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Gas chromatography (GC) is an older analytical technique that is still widely used today. It is a technique that can be used with both inorganic and organic analytes, but the one requirement is that the sample must be volatile, otherwise the analysis won’t work. The technique has seen a lot of use across many industries and it is often coupled with other techniques, most commonly mass spectrometry. Here, we’re going to look at how gas chromatography works.
Gas chromatography is often used to determine the purity of an unknown substance or to separate out the components of an unknown mixture so that each of the different parts can be analyzed. It is a technique that can be used to detect samples in very small quantities. Overall, these instruments have an inlet area, where the carrier gas and sample are injected into the instrument, a chromatographic column which is heated to ensure that all the components are gases, and a detector area which can vary depending on the needs of the analysis.
Like any chromatography instrument, the internal workings consist of a mobile phase and a stationary phase. It does differ from many other chromatography techniques—such as liquid chromatography (LC), high-performance liquid chromatography (HPLC) and thin layer chromatography (TLC)—which have liquid mobile phases, as the mobile phase in gas chromatography is an inert gas. The carrier gases used are generally helium, argon or nitrogen.
The stationary phase can be made up of several materials, but it is commonly a polymer or another inert material—i.e. one that is not going react with any of the gaseous molecules being analyzed, because any reactions cause the stationary phase to degrade. While the stationary phase is a highly packed solid, it is often coated with a liquid stationary phase that has a high boiling point.
The Working Principles of Gas Chromatography
The general working principles are as follows. The sample is injected into the instrument where it is vaporized and mixes with the carrier gas to become a part of the mobile phase. This mobile phase is then carried through the chromatographic column where it interacts with the stationary phase of the column. The interactions between the stationary phase and the analyte determine the elution time of the different molecules in the sample, because if the sample interacts with the stationary phase more, then it will take longer to reach the detector.
Moreover, the stationary phase has molecular pores which different size molecules can get through; so, if the molecule is smaller, then it will have a quicker elution time than the larger molecules. Therefore, the elution time of the molecules in the sample, coupled with the analysis from specific detectors can be used to determine the molecules in a sample as well as their ratios. The detectors which are commonly attached to a gas chromatograph include a mass spectrometer (GC-MS), a flame ionization detector (FID), a thermal conductivity detector (TCD), and an electron capture detector (ECD).
Parameters that Affect the Instrument
There are a number of factors and changeable parameters that can affect how the instrument will perform. These can be split into the properties of the sample and the changeable parameters of the instrument itself.
One factor that can influence the results of the analysis is the vapor pressure of the sample, which in turn is related to the polarity of the molecules in the sample. The polarity of a molecule affects the boiling point of the molecule, which in turn changes the vapor pressure of the molecule. In general, if a molecule has a lower boiling point, it will have a higher vapor pressure and this means that it will elute quicker because it will spend more time in the gas phase than interacting with the stationary phase.
Aside from changing the boiling point, the polarity of the molecules can affect how much it directly interacts with the stationary phase of the column. Therefore, if the stationary phase is polar in nature, polar molecules will take much longer to elute compared to when they’re in a non-polar column.
Other Systematic Parameters
There are also a number of systematic parameters that can affect the results of a gas chromatograph analysis. These include changes to column temperature, carrier gas flow rate, column length, and the amount of material that is injected for the analysis. The different parameters can be tweaked to make the analysis faster (for all the molecules being analyzed), but this can sometimes come at the cost of a lower quality separation—i.e. the peaks are not as distinguishable on the spectra.
In terms of the column temperature, the molecules will elute much quicker, but there will be hardly any separation between the molecules as most of them will stay in the gas phase rather than interacting with the stationary phase. The same scenario is also the case for when the carrier gas flow rate is increased. A longer column, on the other hand, will improve the separation as there are more areas to interact with along the column. While a longer column will increase the elution time proportionally, if a column length with too long a length is used, it will cause the elution peaks on the spectra to broaden due to an increased longitudinal diffusion.
As for the concentration of the sample material used, it is important to not use too much. Gas chromatograph spectra peaks are generally symmetrical and the introduction of excessive amounts of analyte can cause these peaks to tail. A lot of analyte is not needed as it is a highly sensitive technique.
Overall, the consensus is that a high flow rate and temperature can decrease the elution time of all the molecules in the sample, leading to a faster analysis, but it comes at the cost of a quality separation, so compromises need to be made to balance both aspects.