Chromatographic techniques:

This unit in brief gives a chronological development of the important chromatographic techniques. It highlights the fact that the advancements in chromatographic science are prompted to a large extent by the compelling needs of the scientists. Taking a clue from the definition of chromatography, the different ways to classify chromatographic methods are discussed. It emerges that no single criteria seems to be effective to classify all the different techniques logically. The difficulty arises mainly due to diversifications. A simpler approach seems to be by taking the nature of the mobile phase as the main criteria and propose a sub-classification based on the mechanisms responsible for separation. The underlined principles involved in different techniques are explained. A large number of variations are available in liquid chromatography primarily because of different operative mechanisms and availability of a variety of surfaces. Supercritical fluid chromatography combines the advantages of both liquid and gas chromatography but it has to still become popular. The migration of solutes in the column is viewed in terms of fraction of time they spend between the stationary and mobile phases. Depending on this they emerge out of the column at different intervals of time. The position of peak on time axis helps to identify the components and the areas under the peak provide the amount of the component. For separation, the relative migration rates are important and the rates depend upon the equilibrium constants of reaction by which the solutes distribute themselves between the two phases. The significance of retention and selectivity factors is explained. The shape of the peak is generally Gaussian. In order to assess the performance of a column, the concept of theoretical plates or height equivalent to theoretical plates is introduced. This is merely conceptual and there is no actual entity like this. However, it is able to successfully explain the Gaussian shape of the peaks but fails to explain the broadening of peak. A more realistic picture is presented in the form of rate theory. The rate theory, expressed in terms of van Deemter equation, takes into account the eddy diffusion, longitudinal diffusion and mass transfer between the phases. The effect of carrier gas velocity on the three terms is taken into consideration. Based on the conclusions, an optimum carrier gas velocity at which (HETP)min.  is obtained is visualized. Moreover, the conditions which can reduce the peak broadening are concluded. The concept of resolution and its significance in terms of separation/ selectivity factor is discussed.