Gas Chromatography, usually abbreviated to GC, is used in analytical chemistry to separate and analyse compounds that can be vaporised without decomposition.
Within industrial gas production, this applies specifically to the process of separating a gas mixture to determine the presence and relative concentration of gases and impurities in a sample.
When applied correctly, GC is exceptionally accurate and can measure down to part-per-billion concentrations in gaseous samples, making it particularly suitable for use in high-purity processes. It is also atypical as a gas analysis technique in that measurement is made following the individual separation of the gas sample constituents rather than making an analysis within the sample as a mixture.
In GC, the components of the mixture are separated by circulating a gas sample using an inert carrier gas, known as the mobile phase, into a flow-through circular tube known as a column. The different constituents of the gas mixture are separated due to their interaction with the column material, known as the stationary phase, which due to chemical bonding, absorption or solubility causes the different molecules in the sample to elute at different times.
These specific retention times are detected by an electronic sensor at the column exit as the individual molecular properties of each gas cause it to travel through and exit at a different time.
The comparison of retention times allows the user to qualitatively identify gas types by the order in which they elute from the column – most simply, this is enabled by plotting readings on a graph, where the Y-axis represents detector signal and the X-axis represents time. In addition, the composition of each gas sample can also be measured by the detector as each gas elutes from the column.
If conditions are constant, a particular gas will elute with the same retention time, allowing specific gas concentrations to be deduced from the area of the peak.
The conditions by which GC process operates for a given application are invariably different and require individual optimisation. Setting these conditions includes a range of individual considerations including setting column and detector temperatures and specifying carrier gas flow rates, sample loop size and the column’s diameter, type and length.
In addition, some analysers offer the ability to change the route of sample and carrier flow, facilitated by valves that are operated by user-variable timings.
The majority of process GC analysers are therefore preset at the factory with application-specific valve timings, flow, temperature settings and peak detection parameters.
Furthermore, a range of different sensors types are used in GC analysis. These sensors are chosen on the basis of gas type and application, of which the most commonly used are the flame ionisation detector (FID) and thermal conductivity detector (TCD).
While both are sensitive to a wide range of gases over a wide range of concentrations, TCD detectors can be used to detect any component other than the carrier gas, while FID detectors are sensitive to hydrocarbons. Many GC analysers will offer a specific sensor, while others such as Servomex’s SERVOPRO Chroma can be factory-fitted with a combination of sensor technologies for increased flexibility of measurement.
The choice of carrier gas is important, although this is usually determined by which detector method is being used. The carrier is frequently chosen based on the sample itself: for instance, when analysing a mixture in argon, an argon carrier is preferred as the argon in the carrier will not then appear on the chromatogram.
Given the sensitivity of GC, it is most useful in process control to optimise a gas, as well as to qualify the quality of a supplied gas. The most frequent application for GC therefore tends to be the measurement of impurities in bulk gases: H2, O2, N2, CH4, CO, CO2 and non-methane hydrocarbons in H2, O2, N2, Ar and He. Other applications are feasible, like the measurement of both crude argon and medical nitrogen.
When correctly optimised for a particular application, the exceptional accuracy, sensitivity and flexibility of GC make it suitable not only for a range of air separation unit processes, but makes it a vital component in industries and applications as diverse as semiconductor manufacture, steel making and hydrogen production.