LNG Composition ISO 6974 issue

[qasas1]

Problem Description: Baseline not stable and additional peak at end of chromatography.
Correction actions:
1. checked all gas supplies pressure and found OK.
2. On further diagnosis suspect contamination of carrier gas, filters saturated, leak in gas lines.
3. Checked for leaks, isolated the problems by replacing filter (combined filters were not available at site, installed only available moisture filter) and connecting He carrier gas line directly.
4. Problem traced to contaminated/impure Helium carrier gas cylinder/s.
5. Changed cylinder and checked after flush out of gas line. Signal looked stable.
6. Conditioned column for couple of hours.
7. Checked GC by injecting standards and samples. GC is working satisfactorily.
8. Unfortunately, after few days again the same issue repeated and till now we don't know how to fix this!!! by the way, we have 3 GCs for the same application (LNG composition using ISO 6974 method) and when the issue started for one GC the other 2 GCs will be impacted for the same issue. Appreciate any one can help us as soon as possible

GC details: AC#243592, Product No: G3440A, Model: 7890A, Serial No: US10451013

[antonk]

Does the blank injection (truly removed sample, just carrier) brings the last peak?

* somewhere tubings plumbed with rubber instead of brass
* lubrication inside valve
* place indicating oxygen trap just before GC. If it will change colour rapidly - you gas supply is mixed with air.
* connect 1 cylinder directly to 1 GC with 2 meters of copper tube and compare.
* water in carrier - heat the valve and install moisture trap