The Working Principle and Structure of Gas Chromatograph

* The main components of a gas chromatograph

1. Carrier Gas System

The carrier gas system includes a gas source, a gas purifier, and a gas circuit control system. The carrier gas is the mobile phase of the gas chromatography process. In principle, as long as it is not corrosive and does not interfere with the sample analysis, it can be used as the carrier gas. Commonly used are H2, He, N2, Ar and so on. In practical applications, the choice of carrier gas is mainly determined according to the characteristics of the detector, taking into account the separation efficiency and analysis time of the chromatographic column. The purity and flow rate of the carrier gas have a great influence on the separation efficiency of the chromatographic column and the sensitivity of the detector. The function of the gas circuit control system is to stabilize, stabilize and purify the carrier gas and auxiliary gas to meet the requirements of gas chromatography analysis. requirements. When choosing the gas purity, it mainly depends on the analytical object, the packing in the chromatographic column and the detector, and the gas with higher purity should be selected as much as possible. This will not only increase (maintain) the high sensitivity of the instrument, but also prolong the life of the column and the entire instrument (pneumatic control components, gas filters). Practice has proved that, as a medium-sized instrument, it is sometimes difficult to restore the high sensitivity of the instrument when a low-purity gas source is used for a long time. For low-end instruments, high-purity gas is used for constant or semi-trace analysis, which not only increases the operating cost, but also increases the complexity of the gas circuit sometimes, and is more prone to air leakage or other problems that affect the normal operation of the instrument.

2. Sampling System

The sample introduction system includes a sampler and a vaporization chamber, its function is to introduce the sample and vaporize the sample instantaneously. The gas sample can be injected with a six-port valve, and the injection volume is controlled by a quantitative tube, which can be replaced as needed, and the repeatability of the injection volume can reach 0.5%. Liquid samples can be injected with a micro-syringe, and the repeatability is relatively poor. When using, use it under the capacity of the syringe. Autosamplers are commonly used in industrial process chromatographic analysis and routine analysis of large batches of samples, with good repeatability. In capillary column gas chromatography, due to the small sample capacity of the capillary column, a split injector is generally used, and the injection volume is relatively large. After vaporization, only a small part of the sample is carried into the chromatographic column by the carrier gas, and most of it is vented. The function of the vaporization chamber is to instantly heat the liquid sample into vapor, which is then brought into the chromatographic column by the carrier gas.

3. Separation System

The separation system is mainly composed of a chromatographic column and is the heart of a gas chromatograph. Its function is to separate the sample while it is running in the column. There are basically two types of chromatographic columns: packed columns and capillary columns. Packed columns are stationary phases packed in metal or glass tubes (usually 4 mm inner diameter). Capillary columns are hollow tubes drawn from fused silica, also known as elastic quartz capillaries. The inner diameter of the column is usually 0.1 mm to 0.5 mm, the column length is 30 m to 50 m, and the column is wound into a ring shape with a diameter of about 20 cm. Gas chromatography using such a capillary as a separation column is called capillary gas chromatography or open-column gas chromatography, and its separation efficiency is much higher than that of packed columns. It can be divided into open capillary column, packed capillary column, etc. Packed capillary columns are made by filling capillaries with stationary phase, or loose carriers or adsorbents can be loaded into thicker thick-walled glass tubes, and then drawn into capillaries. If the carrier is loaded, apply the fixative solution on the carrier before use to form a packed capillary column for gas-liquid chromatography. If an adsorbent is loaded, it is a packed capillary column for gas-solid chromatography. This capillary column has not been used much in recent years. Open-tube capillary columns are divided into the following four types:

① Wall-coated capillary columns. The inner wall of the hollow quartz capillary with an inner diameter of 0.1mm to 0.3mm is coated with a fixative solution, which is the most used capillary column at present.

② carrier-coated capillary column. First, a layer of diatomaceous earth carrier is attached to the inner wall of the capillary, and then the fixative is coated on the carrier.

③ Small inner diameter capillary column. Capillary columns with an inner diameter of less than 0.1mm are mainly used for rapid analysis.

④ Large inner diameter capillary column. Capillaries with an inner diameter of 0.3 mm to 0.5 mm are often coated with a thick liquid film of 5 μm to 8 μm on their inner walls.

4. Detector

The function of the detector is to convert the information of the components that have been separated after the column into electrical signals that are easy to record, and then identify and measure the composition and content of each component, which is the eye of the chromatograph. In principle, any difference in the properties of the components to be measured and the carrier gas can be used as a basis for designing detectors, but there are only a few detectors commonly used in practice. They are simple in structure, easy to use, and have versatility or selectivity. . The choice of detector depends on the analysis object and purpose.

5. Data Processing System

At present, the data processing system mostly adopts a workstation equipped with an operating software package, and is controlled by a computer, which can not only automatically process the chromatographic data, but also automatically control the parameters of the chromatographic system.