Gas chromatography (GC) is the technique for separating gases and volatile compounds in their gaseous state. In this technique, different chemical compounds are separated based on their volatility using inert gas, like helium, as a mobile phase. The mobile phase (gas) acts as the carrier of the different compounds present in the sample, which then interacts in the column (stationary phase). The different compounds interact differently with the stationary phase based on which separation occurs. The two important properties of gas chromatography are volatility and polarity.
Volatility: the tendency of a substance to evaporate at normal temperatures
The most volatile compounds will move fast and get detected first. The least volatile compounds will move slowly, resulting in their detection at last. The stationary phase can form by polar or non-polar compounds. If the stationary phase and the compound have the exact nature of polarity, it will elute at last due to longer interaction.
For example, if the stationary phase is polar, compound A is polar, and compound B is non-polar. In this case, compound A and the stationary phase have the same polarity, which will interact for more time and elute in the detector late but since compound B has opposite polarity nature to the stationary phase, it will elute first in the detector.
Principle of Gas Chromatography
The basis of gas chromatography is the affinity of the mixture’s components to the stationary phase. The higher the affinity, the longer the retention time because the interaction between the sample and column is much longer, resulting in increased time spent in the column. The carrier gas or mobile phase carries the sample through the column. The column consists of glass or ceramic beads which is gas-solid chromatography (GSC). Sometimes the column forms with volatile liquid, known as gas-liquid chromatography (GLC). GLC is more common than GSC.
The sample is added into the carrier gas stream via injection, which moves through the column. This movement distributes the sample molecules between the gas (mobile phase) and liquid/solid (stationary phase). Each molecule move between the mobile and stationary phase in a dynamic equilibrium. The molecule with higher affinity with the stationary phase will remain dissolved for a longer time resulting in the late emergence of the molecule. Similarly, the higher the volatility of the molecule, the higher the time it will remain in the mobile phase leading to the early emergence of the molecule from the column. Each molecule will separate inside the column and emerge at separate times in a similar way.
The time taken from the injection to the emergence is retention time (Rt). It is the characteristic feature for separation, which depends on the volatility of the substance, the affinity between stationary phase and substance, and the column’s temperature, length, and diameter.
Some substances have a longer retention time at room temperature, heating the column in an oven helps overcome the problem. The detectors like thermal conductivity detectors (TCDs), FID (flame ionization detection), etc., detects the emerged substances in the form of peaks. The recorder records the signals from the detector, producing a chromatogram.
Working Mechanism of Gas Chromatography
The carrier gas passes through a flow regulator to adjust the flow rate and enters the sample injection system. A syringe helps in adding a small amount of sample into the sample injection system. The sample injection system is maintained at a higher temperature than the boiling point of the highest boiling component of the sample. To ensure the rapid vaporization of the liquid sample, the carrier gas entering the sample injection system carries the vaporized sample to the thermostatic column.
The mixture components pass through the column and are distributed between the stationary and mobile phases at different rates. It results in the separation of the components of the sample. The carrier gas with the separated components now enters the detector, which measures the change in the composition of the carrier gas as it passes through it. As each component reaches the detector, the detector provides a peak. The separated components of the sample in the carrier gas illustrate a series of signals which appear as a succession of the peaks above the baseline on the recorded chromatogram.
Parts of Gas Chromatography
The gas chromatography consists of the following parts:
A high-pressure cylinder containing a carrier gas
A high-pressure cylinder containing a carrier gas. The most important requirements of the carrier gas are:
- It should be inert.
- The cost should be low because the required quantities of these are large.
- It should allow the detector to respond adequately (pure).
Typically nitrogen (N2), Helium (He), and Argon (Ar) fulfill the above requirements. For most analytical purposes, H2 (hydrogen) gas as a carrier gas is not applicable because of its explosion hazards and its reactivity toward unsaturated sample components.
The flow rate of the gas dramatically influences the column efficiency. If the flow rate of gas is slow, then it gives a broader peak. If the flow rate of gas is fast, it provides a peak with thinner width.
Sample injection system
The amount of sample required for gas chromatography depends upon:
- The nature and concentration of the components present in the mixture.
- The size of the column
- Sensitivity of the detector
Generally, the amount of sample required is small. For gases, a 0.1 µl to 50 µl sample is used, and for liquids and solids, a few fractions or milligrams are used. A solid sample is first dissolved in ethanol, methanol, etc., and injected as a sample. A syringe may be used to load the sample.
The column length ranges from 120 cm to 5 m, and its diameter ranges from 2-10 mm. Columns are made up of various materials like stainless steel, copper, etc., and may be coiled in U-shaped or W-shaped. Although metal columns are expensive, they are preferred because they are inert and possess good thermal properties.
The column is never operated at room temperature except when dealing with highly volatile liquids. The temperature of the column is controlled by using a circulated air bath, a vapor jacket, or electrically heated metal blocks. The temperature of the columns should be high enough to keep the sample in a vapor state but does not vaporize the stationary phase. Usually, a temperature equal to or slightly more significant than the boiling point of the sample is preferred.
The function of the detector is to measure the small number of separated components present in the carrier gas leaving the column. The output from the detector is then fed to the recorder, which produces a chromatogram. The signals from the detector are recorded on the data handling device. The separated compounds are observed in the form of peaks.
Based on the physical properties of the gases, detectors are of various types:
- Thermal conductivity detector
- Electron capture detector
- Ionization detector
- Gas density detector
Uses of the Gas Chromatography
Gas chromatography applies in the separation of organic molecules and gases. This method usually separates components with a molecular weight of below 1250 dalton. The fields that use GC are as follows:
- GC helps detect food additives, flavor, and aroma components in the food industry. It also helps detect contaminants like pesticides, toxins, fumigants, and environmental pollutants.
- In the pharmaceutical industry, GC contributes to producing large quantities of products in pure form.
- In the automobile industry, it helps detect and measure the chemicals harmful to humans from the car. The car’s carpets, door linings, pedals, seat covers, and other interior materials release these chemicals.
- In the chemical industry, GC helps ensure the consistency and quality of chemicals like emulsifiers, solvents, and co-solvents.
- In forensic science, GC helps determine why a person dies. It detects the reason behind death due to the consumption of different drugs, poisons, and alcohol. It also helps to analyze fibers and blood.
- In microbiology, GC helps determine specific constituents and metabolites called chemical markers from the culture of microorganisms.
- It also helps in determining the quality of air by identifying the types of air pollutants present in the air.
- Finally, GC helps determine blood alcohol level.
Advantages of Gas Chromatography
- This method has high-resolution power than other chromatography.
- It has high sensitivity when used with a thermal detector.
- It gives relatively good accuracy and precision.
- The separation of the sample is relatively faster.
- Samples with less quantity are also separated.
Disadvantages of Gas Chromatography
- It can separate only volatile samples or the samples which can be made volatile.
- The sample about to be injected must be thermally stable so that it does not get degraded when heated.
References and further reading
- Evers, F. R. (2015). Development of a Liquid Chromatography Ion Trap Mass Spectrometer Method for Clinical Drugs of Abuse Testing with Automated On-Line Extraction Using Turbulent Flow Chromatography. September 2014. https://doi.org/10.13140/2.1.2125.1367