Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.
What is gas chromatography?
Gas chromatography is also sometimes known as vapor-phase chromatography (VPC), or gas–liquid partition chromatography (GLPC). These alternative names, as well as their respective abbreviations, are frequently used in scientific literature.
Gas chromatography is the process of separating compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, typically called the carrier gas, and passing the gas through a stationary phase. The mobile phase is usually an inert gas or an unreactive gas such as helium, argon, nitrogen or hydrogen. The stationary phase can be solid or liquid, although most GC systems today use a polymeric liquid stationary phase. The stationary phase is contained inside of a separation column. Today, most GC columns are fused silica capillaries with an inner diameter of 100-320 μm and a length of 5-60 m. The GC column is located inside an oven where the temperature of the gas can be controlled and the effluent coming off the column is monitored by a suitable detector.
Principle of Gas chromatography
The equilibrium for gas chromatography is partitioning, and the components of the sample will partition (i.e. distribute) between the two phases: the stationary phase and the mobile phase.
Compounds that have a greater affinity for the stationary phase spend more time in the column and thus elute later and have a longer retention time (Rt) than samples that have a higher affinity for the mobile phase.
Affinity for the stationary phase is driven mainly by intermolecular interactions and the polarity of the stationary phase can be chosen to maximize interactions and thus the separation.
Ideal peaks are Gaussian distributions and symmetrical, because of the random nature of the analyte interactions with the column.
- The separation is hence accomplished by partitioning the sample between the gas and a thin layer of a nonvolatile liquid held on a solid support.
- A sample containing the solutes is injected into a heated block where it is immediately vaporized and swept as a plug of vapor by the carrier gas stream into the column inlet.
- The solutes are adsorbed by the stationary phase and then desorbed by a fresh carrier gas.
- The process is repeated in each plate as the sample is moved toward the outlet.
- Each solute will travel at its own rate through the column.
- Their bands will separate into distinct zones depending on the partition coefficients, and band spreading.
- The solutes are eluted one after another in the increasing order of their kd, and enter into a detector attached to the exit end of the column.
- Here they register a series of signals resulting from concentration changes and rates of elution on the recorder as a plot of time versus the composition of carrier gas stream.
- The appearance time, height, width, and area of these peaks can be measured to yield quantitative data.
Gas Chromatography Experimental Procedure:
Before starting a gas chromatography experiment, we must recognize the various components that are essential to carry out the process.
It is made up of four main elements.
1. Carrier gas: Since the carrier gas (hydrogen, helium) is used as the mobile phase in GC, it plays an important role in the isolation.
2. Injector: The port is intended to inject samples into the GC by manual or automatic sampling.
3. Oven: The temperature of the GC column is controlled by an oven to manage the separation and the retention time of the analytes.
4. GC column: A column, in which molecules, depending on their affinity for the mobile phase (gas) and the stationary phase, are separated into individual analytes.
5. Detector: TDetector used to determine the composition and concentration of a sample.
Gas Chromatography Procedure:
- Maintain inlet and outlet gas pressure with a regulator mounted on the control panel.
- Install the required column (packed/capillary) and it should not leak.
- Create and download the method and sequence from the software and before the injection starts the flame. Parameters such as injector temperature, detector temperature, oven temperature, gas flow/pressure and sample/vial sequence, etc.
- Prepare samples as needed.
- Download the software method and light the flame before injection.
- Saturate the mobile phase GC column to baseline.
- Inject the sample manually with a syringe or autosampler, filling the vial at least half full.
- Depending on their affinity for the stationary phase, the components of the sample mixture are isolated from the
- At different times, the separated analytes arrive at the detector and are registered by the computerized system.
- From the chromatogram, the retention time (RT), peak area, column efficiency, tailing factor, peak height and number of theoretical plates can be calculated.
Gas Chromatography Applications:
- Gas chromatography is commonly used as a routine analytical technique in pharmaceutical industries.
- It is used for both medical and forensic applications for the quantification of drugs and their metabolites in blood and urine.
- GC is used in the analysis of pesticides and volatiles.
- Gas chromatography techniques are used in many areas of forensic medicine.
- GC is used in the analysis of flavors, fragrances and food products.
- The GC is used to analyze organic compounds in environmental samples.
Advantages of Gas Chromatography Analysis
Gas chromatography (GC) is a widely used analytical technique in chemistry and related fields. It is a powerful separation method that separates and analyzes mixtures of volatile compounds. Gas chromatography analysis offers several advantages over other separation techniques, making it a popular choice for researchers and scientists.
Some of the advantages of gas chromatography analysis.
1. High sensitivity:
One of the significant advantages of gas chromatography is its high sensitivity. GC can detect even trace amounts of compounds in a mixture, making it a powerful tool in analytical chemistry. The high sensitivity of GC is due to the use of small sample sizes and the efficient separation of compounds.
2. High resolution:
GC can separate complex mixtures of compounds with high resolution. The resolution is the ability to distinguish between two adjacent peaks in a chromatogram. The high resolution of GC is due to the use of narrow-bore capillary columns and the ability to control the carrier gas flow rate.
3. Rapid analysis:
GC is a relatively fast analysis method, allowing for the analysis of multiple samples in a short time. This speed is due to the use of narrow-bore columns that have a high surface area-to-volume ratio, allowing for the efficient separation of compounds.
4. Quantitative analysis:
GC can be used to perform quantitative analysis of compounds in a mixture. The area under a peak in a chromatogram is proportional to the amount of the compound present in the sample, allowing for accurate quantification of compounds in a mixture.
5. Minimal sample preparation:
GC requires minimal sample preparation compared to other analytical techniques, such as liquid chromatography. This advantage is due to the volatile nature of the compounds analyzed by GC, which do not require complex extraction or purification steps.
6. Cost-effective:
GC analysis is a relatively cost-effective technique compared to other analytical methods, making it accessible to many researchers and scientists. The low cost of GC analysis is due to the availability of inexpensive equipment and consumables.
Disadvantages of Gas Chromatography GC are:
- Gas chromatography is limited to volatile compounds.
- Non-volatile compounds don’t vaporize.
- Analyte can decompose at high temperature.
- Analyte can also react with stationary phase.
- It is limited to low to medium molecular weight.
- It is incompatible with aqueous samples.
- Thermal stability is required during separation through gas chromatography.
- It is not suitable for high-boiling compounds.
- It is not sutable for polar analytes.
