Composition and basic requirements of gas chromatograph

　　Gas chromatography is suitable for analyzing components with a certain vapor pressure and good thermal stability. It can directly analyze gas samples and organic matter that is easily volatile when heated, while derivatization or cracking methods can be used for substances that are not easily volatile below 500°C or easily decomposed when heated.

　　1. Composition of the instrument

　　The gas chromatograph consists of a carrier gas source, an injection part, a chromatographic column, a column oven, a detector and a data processing system. The temperatures of the injection part, the chromatographic column and the detector are all under control.

　　2. Basic requirements for instruments

　　1. General requirements for instruments

　　(1) Carrier gas source gases Helium, nitrogen and hydrogen can be used as mobile phases in gas chromatography. The carrier gas can be selected according to the properties of the sample and the type of detector. Unless otherwise specified, nitrogen is commonly used as the carrier gas.

　　(2) Injection: The injection method can generally be direct solution injection or headspace injection. When direct solution injection is used, the injection port temperature should be 30-50°C higher than the column temperature. Headspace injection is suitable for the separation and determination of volatile components in solid and liquid samples.

　　(3) Select the chromatographic column according to your needs. New packed columns and capillary columns need to be aged before use to remove residual solvents and low molecular weight polymers. If the chromatographic column has not been used for a long time, it should be aged before use to stabilize the baseline.

　　(4) Column oven The temperature fluctuation of the column oven will affect the reproducibility of the chromatographic analysis results. Therefore, the temperature control accuracy of the column oven should be ±1°C, and the temperature fluctuation should be less than 0.1°C per hour.

　　(5) Detectors Suitable for gas chromatography include flame ionization detector (FID), thermal conductivity detector (TCD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), electron capture detector (ECD), mass spectrometer (MS), etc. Flame ionization detector has a good response to hydrocarbons and is suitable for detecting most drugs; nitrogen phosphorus detector has high sensitivity to compounds containing nitrogen and phosphorus; flame photometric detector has high sensitivity to compounds containing phosphorus and sulfur; electron capture detector is suitable for halogen-containing compounds; mass spectrometer can also provide structural information corresponding to a component of the test sample, which can be used for structural confirmation. Unless otherwise specified, flame ionization detector generally uses hydrogen as fuel gas and air as auxiliary fuel gas. When using a flame ionization detector, the detector temperature should generally be higher than the column temperature and must not be lower than 150°C to avoid condensation of water vapor, usually 250-350°C.

　　(6) Data processing systems currently mostly use computer workstations. The pharmacopoeia stipulates that the chromatographic conditions specified under each item, except for the carrier gas, detector, stationary liquid type and specially specified chromatographic column materials, can be appropriately changed, such as the inner diameter, length, carrier brand, particle size, stationary liquid coating concentration, carrier gas flow rate, column temperature, injection volume, detector sensitivity, etc., to adapt to the specific variety and meet the requirements of the system suitability test. Generally, the chromatogram is recorded within about 30 minutes.

　　2. System suitability test

　　Unless otherwise specified, the method shall be as specified under High Performance Liquid Chromatography.

　　3. Determination method

　　(1) Determination of the content of an impurity or main component in the test sample by internal standard method plus correction factor

　　(2) Determination of the content of an impurity or main component in the test crystal by external standard method

　　(3) Area normalization method

　　The specific contents of methods 1 to 3 above are the same as the corresponding provisions of high performance liquid chromatography.