Gas-liquid chromatography gc, glc, vpc

Gas-liquid chromatography is becoming increasingly popular for reaction monitoring and for analysis of reaction products. It can be used for the analysis of any compounds which are volatile below about 300 C and thermally stable. It is not the intention of this section to give a detailed description of gc instrumentation, but simply to outline some of the uses of the technique for reaction monitoring and related work. 

Gas chromatography is a very sensitive technique requiring only very small amounts of sample (lO g). A solution of about 1% is sufficient and a few microlitres of this is injected into a heated injector block. A stream of carrier gas, usually helium, passes through the injector and sweeps the vapours produced onto the column, which is contained in an oven. The temperature of the oven can be accurately controlled and can either be kept constant or increased at a specified rate. Separation of the components in gc is not based on the principle of adsorption, as it is in liquid chromatography, but on partition. A gc column is rather like an extremely effective distillation column with the relative volatility of the components being the main factor which determines how quickly they travel through the column. 

The development of modem capillary columns has led to improved resolution and has also simplified the process of running gcs considerably. The columns are normally made from fused silica capillary with an inside diameter of between 0.2 and 0.5mm, and are polymer coated. They have no packing, but instead the liquid stationary phase is bonded to the inside wall of the capillary, and this allows gas to flow very easily. Because of this the columns can be made much longer than packed columns (between 12 and 100m) and they are typically ten times as efficient. Capillary columns give extremely high sensitivity and only a very small quantity of material is required. For this reason the injector normally incorporates a splitter, so that only a small portion of the sample injected actually enters the column. 

The development of capillary columns has been largely responsible for the increased use of gc for monitoring organic reactions and for product analysis. The increased sensitivity of these columns is one important reason for this, but the fact that they are very simple to set up and operate is perhaps more significant. The type of stationary phase is not so critical as with packed columns and the gas flow rate is essentially determined by the column, so the instrument can be operated successfully with minimal prior expertise. For most purposes relating to preparative organic chemistry it is sufficient to rely on just two types of column, one non-polar (such as a BPl) and one polar column (such as a BP20). 

Uses ofgcfor reaction monitoring and product analysis Capillary gc instruments are so simple to use that, provided there is one close by, monitoring a reaction by gc is almost as quick as running a tic. It is common to turn to gc monitoring when tic does not provide resolution between starting material and product or between one product and another. Gc will usually separate components which co-run on tic. We also find that some compounds, such as amines, which do not run very well on tic, can be analyzed very easily by capillary gc. 

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