Gas Chromatography Six-way Valve Gas Injection Technique

In gas chromatographic analysis, injection is one of the main sources of quantitative analysis error. Because the principle, structure, materials used, the temperature at the time of sample injection, the sample injection rate, the sample injection speed, and the sample introduction tools all have a direct impact on the qualitative and quantitative repeatability and accuracy of the gas chromatographic analysis.

In the actual analysis, due to the different gas, liquid, solid, and state of the sample, the purpose of the analysis is different, and the requirements are different. There are many kinds of sampling systems for the GC, such as six-way valve gas sampling or injection needle for atmospheric gas samples. Tube injection two. In the following, we will only summarize the points and techniques for gas sample six-way valve sampling techniques, which will be used as references for workers who are often involved in gas analysis.

Atmospheric pressure gas samples are injected through injection septa using a medical syringe (1 ml to 5 ml). This is simple and flexible. However, there are sample backlashes and leaks when the disadvantages occur. The quantitative error is large, and the repeatability is generally above 2.5%. This is because the pre-column pressure is higher than the ambient atmospheric pressure and the sample gas leaks along the inner wall of the injection tube. Although this can be compensated by applying a layer of high-temperature vacuum grease on the inner wall of the pipe to improve the airtightness, the adsorption of organic grease by silicone grease will occur, and the quantification error will still be large. If using a six-way valve for volumetric sampling, it is not only convenient to operate, but also results in rapid and accurate cutting. As long as the operation is reasonable and a certain skill is mastered, the reproducibility may be less than 0.5%. Even if the ambient temperature, pressure changes or different corrections are also easy and convenient. In addition, the six-way valve can also be directly used for high pressure gas injection.

1. Analyze the working principle, structure of the six-way valve that you are using, and whether the sample is directly in contact with the valve material is suitable for your analysis requirements.

2. Because the valve's airtightness is very different (0.1~0.6Mpa), when connecting to your airway system, can you guarantee airtightness? 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 volume of the quantitative tube should be limited to less than 10% of the number of towers during the experiment. Otherwise, further increase the injection volume, only increase the peak width without increasing the peak height, or that the sample volume should be maximized when the peak width is not broadened. For packed columns it is generally not easy to exceed 5 ml.

4. At present, in order not to affect the liquid injection, the six-way valve is often connected in series at the entrance of the vaporization chamber. Obviously, this connection method increases the dead space. When the analysis requirements are high, it is best to directly enter the column through the vaporization chamber or directly insert the six-way valve carrier gas outlet into the column head through the injection pad.

5. When the sample components are likely to condense or contain trace amounts of liquid gas at ambient temperature, consideration should be given to the temperature effect of the six-way valve (including the line leading to the instrument): a) Place the valve in the column compartment; b) Separate temperature control heating .

6. Sample pre-treatment issues: a) should prevent dust, mechanical particles into the valve affect the airtight or normal work; b) to avoid high-boiling impurities on the valve pollution.

7. Sampling method: In order to prevent possible contamination of the sample caused by gas components in the environment, it is best to use a large syringe needle to inject the quantitation tube like a liquid sample. It is not easy to use a variety of hoses or plastic tubes to access this possible: a) the tube itself is not pure; b) all kinds of tubes in principle will have an osmotic effect, which is particularly disadvantageous for trace analysis.

8. Sampling tools: Currently used are metal coated air bags, large syringes or special gas cylinders. Unless the requirements are extremely low, bladders, plastic bags, etc. are rarely used.

9. Quantitative tube sample gas pressure: Because the gas content and gas pressure are directly related, in order to ensure the repeatability of each sample injection, the quantitative tube pressure and the atmospheric pressure should be balanced after sampling. Based on experience, the sample can be equilibrated for 20 to 30 seconds after sampling. .

10. Rinse the sample volume of the dosing tube: Due to the different concentration of the gas sample being analyzed, to prevent the high concentration of gas from entering the lower concentration sample, the interference of the original high-concentration gas remains in the dosing tube. When sampling, it is required to flush the quantitation tube with new sample gas. The amount of flushing gas is not less than 5 times of the quantification volume according to experience. The actual effect can also be judged and selected by the reproducibility of the experimental peaks.

11. At what time after injection, turn the six-way valve back to the sampling position.

Depending on the analysis situation, such as: the volatility of the baseline after injection, qualitative and quantitative reproducibility to determine, according to experience is generally after a few seconds injection (before the first chromatogram peak has not yet appeared), the valve is rotated back The sampling position is better. At this time, it is easy to eliminate the influence of poor airtightness of the valve and excessive volume of the burette on the baseline or peak.

12. If the valve is found to have poor air tightness or is contaminated by experienced operators, the six-way valve may be dismantled, but care should be taken to ensure that the sealing surfaces of the valve body and the disc are only allowed to be wiped with a soft cotton, volatile solvent hexane, Acetone, chloroform, etc., were washed and dried with dry air. However, special attention should be paid to the gas six-valve sampling system for ECD that should avoid the use of halogen-containing hydrocarbons (such as chloroform) as cleaning agents, otherwise these interference solvents will exist at trace levels for a long time. A strange peak appears.