Greenhouse gases_ Negative peak N2O

[fra863]

Hello!

We' set up here a GC for greenhouse gases analysis, and i'm afraid w're experiencing a few troubles with our ECD...

Accuracy is really far from what we were expecting, and something strange is happening with our N2O peaks...

The fact is that a kind of ghost peak appears just before the N2O peak onto the chromatogram.. How can it be possible?

I firstly though that it could be due to oxydation of the detector, so.. Flushed with hydrogen during the hole night at medium temperature. Result : Still not ok...

I also tried to fit it by modulating gas flow rates, Detector T°, and oven temp, but it doesn't go better!

Actual settings are

Carrier gaz N2 20 psi
Makeup gas N2 10 psi
DEtector temp 350 °C
Oven temp. 50 °C
Detect Tension 300 mV

I hope someone could give me some informations

Have a nice day

Guillaume

[Don_Hilton]

What purity grade for the carrier gas?
What column?
Is this in a standard mixture or an atmospheric sample?
If it is a standard mixture, what else is in the mix? Is it a commericial mixture?

I am fishing around here looking for a clue as to something that would elute in the position you describe that enhance rather than trap electon current through the ECD, since you describe this as a negative peak in the caption for this thread.

If you could post a picture, that might help. The shape may give someone a clue.

[BZK]

Hi, negative peaks with ECD could be due to high amount of ionisable (and NO electron capturing) molecules, such as hydrocarbons. Remember that ECD is a "full current" detector in wich without eluted compounds the current of ionized make up gas is at maximun value. If an electron capturing capable molecule appears the current DECREASE (and the software reverse the instrument read). If any ionisable compound (non electron capturing capable) appear in high concentration the amount of ionisation INCREASE and the negative peak appear.
This, probabily, is your case.

Ciao.

Robertino Barcarolo, Italy

[nick576]

I belive the negative peak you are seeing could be CO2, we measure atmospheric greenhouse gases (CH4, CO2 and N2O by FID and ECD). but in our system CO2 only flows to the FID then a valve switches and N2O flows to ECD. If we let CO2 enter the ECD, we also see a small negative peak exactly where I expect CO2 to elute. usually CO2 elutes from column just before N2O, this depends on column of course. have alook at this paper: Zheng et al (2008) Plant and SOil vol. 311, p. 211-311

they show ECD chromatogram with N2O peak and a negative peak just before it (which they claim is CO2).

As to why your accuracy is off is another matter.
Nick

[AICMM]

fra863,

ECD's loose sensitivity for many constituents at high temperatures because they undergo thermal dissociation. This may be the case here and you are ionizing rather than capturing. First thing I would try is to lower the detector temp to 200 or 220 and see what that does.

Best regards.

[kahmark]

Did you get an answer for your N2O problem? It is advisable to keep the detector temp up, N2O is not detectable with an ECD at lower than 300C. Your flow rates also look low. What instrument are you using? Manual or automated? Maybe you should try a P-10 or P-5 gas that will quench the noise more effectively. The ghost peak may also be CO2.

Try making a standard like this:
Purge a 5-10ml vial with N2 for two minutes, add N2O to that vial to get a concentration of 1PPM. Do the same for increments of 1-5PPM. Then observe. Are ghost peaks still active? Can you try a fresh std gas?

Let me know!

Kevin