Gas chromatography

At the heart of many laboratories involved in the analysis of gases and liquids will be a gas chromatograph. A range of high purity specialty gases and most probably also some calibration gas mixtures will be essential for this instrument to function.

Gas chromatography

The science of chromatography is related to separation of chemicals based on their mobility over a static phase. Some species move quickly and others more slowly because they are attracted to the static phase to a greater or lesser extent.

The fact that different chemicals will move at different speeds has two benefits for this analytical technique. Firstly, mixtures may be separated to determine their individual components. Secondly by knowing the speed at which a chemical moves over the stationery phase it is possible to determine what species it is by the retention time that it spent moving through the stationary phase.

In gas chromatography the static phase will be a packed or capillary chromatography column and the mobile phase will be a carrier gas such as helium, hydrogen, nitrogen or argon. To increase the speed at which the chromatographic separation takes place, the column will be placed inside an oven where temperatures up to 350°C are common. The chromatography column can be up to 90 metres in length and to achieve a compact size, it will be coiled around many times in a round bundle inside the gas chromatograph.

At the end of the chromatography column there will be some kind of detector instrument. Some detectors are quantitative, meaning that they are able to measure the concentration of the various species emerging from the gas chromatograph. Examples of quantitative detectors would be a flame ionisation detector (FID) commonly used for a wide range of hydrocarbons or a TCD detector which is more relevant for measuring permanent gases such as nitrogen or oxygen. In contrast, there are also qualitative detectors such as a mass spectrometer. These are required where the species in the sample must be identified but this can not be achieved from knowledge of the chromatographic separation and retention time information.

Many of the detectors used in combination with gas chromatographs require different gases for their operation. For example the FID will require instrument grade air and a fuel gas such as hydrogen 5.0 grade. In some applications, especially in the automotive sector, the hydrogen is diluted to 40% in a balance of helium so that there is a better removal of heat and lower flame temperature intensity. Collectively the detector gases and the carrier gases are known as instrumentation gases. Some are pure and others, such as air, are mixtures.

The analytical samples fed to gas chromatographs may be liquids or gases. When gas samples are used it is common to calibrate the gas chromatograph using a specialty gases calibration gas mixture of a known and certified concentration. For liquid samples the calibration would generally take place using a liquid mixture calibration standard.

FID Detector

A GC-FID setup is very common in petrochemical processing applications and environmental analysis of air samples when VOC pollution is under investigation. Hydrogen and air are burned together to form a very high temperature flame through which the gas from the chromatography column flows. As the various chemicals in the sample are burned their ionisation energy is measured and this can be used to establish their concentration in the gas stream. As a zero reading for the GC-FID setup the chromatography carrier gas will be used. This is often referred to as the baseline measurement. To avoid interference with the analysis, known as a "noisy baseline", it is essential to use high specialty gases for carrier gas applications. Industrial grade gases will not be suitable for this purpose.

TCD Detector

Use of a thermal conductivity detector (TCD) can be combined with other detectors because this is a non-destructive technique. So, GC-TCD-FID is a possible instrumentation setup. The measurement principle relies on differences in thermal conductivity of various gases. As the gases flow over the detector they remove heat at different rates. The cooling of the detector influences its electrical resistance and this can be measured using a Wheatstone Bridge electronic setup. The TCD requires no additional detector gases and the main carrier gas for the chromatography column will be used to transport the sample over the TCD element.

MS Detector

The mass spectrometer detector is a destructive technique. So, similar to an FID it must be the last step in the analytical chain. The measurement principle is to shatter the analytical sample into a range of ions, then detect the amount of energy it takes to bend them through an electromagnetic field and measure the amount of electrical charge the ions transfer with them. The resultant information is the mass-to-charge ratio of the various ions, which can then be built up to create a picture of the original chemical species that is under investigation. The MS detector uses the chromatography carrier gasto transport the sample and requires no additional instrumentation or zero gases. While the FID and TCD detectors are relatively inexpensive to purchase, each being in the order of a few thousand Australian dollars, the MS detector will cost perhaps ten times this sum. For this reason, the TCD and FID detectors are often regarded as the workhorses for GC applications.

Other detectors

There are many other detectors used with GC instrumentation, eg ECD, DID, PID, FPD, AED and SCD to name a few by their often-used acronyms. Some, such as the discharge ionisation detector (DID) require a specific high purity instrumentation gas to function. In the case of the DID this will be 5.0 grade helium. On the other hand, the electron capture detector (ECD) will require a make up gas such as P5 or P10. These are instrumentation gas mixtures of 5% or 10% methane in a balance of argon respectively. The choice of detector will be made based on the nature of the samples that must be analysed.