Purity and selection principles of gases used in t

2022-09-21
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Gas chromatograph uses gas purity and selection principles

how to operate gas chromatograph? How to select gas sources with different gas purity as carrier gas and auxiliary gas is an old technical problem. Although deformation under load - measuring the ability of hard plastics to withstand permanent deformation and the ability of non hard plastics to restore their original shape after deformation, for users who have just come into contact with gas chromatograph, At present, it is difficult to find comprehensive information about this, so they always ask everywhere what kind of gas purity is the best. According to the type of instrument (high, medium and resistant) used by each user, it is indeed a complex problem to choose the purity of gas

in principle, the selection of gas purity mainly depends on ① the analysis object; ② Packing in chromatographic column; ③ Detector. We suggest that on the premise of meeting the analysis requirements, the gas with high purity should be selected as far as possible. This will not only improve (maintain) the high sensitivity of the instrument, but also prolong the life of the chromatographic column and the whole instrument (gas path control components, gas filters). Practice has proved that as a medium and high-grade instrument, it is sometimes very difficult to restore the high sensitivity of the instrument once it is required to analyze samples with low concentration after long-term use of low purity gas source. For low-grade instruments, constant or semi micro analysis and the selection of high-purity gas not only increase the operating cost, but also sometimes increase the complexity of the gas circuit, which is more prone to air leakage or other problems that affect the normal operation of the instrument. In addition, for some special analysis purposes, it is required to deliberately add some "impurities" to the carrier gas, such as adding an appropriate amount of water vapor to analyze polar compounds, and adding trace sulfur to improve the sensitivity of sulfide analysis when operating the flame photometric detector. When operating the helium ionization detector, the neon content must be 5~25ppm, otherwise negative peaks or "W" shaped peaks will be generated when analyzing hydrogen, nitrogen and argon. This article will not discuss in detail here

I. adverse effects of low gas purity

according to the analysis object, the type of chromatographic column, the gear number of the operation instrument and the specific detector, if the unqualified low purity gas is used, the adverse effects may be as follows:

1) sample distortion or disappearance: for example, H2O gas hydrolyzes the silicon chloride sample

2) chromatographic column failure: H2O and CO2 make the molecular sieve column inactive, H2O gas decomposes the polyester fixed liquid, and O2 breaks the chain of PEG

3) sometimes some gas impurities interact with the fixed liquid to produce false peaks

4) influence on the retention characteristics of the column: for example, the retention index of H2O on polyethylene glycol and other hydrophilic fixed liquids will increase. When the oxygen content in the carrier gas is too high, the retention characteristics of both polar and non-polar fixed liquid columns will change, and the longer the service time is, the greater the influence will be

5) detector:

tcd: the signal-to-noise ratio is reduced and cannot be adjusted, which won them the Nobel Prize. In addition, the linearity is narrowed, and the correction factor in the literature cannot be ge9x. The order received is equivalent to the use of 300 aircraft. The oxygen content is too large, which accelerates the aging of components at high temperature and reduces their service life

fid: especially when DT ≤ 1 Ⅹ 10 ˉ ⒒ when operating at ⒒/s, organic impurities such as CH4 will cause the base current to surge, and the noise will increase, so microanalysis cannot be carried out

ecd: oxygen and water in the carrier gas have the greatest impact on the normal operation of the detector. In different power supply modes, pulse power supply has a greater impact than DC voltage power supply, and fixed base current pulse modulation power supply has a greater impact than pulse power supply. This is why at present, many people have to turn the carrier gas purity selector switch from "standard nitrogen" to "general nitrogen" when operating fixed base flow pulse modulated ECD and when the carrier gas purity is low. You will find that in this case, not only the sensitivity becomes lower, but also the linearity becomes narrower. Practice has proved that when operating ECD, ideal performance can be achieved when the water content in the carrier gas is less than 0.02ppm and the oxygen content is less than 1ppm. It is worth pointing out that we have repeatedly found that the secondary pollution of the carrier gas caused by the pollution of the regulating gas circuit system of the instrument has greatly increased the fundamental frequency of ECD and reduced the signal to interference ratio

fpd, NPD and other commonly used detectors, because they are selective detectors, special attention should be paid to the removal of impurities in the tested sensitive substances according to the analysis requirements

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