Gas Chromatography: Principle, Working & Uses
Gas chromatography (GC) is a widely used instrumental method for analytical purposes. GC is an analytical separation and purification technique used to analyze a volatile compound in the gaseous phase. This technique is based on the principles of column chromatography where gas is used as the mobile phase.
The stationary phase in GC can be a liquid or solid and based on this the GC can be categorized as Gas-Liquid Chromatography (GLC) or Gas-Solid Chromatography (GSC). The separation in GLC is based on the principle of partition of the analyte into liquid and gas whereas in GSC the separation is based on the adsorption of the analyte on the solid stationary phase.
GC was first developed by APJ Martin and Anthony T James in 1951 and it was gas-liquid chromatography based on the principle of partition chromatography. The first gas-solid chromatography was developed by Erika Cremer (German Physical chemist) and Fritz Prior (Austrian graduate student) in 1947 and is based on adsorption chromatography.
Principle of Gas Chromatography
The basic principle of the Gas Chromatography is the separation of the analyte based on its affinity with the stationary and mobile phases.
In Gas Solid Chromatography, the separation is based on the laws of Freundlich and Langmuir which is represented by:
x/m = Kc¹⁄n
x/m = K₁c + K₂c
Where, x = mass of the gas
m = mass of the sorbent
K, K₁, K₂ = constant
c = vapour concentration in the gas
Freundlich law describes the adsorption isotherm which is a curve that expresses the variation in the amount of gas adsorbed by the adsorbent with the temperature at constant pressure.
Langmuir law explains the adsorption isotherm which describes the equilibrium between adsorbate and adsorbent systems, where the adsorbate adsorption is limited to one molecular layer at or before a relative pressure of unity is reached.
The principle of Gas-Liquid Chromatography is based on Henry’s law of partition which states that at a constant temperature, the amount of a given gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. It is represented by the equation:
x/m = Kc
Instrumentation and Working of Gas Chromatography
The instrumentation of Gas Chromatography is composed of the following components:
- Carrier gas chamber
- Sample injector
- Separation column
- Thermostat chamber
- Detectors
- Recorder
1. Carrier Gas Chamber
Carrier gas is the soul of this technique. It acts as the mobile phase which carries the vapourised analyte from the separating chamber to the detector.
It plays a very important role in the methods therefore there are certain conditions which are considered to select the carrier gas, which states that a carrier gas should be inert, pure, not causes any hazardous explosion, easily available and cost effective, detectable to detector, and have consistent column speed.
The most frequently used carrier gases are hydrogen, nitrogen, helium and air. These gases can be used as single carrier gases or in combinations.
2. Sample Injector
Sample injector is connected to the head of the separating column which injects a small amount of sample in the column. The sample is injected by a hypodermic syringe through a self-sealing septum into an inlet vaporizing chamber which is heated to vaporise the sample immediately.
The amount of sample injected in the column should be small and must be reproducible so that it can be detected by the detector. The vaporization chamber is typically heated 50°C above the lowest boiling point of the sample and subsequently mixed with the carrier gas to transport the sample into the column.
Liquid samples are directly injected into the chamber whereas the solid samples are first dissolved or mixed in the volatile liquids and then injected in the separating column. A special gas stream is required for the injection of gaseous samples.
3. Separating Column
The separating column is the place where the separation of analytes is carried out. It is a long tube made up of glass or metal or teflon which is wound in a spirally coiled fashion placed in the thermostat chamber. The separating columns used in GC are of two types namely open tubular column and packed column.
Open-tubular columns, also known as capillary columns, are thin, fused silica glass tubes, lined with a liquid phase or adsorbent material or having a chemical bonding layer. They are further classified as WCOT and SCOT.
WCOT, i.e. Wall Coated Open Tubular columns are the capillary tubes coated with a thin layer of stationary phase along the walls. The sample holding capacity of this column is low but still it has better efficiency. FSWC (Fused Silica Wall Coated) is a special WCOT column in which the walls are prepared from the fused silica containing minimal amounts of metal oxides. These columns are highly inert, require small amounts of sample and have higher column efficiency.
SCOT or Support Coated Open Tubular columns are the capillary tubes coated with a thin layer of adsorbent solid such as diatomaceous earth. The sample holding capacity of SCOT is higher but the efficiency is lower than WCOT.
Packed columns are the metal or glass tubings which are densely packed with solid material like diatomaceous earth. The diameter of these columns is usually larger than the open tubular column because they need to be densely packed. This results in their low efficiency in comparison to open tubular columns. In fact there is semi-permanent deposition of impurities in the adsorbent causing deactivation of the column.
4. Thermostat Chamber
The thermostat chamber is responsible for the continuous heating of the columns which is necessary for the maintenance of gaseous phase of the carrier gas and sample throughout the column. The chamber operated either in isothermal programming or temperature based programming.
Isothermal programming maintains a constant temperature throughout the separation process and can be useful for the samples with narrow boiling point difference whereas temperature based programming requires a regular increase of temperature to separate the samples having a broad range of boiling points.
5. Detectors
Detectors are located at the end of the column that gives quantitative measurements of the analyte in the mixture. It is composed of a sensor (placed near the column that detects signal) and electronic equipment (digitizes the received analogue signal).
There is a wide variety of detectors used in GC. Some of them include:
- Flame Ionisation Detectors (FID)
- Thermal Conductivity Detector (TCD)
- Electron-Capture Detector (ECD)
- Atomic Emission Detectors (AED)
- Mass Spectrometry (MS) Detector
- Chemiluminescence (CS) Detector
- Photoionization Detector (PID)
6. Recorder
The last components of a GC instrument are amplifiers and recording systems. The signals detected by the detectors are amplified multiple times by an amplifier and recorded by the computer systems in the form of graphical representations.
Advantages of Gas Chromatography
- High-resolution power
- High sensitivity
- Higher Accuracy
- Quick analysis of the sample
- Easy to use
Limitations of Gas Chromatography
- Only Volatile compounds can be separated.
- Detectors used in this technique are quite destructive.
- We cannot change the mobile phase as it has a constant flow of carrier gas but we can only modify the temperature of the column and oven.
- Need utmost care as the hydrogen gas is highly flammable.
- Impossible to recover individual sample components.
- Only those compounds that are highly stable at high temperatures can be separated.
Uses of Gas Chromatography
- Gas Chromatography is essential for the food industry to ensure the safety of food products through quantitative and qualitative analysis of food.
- GC is effectively used in the pharmaceutical and chemical industries for ensuring the purity of the products.
- GC is used in Research fields for the analysis of meteorites and natural products.
- GC is used in the field of forensics for the analysis of drugs, alcohol, poisons, chemical compounds, etc. to detect and help in the process of investigation.
- GC is used for the purpose of analysis of harmful pollutants in the air to combat the problem of air pollution.
Conclusion
Gas Chromatography is a destructive analytical technique which has a wide application in multiple industries. It is a separation, purification and identification technique, also known as hyphenated analytical method. It is a coupled mass spectroscopy to get enhanced quantifying results.
Though it is an advanced technique it still has a few limitations which needs to be worked upon so that it can give more effective, reliable and robust results.
