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Basic principle and structure of gas chromatograph

current news 2022-03-30 1982

One, the basic principle of gas chromatograph: 

Chromatographic method, also known as chromatography or chromatography, is a physical and chemical separation method using the properties of material solubility and adsorption. The separation principle is based on the difference between the action of each component in the mixture between the mobile phase and the stationary phase. 

The chromatographic method using Gas as mobile phase is called Gas Chromatography (GC). GC is a highly mechanized chromatographic method, which is widely used in the quantitative analysis of small molecular weight complex components. 

Mobile phase: The fluid that carries samples through the system, also known as carrier gas. 

Stationary phase: stationary phase, support, stationary solution, and filler in a column.

Two, gas chromatograph composition: 

Gas chromatograph is mainly composed of gas path system, sampling system, separation system, detection and temperature control system, recording system. 

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FIG. 1. Structure diagram of gas chromatograph

1. Gas path system of gas chromatograph 

Gas chromatograph gas path system includes gas source, purification drying pipe and carrier gas flow rate control device, is a carrier gas continuous operation of the closed pipeline system, through the gas chromatograph gas path system to obtain pure, stable flow rate of carrier gas. The gas tightness of gas path system, the accuracy of flow monitoring and the stability of carrier gas flow rate are the important factors affecting the performance of gas chromatograph. 

Gas chromatograph commonly used in the carrier gas hydrogen, nitrogen and argon, purity requirements of 99.999% or more, good chemical inertia, and do not react with the components to be measured. The selection of carrier gas requires not only the separation effect of the components to be tested, but also the detector sensitivity of the components to be tested under different carrier gas conditions. 

2. Gas chromatograph injection system 

The injection system of gas chromatograph mainly consists of injector and gasification chamber. 

(1) Sampler: According to the phase state of the components to be measured, different sampler is used. Flat head microinjector is generally used for liquid sample injection, as shown in FIG. 2. Gas samples are usually injected using a rotary six-way valve or a pointed microinjector supplied with the chromatograph, as shown in FIG. 2.

气体、液体进样器2.png

Figure 2. Gas and liquid sampler

Solid samples are generally dissolved in an appropriate reagent and then injected in liquid form with a microsyringe. 

(2) gasification chamber: the gasification chamber is generally made of a stainless steel tube, and there is a heating wire around the tube. The function is to instantly completely vaporize the liquid sample into steam. The heat capacity of the gasification chamber should be large enough and there is no catalytic effect to ensure that the sample is instantly vaporized and does not decompose in the gasification chamber. 

3. Separation system of gas chromatograph 

The gas chromatograph separation system is the core part of the gas chromatograph, the role is to be tested in the sample of each component of the separation. 

The separation system consists of column box, chromatographic column and temperature control component. 

Chromatographic columns mainly have two types: packed column and capillary column, column materials include metal, glass, fused quartz, ptfe, etc. 

The separation effect of chromatographic column is not only related to column length, column diameter and column shape, but also related to the type of column packing and operating conditions. 

4. Detection system of gas chromatograph 

The detection system of gas chromatograph is the separation of components in the chromatographic column in accordance with the change of concentration into electrical signals, after the amplifier, the electrical signals to the recorder, by the recorder draw the chromatographic outflow curve. 

The performance of the detector will directly affect the accuracy of the final analysis results of the chromatograph. 

According to the response principle of the detector, it can be divided into concentration detector and mass detector. 

(1) Concentration detector: The transient concentration change of the component to be measured in the carrier gas is measured, that is, the response signal of the Detector is proportional to the concentration of the component to be measured, such as the Thermal Conductivity Detector (TCD) and the Electron Capture Detector (ECD). 

(2) Mass detector: The change of the velocity of the sample in the carrier gas entering the Detector is measured, that is, the response signal of the Detector is proportional to the mass of the component entering the Detector in unit time. For example, the hydrogen Flame Ionization Detector, FID) and Flame Photometric Detector (FPD). 

Temperature control system of gas chromatograph 

In gas chromatograph, temperature control is very important, which directly affects the separation efficiency of chromatograph column, sensitivity and stability of detector. 

The main objects of the temperature control system are the gasification chamber, column and detector. 

In the gasification chamber, the liquid sample should be instantly completely vaporized, and in the column chamber, the components should be completely separated. When there are various components to be measured in the sample, the temperature change of the chamber needs to be controlled by the program. Each component should be separated at the optimal temperature, and ensure that there is no condensation of each component in the sample when passing through the detector. 

The temperature control method of gas chromatograph is divided into two kinds: constant temperature and programmed temperature: 

(1) Constant temperature control mode: for simple samples with narrow boiling range, constant temperature mode can be used. Simple gas analysis and liquid sample analysis are conducted in constant temperature mode. 

(2) temperature-programmed temperature control method: refers to in an analysis cycle, the temperature of the chromatographic column in the gas chromatograph with time from low temperature to high temperature is a step change, so that the components of different boiling points flow out at the optimal column temperature, so as to improve the separation effect and shorten the analysis time. 

For complex samples with wide boiling range, if it is difficult to achieve ideal separation effect at constant temperature, temperature-programmed method should be used. 

6. Gas chromatograph recording system 

Gas chromatograph recording system is mainly used for gas chromatograph recording detector detection signal, and quantitative data processing and recording. Some gas chromatographs are also equipped with electronic computers, which can automatically measure the chromatographic peak area, directly provide accurate data for quantitative analysis, and can automatically reprocess the chromatographic analysis data.

Three, gas chromatograph analysis process 

Gas chromatograph of carrier gas flow in the high pressure cylinder, the required pressure relief valve to gas chromatograph, after purification by drying tube make the carrier gas purification, then through voltage regulator valve and meter, to stabilize the pressure, constant speed through gasification chamber after mixed with samples of gasification has been completed, the sample gas to in the chromatographic column separation. After separation, each component flows into the detector successively for detection. 

The detector converts the concentration or mass change of the components to be measured into electrical signals, which are recorded on the recorder after amplification, and the chromatographic outflow curve can be obtained. Qualitative analysis can be carried out according to the retention time on the chromatographic outflow curve, and quantitative analysis can be carried out according to the peak area or peak height. The specific analysis process is shown in Figure 3. 

气相色谱仪的分析流程3.png

Figure 3. Analysis flow of gas chromatograph

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