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the gas chromatography is a method that seperates the different molecule types according to their weight, therefore size, and their affinity to the column used, since the gas used is inert. the problem with GC separation is that if the molecules are close is weight, so no significant difference in the speed of their trip through the colomn, and if their affinities are close, so the molecules aren't well separated as well, there is no way to identify them according to the retention time, nor to achieve a good separation. the addition of the MS detector makes the identification process easier, as the detector decomposes the molecule into ions identified according to their mass, and using the retention times, since we would have a certain idea about the probable molecules we have, we can know which molecules exist in the solution we analyzed or in the gas we analyzed.
Gas Chromatography (GC) is a commonly used analytic technique in many research and industrial laboratories. A broad variety of samples can be analyzed as long as the compounds are sufficiently thermal stable and volatile enough.How does gas chromatography work? Like for all other chromatographic techniques, a mobile and a stationary phase are required. The mobile phase (=carrier gas) is comprised of an inert gas e.g. helium, argon, nitrogen, etc. The stationary phase consists of a packed column where the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (=high boiling polymer). More commonly used in many instruments are capillary columns, where the stationary phase coats the walls of a small-diameter tube directly (e.g.0.25 mm film in a0.32 mm tube). The main reason why different compounds can be separated this way is the interaction of the compound with the stationary phase“(like-dissolves-like”-rule). The stronger the interaction is the longer the compound remains attached to the stationary phase, and the more time it takes to go through the column (=longer retention time).
What influences the separation?1. Polarity of the stationary phase Polar compounds interact strongly with a polar stationary phase, hence have a longer retention time than non-polar columns. Chiral stationary phases based on amino acid derivatives, cyclodextrins, chiral silanes, etc are capable to separate enantiomers, because one form is slightly stronger bonded than the other one, often due to steric effects.2. Temperature The higher the temperature, the more of the compound is in the gas phase. It does interact less with the stationary phase, hence the retention time is shorter, but the quality of separation deteriorates.3. Carrier gas flow If the carrier gas flow is high, the molecules do not have a chance to interact with the stationary phase. The result is the same as above.4. Column length The longer the column is the better the separation usually is. The trade-off is that the retention time increases proportionally to the column length. There is also a significant broadening of peaks observed, because of increased back diffusion inside the column.5. Amount of material injected If too much of the sample is injected, the peaks show a significant tailing, which causes a poorer separation. Most detectors are relatively sensitive and do not need a lot of material (see below).6. Conclusion High temperatures and high flow rates decrease the retention time, but also deteriorate the quality of the separation.The most common detector used:Mass Spectrometer (GC/MS) Many GC instruments are coupled with a mass spectrometer, which is a very good combination. The GC separates the compounds from each other, while the mass spectrometer helps to identify them based on their fragmentation pattern.
