Abstract: In gas chromatography analysis, sample injection is one of the main sources of quantitative analysis error. Because the principle, structure, materials used, temperature of injection, volume of injection, speed of injection, and tools used for injection will directly affect the qualitative and quantitative repeatability and accuracy of gas chromatography analysis. In the actual analysis, due to the different gas, liquid, solid, and state of the sample, the analysis purpose is different, and the requirements are different, there are a variety of sampling systems for GC, such as: Atmospheric pressure gas samples have a six-way valve gas injection or injection needle Two types of cartridge injection. In the following, we will summarize only a few points with the gas sample six-way valve sampling technology and techniques for the reference of workers who often do gas analysis. Atmospheric pressure gas samples are injected through the injection septum using a medical syringe (1 ml to 5 ml). It is simple and flexible, but the disadvantages are sample backflushing and leakage, large quantitative errors, and repeatability generally above 2.5%. This is because the pre-column pressure is higher than the ambient atmospheric pressure, and the sample gas will leak along the inner wall of the injection tube. At this time, although it can be compensated by coating a layer of high-temperature vacuum silicone grease on the inner wall of the tube to improve the air tightness, there will be adsorption of silicone grease to the organic matter, and the quantitative error is still very large. If a six-port valve is used to inject the sample in a fixed volume, it is not only easy to operate, but also fast and accurate. As long as the operation is reasonable and certain skills are mastered, the reproducibility can be less than 0.5%. Even if the ambient temperature, pressure changes or different corrections are easy and convenient. In addition, the six-way valve can also be directly used for high-pressure gas sampling. 1. Analyze to understand whether the working principle, structure and sample direct contact valve material of the six-way valve you are equipped with is suitable for your analysis requirements. 2. Since the airtightness of the valve varies greatly (0.1 ~ 0.6Mpa), can you ensure that there is no air leakage when connected to your gas circuit? Otherwise, it will not only affect the stability of the instrument, but also cannot guarantee the reproducibility of the instrument injection . 3. Quantitative tube volume: As small as possible when the sensitivity meets the requirements, the maximum quantitative tube volume should be limited to 10% of the number of towers during the experiment. Otherwise, further increase the injection volume, and only increase the peak width without increasing the peak height. In other words, the injection volume when the peak width of the chromatogram is not broadened should be the maximum volume of the quantitative tube. For packed columns it is generally not easy to exceed 5 ml. 4. At present, in order not to affect the injection of liquid injection, the six-way valve is often connected in series at the entrance of the vaporization chamber. Obviously, this connection method adds a certain amount of dead space. When analysis requirements are high, it is best to cross the vaporization chamber directly into the chromatography column or insert the six-way valve carrier gas outlet directly into the column head through the injection pad. 5. At ambient temperature, when the sample components may condense or contain trace liquid gas samples, the temperature effect of the six-way valve (including the pipeline leading to the instrument) should be considered: a) put the valve into the chromatographic column box; b) separate temperature control heating . 6. Sample pre-treatment problems: a) Prevent dust and mechanical particles from entering the valve to affect air tightness or normal operation; b) Avoid contamination of the valve by high-boiling impurities. 7. Sampling method: In order to prevent possible pollution or interference of the gas components in the environment to the sample, it is best to use a large syringe needle to enter the quantitative tube like liquid injection. It is not easy to connect with various rubber or plastic pipes. This may: a) the pipe itself is not pure; b) in principle, all kinds of pipes will have a penetration effect, which is particularly disadvantageous for trace analysis. 8. Sampling tools: At present, metal-coated air bags, large syringes or special gas cylinders are commonly used. Unless the requirements are extremely low, bladders and plastic bags are rarely used for gas extraction. 9. The gas pressure of the sample in the quantitative tube: Because the content of the gas is directly related to the gas pressure, in order to ensure the repeatability of each injection, the pressure of the quantitative tube should be balanced with the atmospheric pressure after sampling. According to experience, it is generally balanced after sampling for 20-30 seconds. . 10. Rinse the sample volume of the quantitative tube: Due to the different concentration of the gas sample being analyzed, in order to prevent the interference of the original high concentration gas in the quantitative tube when entering the higher concentration and then entering the lower concentration sample. When sampling, it is required to flush the quantitative tube with new sample gas, and the flushing gas volume is not less than 5 times the quantitative volume according to experience. The actual impact can also be judged and selected by the reproducibility of the experimental peak. 11. At what time after injection, turn the six-way valve back to the sampling position. Depending on the analysis, such as the baseline volatility after injection, qualitative and quantitative repeatability to determine, according to experience is generally after a few seconds after injection (at this time before the first chromatographic peak has not yet appeared), turn the valve back The sampling location is better. At this time, it is easy to eliminate the influence of the poor air tightness of the valve and the large volume of the quantitative tube on the baseline or peak. 12. If it is found that the valve has poor air tightness or is contaminated, the experienced operator can disassemble and wash the six-way valve, but it should be noted that the sealing surface of the valve body and the valve disc should only be wiped with a soft cotton fabric, and the solvent should use volatile hexane, Acetone, chloroform, etc., after cleaning, blow dry with dry air. However, special care should be taken to avoid the use of halogen-containing hydrocarbons (such as chloroform) as cleaning agents for gas six-way valve sampling systems used in ECD, otherwise these interfering solvents will be present at trace levels for a long time. A strange peak appears. Frying Pan,Skillet Pan,Non Stick Pan,Carbon Steel Pan Jiangmen Junerte Stainless Steel Kitchenware Co.,Ltd , https://www.jetkitchenwarejm.com