Recommend Detectors for trace CH4, CO, O2, N2

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Hi,

I would like to have some advice on choosing detectors for my gas sample. My sample is gaseous, which contains mainly CO2 saturated with H2O vapor, and trace of CH4, CO, O2, N2 (ppm). I have a decent separation of CH4, CO, O2 and N2 using CarbonPlot (Agilent) capillary column (CO2 and H2O come out at the end of the run when oven temperature is high) . I am using Agilent 6890N GC (FID and TCD as detectors) which is coupled with 5975 MS.

Do you have any recommendation how to best run this for quantitation of CH4, CO, O2 and N2?

I am currently using GCMS for this task and running under SIM mode (Selected Ion Monitoring), which works alright for me. But most literature uses TCD for CO and CH4 in gaseous CO2+H2O, which puzzles me a bit.
knmoore,

There are a lot of postings on measuring fixed gases on the site so it is worth searching through for those. Basically, MS is not the ideal solution for several reasons, low M/Z, air interference, small bore columns, etc.... This is why a lot of people use TCD to measure fixed gases and, as long as you don't need to also measure hydrogen or go too low, it works really well. You can expect DL's on the order of 10's - 100's of ppm depending on the injection volume. You should be using a gas sampling valve and you can probably order a mix of components from your local gas bob's that you can use for quantitation. The TCD is very linear so I would order on the upper end of the expected levels and then assume through zero if you are pressed for cash. If you are using a gas sampling loop, remember that pressure will come into play and you want to measure sample and standards at the same pressure.

For higher level analysis where you also have to measure hydrogen, I would recommend an HID for which my standard disclaimer applies.

For trace level gas analysis, you should probably use a helium ionization detector (HID) either AIC, Valco or Gow-Mac and of course I have opinions on that..... This is really the detector that is capable of low level fixed gas analysis.

So, when you say trace, how low do you really need to go?

Best regards,

AICMM
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