Gas Chromatography
Gas chromatography, also known as vapor-phase chromatography (VPC), or gas-liquid partition chromatography (GLPC), is a technique used to detect and separate small molecular weight compounds from a mixture. Gas chromatography (GC) is an analytical methodology, which was devised by Nobel Laureate, Martin, et al. in 1952.
Gas chromatography is of two types:
- gas-solid chromatography (GSC), and
- gas-liquid chromatography (GLC).
The stationary phase in GLC and GSC is liquid and solid respectively. Gas-liquid chromatography method is widely used as compared to gas-solid chromatography.
Term | Definition |
---|---|
Chromatography | A separation technique that divides a mixture into its components based on their partitioning behavior between a mobile phase (flowing) and a stationary phase. |
Chromatograph | An instrument used in chromatography to separate, identify, and quantify components of a mixture. |
Stationary Phase | The fixed immotile phase in chromatography where separation of components occurs based on their interactions. |
Mobile Phase | The moving phase in chromatography that carries the sample through the stationary phase. |
Sample | The mixture to be separated into its components in chromatography. |
Injection | The process of introducing the sample into the gas chromatograph. |
Detector | The device in a gas chromatograph that detects the separated components and generates a signal proportional to their amount. |
Chromatogram | A graphical representation of the detector signal versus time, showing peaks corresponding to the separated components. |
Retention Time | The time it takes for a component to travel through the column and be detected. |
Peak Area | The area under a peak in a chromatogram, which is proportional to the amount of the corresponding component in the sample. |
Gas-solid chromatography
Gas-solid chromatography is a technique in which the separation of the mixture takes place through the adsorption process. It is used for the separation of low molecular gases, namely, H2S, CO2, and CO. It is a technique in which the stationary phase (liquid) is first converted into vapours. Then, the separation of mixtures is done which depends upon the relative vapour pressure and affinities for the stationary phase. The affinity of the substance towards the stationary phase can be described in chemical terms as partition coefficient, also known as distribution constant, as KC = [A]S / [A]m, where [A]s is the concentration of compound A in the stationary phase and [A]m is the concentration of compound A in the stationary phase.
Working Principle of Gas chromatography
Gas chromatography works on the principle of separation/partition of the mobile phase and the stationary phase in which the mobile phase is the carrier gas. It is a process in which a gaseous sample is injected into the injection port with the help of a GC syringe, where it gets vaporized and is further carried by a gas flow regulator (carrier gas) to pass through the stationary phase (viscous liquid). Inert gases like helium, argon, nitrogen, and hydrogen are usually used as mobile phase/carrier gas in the gas chromatography technique. This sample is then separated electronically at the detection port with the help of suitable temperature programming. Clear visualization of this process can be seen on recorders/computers in the form of peaks.
Conditions
- Carrier gas (mobile phase) does NOTHING in GC but transport the compounds. Not involved in separation mechanism (H2 and He common).
- Injection volume (0.1 – 10 μL generally). Temperature of injector is 50 oC greater than least volatile (highest boiling point compound). All compounds must be vaporized before transport onto column.
- Fixed temperature separation – average boiling point of all analytes is a good starting point.
- Carrier gas is often dried by passage over molecular sieves as they strongly retain water. Activated by heating to 300 oC in vacuum.
- Gaseous mobile phase carries gaseous compounds (analytes) through a long column with a stationary phase
Instrumentation of Gas chromatography
Gas chromatography, both GSC and GLC, is mainly composed of the following components-
1. Carrier gas
A carrier gas is a high-pressure cylinder equipped with attendant pressure regulators and flow meters. Carrier gases used in GC are inert oxygen-free gases that act as a gas flow regulator/carrier for the mobile phase to pass through the stationary phase/column. Helium, Nitrogen, argon, and hydrogen gases are used as carriers in GC, depending upon the desired performance and the detector being used. Hydrogen, as a carrier gas, is highly efficient and provides the best separation. However, helium is preferred over hydrogen because of its inflammable properties, higher flow rates, and its compatibility to work with a greater number of detectors. For example, helium is preferred for thermal conductivity detectors because of its high thermal conductivity relative to most organic vapors.
Carrier Gas | Advantages | Disadvantages |
---|---|---|
Helium (He) | Safe (inert): Relatively wide optimum linear velocity range (faster analysis) * High diffusivity (good for separating low molecular weight compounds) | * Expensive * Limited supply * Low thermal conductivity (may require higher detector temperature) |
Nitrogen (N₂) | * Cheap * Readily available * Safe (inert) | * Narrow optimum linear velocity range (slower analysis) * Low diffusivity (may not be suitable for all separations) |
Hydrogen (H₂) | * Very fast analysis times due to high linear velocity * High diffusivity (good for separating low molecular weight compounds) | * Highly flammable (safety hazard) * Can damage some detectors |
2. Sample-injection system
It is essential for introducing the sample at the head of the column. The sample is injected into the injection port with the help of a micro-syringe/GC syringe. The sample gets volatilized here, and the resulting gas is then carried to the column with the help of carrier gas. Many inlet types exist including, Split/Splitless, Programmed Thermal Vaporizing (PTV), Cool-on-column (COC), etc. In the case of the COC injector, the sample is introduced as liquids to avoid thermal decomposition.
There are two main injection systems, depending on the nature of sample:
- Injection port: for introduction of liquids and solutions.
- Sampling loop injection: for introduction of gas samples.
Injection techniques
- Syringe injection.
- Gas sampling loop/valve.
- Purge and trap.
- Solid phase microextraction (SPME).
Syringe injection
The syringe used to introduce an accurate volume of the liquid or gas sample in the injector. Several syringe models are available: from 1 mL to several millilitres, with various options: fixed or removable needle, adaptor, sharp or round needle.
3. The separation column
The heart of gas chromatography is the column which is made up of metals, bent in a U-shaped or coiled into an open spiral. The sample mixture is separated here and is then carried to the detector. Different-sized columns are used, depending upon the requirement. For example, Open tubular columns or capillary columns and packed columns are most commonly used. Two types of columns are used in GC, namely, packed columns and capillary columns.
Columns can be classified by tubing diameter and packing type.
- Packed columns.
- Open tubular capillary columns.
- Wall-coated open tubular (WCOT)
- Support-coated open tubular (SCOT)
- Porous layer open tubular (PLOT)
4. Column oven or thermostat chambers
These thermostat chambers help to control the temperature to conduct precise work. The temperature is controlled via two methods, isothermal programming, and temperature programming. In isothermal programming, the temperature is kept steady throughout the analysis, while in the other temperature can be increased or controlled according to the requirement.
5. Detector
Detectors provide signals, to give us the quantitative measurement of the components of mixtures, by sensing the arrival of separated components of the mixture. Various detectors used in GC are Mass Spectrometer, Flame ionization detector (FID), Electron capture detector (ECD), Thermal conductivity detector (TCD), Atomic emission detector (AED), Photoionization detector (PID), and chemiluminescence detector.
The detector indicates what and how much is in the carrier gas that exits the column. Detectors need to have the following characteristics:
- high sensitivity
- low noise
- linear response to concentration
- response to the types of chemicals the user is interested in
- cost effectivenes
6. Recorder
The recorder is meant to record the signals that come from detectors and amplify them to generate an electronic response in the form of a graph, known as a chromatogram.
The routine steps involved in Gas Chromatography are
- Selection of column, stationary phase, inlet types, carrier gas, and their flow rates
- Addition of the sample to be injected to the syringe
- Injection of the sample into the injector port
- Data reduction and analysis
Applications of Gas Chromatography
Gas chromatography has a wide range of applications in various fields, such as-
- It has medicinal and pharmaceutical applications
- GLC is combined with mass spectroscopy (GC/MS) for drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples.
- It is also used in environmental analysis and monitoring of pollutants like carbon monoxide, benzene, DDT, etc.
- Doping of drugs is detected using this method.
- Miscellaneous analysis of foods like proteins, carbohydrates, lipids, fats, steroids, etc.
- usually used to separate and measure organic molecules and gases.
- the components being analyzed must be volatile.
- Forensically used in drug analysis and toxicological analysis.
- the molecular weight of components should be less than 1250 Da
- GC is also used in catalysis
- Dairy product analysis- rancidity
- Identification of hazardous compounds in waste dumps
- useful in the isolation of RNA etc.
Industry Type | Analysis Performed |
---|---|
Pharmaceutical | Residual solvent analysis, Assay, Excipient analysis |
Food and Beverages | Component analysis, Food safety analysis, Halal analysis of alcohol |
Environmental | Air, water, soil analysis (pollutants, microplastics) |
Petrochemicals | Simulated distillation, Component analysis |
Chemicals | Material, polymer, additive analysis, Gas purity analysis |
Automotive | Gas emission analysis |
Energy and Gas | Artificial photosynthesis research |
References
- McNair, H.M. and J.M. Miller, Basic Gas Chromatography. Techniques in Analytical Chemistry. 1998, New York, NY: John Wiley & Sons, Inc.
- McNair, H.M. and E.J. Bonelli, Basic Gas Chromatography. 1969, Berkely, California: Consolidated Printers.
- Skoog, D.A., F.J. Holler, and T.A. Nieman, Principles of Instrumental Analysis, Fifth Edition. 1998, Thomson Learning, Inc.
- https://www.shimadzu.com/