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

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

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

Early Gas Chromatography Detectors The first GC detector was invented by James and Martin [4] in 1952, and used for the separation of some fatty acids. It consisted of a titration apparatus situated at the end of the column and the eluent gas was bubbled through a suitable aqueous liquid to absorb the solutes. The solution contained an indicator and, as each solute was eluted, the solution was manually titrated. The titration process was eventually automated and an integral chromatogram was obtained by plotting the volume of base solution added against time. The integram consisted of a series of steps, one for each solute. This rather primitive arrangement validated the gas chromatographic concept but also indicated that a detector with greater sensitivity and a more

(ca 100 nsec) so that all the ions in the source are accelerated virtually simultaneously. The ions then pass through the third electrode into the drift zone and are eventually collected by the sensor electrode. The time of flight mass spectrometer is not employed extensively in gas chromatography/mass spectroscopy combination systems as it is more commonly used to examine high molecular weight materials Many analysts that use GC/Mass Spectrometer combined systems are neither specialists in gas chromatography or mass spectrometry and may need the support of experienced gas chromatographers or mass spectroscopists for particularly challenging samples. For those who wish to study mass spectrometry further, an excellent discussion on general organic mass spectrometry is given in Practical Organic Mass Spectrometry edited by Chapman (9).   Gas Chromatography IR Spectroscopy (GC/IR) Systems   IR spectra were initially obtained off-line, by condensing the eluted solute in a

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