Gas chromatography (GC), like any other type of chromatography, is a technique that makes it possible to separate different types of molecules present in a complex mixture. GC applies essentially to gaseous compounds or substances that can be vaporized without being decomposed.
A sample of the mixture is vaporized at the entry point of a column packed with a solid or liquid substance called the stationary phase. The sample is then transported through the stationary phase thanks to a carrier gas (also called vector gas). The different molecules contained in the mixture are separated and leave the column at different times, as a function of their affinity with the stationary phase.
Gas chromatographs are usually composed of the following parts:
- A rotating heat oven with temperature programming ranging from 20°C (-100°C in some systems) to 450°C and which includes a rapid cooling system.
- An injection system which makes it possible to introduce and vaporize the sample of interest. Injection may be carried out manually or with the help of a sampler.
- A capillary or packed column capable of separating the different molecules in the injected sample as a function of their affinity with the stationary phase.
- A detection system capable of measuring and identifying the signal produced by the different molecules. The signal may be conveniently recorded using a computer and appropriate software (instead of the older analogical paper chart recorders).
- A gas-pressure regulator for the gases used in the system (essentially helium). Modern GC systems contain electronic gas pressure regulators and filtering cartridges that purify the circulating gases.
Principle of action
The sample (a volatile liquid) is first introduced into the column head with a microsyringe through a small rubber disk called a septum. Upon introduction, the sample finds itself in the injector, a small chamber upstream of the column and which is swept by the carrier gas . The temperature of the injector corresponds to the sample’s volatility. Injected sample volumes may range from 0.2 to 5.0 µl.
Once the different components of the mixture have been rendered volatile, they are transported by the carrier gas (or vector gas) through the column and get separated from each other thanks to differing affinities with the stationary phase. This phenomenon, called chromatographic retention, affects different compounds (solutes) differently. The higher the affinity with the stationary phase, the longer the time needed to traverse the column. The raw experimental measurement of this time is called retention time. It corresponds to the time lapse between the moment a sample is injected and the peak of detection of a particular solute. In order to optimize the transport of all of the solutes through the column (elution), the temperature of the oven must be adjusted, As a rule, this temperature must be higher than the boiling temperatures of the compounds under study. It is possible to run gas chromatography isothermically (at a single temperature throughout the analysis) or at varying temperatures.
The carrier gas (or vector gas) is the dynamic, mobile phase in gas chromatography. It carries in its flow the injected mixture through the column all the way to the detector. In most cases, the carrier gas must be inert to avoid any interaction with the solutes or the stationary phase. Four different gases can be used: helium, hydrogen, nitrogen and argon.
In order to avoid interactions with the solutes or the stationary phase, carrier gases must be pure and free of water, oxygen and light hydrocarbons.
It is also essential that carrier gases must be insoluble in liquids to avoid generating any carrier-induced electrical signal in the chromatogram.
After traversing the column, the solutes reach the detector, an essential component of the gas chromatograph. The detector’s role is to measure the quantity of each solute as it travels within the carrier gas, based on the varying physical properties of the gas mixture. The detector generates an electronic signal which is processed by software that integrates the curves of all the peaks as a function of their intensity. The result, a set of Gaussian curves, is called a chromatogram. In the past, the signals sent by the recorder were interpreted and printed by paper chart recorders. These devices are nowadays replaced by computers and specific software.
Many different types of detectors can be coupled to a chromatograph. The most common detectors are:
- NPD-FID : nitrogen phosphorus flame ionization detector. An electrical tension of approximately 100 volts is maintained between the flame nozzle and a surrounding electrode. When the molecules enter the flame, they are ionized, resulting in an electrical current between the electrodes which is then amplified.
- MS or mass spectrometer: A mass spectrometer relies essentially on either electron impact ionization or chemical ionization as a means of detection.
Others information and illustrations are available in the presentation page.
GC oven and capillary column