Double injection on a 7890B

I've experience, something unusual in one of our GC's which is used for GPA2286 Hydrocarbon Analysis. Recently our vendor done some PMs and since then we noticed an unknown peak started to appear prior to a valve switch (which direct majority of methane) in a separated column. I checked the valve head and there has been no leaks found on it. When I ran a sample which didn't have any methane, that peak wasn't visible. This prompt me to think this is more likely to be Methane. But only way that can occur is having a double/partial injection with the rotor's movements. This is still one of the theories. 

Has anyone experience something like this? Any thoughts?



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