gas chromatography working principle

Gas chromatography is an experimental technique that separates the mixture into individual components. It is used to identify and quantify the components of the sample. After the sample passes through the detector, the chromatogram is recorded. Each peak represents a different component from the initial mixed sample. The time at which a peak appears is called retention time and can be used to characterize each component, and the size of the peak (peak height or area) is a measure of the amount of component content.

Basic structure

The basic structure of a gas chromatograph consists of two parts, an analysis unit and a display unit. The former mainly include gas sources and control metering devices, sample feeding devices, thermostats and columns. The latter mainly includes the detector and the automatic recorder. Columns and calibrators are the core components of a gas chromatograph.

（1）Pneumatic system The gas path in the gas chromatograph is a closed circuit system in which the carrier gas continuously operates. The entire gas circuit system requires pure carrier gas, good hermeticity, stable flow velocity, and accurate flow rate measurement.

（2）Injection System Injection is the uniform and quantitative addition of a gas or liquid sample to the top of the column.

（3）At the heart of the separation system is the column, which acts to separate the multi-component sample into individual components. Columns are divided into packed and capillary columns.

（4）The function of the detection system detector is to convert the components of the sample separated by the chromatographic column into electrical signals according to their characteristics and contents. After amplification, they are recorded by the recorder as chromatograms.

（5）Signal recording or microcomputer data processing systems In recent years, chromatographic data processors have mainly used chromatographs. Chromatographic data processors can print the recorded chromatograms and print the processed results on the same recording paper, such as retention time, mass fraction of measured components, and more.

（6）Temperature Control System Used to control and measure the temperature of the column, detector, and gasification chamber is an important part of the gas chromatograph. Gas chromatograph is divided into two categories: one is gas-solid chromatograph, and the other is gas-liquid chromatograph. The two types of chromatographs have different stationary phases, but the structure of the instrument is universal.

Working principle

The chromatograph uses a column to separate the mixture first, and then uses the detector to sequentially detect the separated components. Columns are several millimeters in diameter, filled with a solid adsorbent or liquid solvent, and the adsorbent or solvent filled is called the stationary phase. Corresponding to the stationary phase there is also a mobile phase. The mobile phase is a gas that does not react with both the sample and the stationary phase and is generally nitrogen or hydrogen.　 The sample to be analyzed is injected into the mobile phase at the top of the column. The mobile phase carries the sample into the column, so the mobile phase is also called the carrier gas. During the analysis, the carrier gas flows continuously through the column at a constant flow rate; the sample is injected only once at a time, and the analysis results are obtained for each injection. The separation of the sample in the column is based on the difference in thermodynamic properties. The stationary phase and the components in the sample have different affinities (different adsorptive forces for gas-solid chromatograph and different solubility for gas-liquid chromatograph). When the carrier gas continues to pass through the column with the sample, the components with high affinity move slowly in the column because the high affinity means that the stationary phase has a large force to pull it. Affinity is faster when moving. The four column tubes are actually one, just used to indicate the state of each component in the sample at different instants. The sample is a mixture of A, B, and C components. When the carrier gas just brought them into the column, the three were completely mixed. After a certain period of time, after the carrier gas has taken them a certain distance in the column, the three begin to separate. Then proceed, the three will be separated.

The signal given by the detector for each component appears on the recorder as a single peak, called the chromatographic peak. The maximum value on the chromatographic peak is the basis for qualitative analysis, and the area covered by the chromatographic peak depends on the content of the corresponding component, so the peak area is the basis for quantitative analysis. After a mixture sample is injected, the curve recorded by the recorder is called a chromatogram. Analyze the chromatogram to obtain qualitative analysis and quantitative analysis results.

The carrier gas is provided by the carrier gas cylinder and is passed through the carrier gas flow control valve to stabilize the flow and the rotameter to detect the flow rate to the sample vaporization chamber. The sample vaporizer has a heating coil to vaporize the liquid sample. If the sample to be analyzed is gas, the vaporization chamber does not have to be heated. The vaporization chamber itself is the injection chamber, and the sample can be injected into the carrier gas. The carrier gas enters the column with the injected sample from the injection port, enters the detector after separation, and is then vented. After the signal from the detector is amplified, the chromatogram of the sample is recorded by the recorder.

Gas chromatograph is a multi-component mixture separation and analysis tool. It is a column chromatographic technique using gas as mobile phase and flushing method. When the multi-component analysis material enters the column, the partition coefficient between the gas phase and the stationary liquid phase of the components in the column is different, so the running speed of each component in the column is different. After the column is long, it leaves the column in sequence and enters the detector. After being detected, it is converted into an electrical signal and sent to the data processing station. This completes the qualitative and quantitative analysis of the measured substance.