Chromatography Forum

Hi Guys,

First off, the setup:

GC Model: Agilent 7890B
Columns: HP-Plot Q 0.53 mm x 15 m, CP-Molsieve 0.53 mm x 25 m
Detectors: TCD-Methanizer-FID
Carrier: Argon (30 ml/min)

We are using this setup to detect trace hydrocarbons from the electrochemical reduction of CO2 in the gas phase. Products concentration is very low so the main component in the sample is CO2. Products we detect are CO and H2 (mainly) and then light hydrocarbons (methane, ethane, ethylene). With some valve switching, we can exhaust all the CO2 before the methanizer and with the current method, we can resolve all peaks quite well. The method takes 30 min roughly (this is without any oven ramp).

The issue I face is that every time I ramp oven temperatures with the FID in-line, the FID flame extinguishes. This also happens with increased column flows so I'm guessing it may be a column pressure issue? The FID baseline is quite low, ~0.5-1 pA when the TCD and FID are in-line. ~1.5 pA when the FID is isolated. It has been like this since the installation (about a month ago). All gases are UHP from cylinders with H2 flow = 45 ml/min and Air flow = 500 ml/min (Agilent recommended for methanizer-FID).

Any suggestions on how I can stabilize the flame?

Cheers

Reduce lit offset in FID configuration.
Make-up to FID = ?

You may need a flame jet with a bigger diameter.

Peter

Thanks for the responses!

The lit offset is already set at 0.3 pA so I don't think that's the issue.

Thanks Peter,

I have a larger jet in my tool box that I could possibly try

... This also happens with increased column flows so I'm guessing it may be a column pressure issue? ...

Nope. Pressure can not extinguish FID flame. The reason might be column flow, exactly linear velocity of gases in jet orifice. Increasing jet orifice diameter leads to decreasing linear velocity of gases leaving jet. There may be other reasons of flame instability thou.