiberis rhizoma and chemical change of constituents. Yakugaku Zasshi 1993 113(4) 307-315. 

Adsorption Chromatography. The principle of gas-solid or liquid-solid chromatography may be easily understood from equation 35. In a linear multicomponent system (several sorbates at low concentration in an inert carrier) the wave velocity for each component depends on its adsoiption equilibrium constant. Thus, if a pulse of the mixed sorbate is injected at the column inlet, the different species separate into bands wliich travel through the column at their characteristic velocities, and at the outlet of the column a sequence of peaks corresponding to the different species is detected. Measurement of the retention time (tl) under known flow conditions thus provides a simple means of determining the equilibrium constant (Henry constant) ... 

The present Chapter includes a description of the general methods and basic principles of gas-liquid chromatography. The development of the method as applied to various classes of carbohydrate derivatives is surveyed, and this treatment is followed by a discussion of mobilities of compounds in relation to their structure. Applications of the method to specific problems are described, and the liquid phases used in gas-liquid chromatography of carbohydrate derivatives are summarized. Finally, an attempt is made to assess the relative merits of gas-liquid chromatography and other chromatographic methods as applied to carbohydrate derivatives. 

The preceding discussion was provided to give a basic understanding of the principles of gas-liquid chromatography and of the various factors that contribute to the separations achieved on packed columns. Much of the discussion was drawn from a number of textbooks or monographs, to... 

The headspace technique, a static gas extraction method, is particularly suitable for the enrichment of volatile compounds. It enables the analysis of solid and liquid samples by direct sampling from the gas phase, and can be directly combined with gas chromatography. This principle is based on the distribution of analyte between the matrix and gas phase. It has been used successfully to determine volatile sulfur compounds from various matrices, such as wastewater, body fluids, plants, and animal fatty tissue. 

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