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- Posts: 3
- Joined: Fri Aug 23, 2019 9:53 am
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.