Chromatography Forum

I am currently setting up an older HP 6890 GC with a stock TCD detector. 30m capillary column, split/splitless injector, no FID on board.

My question deals with the characteristic signal of this (or any other) TCD detector as a function of oven temperature. I have my reference flow set equal to my (column flow + makeup) flow at about 12cc / min (constant flow mode on the column, 2cc/min). As the oven's temperature ramp begins at about 3 minutes into my program, the TCD baseline signal drops off precipitously, finally bottoming out at about 10 minutes (~200C). I see this phenomenon whether I set the TCD block at 250 or 350 or whatever, or if I switch to constant pressure mode.

Why does the baseline drift in TCD detection even when a matched reference flow is used? I assumed that the reference flow would compensate for changes in thermal conductivity, etc in the column effluent and give me a flat baseline throughout the entire run.

When you say that the detector is a 'stock TCD' do you mean this is a standard Agilent TCD or the traditional type of Wheatstone bridge type of TCD. There is a BIG difference.

Gasman

I believe this is a stock Agilent / HP TCD detector. Is there a pathology associated with one or the other?

The TCD ( both types) are flow sensitive. If your TCD signal is drifting DOWN when you temperature program, there is a good chance that you have leak at one of your column connections. It is difficult to trouble shoot in a hot oven, so you can not use the traditional leak testing methods. Check ALL connections in the oven area or remake them with new ferrules.

You may well find that at room temp there is no leak, but as the temperature increases, expansion starts to take place and you have a leak.

Gasman