Static Gas Standard Mixing Issues

Discussions about GC and other "gas phase" separation techniques.

4 posts Page 1 of 1
Hello,

I have been mixing my own gas standards for a while now, without issues. Usually I am mixing H2, CH4, C2H6, C2H4, N2O in Helium, either all together or a selection thereof. I usually run calibration curves from 0.1 to 100 ppm in duplicate or triplicate. I mix in 100 mL serum bottles which I flush with helium, release over pressure to bring to atmospheric, then I use gas tight syringes to add my gases of interest (which should also be at atmos. pressure in the syringes). I then dilute the standards in 20 mL vials flushed with helium and released of over pressure. I know the true volume of my serum bottles and analysis vials (filling with water in triplicate and weighing). I hand mix two different stocks, one at 10000 ppm and one at 1000 ppm, so the final standards are made from both hand mixed stocks, which I figure would tell me if I've got mixing issues. I've always gotten very good calibration curves R2>0.99.

I have a GC with two PDDs optimized for different gases than can run in tandem on the same sample, as well as an FID that can run in tandem with one of the PDDs.

Recently I added CO2 to the mix and it isn't going well. Not only is my CO2 all over the place, it seems to be affecting my other gases as well.

Thinking it was a mixing issue, I tried a suggestion in Modern Practices of Gas Chromatography, which is to add small pieces of aluminum foil into the mixing bottles to supplement diffusion mixing. This helped my upper level standards (10, 100, 1000 ppm) but the lower standards are still all over the place. The CO2 calibration curve is just complete garbage in the standards with Aluminum foil, but is ok in the comparison standards I made normally, making me think that perhaps the CO2 is reacting with the foil. The low CO2 normal standards were still wrong though.

The reason I want everything mixed together is because 1) analysis time will be ridiculous if I run separate standards and 2) my environmental samples are a mix of these gases, so for the best calibration of the samples I really should have a mixed standard.

Does anyone have any suggestions for what is going on with my standards? I'm hoping to get a certified mixed standard of these gases at some point, but it's expensive and currently my project is unfunded.
The only thing that comes to mind is that your original gases are not present in the atmosphere to any extent but CO2 is present, and this is the source of your problem.

Gasman
GasMan wrote:
The only thing that comes to mind is that your original gases are not present in the atmosphere to any extent but CO2 is present, and this is the source of your problem.

Gasman


That sounds reasonable and would be a problem if the Helium is not flushing all the CO2 out of the container. To fix I can see needing to either use a lot more helium to flush the container, use a container you can pull a strong vacuum on before filling with helium, or possibly use Argon instead of Helium since it is more dense and may force the CO2 out better than the Helium. Of course I am just working the problem in my head since it has been a long, long time since I did any work with gasses.
The past is there to guide us into the future, not to dwell in.
Hi GasMan and James

Thanks for the responses! Apparently the notification I set up for this wasn't working.

I also prepare and analyze Helium blanks the same way as my standards (and at the same time), and they are always clean, both in serum bottles and in vials. I also flush the vials for 10 minutes after capping, so the pressure inside is around 1 bar, the serum bottles are flushed for 15 minutes, also at 1 bar pressure. Both are over-pressurized before removing from the helium. I also randomize which bottles/vials I use for He blanks vs standards so that there isn't a subconscious bias in how I'm preparing the blanks vs the standards.

There is some background CO2 and Methane in the system itself, most likely from the instrument, however, those background levels remain consistent, and they don't change when I try hand injections either, so it seems to be the normal background for this instrument (it's a custom built multi-valve, multi-detector).

However, I do think this may actually be a problem with the syringes I am using after further tests. This is rather problematic because I don't have much reason to doubt either of the syringes I am using, but they don't agree with each other. I recently switched the lower volume syringe due to moving to a different working group. My higher volume syringe is the same. Previously the standards made with the higher and lower volume were in perfect agreement. I also have more reason to doubt the previous low range syringe since it was ancient and cumbersome. So now I guess I get to figure out which one is more accurate without a standard.

It's also nice to know that you haven't found any major faults with my procedure. Then again a major flaw might be easier to fix then what I have now... I'm, somewhat unfortunately, one of the few people with experience working with gases at my institute, and I don't have a ton of people I can bounce ideas and problems off of.

Thanks again!
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