Chromatography Forum

Negative signal of the GC: I am using Hewlett 5890 Series II Gas Chromatography with FID detector and fused silica column (Rtx-200). It is giving negative signal with or without injection. Beginning of the run the signal is positive then turns to negative; during the running time the signal does not go back to positive, after end of the run the signal goes back to positive.

It sounds like your FID is detecting a bleed from your column which is decreasing with time. It then resets the baseline at the next start of run (autozero function)

Your bleed could be from the column or from somewhere upstream.

best wishes,

Rod

Are you running the carrier gas a constant flow or constant pressure ? If at constant pressure the volume flow rate will decrease as temperature increases, but this effect is usually cancelled by bleed from the column.

Peter

Thanks for responses.
I tested with different column but still the signal is negative.
The carrier gas flow is constant.

By the signal going to negative, do you mean that it decreases below the original zero, or that you see an upside down chromatogram ?

How fast does this change to a negative signal happen, and how far into the run ?

Peter

No, I did not see upside down chromatogram but base line shifted below to the original base line. For example at the beginning of the run (oven temperature = 80 C) the signal is +3.4 after 15 min the signal is -95 (oven temperature=210).

If you have the gas saver activated try deactivating it.

Peter

Thanks for your response. Our GC does not have gas saver.

I would suspect a leak somewhere that goes away as the oven heats up.

Either carrier gas contamination (a rather volatile one) from upstream or from a lubricant in the GC or a cracked column could cause the problem described. A problem in the FID itself is a less likely but possible issue.

good hunting,

Rod

You are working with constant carrier flow - what is the flow setting (in ml/min or cm/s) ?

Peter

Column flow= 2.9ml/min
He + column flow= 48.8ml/min
Air + column flow= 320.9ml/min
H2 + column flow= 43.3ml/min

I removed the column from the detector side. Seal the side of the detector. Start isothermal run: As soon as I press “Start”, base line moves to original zero to negative side than if I press, “STOP” it go back to original base line.
All over the process temperature, gas flow all other condition are same only I pressed start.
Any more idea…………

I would guess that your pneumatics are contaminated and the bleed is slowly decreasing with time.

Time to perform a thorough cleaning of your pneumatics.

best wishes,

Rod

chromatography1,

Do you have column comp on? Do you have a zero set on your GC?

Best regards,

AICMM