A gas chromatograph temporarily separates the mixture on a column and detects the separated components in turn with a detector. The column is several millimeters in diameter and is filled with a solid adsorbent and a liquid solvent called a stationary phase.
There is also a mobile phase corresponding to immobilization. The mobile phase is a gas in which neither the sample nor the stationary phase reacts, generally nitrogen and hydrogen. The sample of the analytical object is injected into the front of the column by the mobile phase, which carries the sample into the column, so the mobile phase is also called carrier gas.
Carrier gas flows through the column at a continuous flow rate during analysis. The sample is injected once each time, and the analysis results are obtained each time.
The separation of samples through a column is based on differences in thermodynamic properties. The stationary phase has different affinity with the components in the sample (different adsorption capacity in gas chromatography, different solubility in gas liquid partition chromatography). When the carrier gas carries the sample through the column continuously, the components with high affinity move slowly in the column. Because a high affinity means a high tensile force relative to the fixation. Small affinity fast movement. The four cylindrical tubes are actually one, and are only used to represent the states of various components in the sample at different moments.
The sample is a mixture of a, B and C. After the carrier gas is brought into the column, the three are completely mixed as shown in state (I). After a certain time, that is, after the carrier gas has carried them a certain distance in the column, the three begin to separate, as in state (ⅱ).
Further, like states (ⅲ) and (ⅳ), the three are separated. The fixed affinity for them is A>B>C, so the moving speed is C>B>A. The initial component C enters the detector at the back of the column, such as state (ⅳ), followed by B and A.
The detector provides signals corresponding to each component of the input. The sample is injected with carrier gas as the starting point of timing. After separation, each component enters the detector in turn. The maximum signal corresponding to each component (often called the peak value) provided by the detector is called the retention time of each component TR.
In fact, it is proved that the retention time (TR) of different components is also certain under certain conditions (including carrier gas flow rate, material and nature of stationary phase, column length and temperature, etc.).
Therefore, it is possible instead to infer what kind of substance its composition is based on the retention time. Therefore, retention time is the basis for qualitative analysis of chromatographic instruments.
The signal given by the detector to each component shows a peak value in the recorder, which is called chromatographic peak. The maximum value on the chromatographic peak is the basis of qualitative analysis, but the area of the chromatographic peak depends on the content of the corresponding components, so the peak area is the basis of quantitative analysis.
After the injection of a mixture sample, the curve recorded by a recorder is called a chromatogram. Qualitative and quantitative results can be obtained by analyzing chromatograms.
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