The analytical method used to segregate the chemical compounds of a given sample and then detect them to find their presence or absence is known as gas chromatography. It is also used to determine how many chemical compounds are present in a sample mixture.

These chemical compounds are generally organic gases or molecules. For successful analysis, these compounds have to be volatile, generally with a molecular weight less than 1250 Da. They also have to be thermally stable so that they don’t degrade.

Gas chromatography is a commonly used method across most industries for quality assurance in manufacturing products like pharmaceuticals, cars, chemicals, and others. It is also used for purposes of research like analysis of natural products and meteorites. Some of the sample preparation methods for this chromatography are enumerated below.

1.   Pressurized liquid and extraction

The extraction is done with solvents at high temperatures and pressures by not reaching their critical point. It is to achieve effective and rapid extraction of analytes from a solid sample. This extraction is also known as pressurized fluid extraction or accelerated solvent extraction.

Various reviews have been issued, and this technique proves to have notable advantages over other methods. Typically, pressurized liquid extraction focuses mainly on the separation of organic micro-contaminants from aspects in the environment like sewage sludge, soil, and sediment.

These days, the technique is utilized for the analysis of biological and food samples. Generally, higher temperatures will increase the efficiency of extraction due to improved sample wetting, greater diffusion, enhanced penetration of the solvent, and the analyte’s desorption rates from the source to the solvent. Pressure plays the role of keeping the solvent liquid at high temperatures.

2.   Subcritical hot water extraction

Pure water can also be utilized as an alternative to organic solvent for extraction. In such cases, the method is usually known as subcritical hot water extraction. It happens at temperatures between 100 and 374 degrees Celsius and at pressures enough to retain it in the liquid phase. Under these conditions, the polarity can be easily reduced by raising the temperature.

At room temperature and pressure, pure water has a polarity of 79, while raising the temperature to 250 degrees Celsius at a pressure up to 5 MPa results in a significant decline to about 27. It is important to note that water is not a gas chromatography-compatible solvent.

Therefore, the analytes present in the extract should be shifted to a GC-compatible medium, for example, by solid-phase extraction, liquid-liquid extraction, or stir-bar sorptive extraction.

3.   Microwave-assisted extraction

This extraction is widely acknowledged as a versatile extraction method, particularly for solid samples. Micro-assisted extraction uses electromagnetic radiation to enable the desorption of analytes from their matrices.

The region of the microwave is considered to be present at frequencies between 300 MHz to 100 GHz. Even though the entire area is possibly available for usage, all ovens function at 2.45 GHz only. The significant benefits of microwave-assisted extraction are the increased extraction rates owing to the raised temperatures and the rapid heating and the effortless instrument operation.

The primary procedures for energy absorption when it comes to micro-assisted extraction are dipole rotation and ionic conductance. Owing to the electrophoretic movement of ions when there is an application of the microwave field, ionic-conductance heating happens.

The matter’s resistance to this flow will create heat as a result of friction. The dipolar molecules pair electro-statically to the electric field induced by the microwave and tend to line up themselves with it.

Considering that the microwave field is altering in time, the dipole molecules will realign when the field reverses. Therefore, they are in a continuous oscillation at the frequency of the microwave. Frictional forces cause the development of heat because of the movement of dipoles.

4.   Supercritical fluid extraction

The only area that accelerated interest in improved fluid extractions is supercritical fluid extraction. It is a long-established technique, which has been utilized industrially for a considerable number of years.

But it was not before interest was exhibited in supercritical liquids as chromatographic media, it started to be studied seriously as an extraction method on an analytical scale. There have been several reviews and books based on this subject since then.

Almost every supercritical fluid extraction utilizes carbon dioxide as a supercritical fluid. It is an ideal solvent as it merges elevated analyte diffusivities and low viscosity with increased volatility. It makes analyte recovery simple and offers solvent-free concentrates. This process is eco-friendly and also inexpensive.

The essential parts of a supercritical fluid extraction are carbon dioxide pipe, oven for the vessel, supply of higher purity carbon dioxide, pressure outlet, and apt collection vessel for recovering the extracted analytes. Trial collection can be done by purifying the extract with a suitable absorbent like Florisil.

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