![]() ![]() ![]() If too much of the sample is injected, the peaks show a significant tailing, which causes a poorer separation. Ideally, the peaks in the chromatogram display a symmetric shape (Gaussian curve). Broadening is also observed because of the finite rate of mass transfer between the phases and because the molecules are taking different paths through the column. This broadening is inversely proportional to the flow rate. One has to keep in mind that the gas molecules are not only traveling in one direction but also sideways and backwards. The trade-off is that the retention time increases proportionally to the column length and a significant peak broadening will be observed as well because of increased longitudinal diffusion inside the column. Like above, the components have very little time to interact with the stationary phase and are just being pushed through the column.Ī longer column generally improves the separation. Temperature gradients, because the differences in polarity and inĪ high flow rate reduces retention times, but a poor separation would be observed as well. The best separations are usually observed for Separation deteriorates, because the differences in retention times are If the compound does not interact with the stationary phase, the Occur the components need to be able to interact with the stationary Retention time but also in a very poor separation because all components ![]() Modified -cyclodextrin column is used in the determination of the enantiomeric excess in the chiral epoxidation experiment (Chem 30CL).Ī excessively high column temperature results in very short Chiral stationary phases that are based on amino acid derivatives, cyclodextrins and chiral silanes are capable of separating enantiomers because one enantiomer interacts slightly stronger than the other one with the stationary phase, often due to steric effects or other very specific interactions. As a result, polar compounds have long retention times on polar stationary phases and shorter retention times on non-polar columns using the same temperature. If the polarity of the stationary phase and compound are similar, the retention time increases because the compound interacts stronger with the stationary phase. The polarity of components versus the polarity of stationary phase on column However, their vapor pressures are low compared to liquids (i.e., water (24 mmHg/25 oC), ethyl acetate (95 mmHg/25 oC), diethyl ether (520 mmHg/25 oC)).Ģ. That is the reason why we can smell compounds like camphor (0.065 mmHg/25 oC), isoborneol (0.0035 mmHg/25 oC), naphthalene (0.084 mmHg/25 oC), etc. The temperature of the column does not have to be above the boiling point because every compound has a non-zero vapor pressure at any given temperature, even solids. That is one of the main reasons why low boiling solvents (i.e., diethyl ether, dichloromethane) are used as solvents to dissolve the sample. The higher the vapor pressure of the compound and the shorter retention time usually is because the compound will spent more time in the gas phase. The boiling point of a compound is often related to its polarity (see also polarity chapter). Which factors influence the separation of the components? As a result, compound O has a much shorter retention time than compound X. In the example above, compound X interacts stronger with the stationary phase, and therefore lacks behind compound O in its movement through the column. The stronger the interaction is, the longer the compound interacts with the stationary phase, and the more time it takes to migrate through the column (=longer retention time). The separation of compounds is based on the different strengths of interaction of the compounds with the stationary phase (“like-dissolves-like”-rule). Most analytical gas chromatographs use capillary columns, where the stationary phase coats the walls of a small-diameter tube directly (i.e., 0.25 μm film in a 0.32 mm tube). In which the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (=high boiling polymer). The stationary phase consists of a packed column The mobile phase (=carrier gas) is comprised of an inert gas i.e., helium, argon, or nitrogen. Like for all other chromatographic techniques, a mobile and a stationary phase are required for this technique. A broad variety of samples can be analyzed as long as the compounds are sufficiently thermally stable and GC is also a frequently used technique in many environmental and forensic laboratories because it allows for the detection of very small quantities. Gas Chromatography (GC or GLC) is a commonly used analytic technique in many research and industrial laboratories for quality control as well as identification and quantitation of compounds in a mixture. ![]()
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