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Gas chromatography Definition, principle, working, uses

The gas chromatography process differs from other types of chromatography because it is gas, and the components are separated into vapours. This...

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This article writter by MN Editors on January 28, 2022

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Gas chromatography Definition, principle, working, uses
Gas chromatography Definition, principle, working, uses

What is gas chromatography?

The gas chromatography process differs from other types of chromatography because it is gas, and the components are separated into vapours. This allows it to distinguish and identify smaller molecular weight compounds inside the gas phase. The sample is either liquid or gas which is then vaporized at the port for injection. The mobile phase in gas chromatography is a transporter gas, most often helium, because due to its molecular weight being low and its chemical inert. It is pressed, and the mobile phase is able to move the analyte along the column. The separation process is carried out using a column coated stationary phase.

What is gas chromatography?
A gas chromatograph with a headspace sampler | Source: https://en.wikipedia.org/wiki/Gas_chromatography

Principle of Gas chromatography (how does gas chromatography work)

The gas chromatography equilibrium is partitioning. This means that the constituents of the specimen will split (i.e. divide) between two phases, both the stationary and mobile phase. Chemicals with a higher affinity to the stationary phase are able to spend longer in the column and will elute more slowly and have a greater period of time to retain (Rt) as compared to samples which have a stronger attraction to mobility.

The affinity for the stationary phase is primarily driven by intermolecular interactions . Furthermore, the nature that the phase is stationary could be selected to enhance interactions, and consequently the separation. The ideal for peaks are Gaussian distributions, and symmetrical due to their randomness. analyte’s interactions with the column.

The separation is achieved by dividing the sample between gases and an extremely thin coating an inert liquid that is firmly supported on a solid base. The sample that contains the solutes is then injected into a hot block, that is then immediately evaporated and swept into an oblique vapor plug by the gas carrier stream that flows into the column’s inlet.

The solutes are then absorbed in the stationary part, and later, they are removed by a new carrier gas. The process repeats in every plate as the sample moves towards the outlet. Each sample will move at its own speed throughout the column.

The bands of the molecule will split into distinct zones based on the partition coefficients and bands will spread out. Solutes are eluted one after another , in the ascending number of their kds, and are then absorbed into an detector that is that is attached to the exit of the column.

They record a sequence of signals caused by fluctuations in concentration as well as rates of elution to the recorder, as a plot of time against the composition of the gas stream. The time to appear, the width, height and the area of these peaks are determined to provide precise data.

Parts of Gas chromatography

The gas chromatography process is made up of the following components:

Parts of Gas chromatography
Parts of Gas chromatography | Image Source: Bitesize Bio.

1. Carrier gas in a high-pressure cylinder with attendant pressure regulators and flow meters

Helium N2 Helium, N2, Argon and Argon are utilized as carriers for gases. Helium is a popular choice in thermal conductivity detectors due to due to its superior thermal conductivity in comparison to organic gases. N2 is a good choice in situations where a significant amount of gas carriers is required.

Carrier gas that is released from the tank is pumped through the toggle valve the flow meter (1-1000 milliliters/min) capillary restrictors and an air tension measurement (1-4 atm). The flow rate is controlled through an adjustable needle valve that is mounted at the bottom of the flowmeter, and managed by the capillary limiters. The efficiency of operation that the gaschromatograph achieves is directly dependent on the steady gas flow.

2. Sample injection system

The liquid samples are injected with an injection via a microsyringe that has a needle that is placed through a self-scaling silicon-rubber septum and into an insulated block of metal heated by an resistance heater. Samples of gas are injected through the use of a gas-tight syringe, or the by-pass loop and valves. Sample volumes typically range between 0.1 to 0.2 milliliters.

3. The separation column

The core of gas chromatography process is the column made of metals that are bent into a U shape, or twisted to form an open spiral flake shape. Copper can be used up to 2500. Lock fittings made of Swege allow column installation to be simple. Different sizes of columns can be employed based on the specifications.

4. Liquid phases

An endless variety of liquid phases is available and limited by their thermal stability, volatility and the ability to moisten the support. A single phase cannot be used for all separation issues at all temperatures.

Non-Polar

Parafin, Squalane, silicone greases and apiezon and silica gum rubber. These substances separate the constituents according to their boiling points.

Intermediate Polarity 

They are composed of an polar or polarizable group within a long, non-polar skeleton that can dissolve both non-polar and polar solutes. For example, diethylhexylphthalate is used to separate alcohols with high boiling temperatures.

Polar – Carbowaxes 

Liquid phases with a significant portion of the polar group. Separation of non-polar and polar substance.

Hydrogen bonding 

Polar liquid phases that have strong hydrogen bonds e.g. Glycol.

Specific purpose phases 

Relying on chemical reactions using a solute to create separations. e.g AgNO3 in glycol separates unsaturated hydrocarbons.

5. Supports

The surface and structure of the materials used for support are crucial variables that affect the effectiveness of the support and its level of separation. The support must be impermeable, however, it should be capable of retaining an enormous amount of liquid phase in thin films on its surface.

The surface area must be substantial to allow for that equilibrium is quickly achieved between the stationary and mobile phases. The support must be robust enough to withstand the stress of handling, and also be able of being creating a uniform bed. Diatomaceous earth kieselguhr treated with Na 2CO 3 at 9000 C results in the particle to fuse into finer aggregates.

Glass beads that have a small volume and low porosity may be used to cover as much as 3% of stationary phases. Polymer beads with pores that differ in the amount of cross-linking between styrene and alkyl-vinylbenzene are also utilized that are stable for up to 2500

6. Detector

Detectors detect the presence of the components that are separated and then provide signals. These are either concentration-dependent or mass dependant. The detector must be located close to the column’s exit point and at the proper temperature to prevent decomposition.

7. Recorder

The recorder should generally be 10 mV (full scale) equipped with a quick response pen (1 second at most). The recorder must be connected to a set of high quality resistors to the inputs in order to reduce the big signals. An integrator could be an ideal option.

The procedure of Gas Chromatography

Step 1: Sample Injection and Vapourization

  1. A small amount of the liquid sample to be examined is drained into the Syringe.
  2. The needle in the syringe is set in the hot injector channel of the gaschromatograph. the sample is quickly injected.
  3. The time of the sample’s injection is thought of as an “point” in time, which means it is believed that every sample will enter the gas chromatograph simultaneously and therefore the sample needs to be injected rapidly.
  4. It is designed to have a temperature greater than the boiling point of the components in the mixture, so that the components begin to evaporate.
  5. The vaporized components mix with the gas mobile phase of inert gas and are then transported into the column of gas chromatography, where they will be separated.

Step 2: Separation in the Column

  1. The components in the mix are separated according to their ability to adsorb onto, or bond to in the stationary phase.
  2. A substance that adsorbs the strongest heavily to stationary phases will stay most duration in the column (will be kept within the column the most amount of time) and, consequently be the most retentive duration (Rt). It will be released from the gas chromatograph at the end of the process.
  3. The component that adsorbs most strongly towards the stationary phase is likely to stay most time inside the column (will remain by the column over the most period of time) and, consequently be the one with the shortest period of retention (Rt). It will come out of the gas chromatograph the first.
  4. If we look at a 2- components mixture in which components A and B are more extreme than B, then:
  • component A will have a longer retention time in a polar column than component B
  • component A will have a shorter retention time in a non-polar column than component B

Step 3: Detecting and Recording Results

  1. The constituents of the mixture enter into the detector with different timings because of variations in the length of time they remain within the column.
  2. The part that is retained with the shortest amount of period in the column is discovered first. The part that is retained for longest inside the column will be discovered the last.
  3. The detector transmits a signal for the chart recorder, which produces a peak appearing on the paper chart. The part that is first detected gets recorded as the first. The component that is discovered the last time is recorded as the last.

Applications

  • The analysis of GC is utilized to calculate the quantity of a chemical substance such as in ensuring the quality of chemicals used that are used in the chemical industry or to determine the presence of harmful substances in soils or air.
  • The use of gas chromatography can be found for the analysis of:
    • air-borne pollutants 
    • performance-enhancing drugs in athlete’s urine samples 
    • oil spills 
    • essential oils in perfume preparation
  • The GC method is extremely accurate when utilized correctly, and it can be used to measure picomoles of a substance within 1 ml of liquid samples or parts-per-billion concentrations of gaseous samples.
  • Gas Chromatography is widely used in the field of Forensic Science. Different disciplines, such as the solid drug dosage (pre-consumption form) identification and quantification arson investigation as well as paint chip analysis and toxicology investigations, use GC to determine and quantify different biological specimens and crime scene evidence.

Advantages

  • The usage of longer columns as well as greater velocity of the carrier gas makes it possible to separate the gas within several minutes.
  • Working temperatures that can reach 5000C as well as the potential of turning any substance to a volatile element makes gas chromatography among the most flexible methods.
  • GC is used extensively in industrial and environmental monitoring applications due to the fact that it is extremely robust and can utilized for nearly continuous.
  • The most common use for GC is in situations where small, volatile molecules are detected , and using non-aqueous solutions.
  • GC is preferred to be used for molecules that are non-polar.

Limitations

  • The compound to be studied must remain stable in GC operating conditions.
  • They must possess a vapour pressure that is significantly more than zero.
  • The majority of the compounds studied typically have less than 1,000 Da due to the fact that it is difficult to evaporate larger substances.
  • The samples also have to be salt-freeand must not contain ions.
  • Small amounts of a chemical can be measured however, it is usually essential that the specimen be compared to the sample that contains the substance that is pure and believed to be also known as the reference standard.

Common problems with gas chromatography

The most frequent issue that occurs in GC can be caused by leaks. Mobile phase is a gas that flows through the entire system, so the proper installation of components and consumables is essential as is regular leak testing.

Activity is a different issue for more polar analytical substances, particularly at the trace level. Silanol-based groups on glass column and liners, as well as a build-up of dirt within the system may cause tailing peaksand irreparable catalytic breakdown, or adsorption. Inlet is the region which causes the most issues since it is where when the test sample gets injected, evaporated and transferred to the GC column. So, regular maintenance of the inlet and using the right consumables, like an inlet liner that is deactivated, is essential to ensure that the instrument in good working order.

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