when should you use flame ionization detection vs. thermal conductivity?

Hello,
You should use flame ionization detector for organic compound analysis with more than one carbon. TCD is good for the analysis of gas like hydrogen or helium.
Sincerly yours
Isabelle Denis

The major use of the TCD is for permanent gases. It is less sensitive than most other GC detectors, so it is often used with packed columns or PLOT columns so that large sample volumes may be injected.

The FID is a good general purpose detector for the determination of hydrocarbons. Not all organics will respond on the FID, but the vast majority will.

There are other detectors that you may need to consider based on your application. Halogenated compounds are often determined on an ECD, while organonitrogen and organophosphorus compounds may be analyzed by an NPD.

Check out one of the column manufacturers catalogs or web sites to get an idea of what detectors are used for a specific analysis.

Briefly put, use an FID for anything that burns well, like those compounds mentioned above. FIDs are very insensitive to things like formic acid. Use a TCD when you want to detect anything, with water being the most common nondetectable by an FID. TCDs have an feature that might be of interest as far as quick and dirty weight percent determinations are concerned: response factors for most organic compounds are more similar on a TCD than and FID, so an area percent analysis with a TCD will give a reasonable weight percent approximation. FIDs are usually an order of magnitude or two more sensitive than TCDs, so use an FID for trace analysis.

I am trying to detect small water concentrations in a nitrogen stream by means of a TCD. The dew point is smaller than -40°C. I am observing a negative signal when the water concentration gets very low. Does anybody have an explanation for it?