TCD Problem

[Virnia]

Please need your help!!

I´m working with natural gas samples. The concentration for the nitrogen has been around 0.51%; the standard gas has 0.51% nitrogen. But in the last week, this value has been very inestable, going from 0.1 to 0.8, with the patron, and even worse with the samples.
The parameters are the following:

Heater=200C
Reference flow= 45 ml/min
Meke up flow (He) = 2 ml/min

Please, some idea; I´m brand new in GC.

[GasMan]

Is it just the nitrogen that is varying or are all other compounds varying.

Gasman

[Virnia]

Just the nitrogen is varying.
Also, after recalibrating, it maintains its value for almost two or three samples, for the next samples it starts to decrease.
Thanks for the response =)

[GasMan]

This suggests that you are getting air into the system. Do not use any plastic tubing to connect the sample/calibration to the GC. Another source can be the regulator on the gas tank. If the regulator is not a high quality version with stainless steel diaphragm but uses a rubber diaphragm, air will diffuse through the rubber into the regulator. You can try purging the regulator each morning or after the system stands idle for any length of time by the following technique. Open the main valve on the gas tank and set the pressure to a higher value than you normally use. If the regulator has an ON/OFF valve at the outlet set it to on. Close the main valve on the tank and wait until all pressures have dropped to zero. Repeat this 3 or 4 times by opening the main valve on the tank, closing it and waiting for the pressures to drop to zero.

Gasman

[Virnia]

Hi Gasman!

Thank you very much for the reply; but I don´t think that´s the problem, we´ve already checked the fittings, and we use a high quality regulator; also, the TCD is not detecting the nitrogen anymore, it seems the peak is to small, but it the daniell cromatrograph detects it in normal way. So... what could it be?
We´re trying to adjust the sensitivity value for the TCD, but it seems no to be helping.
Also, have to mention that the equipment it´s on 24 hour a day, because of the number of samples we´ve to run; so, the time for adjusting and checking problems is very limited.

Could it be some hardware problem?
Thanks for the time.

[aidnai]

Hi,
Gasman could be on to something. The fact that only the nitrogen is changing is a big clue that could point to air in the carrier gas. How did you check the fittings? do you have a helium leak sniffer? If you perform the regulator purge as Gasman described, does that change the results at all?

Also, does the baseline drift at all? that would be another important clue.

Aidan

[cravo1975]

Hi,
You probably have an internal little leak in pneumatic system, check using a leak detector or foam itself to the service.

[Virnia]

Well.... after as my turn was over (fourteen working days regimen), they continued working with the error we´ve talked about.

Today, we changed the Helium, and now.... I see.. the nitrogen has gone!!, it´s not detected anymore. Apparently, the base line increases at that point, and the nitrogen it´s not detected.

What would be the problem?

[aidnai]

Hi Virnia,
Have you tried any of the suggestions made so far?
Aidan