negative peak in front of O2 peak - TCD

[Thomas85]

Hello everyone,

I have a special phenomenon on my TCD.
In front of the O2 peak the base line goes negative (negative area of ca. 200). The problem is that at this retention time I expect the H2 peak.
Because of that, it is problematic to detect low concentrations of H2 (e.g. 50ppm)

TCD chromatogram:
http://s14.directupload.net/file/d/3393 ... i8_jpg.htm

red: gas sample with 5000ppm H2, 25% O2, 72% N2 (H2 peak overlaps negative peak)
black: without sample injection (no negative peak)
blue: air injected (negative peak)

carrier gas: Argon 5.0
column: Mole Sieve 5A 1m, micropack

Is it possible to get a different separation of these three gases with another column?

Thanks and best regards

Thomas

[muGC]

My network is blocking your image, but I think I can answer this without seeing the chromatogram.

The negative peak is your H2 peak. You need to set up in the run time table to reverse the polarity of the TCD before the H2 peak elutes if you want it to be a positive peak, and then return to normal polarity after the peak elutes.

[Wissenschaft]

Just throwing this out there because I've encountered it before, but is your carrier gas purifier working properly? I've noticed the N2 peak goes negative when the purifier stops working.

[Johnny Rod]

If you have some hydrogen (or helium if it co-elutes) in your carrier then you'll get a negative peak when you inject a samples containing less/no hydrogen, it's called vacancy chromatography.