Odd oscillating baseline signal on 5890 (FID)

[ForestryUBC]

Hi all

A bit of a puzzle here. I recently installed a new methanizer catalyst tube since the last one died (no CH4 or CO2 peaks). I now see CH4 and CO2 peaks, however the baseline oscillates, but at least the methanizer works.

After calibrating the gas flow rates and letting the system settle for an hour, the Signal 1 baseline oscillates at ~69 +/- 5 over around 30 seconds from crest to crest (very regular!) and I can see it on ChemStation. The service tech thought it might be a dirty FID, or faulty gas cylinder regulator, or the wrong delivery pressure set on the regulator. The gas cylinders and regulators are the same since before the old methanizer tube died, and the delivery pressures have not changed (all the flow rates are correct and properly calibrated).

I've cleaned the FID collector housing and installed a new FID jet (packed column) but am still seeing the oscillating baseline. When room air is injected I can see the baseline oscillating beneath the CH4 and CO2 peaks. My concern is for anyone wanting to analyse low levels of CH4, and for reproduceability of data, i.e., if I inject a standard and the peak elutes during an oscillating baseline "peak" compared to the next standard injected where the peak elutes during an oscillating baseline "valley". The room air CH4 peak accounts for 34% of the peak area, CO2 57%, and 4 visible baseline oscillation peaks accounting for 1% each.

My He carrier gas is running low (400 psi left) but I can't imagine that would cause the regular oscillation. So I don't know if the oscillation is being caused by some physical problem, an issue with chemistry (how the gas is ionizing inside the FID), or if this is some electronic issue. Any thoughts are appreciated!

Thanks!

[osp001]

My He carrier gas is running low (400 psi left) but I can't imagine that would cause the regular oscillation.

Just a longshot- any chance you're using a single-stage regulator?

[Don_Hilton]

A quick double check. Do you have at least a 20 lb pressure drop across each regulator - the one on the bottle and (probably more important) the one(s) measuring gas pressure in the GC? That 30 second oscilation sounds pretty familiar - and is often caused by trying to drive a regulator at too low an inlet pressure.

[ForestryUBC]

Hi osp001, Don: The He regulator is two-stage (I believe!). I am able to adjust the delivery pressure... How do I determine if there is a 20 lb pressure drop across the regulator? Does it matter which gas regulator, or should I consider all gas regulators in use? Which pressure in the GC should I be comparing the regulator pressure(s) to? Thanks!

[Yama001]

I usually try to ensure the bottle supply pressure is 80 to 100 psi so this (potential) issue never comes up. I hate tank regulators that will not allow a 200 psi max - there are a lot that only go to 60 psi or so.

You would want the supply pressure at the instrument to be ~20 psi above the highest pressure your instrument displays (it will be highest at the max program temp for a contant flow system). You can have pressure drops between the bottle and the instrument from bends, tees, filters, etc. A single stage regulator just before the instrument is nice to have, but if you keep the supply pressure high enough, it will not be needed.

Note I would think you may have retention time shifts as well if this is the issue.

[ForestryUBC]

After leaving the system on to equilibrate overnight, and relighting the flame this morning, the signal quickly dropped to 14 and stopped oscillating (!!??). I did not adjust any of the regulator pressures. I also noticed that in ChemStation in the Inlet Pressure Programs, the oven temp was programmed at a lower temperature (50 degrees) than what we actually use (80 degrees). I input the actual temperature, which automatically reduced the flow rate inside the GC. Not sure if that had any bearing on my problem... Very strange but the system seemed to heal itself! I guess it just needed a longer time to settle than usual.

Injection of room air looked fine. CH4 and CO2 peaks now make up all of the peak areas. I don't recall what the RTs used to be but will keep an eye on it.

Thanks, all, for your help and suggestions. Much appreciated.