Essential Oils Without the use of gas chromatography the analysis of essential oils would be extremely difficult. Prior to the technique being developed, only the major components of the oils could be separated, achieved by distillation with high efficiency columns. Even so, such columns rarely contained more than 100 theoretical plates (distillation plates), were very slow in operation, and took many days to complete an analysis. Due to the limited separation efficiency of the distillation column, even the major components were contaminated with traces of materials, many of which had strong olfactory intensity and thus confused the olfactory character of the major component. The gas

for medicinal purposes. Later, with the advent of the industrial revolution, petroleum fractions, solvents and coal products, such as coal tar and benzole mixtures were added to the list. Prior to the introduction of gas chromatography (GC) by James and Martin (1) in the early 1950s, the analysis of complex mixtures of volatile substances was extremely difficult and time consuming to carry out.   At that time, the only effective procedure for separating and analyzing such materials was by distillation using (what was then) high efficiency fractionating columns which, due to their very long equilibrium times, many days (sometimes weeks) were necessary to complete a separation. In addition, the distillation process was only really effective for isolating the major component of the mixture in pure form, the fractions containing the minor components still consisted of complex mixtures. As a consequence, this type of separation was virtually useless as it was often the minor component

nbsp;   It is seen that there are a number of low-level contaminants in the crude material, and by the use of single ion monitoring, very small traces of contaminants can easily be identified. The chromatogram obtained by single ion monitoring at a m/z value of 76 discloses the presence of arsine at a level of 4 ppb in the original sample.   GC/MS has also been extensively used in environmental analysis. Hiatt et al. (24) developed a vacuum distillation procedure for the isolation of volatile materials from natural sources, such as water, soil, oil, fish samples etc. The layout of their sample collection apparatus is shown in figure 63. The samples were placed in a flask which was connected in sequence to two valves, the first valve connected a pump to the sample vessel either directly, or by a second valve and through a cryo-trap. The sample chamber valve was closed and the condenser, cryo-trap, and gas lines were evacuated. The

Mechanism of Chromatographic Retention and in this book the processes of peak dispersion will be considered together with the means by which peak dispersion can be minimized. Solute equilibrium between the mobile and stationary phases is never achieved in the chromatographic column except possibly at the maximum of a peak. To circumvent this non equilibrium condition and allow a simple mathematical treatment of the chromatographic process, Martin and Synge (1) borrowed the plate concept fromdistillation theory and considered the column consisted of a series of theoretical plates in which equilibrium could be assumed to occur. In fact each plate represented a 'dwell time' for the solute to achieve equilibrium at that point in the column and the process of distribution could be considered as incremental. This approach has been discussed in Plate Theory and Extensions . Employing this concept an equation for the elution curve can be easily obtained and, from that basic equation

and inversely proportional to the surface area of the original silica gel.   The method of synthesis is very similar for the alkoxysilane reagents. The same solvents can be used but, as no hydrochloric acid is generated, there is no need to have pyridine present as a scavenger. The most reactive alkoxy reagents are the methoxy and ethoxysilanes and their reaction with a hydroxyl group is accompanied by the release of methanol or ethanol. The reaction is best carried out in a distillation flask and the methanol removed as it is formed, the heating rate is adjusted to ensure that the aromatic solvent is not removed as well. The reaction is allowed to proceed for about 5 hours and the product then filtered through a sintered glass filter and washed with the same sequence of solvents as those used in the chlorosilane synthesis. The product is then refluxed with the THF/water mixture, filtered and again washed with the appropriate solvents and dried. The final capping

column outlet is passed to a selection valve that diverts the eluent to its appropriate collecting vessel. The collecting vessel may be cooled in ice, solid carbon dioxide or if necessary liquid nitrogen (liquid nitrogen can only be used if a low boiling gas such as helium is employed as the carrier gas). In some cases the solutes contained in the eluent can be extracted into an appropriate liquid or onto the surface of a suitable adsorbent. the desired fractions are then recovered by distillation or desorption