Beginner Question: GC-EAD - why use FID and not MS?

[James_K]

Hi there,

This is a really basic question I've been trying to figure out but so far have had no luck.

Why do people doing GC-EAD (gas chromatography linked to Electroantennogram) always (I've never seen it done another way) pair their EAD with FID to see when compounds elute? It seems to me that you could have a y-splitter that sends 50% of the sample to an MS instead of the FID - which should allow you to detect lower concentrations of compounds of interest (at least I think that would be true) - but I never see this done in the literature. If anyone could clarify this for me it would be much appreciated!

Cheers,
James

[Peter Apps]

Hi James

It's down to plumbing. Splitting between an antenna and an MS is more complicated than just putting a Y at the end of the column. Because the MS has a vacuum in the source and the antenna is at atmospheric pressure the MS takes all the column effluent and also sucks air backwards through the line to the antenna. You can put a restrictor in the line to the MS, but its resistance changes as the oven temperature changes, and so the split between antenna and MS changes during a run. Adding make-up at the split point can help, but it needs independent pressure control, and probably pressure programming.

And unless you know what you are looking for and can run the MS in SIM (and if you know what you are looking for why would you be doing antennography of column effluent ?) a benchtop MS is not more sensitive than an FID.

There are some atmospheric pressure MS sources coming onto the market that would be a very expensive way of solving the plumbing problem.

Peter

[James_K]

Ah ok, that clarifies things - thanks Peter!