[curiouscat]

GC-MS is always done on peaks that can be resolved on a column, correct? I was just wondering, in case of close-boiling or other hard to resolve peaks is there any way of interpreting a mixed GC-MS spectrum? i.e. you'd see fragments from both molecules, I expect?

Or is that not done.

[marquis]

You need to examine the spectrum of the two materials.  If you can find
an ion present in one, but not the other, you can usually do an extracted
ion chromatogram(i.e. you run a chromatogram looking for only the
ion present in one of the materials).  You should be able to find some help
files in you software or manual.

Usually, you can start by reprocessing a total ion chromatogram (TIC) to see if this
approach will work.  If it does, then you can work on setting up the individual
run parameters optimized to the spectrum fragments you're after.
 
The extracted ion is usually of limited help.  You'll often find the two materials
eluting at the same time have closely related spectrum, but it is worth a try.

Good luck