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phase, the primary classification of chromatography is based on the physical nature of the mobile phase. The mobile phase can be a gas or a liquid which gives rise to the two basic forms of chromatography, namely, gas chromatography (GC) and liquid chromatography (LC). The stationary phase can also take two forms, solid and liquid, which provides two subgroups of GC and LC, namely; gas–solid chromatography (GSC) and gas–liquid chromatography (GLC), together with liquid solid chromatography (LSC) and liquid chromatography (LLC). The different forms of chromatography are summarized in Table 1. Most thin layer chromatography techniques are considered liquid-solid systems although the solute normally interacts with a liquid-like surface coating on the adsorbent or support or, in some cases an actual liquid coating. Table 1 The Classification of Chromatography chromatography systems

Preparative Gas Chromatography Gas chromatography has not been used extensively for preparative work although its counterpart, liquid chromatography, has been broadly used in the pharmaceutical industry for the isolation and purification of physiologically active substances. There are a number of unique problems associated with preparative gas chromatography. Firstly, it is difficult to recycle the mobile phase and thus large volume of gas are necessary. Secondly, the sample must be fully vaporized onto the column to ensure radial distribution of the sample across the column. Thirdly, the materials of interest are eluted largely in a very dilute form from the column and therefore must be extracted or condensed from the gas stream which is also difficult to achieve efficiently. Finally, the efficient packing of large GC columns

Introduction Chromatography, in one of its several forms, is the most commonly used procedure in contemporary chemical analysis and the first configuration of chromatography equipment to be produced in a single composite unit and made commercially available was the gas chromatograph. Gas chromatography was invented by A. J. P. Martin who, with R. L. M. Synge, suggested its possibility in a paper on liquid chromatography published in 1941 (1). Martin and Synge recommended that the liquid mobile phase used in liquid chromatography could be replaced by a suitable gas. The basis for this recommendation was that, due to much higher diffusivities of solutes in gases compared with liquids, the equilibrium processes involved in a chromatographic process (see Principles and Practice of Chromatography) would be much faster and thus, the columns much more efficient and separation times much shorter. So the concept of gas chromatography was envisioned more than fifty years

the work of Tswett, impeded the recognition of chromatography as a useful separation technique for nearly 20 years. In the late 1930s and early 1940s Martin and Synge introduced a form of liquid-liquid chromatography by supporting the stationary phase, in this case water, on silica gel in the form of a packed bed and used it to separate some acetyl amino acids. They published their work in 1941 (3) and in their paper recommended the replacement of the liquid mobile phase with a suitable gas which would accelerate the transfer between the two phases and provide more efficient separations. Thus, the concept of gas chromatography was born. In the same paper in 1941, Martin and Synge suggested the use of small particles and high pressures in LC to improve the separation which proved to the critical factors that initiated the development of high performance liquid chromatography(HPLC). "Thus, the smallest H.E.T.P. (the highest efficiency) should be obtainable by using

essential requisites for a chromatographic separation,viz. a mobile phase and a stationary phase. The technique, as described by Tswett was largely ignored for a along time and it was not until the late 1930s and early 1940s that Martin and Synge(2) introduced liquid-liquid chromatography by supporting the stationary phase, in this case water, on silica in a packed bed and used it to separate some acetyl amino acids. In their paper, they recommended replacing the liquid mobile phase by a suitable gas, as the transfer of sample between the two phases would be faster, and thus provide more efficient separations. In this manner, the concept of gas chromatography was created but again, little notice was taken of the suggestion and it was left to Martin himself and A. T. James to bring the concept to practical reality nearly a decade later. In the same publication in 1941, the essential requirements for HPLC (High Performance Liquid Chromatography

indeed improve the performance of the TLC analysis but are also expensive and in many cases tend to make the TLC system more like a liquid chromatograph. The great advantage of TLC is its low cost and its relatively high separating capability. If the required performance required is at the limit or beyond the capability of the technique, there is no point in trying to stretch it. The rational solution for the chemist or analyst would be to change to an alternative procedure such as liquid chromatography or to some other technique if more appropriate. Chromatography Applications Gas chromatography has an entirely different field of applications to that of liquid chromatography. In general, gas chromatography is used for the separation of volatile materials and liquid chromatography for the separation of involatile liquids and solids. There are certain compounds, however, that can be separated with either techniques, and more importantly, many involatile