strang - signal keep decrease using CI- GC-MS

[s2008]

Hello Guys:

I have less experince in using CI-GC-MS in negative mode. Currently , I met a strange problem. When I repeat to inject a same standard solution. The signal intensity was kepting on decreasing(significantly). After I changed the ion volume, the signal intenisty of first injection went back to the higest level. However, the signal intensity of following injections was repeated to keep on decreasing ( significanrly after each injection). Any suggestions I should check where this kind of problm comes from?

Thank you so much

[jh1]

Did you happen to clean the source recently?

Typically when I clean our CI source it takes quite a few injections for the system to equilibrate and decent integrity to return.

Also how's your PCI 19 m/z peak? Too much water can result in significant decreases in area counts between runs.