I am using an HP 6890, FID. Without making any injections, the strip chart and computer integration program are displaying a rather excellent sine wave. Any ideas as to the cause of this or how to go about troubleshooting the problem? I'm ruling out gases because other systems are running off the same gases without any problems. Could it be electricity? Any ideas would be greatly appreciated.

Dale, sounds like an electrical problem. In past experience, if your sine wave (or other pattern) is regular/repeateing, it usually is something systemic. Do you have a timetable or some other occurence that has the same peroid as you sine wave? Are all you cables shielded?
With LC's I know we've seen a sine wave with a peroid of about 20 min. This is from the air condition cycling. I don't think that you'd see it with an GC, but perhaps that will get you thinking.

You can NOT arbitrarily rule out gas regulator problems, because each 6890 may have it's own built-in set. We have 6890's with MSD's, not FID's, so I don't know for sure how your machine handles gas flows, or if they all use electronic flow control.

Sounds like a classic case of mis-matched regulator settings, or some other regulation problem, probably with the FID flame gases.

Turn them up or down, and see if the period changes. Might tell you something.

Dale,

We have just had exactly the same problem - it is a fault in the flow controller for the make-up gas. Cost to repair was £2000!


Regards,

Ralph

Dear Dale,
Are you analysing on methanation catalyst? Are you doing PPM level analysis of CO CO2 in poluted air, then it is a phenomena of controller used for maintaining temperature of catalyst. The controller specifications are + or - 0.2 Deg C
if it exceeds ) 0.2 deg C it is observed as sin wave

Dale:
You said this was on an HP with FID. Have you tried disconnecting the column from the FID, and puting a cap on the inlet to the detector? You can use the standard HP nut and a graphite ferrule. Overtighten the nut slightly, so that the graphite ferrule closes to make a tight seal. For a packed column inlet, use a Swagelok plug.

Re-light the FID, on flame air and H2 fuel only. If the baseline still wanders in a sine wave, the problem is likely to be electronic, or associated with the air/H2 flame flow controls, and NOT the column, oven temp or carrier flow controls. Simple test, to narrow the possible sources of the problem.

Incidentally, do you see the same sine wave if you flame out??? If so, definately electronic.

Good luck!
Bill Jenko
Sr. Chemist
Siemens Applied Automation, Inc.