Practical gas chromatographic working principle

The chromatograph uses a column to separate the mixture first, and then uses the detector to sequentially detect the separated components. The gas chromatograph of the column is several millimeters in diameter, filled with a solid adsorbent or a liquid solvent, and the adsorbent or solvent filled is called a 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 chromatography 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 moments. 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, as in state (I). After a certain period of time, after the carrier gas has taken them a certain distance in the column, the three begin to separate, as in state (II). Then move on, the three will be separated, as states (III) and (IV). The fixed relative to their affinity is A>B>C, so the moving speed is C>B>A. The most advanced component C first enters the detector immediately after the column, such as state (IV), and then A and B also sequentially enter the detector. The detector gives a corresponding signal for each incoming component. The carrier gas is injected from the sample as a timing starting point, and after the components are separated, they enter the detector in sequence. The detector gives the time corresponding to the maximum signal of each component (often referred to as the peak value) and is called the retention of each component. Time tr. Practice has proved that when the conditions (including carrier gas flow rate, stationary phase materials and properties, column length and temperature, etc.) are certain, the retention time tr of different components is also certain. Therefore, in turn, it can be inferred from the retention time what the component is. Therefore, retention time can be used as a basis for qualitative analysis of chromatographic instruments.

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 structure of c in the figure. 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 measure 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 with carrier gas via it. The carrier gas enters the column with the injected sample from the inlet, 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.