How to operate a gas chromatograph using gas sources with different gas purities as carrier gas and auxiliary gas is an old technical problem, but it is difficult for users just touching the gas chromatograph to find comprehensive information on this aspect, so they Always ask questions about what kind of gas purity is good. According to the type of instrument that each user uses, what kind of purity gas is chosen is indeed a more complicated problem. In principle, when selecting the purity of the gas, it mainly depends on 1 analysis object; 2 the filler in the column; 3 detector. We recommend using a higher purity gas as much as possible while meeting the analytical requirements. This will not only increase the sensitivity of the instrument, but also extend the life of the column and the entire instrument. Bad effects of low gas purity Depending on the object of analysis, the type of column, the order of the instrument, and the specific detector, if an undesired low-purity gas is used, there are several possibilities for adverse effects:
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Practice has proved that as a medium-to-high-end instrument, a low-purity gas source is used for a long time. Once a low-concentration sample is required to be analyzed, it is sometimes difficult to restore the high sensitivity of the instrument. For low-end instruments, for high- or semi-micro analysis, the use of high-purity gas not only increases operating costs, but also increases the complexity of the gas path, and is more prone to air leaks or other problems that affect the normal operation of the instrument. In addition, for some special analytical purposes, it is required to add some "impurities" to the carrier gas, such as: analysis of polar compounds, adding appropriate amount of water vapor, in order to improve the sensitivity of the analysis of sulfides when operating the flame photometric detector. And add a trace of sulfur. The content of the helium ionization detector must be 5 to 25 ppm, otherwise a negative or "W" peak will be generated when analyzing hydrogen, nitrogen and argon. This article will not be discussed in detail here.
1) The sample is distorted or disappeared: such as H2O gas, the chlorine silicon sample is hydrolyzed;
2) Capillary column failure: H2O, CO2 deactivates the molecular sieve column, H2O gas decomposes the polyester fixing solution, and O2 breaks the PEG.
3) Sometimes some gas impurities interact with the fixative to produce a false peak;
4) Effect on column retention characteristics: For example, H2O will increase the retention index of hydrophilic fixatives such as polyethylene glycol, and when the oxygen content in the carrier gas is too high, whether it is a polar or non-polar fixative The retention characteristics of the column will change, and the longer the use time, the greater the impact. 5) Detector:
TCD: The signal-to-noise ratio is reduced, it can't be zeroed, the linearity is narrowed, the correction factor in the literature can't be used, and the oxygen content is too large, which makes the component accelerate aging at high temperature and reduce the life.
FID: Especially when Dt≤1X10ˉ11./sec, organic impurities such as CH4 will increase the base flow and increase the noise.
ECD: The influence of oxygen and water in the carrier gas on the normal operation of the detector is large. In different power supply modes, the pulse power supply has a greater influence than the DC voltage supply, and the fixed base current pulse modulation power supply has a greater influence than the pulse power supply. This is why many current fixed-flow pulse-modulated ECDs must be used to shift the carrier gas purity selector switch from “standard nitrogen†to “general nitrogen†when the carrier gas purity is low. You will find that operating in this situation, not only the sensitivity is low, but the linearity is also narrowed. Practice has proved that when operating ECD, the water content in the carrier gas is less than 0.02ppm, and the oxygen is lower than 1ppm to achieve better performance. It is worth noting that we have repeatedly found that the secondary pollution of the carrier gas caused by the pollution of the instrument's regulating gas path system has caused the ESD fundamental frequency to increase significantly to reduce the signal-to-dry ratio. Commonly used detectors such as FPD and NPD, because they are selective detectors, should be handled according to the analysis requirements, paying special attention to the removal of impurities in the sensitive substances being tested.
6) During the temperature-programming operation, some impurities in the carrier gas remain in the column at low temperatures. When the temperature rises, not only the baseline drift but also a wide "false peak" may appear on the spectrum. .
7) Instrument impact a. Various types of filters accelerate failure;
b. The regulating valve (regulating valve, steady flow valve, needle valve) is polluted, the air resistance is blocked, and the adjustment accuracy is reduced or failed;
c. The pneumatic system is polluted. If the instrument is to be operated under high sensitivity, it may sometimes be washed for a long time (maybe more than a week).
d. The life of the detector, the practice shows that the impact on the life of ECD and TCD is obvious, should cause special attention to the user