Chromatography

Chromotography Chromotography is a technique of resolution or separation. Chromatography involves a sample being converted into a solution. The solvent is called mobile phase (which may be a gas, a liquid or a supercritical fluid). The mobile phase is then forced through a stationary phase. The phases are chosen such that components of the sample have differing solubility in each phase. A component which is quite soluble in the stationary phase will take longer to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase. Different types of chromatography are discussed in the following paragraphs. Solid to Liquid Column chromatography is a separation technique in which the stationary bed is within a tube. The particles of the solid stationary phase or the support coated with a liquid stationary phase may fill whole inside volume of the tube (packed column) or be concentrated on or along the inside tube wall leaving an open, unrestricted path for the mobile phase (open column). Differences in rates of movement through the medium are calculated to different retention times of the sample. Each component can be subsequently be collected as it elutes with solvent and drips down the coloumn. Paper Chromotography A small concentrated spot of solution preferably colored that contains the sample is applied to a strip of chromatography paper about 2 cm away from the base of the plate. This is absorbed

onto the paper and may form interactions with it. Any substance that reacts or bonds with the paper cannot be measured using this technique. The paper is then dipped in to a suitable solvent, such as ethanol or water, taking care that the spot is above the surface of the solvent, and placed in a sealed container. The solvent moves up the paper by capillary action, which occurs as a result of the attraction of the solvent molecules to the paper and to one another. As the solvent rises through the paper it meets and and dissolves the sample mixture, which will then travel up the paper with the solvent. Different compounds in the sample mixture travel at different rates due to differences in solubility in the solvent, and due to differences in their attraction to the fibers in the paper. Paper chromatography takes anywhere from several minutes to several hours. Result is is a series of colored dots representing different components of the sample which can be noticed on the paper. The distance between the different dots is indication of their property of solubility and affinity to paper in comparison to solvent. The thin layer chromotogrphy is similar to paper chromotography except that a coated glass is used and the results are analysed by iodine vapor chamber.

Gas to liquid. Gas chromatography - specifically gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous (mobile phase). The column itself contains a liquid (stationary phase) which is adsorbed onto the surface of an inert solid. The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The

choice of carrier gas is often dependent upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities. Compounds in the mixture interact with liquid at different rate and elute as individual component at the exit port. **********************************