Gas sampling standards appear to have humidity

[Analisis]

Hello,

We are working with two equipments for CO2 Analysis. They are in two different locations and to make the calibration, we have one set of seven cylinders that are in an office that is between the two sites. For years the calibration was made with Tedlar Sampling bags. The procedure is to take the sample from the standard cylinder into the Tedlar Bags. It involves a set of 7 standards. We do so because of the costs of having two sets of cylinders in each site. This was the procedure. I was not involved with this procedure until last week. Now I am in charge of the calibration of this equipments. It was said to be an easy procedure. Our equipment is a GC with MSD, FID and TCD from Wasson. It has six columns and 5 valves. If you are interested I can attach the diagram of it.
Well when we injected one of the standard we got two peaks instead of only one peak. The standard was at 1 ppm. I rembeber that it was benzene, so because we have an MSD I have run an SCAN to see what is in the bag. The surprise was that I found a very large peak of water in the bag. We have then checked all the standards, And all have water. We have for example an Standard that have mercaptans at 0.1ppm and we have this huge peak of water almost the same size of the Nitrogen peak that is the balance of the standard (The components are in Nitrogen). Well then we supposed that the problem was in the taking of the sample into the bag. We sand two of the technicians with years of experience and they took the samples again. Again we got water in the bags. We have also checked the humidity in the bags with a moisture analyzer and it have in some cases 5 ppm. We were about 100 km from the standard cylinders so we coudn't check if there was humidity in the cylinders. So we have taked a sample from one CO2 tank and have analyzed it in the GC, the answer was humidity again. We have checked the bag with the moisture analyzer and gave 5ppm of H2O then we checked the tank of CO2 and it gave us 1.5 ppm. So the problem apparently is in the sampling and we are taking some water from the athmosphere.
Somebody knows what is the correct form of taking a sample with Tedlar bags so that we don't get moisture from the ambient.

Thank you for your answer.

Regards,

Adolfo Alvarez

[Yama001]

It is possible the gas standards are humidified. This may not apply to your application, but Summa cannisters typically use humidified air. It has something to do with achieving a stainless steel surface with a layer of water molecules - this reduces interactions with active analyes.

[AICMM]

analisis,

Several comments. First, are you sampling with a valve or a syringe? It could be the process of transferring with a syringe that is getting contaminated. Second, you never saw the peak before? This would suggest that your current valve timing may have been adjusted and now water is coming off when before it may have been backflushed. Third, I would not expect Tedlar to be perfectly impermeable to water. I know for a fact things get out of Tedlar, it would not surprise me at all that some things get in as well over a period of time. Finally, you might consider an aluminized tedlar as an option. Alltech used to carry these in their catalog (looked at an older catalog tonight) and they might still. Or Summa's or Whitey bottles if you have enough pressure.

Best regards,

AICMM

[Analisis]

Hi,

Well it is some time since I have posted this issue. Well we have the cylinder standards in one of the labs now. When using the standards we don't have water. And the problem was resolved. In this issue.