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Gas chromatography (GC) is used for the analytical separation of volatile substances in the gaseous phase. This chromatographic technique is widely used in various fields of research and industrial sectors such as pharmaceuticals, cosmetics, and even for the identification of environmental toxins. It also helps in the identification and quantitation of compounds in a mixture. The two important features that are required for sample analysis are that the compounds under consideration must be thermally stable and volatile.
The GC technique requires a mobile and a stationary phase. The mobile phase is associated with chemically inert carrier gases, for example, argon, helium, or nitrogen. The mobile phase carries the analyte molecules through a heated column. The stationary phase consists of a packed column (such as capillary columns). The stationary phase contains either a solid adsorbent, which is known as gas-solid chromatography or a liquid on inert support called gas-liquid chromatography.
On completion of the analysis, GC produces a graph called a chromatogram. This graph contains a peak for each separated component of the sample with respect to its retention time. The number of peaks represents the number of compounds separated in the sample.
Factors on which separation of compounds in GC depends:
- Vapor pressure: A compound with a lower boiling point possesses higher vapor pressure. This in turn shortens the retention time of the compound because the compound spends more time in the gaseous phase.
- The polarity of the stationary phase on a column and the sample: When the polarity of the compounds and the stationary phase are similar, the retention time increases. This is because a strong interaction between the stationary phase and compounds takes place. Thus, polar compounds show longer retention times in polar stationary phases and retention times are shorter in the case of non-polar columns using the same temperature.
- Temperature of the column: A very high column temperature reduces the retention time. This high temperature deteriorates the quality of separation. The best separation is obtained by using a temperature gradient. This is because of the difference in polarity and boiling points of samples and the stationary phase.
- The flow rate of the carrier gas: A high flow results in the shortening of the retention time and also deteriorates the quality of separation.
- Column length: Typically, the longer the length of the column, the better the separation of compounds. This is because the retention time is directly proportional to the column length.
- The amount of sample injected also plays a vital role in the separation of the compounds and the development of chromatograms.
General Working of Gas Chromatography
A gas chromatograph contains a heated inlet port, an oven, and a fused silica column. The silica column is typically a coiled glass tube that is coated with appropriate packing material (the stationary phase). The following are the typical sequential steps related to the identification of compounds:
Sample preparation: GC samples are mostly diluted using an appropriate solvent system. This sample is then injected into the inlet port.
Vaporization: The liquid sample is vaporized in the hot inlet.
Separation: The mobile phase carries the vaporized sample through the column. Different analytes undergo different types of interaction with the stationary phase (column), which depends on the structure, molecular weight, and polarity of the sample and the stationary phase. When the interaction between a compound and stationary phase is stronger, the compound will take more time to migrate through the column. Thereby, analytes are separated based on the difference in the traveling time and speed. The time taken for a compound to travel through the column is known as its retention time.
Detection: The detector is the device placed at the end of the column. It helps to identify and quantify components of the mixture as they elute in combination along with the carrier gas. There are two parts of a detector. The first part is the sensor, located at the end of the column to optimize the detection. The second part is the electronic equipment which converts the detected property changes into an electrical signal. This signal is recorded as a chromatogram. There are different kinds of detectors, some of which are described below:
- Mass Spectrometer (MS): This detector is regarded as the most powerful of all GC detectors. A GC/MS system scans the samples continuously throughout the separation and, ultimately, the chromatogram shows the retention times. The chromatogram peak is analyzed by the mass spectrometer which leads to the identification of the separated components.
- Flame Ionization Detectors (FID): This is a widely used detector, where the sample is introduced to an air-hydrogen flame at the end of the column. The sample undergoes pyrolysis when directed to the high temperature of the air-hydrogen flame. The pyrolyzed hydrocarbons release ions and electrons which are detected by high impedance picoammeter. The main advantage of FID is that the detector remains unaffected by the flow rate and non-combustible gases.
- Photoionization Detectors (PID): This detector utilizes the properties of chemiluminescence spectroscopy. It helps to detect aromatic hydrocarbons, organo-heteroatom, and other organic compounds. This detector consists of an ultraviolet lamp to emit photons that are absorbed by the compounds, in an ionization chamber, exiting from a GC column.
- Atomic Emission Detectors (AED): This detector is the latest addition to the gas chromatographer's arsenal. It is based on the detection of atomic emissions.
- Thermal Conductivity Detectors (TCD): This one of the oldest detectors developed for GC. The main principle of this detector is to quantify the change in the carrier gas’s thermal conductivity, which is altered due to the presence of the sample. This detector is used to identify both organic and inorganic samples. However, one of the main drawbacks of this detector is the low sensitivity of this instrument when compared to other detectors.
References and Further Reading
Anon, 2020. Gas Chromatography. [Online] Available at: https://chem.libretexts.org/@go/page/301 [Accessed March 31, 2021].
Pang, T. et al. (2007). Identification of unknown compounds on the basis of retention index data in comprehensive two‐dimensional gas chromatography. Journal of Separation Science, 30(6), pp. 868-874. https://doi.org/10.1002/jssc.200600471
Brown, I. (1960). Identification of Organic Compounds by Gas Chromatography. Nature, 188, pp.1021–1022. https://doi.org/10.1038/1881021b0