We are trying to quantitate less than 25ppm CO2 in a gas sample. We have an AI94 GC fitted with a methaniser at the base of the FID.
I can obtain good peaks for CO but not CO2, when I have obtained a CO2 peak then it has a massive tail. I have tried Porapak Q,QS,N,and T but none improves the peak shape. I am sure I have a catalyst problem.
I would be obliged if someone could give me some operating hints and advise me of a catalyst supplier.
By Dave on Thursday, December 16, 1999 - 09:35 am:
I am not familiar with your GC, but we made our own methanizer using nickle catalyst from HP (Part Number 5080-8761). Heated at 380C. Use a 14 foot Molesieve 5A column (45/60 mesh) from Resteck (special order it that length). But we only see CO, not CO2 (well). I am sure you could see both using a Rt-QPLOT at subambient temps. You may just need to increase the temp. of your catalyst?
By Bill Jenko on Saturday, December 18, 1999 - 11:05 pm:
Virtually all methanizers I know of are the same: a nickel catalyst packed in a tube and heated to 380 C or hotter with a steady supply of hydrogen. The most common problem with methanizers is keeping the catalyst hot. If the packing is allowed to cool, usually near the ends of the column, it will stop working. Generally, they either work great, or don't work at all. They are poisoned by Sulfur compounds and can be deactivated with large quantities of some higher hydrocarbons, especially alkenes and alkynes. It's unusual to have a methanizer that will convert CO, but not CO2.
What column(s) are you using to separate CO and CO2 from the balance of the sample? If you are using a Molecular Sieve, the Mole Sieve will not elute CO2 except at very high temperatures and then with a nasty tail.
Porous polymers, such as Hayesep or Porapak will separate CO2 nicely, but CO is hard to separate from H2, N2 and O2. One trick we do to separate CO from H2, N2 and O2 is to use H2 carrier, separate CO2 from N2, O2 and Methane on Hayesep Q (2 or 3m x 1/8" at 60C is plenty) then put a longer Hayesep Q column downstream of the methanizer (we call ours a methanator). Typically, 3m x 1/8" at 60C is enough. The CO is converted to Methane in the methanizer, and the CO peak (now Methane) is separated from the N2 and O2 in the column downstream of the methanizer. Works for us.
Seimens Applied Automation, Inc.
Posting is currently disabled in this topic. Contact your discussion moderator for more information.