Chromatography Forum

We have an SRI GC with FID that we use for C1-C7 mixture assays.

I've been having a re-occuring problem with dropping response over the day. The response will recover if no injections are made for a long (1 hour or more) period of time.

I discovered a few weeks ago that this instrument has a methanizer on it. As far as I can tell, the methanizer is integrated into the FID. (The instrument's manual only discusses a seperate methanizer, which the actual instrument does not have. But, while the FID does not respond to N2, it does to CO2. So, there is a methanizer in there somewhere. A label on the instrument says "METHANIZER JET IN DETECTOR BODY...")

Anyway, as an experiment, I tried lowering the FID's temperature from 385 C to 230 C (the oven's temperature ramp goes to 220 C). The response went up for C1 significantly, up for C2 slightly, and down for C3-C4. (The standard I'm running at the moment only goes up to C4.)

Is this change in response typical for changing the temperature of the FID? Or is this an indication that the methanizer may be the cause of the dropping repsonse?

(I'm thinking about replacing the "methanizer jet" with a non-methanizing one, but I don't want to spend the money and the time if there's no evidence that the methanizer is the problem.)

Your post was quite interesting.

YOur unit has a nickel catalyst in the FID jet assembly and this could be the reason for your problem.

See http://www.srigc.com/2005catalog/cat85.htm

I would carefully use air as the carrier gas (remove your column first if needed) and raise the temperature of your detector back to 380°C.

Burn off any carbon deposits in your methanizer and see then if your response returns.

Otherwise I would replace the FID jet with a normal FID jet. Didn't you discuss this problem earlier on the forum?

best wishes,

Rod

Hi Michael

An FID is about the most robust detector there is with respect to operating conditions and although you might see a small change in response with a change in temperature I would definitely not expect a different change for different hydrocarbons.

That only leaves the methanizer as the culprit. I would replace it with a plain FID jet.

Peter