I would like to ask for help regarding the problem that we have encountered with our GC system. Instead of cylindrical gases, we have acquired two gas generators, one for hydrogen and the other one delivers compressed air and nitrogen. We used NPD as our detector. The problem that we have noticed si with our baseline. We have observed a cyclical noise pattern in our baseline. The noise look pretty much like a symmetrial wave form that goes up and down, mostly in the same amplitude. This is problematic for us since we need to detect very low sample concentrations and this noisy baseline just is not acceptable. I will appreciate any comment/advice that you can give me regarding this problem of ours. Thank you all in advance.
By Anonymous on Thursday, August 29, 2002 - 07:53 am:
Obviously, first substitute cylinder gas for one generator, then the other, to isolate if these are involved. Then you'll know if the generators are the issue.
By Anonymous on Thursday, August 29, 2002 - 05:32 pm:
We have traced the problem to be coming from the nitrogen/compressed air generator. What is the possible sources of this problem in the generator?
By R. Plausinaitis on Friday, August 30, 2002 - 04:07 am:
I presume air suply is more likely to cause problems.
Thing is, that compresor pumps air to some presure value then stops until low presure value is reached and switches on again. So the presure is all the time cycling. NPD response is dependent on air flow in detector. If your GC is not able to filter presure surges and suplies detector with cycling flow of air detector signal will cycle as well.
If it is so, you might solve this by:
1. trying to set other value of the air flow to detector - they usualy have a plateau (flow/response graph might be in instrument manual);
2. incorporating in air suply system some kind of "buffer" container which could make presure fluctuations smaler;
3. ajusting compressor switch on/off values to be closer which also will make presure fluctuations smaler.
Sory if my explanation is not clear - I'm not English native.
By Anonymous on Friday, August 30, 2002 - 07:52 am:
OK, I was first replier. I'd add an in-line adjustable pressure regulator just before your nitrogen/compressed air generator, setting its pressure just below the lowest pressure delivered by your air compressor. This is easy to do, and quite inexpensive, you won't need a licensed plumber to do this if you're handy at all; we've done this several times ourselves for similar reasons, as well as for reducing house air pressure to the desired pressure.
By Anonymous on Friday, August 30, 2002 - 06:45 pm:
Dear Mr. Plausinaitis and Mr. Anon,
Thank you for your advice. I will try your suggestions and hope it will solve our problem. Mr. Plausinaitis, your explanation was very clear and I understood what you wrote very well. I will get back to you both with the results later. Thank you for your advices!
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