Steve
I agree with our collegues regarding the teflon valves. Some do tend to "breathe". Our carbonate vessels are a straight tube (Rotulex 19/9 Joint) with a side arm. The valves are ACE but with the glass stem. As for the vacuum greases we do use th Apiezon in the following way:
Apiezon M: Day to day use at room temperature and for 25oC reaction of calcite. We use a very thin coat on the viton o-ring at the valve to reaction vessel seal and on the twin outer seals of the valve stem but no grease on the inner ring.
Apiezon T: Is fine for 50oC for a period of 2 to 4 days. The M is too liquid at this temp.
Apiezon H: Is good for longer reactions at 50oC such as magnesite (3 weeks min) with no atmospheric leakage.
Apiezon N: The particularity of this one is that it seems to rubberize. It is good for long term seals such as vacuum traps at the pump. When attempting to remove it it "cracks" creating branching air leaks much like the silicon vac. grease.. However we don't use it anymore
We tend now to only use the H and M. Almost all of our extraction techniques are greasless including the sample trapping (breakseals). This includes hydrogen reduction (Zn or Mn technique), CO2 and SO2 gases as well as MS connections.
I hope this is of some help
Gilles
Gilles St-Jean
Technologue de Recherche
U d'Ottawa/ Géologie/ Laboratoire d'Isotope G.G. Hatch
140 Louis Pasteur, Ottawa, Ontario, Canada, K1N 6N5
Bureau/Office: 1-(613) 562-5800 ext. 6839
Lab: ext. 6836
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-----Message d'origine-----
De: Steve Nelson [SMTP:[log in to unmask]]
Date: 2 juillet, 1997 14:19
À: [log in to unmask]
Objet: high residual gas pressure
Dear Listmembers,
Once again I hope to enlist your help. I have built a conventional
carbonate line and all seems to be working well now except one item--the
reaction vessels--which seem to contain high residual gas pressures [about
40 microns upon expansion into the manifold]. The pressures are high
enough that the gas transfer rates are slow. It takes 7-8 minutes to
freeze down 100 micromoles of CO2 and clearly that is too long. This is
after rough pumping overnight on the acid/sample and then pumping for at
least a couple of hours with the diffusion pump and a cold trap before
reaction. I have been letting samples react overnight. Once the CO2 is
trapped and the residual gas is pumped away, the CO2 will freeze again very
rapidly if I re-expand it.
My two legged reaction vessel design was copied from a colleage.
The upper half contains the teflon/o-ring stopcock and the lower half
contains the two legs for reagent and sample. They are joined with a ACE
o-ring connection and a clamp rather than a ground glass fitting.
I ran blank vessels to determine whether the residual gas was
related to the reagent or the reaction. I rough pumped on the vessels
overnight and then with the diffusion pump and a cold trap, then I closed
the stopcocks. I left the vessels on the manifold but let them sit
overnight [assuming the problem is not in the stopcocks]. I also torched
the bottom of the vessels before closing the stopcocks. Overnight a
residual pressure of about 40 microns resulted. Hence, the source of the
residual gas appears to be the reaction vessel itself.
It seems to me that if the problem were leakage, that it would be
unlikely that all of the vessels would produce such similar residual
pressures or leak the same amount. Any ideas as to the cause or a possible
solution would be appreciated. I've struggled with this for quite some
time now.
********************************
Steve Nelson
Dept. of Geology
673 WIDB
Brigham Young University
Provo. Utah 84602
voice: 1-801-378-8688
FAX: 1-801-378-8143
"INTEL INSIDE" is a warning label
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