The high and inconstant base signal

[chau pha]

I'm using a Gas Chromatography with FID that uses hydro and oilless air.
The base signal is higher 30 pA, sometimes higher 50 pA. The base line goes up and down routinely each 15 minutes with a 6-pA-drift for each.
I don't kown how to correct it.
Help me.

[LCbob]

Hi

You say oilless air, do you mean cylinder?

If you are using a compressor you can get a wave on the baseline due to oil residue present in the compressor. Adding a filter helps.

Assumming you have an Agilent GC 30pA sounds very high for pure gases. Typically I see 5-10pA for a good system with 'pure' gases.

Hope this helps

[chau pha]

Thanks for your reply.
I use an oil-less air compressor, so this except the influence of oil residue.
May be dirty FID and collector cause high signal. But I don't know why the base line is very inconstant.

[Don_Hilton]

Do you have a carbon trap between the compressor and the back of the GC? The oilless compressor compresses whatever is in the air - including hydrocarbon contaminants. Do you have a pressure regulator between the compressor and the instrument? You need to know that the pressure coming to the back of the GC is at least 20 psi above the pressure set for the air inside the instrument - and is fairly constant. Be sure you have adequate pressure drop across regulators and a constant pressure soruce for fuel gas as well.

[chau pha]

I hace a zero air generator for the compressor so the air isn't hydrocarbon and water residue. I also have a pressure generator between the compressor and the instrument.
The signal goes down as soon as the oil-less compressor works on to make up for the lack pressure.

[DSP007]

Do you have a carbon trap between the compressor and the back of the GC? The oilless compressor compresses whatever is in the air - including hydrocarbon contaminants. Do you have a pressure regulator between the compressor and the instrument? You need to know that the pressure coming to the back of the GC is at least 20 psi above the pressure set for the air inside the instrument - and is fairly constant. Be sure you have adequate pressure drop across regulators and a constant pressure soruce for fuel gas as well.

Hi , sorry from "poiging inglish" , naturally Russian langvige.
1) If you have oilless compessor & carbon trap - some problems desolvied
2) What is air? If car smoke in windows or smoker smoke in hood- its a bad air
3) If you use Parker "zero air system" many years - catalisators dead.
4) We dont know your samples. May by "guano" (high boiling components) was sorbed in column & ingector and slowly desorbed many hours.
5) Realy actual ratio H2/air/make up gase on FID detector ? Can be drift gase ratio or rich gase mixture, as we sey "automatics trust but verify yourself "

[Peter Apps]

I hace a zero air generator for the compressor so the air isn't hydrocarbon and water residue. I also have a pressure generator between the compressor and the instrument.
The signal goes down as soon as the oil-less compressor works on to make up for the lack pressure.

You have answered your own question - there is something wrong with the air system, and you need to fix it.

Peter

[Don_Hilton]

I assume that it is a pressure regulator you have between the compressor and the instrument? If you do not have enough pressure across the regulator to drive the regulator properly, you will have changes in pressure downstream from the regulator that reflect changes in pressure upstream from the regulator.

Also, be sure of the tubing you have used for putting the system together. I have seen people use tubing that has not been cleaned for GC application. Such tubing may have a high background of oils in it, I presume from manufacturing.

[DSP007]

Also, be sure of the tubing you have used for putting the system together. I have seen people use tubing that has not been cleaned for GC application. Such tubing may have a high background of oils in it, I presume from manufacturing.

Hi Don Hilton
Tubing ?! Unlakely.
If I understand correctly, the problems were not from installing the device, but start now day ( after a long time).

PS
During Gorbachev prohibition times, for cleaning pipelines discharged (issied on hands) 96% ethanol. I have heard many cases where workers drank it, but I have not heard of its use for cleaning chromatograph tubing.

[Don_Hilton]

If you need to clean new copper tubing for use with a GC s ystem, I would suggest rinsing with acetone and DCM. I've done this before with new "clean" tubing and watch the washes come out of the tubing with a yellow color. Significant contamination.

(And for anyone trying this - you need to purge all of the solvents out of the line before attachign it to the GC. I would purge with nitrogen or hellium and use a heat gun to drive off the residues.)

[chau pha]

Finally, I found the error that cause the high & inconstant base signal is the oilless air compressor.
As usual:
- The inside pressure is set at 8 bar (P1).
- The outside pressure is set at 5 bar (P2)
When P1 goes down to about 7 bar (P3), the compressor work on automatically to make up the lack pressure. At that moment, P2 is also lower than P3, so this operation doesn't influence to the air line coming to GC system.

But now, P3 goes down to 4 bar that is lower P2 and this circumstance influences to GC system.

My solution: set the inside pressure at 3.5 bar (fID: H2 30 ml/min, Air 300 ml/min)
Results: constant baseline and a 20pA-base signal.

[Peter Apps]

Thanks for the feedback - I'm glad that it's fixed.

Peter

[chau pha]

I'm sorry.
I correct "My solution":
My solution: set the outside (I wrote incorrectly: inside) pressure at 3.5 bar (fID: H2 30 ml/min, Air 300 ml/min)
Results: constant baseline and a 20pA-base signal.