By cecile on Tuesday, July 13, 2004 - 07:32 pm:

Hi everyone!

I am using a GC set up with capillary column, nickel catalyst and FID detector to detect CH4 and CO2.

I inject manually (syringe) the samples into the GC. They are samples from an atmosphere in equilibrium with sea water.

I got a negative peak followed immediatly by a positive one (image in a mirror) just before the CH4 peak at about 2min time. It doesn't change with the volume injected and I get it as well if I use tap water or superQ instead of sea water.

Doesn't appear with calibration gas mixture.

Does anyone have an idea what could it be???

Cheers
cecile

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By jay on Thursday, July 15, 2004 - 10:47 pm:

Generally I have seen negative deep anything injected with GSV or even some times GAS injected with manual injection (syring ) may be due to flow disturbance which reflect as a negative peak.
some times if carrier gas is HE & calmix may be in He so you may not get-ve peak while your sample contain N2, moisture.

Considering your dought in calibration mix no -ve peak reason may be concentration of calmix is high may be -ve peak is there just lower the scale .

This _ve peak really affects your integration ? change flow .

jay