lec 9 Dr.thukaa.pdf

Gas – liquid chromatography (GLC) Dr. Thukaa Z. Abdul-Jalil Lec 9 Gas-liquid chromatography is a partition type of chromatography where by the solute is partitioned between two phases a liquid stationary phase & a gas mobile phase. The inert carrier gas passes through a pressure regulator into a sam...

Gas – liquid chromatography (GLC) Dr. Thukaa Z. Abdul-Jalil Lec 9 Gas-liquid chromatography is a partition type of chromatography where by the solute is partitioned between two phases a liquid stationary phase & a gas mobile phase. The inert carrier gas passes through a pressure regulator into a sample injector & mixing chamber. From here the carrier gas (mobile phase) carries the sample into the column. The column may be packed with a porous solid coated with a thin film of a non-volatile liquid (stationary phase) , or it may be a long capillary tube with a thin coating of liquid on its wall. The sample components are separated as they pass through the column & one after the other pass through a detector which sends a signal to a recorder. Finally , the gas passes through a flow meter & is exhausted to the atmosphere. A thermo stated oven is provided for the column , injector , & detector , although the last two may be heated separately. The instrument of GLC is composed of the following parts : Carrier gas supply : In GLC the carrier gas (which is supplied by a cylinder) provides transportation for the sample components through the apparatus. It must be chemically inert & available in pure form & reasonable cost.The most commonly used gases are helium , nitrogen, argon, hydrogen & carbon dioxide. A high density is preferred for best efficiency & a low density for maximum speed. The flow control is obtained from a standard reducing valve & a soap film flow-meter is used almost exclusively to measure the flow rate. The sample injection point in the column is important part & usually there is a rubber stopper at the top of the column through which the syringe needle containing the sample passes to introduce the sample at the top of the column. The whole of the injection point unit should be at a higher degree than the rest of the column & it is usually kept 10 C above the temperature needed for the rest of the column , this is important to ensure the immediate evaporation of the liquid sample once it enters the column. The column oven may be set at any temperature required between 0-400 C. The injection unit has a separated heating control than the rest of the column. Columns : There are two distinct types of columns in common use i.e. packed & open tubular (capillary). They are either stainless steal or glass column. The column is packed with the inert support which has been already coated with the liquid phase. One end of the column is connected to the sample injector that means the place through which the sample or solute is introduced , the other end of the column is connected to the detector which will analyze the signal received. Solid support : The ideal solid support should have a high specific surface (1m2/gm) , & the surface must be chemically inert also wet able by the liquid phase so that it will spread in a thin layer of uniform thickness. In addition it must have thermal stability , mechanical strength & available in a uniform size, nearly spherical shaped particles