GAS CHROMATOGRAPHY: HOW IT WORKS, TYPES, PARTS, USES - CHEMISTRY - 2024

The gas chromatography (GC) is an instrumental analytical technique for separating and analyzing components of a mixture. It is also known as gas-liquid partition chromatography, which, as will be seen later, is the most appropriate to refer to this technique.

In many areas of scientific life, it is an indispensable tool in laboratory studies, since it is a microscopic version of a distillation tower, capable of generating high quality results.

Source: Gabriel Bolívar

As its name indicates, it uses gases in the development of its functions; more precisely, they are the mobile phase that carries the components of the mixture.

This carrier gas, which in most cases is helium, travels through the interior of a chromatographic column, while at the same time all the components end up separating.

Other carrier gases used for this purpose are nitrogen, hydrogen, argon, and methane. The selection of these will depend on the analysis and the detector coupled to the system. In organic chemistry, one of the main detectors is the mass spectrophotometer (MS); therefore, the technique acquires the CG / EM nomenclature.

Thus, not only are all the components of the mixture separated, but their molecular masses are known, and from there, their identification and quantification.

All samples contain their own matrices, and as chromatography is capable of "clarifying" it for study, it has been an invaluable aid for the advancement and development of analytical methods. And in addition, together with multivariate tools, its scope could be raised to unsuspected levels.

How does gas chromatography work?

How does this technique work? The mobile phase, whose maximum composition is that of the carrier gas, drags the sample through the interior of the chromatographic column. The liquid sample needs to be vaporized, and to ensure this, its components must have high vapor pressures.

Thus, the carrier gas and the gaseous sample, volatilized from the original liquid mixture, constitute the mobile phase. But what is the stationary phase?

The answer depends on the type of column with which the team works or demands the analysis; and in fact, this stationary phase defines the type of CG considered.

Separation

The central image represents in a simple way the operation of separation of the components within a column in CG.

The carrier gas molecules were omitted so as not to be confused with those of the vaporized sample. Each color corresponds to a different molecule.

The stationary phase, although it appears to be the orange spheres, is actually a thin film of liquid that wets the inner walls of the column.

Each molecule will dissolve or distribute differently in said liquid; those that interact the most with it are left behind, and those that do not, advance more quickly.

Consequently, the separation of the molecules occurs, as shown by the colored dots. It is then said that the purple dots or molecules will elude first, while the blue ones will come out last.

Another way of saying the above is this: the molecule that eludes first has the shortest retention time (T _R).

Thus, one can identify these molecules are by direct comparison of their T _R. The efficiency of the column is directly proportional to its ability to separate molecules with similar affinities for the stationary phase.

Detection

Once the separation is finished as shown in the image, the points will elude and will be detected. For this, the detector must be sensitive to disturbance or physical or chemical changes caused by these molecules; and after this, it will respond with a signal which is amplified and represented through a chromatogram.

It is then in the chromatograms where the signals, their shapes and heights as a function of time can be analyzed. The example of the colored dots should originate four signals: one for the purple molecules, one for the green ones, another for the mustard colored ones, and a last signal, with a higher T _R, for the blue ones.

Suppose the column is poor and cannot separate the blueish and mustard colored molecules properly. What would happen? In this case, not four elution bands would be obtained, but three, since the last two overlap.

This can also happen if the chromatography is performed at too high a temperature. Why? Because the higher the temperature, the higher the speed of migration of the gaseous molecules, and the lower their solubility; and therefore its interactions with the stationary phase.

Types

There are essentially two types of gas chromatography: CGS and CGL.

CGS

CGS is the acronym for Gas-Solid Chromatography. It is characterized by having a solid stationary phase instead of a liquid one.

The solid must have pores of a diameter controlled by where the molecules are retained as they migrate through the column. This solid is usually molecular sieves, like zeolites.

It is used for very specific molecules, since CGS generally faces several experimental complications; for example, the solid can irreversibly retain one of the molecules, completely altering the shape of the chromatograms and their analytical value.

CGL

The CGL is Gas-Liquid Chromatography. It is this type of gas chromatography that covers the vast majority of all applications, and is therefore the more useful of the two types.

In fact, the CGL is synonymous with gas chromatography, even though it is not specified which one is talking about. Hereinafter only mention will be made of this type of CG.

Parts of a gas chromatograph

Source: No machine-readable author provided. Dz assumed (based on copyright claims)., via Wikimedia Commons

A simplified schematic of the parts of a gas chromatograph is shown in the image above. Note that the pressure and flow of the carrier gas stream can be regulated, as well as the temperature of the furnace that heats the column.

From this image you can summarize the CG. A stream of He flows from the cylinder, which depending on the detector, one part is diverted towards it and the other is directed to the injector.

A microsyringe is placed in the injector with which a volume of sample in the order of µL is released immediately (not gradually).

The heat from the furnace and the injector must be high enough to instantly evaporate the sample; unless a gaseous sample is injected directly.

However, the temperature cannot be too high either, since it could evaporate the liquid in the column, which functions as a stationary phase.

The column is packed as a spiral, although it can also have a U-shape. As the sample travels the entire length of the column, it reaches the detector, whose signals are amplified, thus obtaining the chromatograms.

Column

In the market there are an infinity of catalogs with multiple options for chromatographic columns. The selection of these will depend on the polarity of the components to be separated and analyzed; if the sample is apolar, then a column with a stationary phase that is least polar will be chosen.

The columns can be of the packed or capillary type. The column of the central image is capillary, since the stationary phase covers its internal diameter but not the entire interior of it.

In the packed column, the entire interior has been filled with a solid which is usually firebrick dust or diatomaceous earth.

Its outer material consists of either copper, stainless steel, or even glass or plastic. Each one has its distinctive characteristics: its mode of use, length, the components that it best manages to separate, the optimal working temperature, the internal diameter, the percentage of stationary phase adsorbed on the solid support, etc.

Detector

If the column and furnace are the heart of the GC (either CGS or CGL), the detector is its brain. If the detector does not work, there is no point in separating the components of the sample, as you will not know what they are. A good detector must be sensitive to the presence of the analyte and respond to most of the components.

One of the most used is thermal conductivity (TCD), it will respond to all components, although not with the same efficiency as other detectors designed for a specific set of analytes.

For example, the flame ionization detector (FID) is intended for samples of hydrocarbons or other organic molecules.

Applications

-A gas chromatograph cannot be missing in a forensic or criminal investigations laboratory.

-In the pharmaceutical industry it is used as a quality analysis tool in search of impurities in the batches of manufactured drugs.

-Helps to detect and quantify drug samples, or allows analysis to check if an athlete was doped.

-It is used to analyze the amount of halogenated compounds in water sources. Likewise, the level of contamination by pesticides can be determined from soils.

-Analyze the fatty acid profile of samples of different origins, whether vegetable or animal.

-Transforming biomolecules into volatile derivatives, they can be studied by this technique. Thus, the content of alcohols, fats, carbohydrates, amino acids, enzymes and nucleic acids can be studied.

References

1. Day, R., & Underwood, A. (1986). Quantitative Analytical Chemistry. Gas-liquid chromatography. (Fifth ed.). PEARSON Prentice Hall.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill, p577-578.
3. Skoog DA & West DM (1986). Instrumental Analysis. (Second edition). Interamerican.
4. Wikipedia. (2018). Gas chromatography. Recovered from: en.wikipedia.org
5. Thet K. & Woo N. (June 30, 2018). Gas chromatography. Chemistry LibreTexts. Recovered from: chem.libretexts.org
6. Sheffield Hallam University. (sf). Gas chromatography. Recovered from: teaching.shu.ac.uk