author:seohtm Add the Time:2015-7-7 16:16:04 Click: 900

    Gas chromatograph to choose different gas purity air as carrier gas and auxiliary gas, although it is an old problem, but just contact for users of the gas chromatography. At present, it is difficult to find comprehensive information about this, and so they are always asking around how to choose what kind of gas purity best this kind of problem. According to each user specific use of that class (high, medium and low) instrument, choose what kind of gas, is indeed a more complex problem. In principle, when the purity of the gas is ed, it is mainly determined by the analysis of the object. We suggest that we can use the high purity gas as far as possible to meet the requirements of the analysis. This will not only improve the high sensitivity of the instrument, but also extend the life of the column, the whole instrument (gas path control unit, gas filter).
The practice has proved that, as a medium - grade instrument, it is very difficult to recover the high sensitivity of the instrument. For low-grade instrument, constant or semi micro analysis, ion of high purity gas, which not only increases the operation cost, sometimes also increased the complexity of gas path, more prone to leakage or other problems and the impact of the instrument in normal operation. Also for some special analysis objective requirements specifically in the carrier gas added some "impurity", such as: analysis of polar compounds add appropriate amount of water vapor, operate a flame photometric detector, in order to improve the sensitivity of the analysis of sulfide, and add trace sulfur. The content of NE must be in the 5~25ppm, otherwise it will produce negative or "W" - shaped peaks in the analysis of hydrogen, nitrogen and argon. This article will not be discussed in detail in this article.
The bad influence of gas purity
According to the analysis object, the type of column, operation instrument of the stop and specific detector, if the use of substandard low purity gas, adverse effects have the following several possible:
1) the sample distortion or disappearance: such as H2O gas to make the sample of chlorine silicon;
2) column failure: H2O, CO2 molecular sieve column lose activity, H2O gas polyester fixed liquid decomposition, O2 PEG broken chain.
3) at times, some gas impurities and fixed liquid interact with each other and produce false peaks;
4) effect of column retention characteristics of: such as: H2O on polyethylene glycol hyhilic stationary phase retention index will increase, load gas oxygen content is too high, whether polar or nonpolar stationary liquid column retention characteristics will change, use time longer influence is bigger.
5) detector:
TCD: the signal to noise ratio is reduced, can not be adjusted to zero, linear narrowing, the correction factor in the literature can not be used, oxygen content is too large, so that the element in high temperature accelerated aging, reduce life.
FID: especially in DT is less than or equal to 1 x 10 - News / seconds to do gymnastics, CH4 and other organic impurities, the base surge current, noise increase can not trace analysis.
ECD: has the greatest impact on a carrier of oxygen and water in the gas of the detector work normally, in different power supply mode, pulse power supply than the DC voltage supply effect, fixed base flow pulse modulated power supply than the pulse power supply of high. Which is why at present, a lot of in the operation of fixed base flow pulse modulated ECD, at low carrier gas purity must the carrier gas purity ion switch from "standard nitrogen" dial to "general nitrogen" position. It will be found that, in this case, not only the sensitivity is low, but also the linear is narrow. The practice shows that the water content in the carrier gas is lower than that of 1ppm and the oxygen is less than 0.02ppm when the ECD is operated. It is worth pointing out that we have found that the two pollution to the carrier gas caused by the instruments adjustment of the gas path system has caused the increase of the fundamental frequency of the ECD to increase the ratio of the signal to dry.
FPD and NPD, and other commonly used detectors, because they belong to the ive detector, when the analysis requires that special attention is being measured in the removal of impurities in sensitive material.
6) in program heating up operation and load some of the impurities in gas, at a low temperature, retention in chromatographic column, when leaning on temperature rises caused not only the baseline drift may also in the spectral diagram appears relatively wide "spurious peaks".
7) instrument
A. various types of filter acceleration failure;
B. valve (valve, flow valve, needle valve) contaminated, gas barrier blocking, reduce regulation accuracy or failure;
C. gas path system to be contaminated, to reinstate instrument in high sensitivity have drills, sometimes washing blowing very long time, maybe a week or more) pollution is serious and sometimes beyond recovery.
The life of D. detector, the practice shows that the life of ECD and TCD is the most obvious, which should be paid special attention to.
Two general principles for the ion of gas purity
1 from the point of view, the analysis of the micro analysis is higher than the constant analysis. That is to say, the gas impurity content must be less than by content analysis, if by TCD analysis of 10 ppm Co, carrying gas in the total impurity content shall not exceed 10 ppm, because the purity of 99.999% of gas containing 0.001% of impurities, quite to 10 ppm so for 10 ppm trace analysis, the purity of carrier gas should be higher than 99.999%; for FID using gas, hydrocarbon content must be very low, the carrier gas in a large number of oxygen impurities as long as the wrong column impact, the performance will not influence the FID and ECD, carrier gas, oxygen and water content must be very low.
2 capillary column analysis is better than the packed column