On-line gas chromatographThe mixture is first separated using a chromatographic column, and then the separated components are sequentially detected using a detector. The column is a few millimeters in diameter, and the adsorbent or solvent filled with solid adsorbent or liquid solvent is called the stationary phase.

The other mobile phase corresponds to the stationary phase. The mobile phase is a gas, usually nitrogen or hydrogen, that reacts with the sample and the stationary phase. The sample to be analyzed injects the mobile phase into the top of the column, and the mobile phase brings the sample into the column, so the mobile phase is also called a carrier gas.

In the on-line gas chromatograph analysis process, the carrier gas flows continuously through the column at a certain flow rate; however, the sample is only injected one at a time, and the analysis results will be obtained with each injection.

Depending on the thermodynamic properties, the sample can be separated in a chromatographic column. The stationary phase has different affinities with the components in the sample (different adsorption forces for gas-solid chromatographs and different solubilities for gas-liquid partition chromatographs).

When the carrier gas is continuously passed through the column with the sample, the component with greater affinity moves slowly in the column, because greater affinity means greater force of the stationary phase pull. People with little affinity move quickly. The four column tubes are actually one, just used to represent the state of each component in the sample at different times.

The sample is a mixture of three components consisting of A,B, and C. When the carrier gas carries them into the column, the three are completely mixed, such as state, (I). After a period of time, that is, after the carrier gas takes them for a distance in the column, the three begin to separate, such as state (II).

If we move on, the three will separate, such as states (III) and (IV). Fixation is relative to their affinity A>B>C, so the speed of movement is C>B>A. The leading component C first enters the detector behind the column, for example state (IV), and then B and A enter the detector in turn.

The detector sends a corresponding signal to each component that enters. From the time the sample is injected into the gas as the starting point of the timing, the detector gives a signal (usually called the peak value) for each component, called the retention time tr of each component.

Practice has proved that the retention time of different components is also positive under certain conditions (including airflow velocity, materials and properties of stationary phase, length and temperature of chromatographic column, etc.).

So, in turn, you can infer from the retention time what kind of substance the composition is. Therefore, the retention time can be used as a basis for qualitative analysis.

The detector displays the signal of each component on a recorder, called a chromatographic peak. The value of the chromatographic peak is the basis of qualitative analysis, and the area of the chromatographic peak depends on the content of the corresponding component, so the peak area is the basis of quantitative analysis.

After a mixed sample is injected, the curve recorded by the tape recorder is called the chromatogram. Qualitative analysis and quantitative analysis results can be obtained by analyzing the chromatogram.