What Is Gas Chromatography and Why Is It Important?
Gas liquid chromatography (GC) is an analytical technique for separating chemical components in a sample mixture and then detecting them to ascertain their presence or absence, as well as how much of each component is present. Organic molecules or gases are the most common chemical elements. These components must be volatile, with a molecular weight below 1250 Da, and thermally stable so that they do not degrade in the Gas Liquid Chromatography system, in order for GC to be effective in their study.
Gas Chromatography Mass Spectrometry (GC-MS)
Glc chromatography is a commonly used technique in almost every industry: for quality control in the manufacturing of a wide range of products, from automobiles to chemicals to pharmaceuticals; for testing, from meteorite analysis to natural product analysis; and for protection, from environmental to food to forensics. To allow the identification of chemical components, gas chromatographs are frequently hyphenated to mass spectrometers (GC-MS).
Strengths and Limitations of Gas Chromatography
Gas solid chromatography is a commonly used technique in almost every industry. It's used for anything from routine analysis to testing, analyzing a few to hundreds (or thousands with GC x GC) of compounds in a variety of matrices, from solids to gases. It's a reliable method that can be combined with other methods, such as mass spectrometry.
GC can only analyze volatile compounds ranging from helium/hydrogen to about 1250 u in molecular weight. Thermally labile compounds can degrade in a hot GC, so use cold injection techniques and low temperatures to prevent this. Since more polar analytes can get trapped or lost in the GC, the device should be deactivated and well-maintained, or the analytes should be derivatized.
What are the Different Types of Gas Chromatography?
There are many different ways of using chromatography. These are some of the best known:
Paper Chromatography
This is the classic "spot of ink on paper" experiment from school (also the effect we described at the start when you get your papers wet). Typically, a spot of ink is placed on one of the filter paper's edges, and the paper is then hung vertically with the lower edge (nearest the spot) dipped in a solvent such as alcohol or water. The solvent travels up the paper due to capillary action, where it reaches and dissolves the ink. The dissolved ink (the mobile phase) moves up the stationary phase (the stationary phase) and divides into various components. These are often colored; other times, you must color them by adding other substances (referred to as developers or developing fluids) that aid in identification.
Column Chromatography
Instead of paper, the stationary step is a vertical glass jar (the column) filled with a highly absorbent solid like silica crystals or silica gel, or a solid covered in a liquid. The mobile phase drips (or is pumped at high pressure) through the column, splitting into its constituents, which are then extracted and analyzed.
There are Quite a Few Variations, Including:
Liquid-column chromatography is a form of chromatography in which the mixture being examined is put at one end of the column and an eluent (also written eluent) is poured in to help it move through.
Thin-film chromatography is a variant of this technique in which the "column" is simply a thin layer of adsorbent material coated on a film of glass, plastic, or metal.
The mixture is pushed through the column at high pressure in high-performance liquid chromatography (HPLC) (roughly 400 times atmospheric pressure). This is more concise, quicker, and responsive.
Gas Chromatography
We've spoken about chromatography of liquids passing through solids so far, but one of the most common techniques is a form of column chromatography that uses gases as the mobile step. Gas chromatography is a method of chemical analysis that is largely automated and performed using a sophisticated piece of laboratory equipment called, unsurprisingly, a gas chromatograph machine.
Gas Liquid Chromatography
Gas-liquid chromatography (also known as gas chromatography) is an analytical technique with many applications. A stationary phase and a mobile phase are used in all types of chromatography. The mobile phase is a liquid in all of the other types of chromatography you'll encounter at this stage.
The mobile phase of gas-liquid chromatography is a gas such as helium, and the stationary phase is a liquid with a high boiling point adsorbed onto a solid. The amount of time a compound spends traveling with the gas as opposed to being bound to the liquid in any way determines how quickly it moves through the pump.
Stationary Phase in Gas Chromatography
First, a small sample of the mixture of substances being analyzed is pumped into the machine using a syringe. The components of the mixture are heated and vaporize almost immediately. Then we add an eluent (carrier), which is simply a neutral gas like hydrogen or helium that helps the gases in our sample pass through the column. In this case, the column is a thin glass or metal tube filled with a high-boiling-point liquid (or sometimes a gel or an adsorbent solid). The mixture is adsorbed and separated into its constituents as it passes through the column. Each part emerges from the column's end and passes through an electronic detector (sometimes a mass spectrometer) that recognizes it and prints a peak on a map. The final graph has a set of peaks that correspond to each of the mixture's constituents. Vapor-phase chromatography (VPC) or gas-liquid partition chromatography are two terms used to describe gas chromatography (GLPC).
Carrier Gas in Gas Chromatography
For gas chromatography, the carrier gas (mobile phase) should be an inert gas that does not interfere with the sample components. The contribution of the GC carrier gas to the partitioning phase should be negligible. In liquid chromatography, this is different from the mobile phase. The carrier gas in GC is simply described as a transporter for vaporized solute molecules through the column during the partitioning process. Compressible gases that grow as the temperature rises are known as carrier gases. The viscosity of the gas changes as a result of this. The carrier gas's linear velocity and selection can influence resolution and retention times. Carrier gases must be inert to the stationary phase and free of pollutants that can be detected.
FAQs on Gas Chromatography: Principles, Types, and Applications
1. What is the main principle behind Gas Chromatography (GC)?
The core principle of Gas Chromatography is the separation of components in a mixture. This happens based on how the components distribute themselves between two phases. The mobile phase is an inert gas that carries the sample, and the stationary phase is a solid or liquid lining inside a column. Components that interact less with the stationary phase travel faster and are detected first.
2. What are the common real-world applications of Gas Chromatography?
Gas Chromatography is a powerful analytical technique used in many fields. Some common applications include:
- Environmental testing: To detect pollutants in the air, water, and soil.
- Forensic science: To analyse blood and fibre samples from crime scenes.
- Food industry: To check for spoilage, identify flavour and aroma components, and analyse nutritional content.
- Pharmaceuticals: To ensure the purity of drugs and detect any impurities.
- Security: To detect explosives at airports.
3. What are the main components of a Gas Chromatograph?
A typical Gas Chromatograph consists of several key parts working together. These are:
- Gas Supply: Provides the inert carrier gas (mobile phase).
- Injector: Where the sample is introduced and vaporised.
- Column: A long tube containing the stationary phase, where separation occurs. This is housed in an oven to control the temperature.
- Detector: Senses the components as they exit the column and sends a signal.
- Data System: Records the signal from the detector and generates a graph called a chromatogram.
4. What kind of samples can be analysed using Gas Chromatography?
Gas Chromatography is primarily used to analyse chemical compounds that are volatile or can be made volatile. This means the substances must be able to turn into a gas at a reasonable temperature without decomposing. It is ideal for separating and analysing complex mixtures of small organic molecules.
5. What are the two main types of Gas Chromatography?
The two primary types of Gas Chromatography are distinguished by their stationary phase:
1. Gas-Liquid Chromatography (GLC): In GLC, the stationary phase is a non-volatile liquid coated onto a solid support. This is the most common type of GC.
2. Gas-Solid Chromatography (GSC): In GSC, the stationary phase is a solid adsorbent material. This type has more limited applications but is useful for separating low molecular weight gases.
6. Why is an inert carrier gas like Helium or Nitrogen used in Gas Chromatography?
An inert gas is used as the mobile phase because its only job is to carry the sample's components through the column. It must not react with the sample, the stationary phase, or the instrument components. If the carrier gas was reactive, it would alter the chemical nature of the substances being analysed, making the separation and identification impossible.
7. How does the choice of stationary phase affect the separation in GC?
The stationary phase is crucial because separation depends on the different interactions between the sample components and this phase. A polar stationary phase will retain polar components for longer, while a non-polar stationary phase will retain non-polar components. By choosing a stationary phase with properties similar to the components you want to separate, you can achieve a more effective and clear separation.
8. Can Gas Chromatography be used for non-volatile substances like proteins or salts?
No, Gas Chromatography is generally not suitable for non-volatile substances like proteins or salts. The technique relies on the sample being vaporised into a gas to be carried by the mobile phase. Non-volatile substances cannot be turned into gas without decomposing at high temperatures, which would destroy the sample and prevent any meaningful analysis.
9. What is the difference between Gas-Solid Chromatography (GSC) and Gas-Liquid Chromatography (GLC)?
The key difference lies in the stationary phase. In GLC, the stationary phase is a liquid coated on a solid support, and separation occurs based on how components partition between the gas and liquid phases. In GSC, the stationary phase is a solid adsorbent, and separation occurs based on how strongly components are adsorbed onto the solid surface. GLC is more widely used due to its versatility.
10. How is a chromatogram interpreted to identify and quantify substances?
A chromatogram is a graph that plots the detector's response against time. Each peak on the graph represents a different component.
- Identification: The time it takes for a component to reach the detector is its retention time. Comparing this to the retention time of a known standard helps identify the substance.
- Quantification: The area under each peak is proportional to the amount of that component. A larger peak area means a higher concentration of that substance.
