Classification and selection of gas chromatographic columns

Gas chromatographic column is one of the core components of gas chromatograph. In gas chromatography analysis, the selection of chromatographic column is very important. The properties of components to be tested and experimental conditions (such as column temperature and column pressure) should be considered.
When we look up data, we often see that gas chromatography is divided into gas-solid chromatography and gas-liquid chromatography, and the gas chromatography column is divided into capillary column and packed column. What conditions are they divided into? What's the difference between them? How to choose capillary column and packed column in analysis?
The classification of gas-solid chromatography and gas-liquid chromatography is based on the physical state of stationary phase in chromatographic column. Solid adsorbent is used as stationary phase in gas-solid chromatography, and its separation is mainly based on adsorption mechanism; while in gas-liquid chromatography, liquid is used as stationary phase, and its separation is mainly based on distribution mechanism.
In the work, some analysts equate gas-solid chromatography with packed column chromatography and gas-liquid chromatography with capillary chromatography, which is a very wrong understanding.
In fact, gas-solid chromatography can be either packed column chromatography or capillary chromatography, as well as gas-liquid chromatography. The stationary phase of packed gas-solid chromatographic column is granular solid adsorbent, while the stationary phase of packed gas-liquid chromatographic column is fixed liquid film coated on inert solid particles (carrier or supporter). Capillary gas-solid chromatographic column generally refers to the porous layer open tubular column (plot), the inner wall of which is only coated with a layer of porous adsorbent particles. Other kinds of capillary column belong to gas-liquid chromatography.
Most textbooks and Monographs on the classification of gas chromatographic column are simply divided into packed gas chromatographic column and capillary gas chromatographic column according to the size and length of chromatographic column inner diameter. It is pointed out that the inner diameter of the former is 2-4mm and the length is 1-10m; the latter is 0.2-0.5mm, and the length is generally 25-100m. But this kind of classification is actually not rigorous, only a kind of superficial distinction.
In a strict sense, the corresponding packed gas chromatographic column should be an open tube gas chromatographic column. The difference between the two lies in the different loading methods of the stationary phase in the chromatographic column. In the packed column, the stationary phase is the granular adsorbent loaded in the column (gas-solid chromatography) or the stationary liquid coated on the inert solid particles (gas-liquid chromatography); while in the open tube gas chromatography column, the solid phase is coated or chemically crosslinked on the inner wall of the capillary tube, so the open tube column is also commonly called It is a capillary column, but the capillary column is not always open.
As a matter of fact, capillary column can be divided into packed column and open column. The packed capillary column was developed later. Some porous solid particles were put into thick wall glass tube, and then heated and drawn into capillary tube. The inner diameter of capillary column is 0.25-0.5mm.
In view of these differences between packed column and capillary column, their application scope in analytical application is also very different. When to use packed column and when to use capillary column, it must be considered seriously for analysts.
Compared with packed column, capillary column has higher separation efficiency
① The inner diameter of the capillary column is small, generally 0.1-0.7mm. The fixed liquid film on the inner wall is very thin and the center is empty, so the resistance is very small, and the vortex diffusion term does not exist, and the spectral band broadening becomes smaller. Because the resistance of capillary column is very small, its length can be dozens of times of that of packed column, and its total column efficiency is much higher than that of packed column. Generally speaking, a 30 m long capillary column can easily achieve 100000 plate count, while a 3 m long packed column can only achieve a total column efficiency of 4500 at most.
② The analysis speed of capillary column is about ten times of that of packed column. Because the liquid film is very thin, the distribution ratio K is very small and the ratio is large, the mass transfer rate of components in the stationary phase is very fast, so it is beneficial to improve the column efficiency and analysis speed. It can separate more than 100 kinds of gasoline components in one hour and more than ten kinds of compounds in a few minutes. Therefore, capillary column is more advantageous to the separation of complex samples, such as simultaneous detection of pesticide residues in agricultural products, detection of organic pollutants in environmental samples, and detection of flavors in cosmetics.
Of course, capillary column also has its limitations. Because of its small inner diameter, small column capacity, and higher requirements for injection technology, carrier gas flow rate control requirements are more accurate. The smaller the injection volume is, the higher the sensitivity of the detector is required. Therefore, in consideration of the cost and economic benefits of the analytical work, it is recommended to use a packed gas chromatographic column for simple permanent gas analysis and low molecular weight organic compounds analysis.
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