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 a 0.32 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. Thickness and type of of the stationary phase
The thickness of the layer on the column infmuences the retention time: a thicker film prolongs the retention time. Some materials have special interaction with some compounds. Water solveable compounds, also called polar compounds interact strongly with a polar stationary phase, hence have a longer retention time than non-polar columns. Some column materials have very specific interactions, like aluminium oxide columns with unsatrurated hydrocarbons like alkenes. But in general the separation is by boiling point.
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).