Seeking help with gas chromatography issues! ! !
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Our unit uses an Agilent gas chromatograph, model: 7890A. It has been in use for over three years with basically no problems, but recently the following phenomenon has occurred: the total percentage sometimes amounts to 95% or 101%. The main components of the gas are H2 and N2, which are used as raw materials for synthesizing ammonia. Occasionally, the ratio is 70% H2 and 24% N2, with trace amounts of other components; in such cases the total percentage of the gases is 94%, whereas it should be 100%. From a process perspective, there should be no significant changes in the gas composition; the online analyzer is also operating stably, and the operator has 3 years of experience, so there should be no issues with the sampling process. Therefore, it is suspected that the problem might lie with the chromatograph. Please help analyze it. Normal analysis parameters: CO ppm ≤1, H2 % 74-76, N2 % 24-26, CH4 ppm ≤10
It depends on what kind of chromatographic quantification method you are using. If full normalization is used, it is very likely that some components are included in the denominator during the normalization process, but they are not counted as separate components due to their low content or response values. If it is the external standard method, it may be that the set analysis conditions or parameters are not appropriate; in such cases, re-calibration should resolve the issue. If the components analyzed include hydrogen, nitrogen, ammonia, methane, and argon, the response of ammonia may lack good repeatability, so strict attention must be paid.
The quantitative method has changed; previously, the corrected normalization method could be used, while now the external standard method is likely to be used
This is what happens with the external standard method: there are components in the gas sample that do not produce peaks.
This should occur in the external standard method; area normalization will not arise in that case.
Method setup issue: those with low contents at the ppm level were likely excluded
In the ammonia synthesis gas, there may also be significant amounts of argon and methane, as in addition to fresh hydrogen and nitrogen, argon and methane gradually accumulate and need to be periodically vented. The occasional occurrences of **values below 100% are due to the lack of quantitative calculation and normalization for argon and methane.