The determination steps of gas chromatography include sample preparation and pretreatment, determining instrument configuration, setting initial operating conditions, and conducting analysis and recording data.

1. Sample preparation and pretreatment

-Understand the properties of the sample: clarify the source, composition, and whether it contains substances that cannot be directly analyzed by GC (such as inorganic salts). If the sample system is simple and the sample components can vaporize, direct analysis can be carried out; If there are interfering substances or inappropriate concentrations, pre-treatment is required. Common pretreatment methods include adsorption, desorption, extraction, concentration, dilution, purification, derivatization, etc.

-Choosing the appropriate solvent: For solid samples, they should be dissolved in a suitable solvent, while ensuring that the selected solvent has good compatibility with the chromatographic column and does not affect the separation effect and instrument performance.

-Control sample volume: Determine the appropriate injection volume based on the requirements of the instrument and the actual concentration of the sample. Generally speaking, when the sample concentration does not exceed 10mg/mL, the injection volume of the packed column is usually 1-5 μ L; For capillary columns, when the split ratio is 50:1, the injection volume generally does not exceed 2 μ L.

2. Determine the instrument configuration

-Choose detector type: Different detectors are suitable for different types of compounds. For example, flame ionization detectors (FIDs) are often chosen for hydrocarbons; Substances with more electronegative groups (F, Cl, etc.) and less carbon hydrogen content are more likely to choose electron capture detectors (ECD); When the sensitivity requirement is not high or when there are non hydrocarbon components, a thermal conductivity detector (TCD) can be selected; For samples containing sulfur and phosphorus, a flame photometric detector (FPD) can be selected.

-Selection of injection device: Diaphragm pad injection method is commonly used for liquid samples, while six way valve or adsorption thermal analysis injection method can be used for gas samples. However, general chromatography only requires membrane pad injection, so gas samples can also be analyzed using adsorption solvent analysis membrane pad injection.

-Selection of chromatographic columns: According to the principle of "similar compatibility", non-polar chromatographic columns are selected for separating non-polar substances, and polar chromatographic columns are selected for separating polar substances. After determining the chromatographic column, it is necessary to set the operating temperature of the column based on the difference in distribution coefficients of the components to be tested in the sample. Simple systems can be analyzed using isothermal methods, while complex systems with significantly different distribution coefficients can be analyzed using programmed heating methods.

-Select carrier gas: Common carrier gases include hydrogen, nitrogen, helium, etc. Among them, hydrogen and helium have smaller molecular weights and are often used as carrier gases for packed column chromatography; Nitrogen has a relatively high molecular weight and is often used as a carrier gas for capillary gas chromatography; When gas chromatography-mass spectrometry is used, helium gas is usually used as the carrier gas.

3. Initial operating conditions for gas chromatography

-Injection volume: determined by comprehensive consideration of factors such as sample concentration, chromatographic column capacity, and detector sensitivity.

-Injection port temperature: It should be able to quickly vaporize the sample, but not too high to cause sample decomposition.

-Detector temperature: Ensure the normal operation and stable performance of the detector.

-Column temperature: This is one of the key factors affecting separation efficiency and requires careful optimization of settings.

-Carrier gas flow rate: An appropriate carrier gas flow rate can help improve separation efficiency and shorten analysis time. The automatic testing device on the high-end chromatograph can be used for measurement. If this function is not available, a soap film flowmeter can be used for measurement.

4. Conduct analysis and record data

-Inject sample: Introduce the processed sample into the gas chromatograph through the injection system.

-Running the instrument: Start the instrument and allow the carrier gas to carry the sample into the chromatographic column for separation. The different distribution coefficients between the stationary phase and the mobile phase result in varying operating speeds of each component within the column, thus achieving mutual separation.

-Detection and recording: The separated components enter the detector in sequence, and the generated signal is amplified and recorded by the recording system to obtain a chromatogram. Information such as retention time and peak area can be obtained from the graph for qualitative and quantitative analysis.