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. 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.
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).
High temperatures and high flow rates decrease the retention time, but also deteriorate the quality of the separation.
Which detectors are commonly used?
1. 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.
2. Flame Ionization Detector (FID)
The detector is very sensitive towards organic molecules (10-12 g/s, linear range: 106 107), but relative insensitive to a few small molecules e.g. N2, NOx, H2S, CO, CO2, H2O. If proper amounts of hydrogen/air are mixed, the combustion does not afford any ions. If other components are introduced that contain carbon atoms cations are produced in the effluent stream. The more carbon atoms are in the molecule, the more fragments are formed and the more sensitive the detector is for this compound (-- > response factor). However, due to the fact that the sample is burnt (pyrolysis), this technique is not suitable for preparative GC. In addition, several gases are usually required to operate a FID: hydrogen, oxygen (compressed air), and carrier gas.
3. Thermal Conductivity Detector (TCD)
This detector is less sensitive than the FID (10-5-10-6g/s, linear range: 103-104), but is well suited for preparative applications, because the sample is not destroyed. It is based on the comparison of two gas streams, one containing only the carrier gas, the other one the carrier gas and the compound. Naturally, a carrier gas with a high thermal conductivity e.g. helium or hydrogen is used in order to maximize the temperature difference (and therefore the difference in resistance) between two thin tungsten wires. The large surface-to-mass ratio permits a fast equilibration to a steady state. The temperature difference between the reference cell and the sample cell filaments is monitored by a Wheatstone bridge circuit.
4. Electron Capture Detector (ECD)
The detector consists of a cavity that contains two electrodes and a radiation source that emits b-radiation (e.g. 63Ni, 3H). The collision between electrons and the carrier gas (methane plus an inert gas) produces a plasma containing electrons and positive ions. If a compound is present that contains electronegative atoms, those electrons are captured and negative ions are formed, and the rate of electron collection decreases. The detector is extremely selective for compounds with atoms of high electron affinity (10-14 g/s), but has a relatively small linear range (~102-103). This detector is frequently used in the analysis of chlorinated compounds e.g. pesticides, polychlorinated biphenyls, which show are very high sensitivity.