I am injecting a known substance on my GC-MS with Chemstation. Peaks look reasonable but the ion fragments can't be correct. Consistently, ion fragments 113, 163, 213 and 232 show up when the actual compound has a MW of much less. I can't ask the software to ignore all over say 100 MW because it is possible I have some impurities with higher MW. The actual compounds MW usually does show up with higher abundance, but the library search is routinely thrown off by the incorrect higher MW fragments.
I've done a search on these MW and contamination issues and those particular fragments don't appear to correspond to anything.
I did in the not too distant past have air contamination but now my tune file looks good.
Any ideas are appreciated.