Greg,
In my manual, and following the instruction of the Thermo service engineer
this is what I have:
Costech ZBA nut, 1 viton o ring bevel up, flat washer.
I never tighten anything more than hand tight. It helps to double check
that all mating surfaces are clean and to loosen the legs on the autosampler
before tightening the nut. This lets the AS go where it needs to seal
properly. Adjust the legs for support later.
I'm curious to hear more about the new He flow setup and if it is related to
your problem. Is the top He fitting plugged or absent altogether? If you
have an isolation valve, are you leak checking with it closed?
Good luck,
Benjamin Harlow
Washington State University
School of Biological Sciences
P.O. Box 644236
Pullman, WA 99164-4236
Office: 509-335-6161
Lab: 509-335-6154
Fax: 509-335-3184
Laboratory for Biotechnology and Bioanalysis Stable Isotope Core
[log in to unmask]
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Greg Cane
Sent: Tuesday, August 30, 2005 3:35 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] TC/EA leaks
Dear TC/EAers,
At the risk of sounding frustrated, which I am, here's a simple
question for those screwing the fittings at the top and bottom of a
TC/EA column. I am having leak issues at the top fitting of our
column. This is a relatively new instrument purchased through
Finnigan along with a Delta Plus MS. I am monitoring mass 40 while
purging argon around the fitting and despite my triple checking the
order in which the metal washer and viton O-ring are placed (based on
Thermo's manual) and screwing the fitting as tight as I can get, I am
still having leak issues. I have performed leak checks both while the
column is at room temp. as suggested in the manual and at sample
operating temperatures. Despite having leaks at BOTH bottom and top
fittings at room temp., I figured the viton O-rings would be happier
making a seal with an increase in temperature. So, I've checked for
leaks again, at T=1350 C (for liquids) and sure enough, the leak at
the bottom fitting is no longer. However, the top is still giving me
problems as M40 rises steadily while applying argon. How tight is the
top fitting supposed to be? I can't get it any tighter by hand, and
I'm assuming this is not normal to be tightening it so much. My
background on M40 has been as low as 18mV (on previous column
installations), which I still find high. And I'm still not happy with
backgrounds on M30, which are around 200 mV. Thermo recommends limits
of less than 10mV on M40 (which is what I'm used to seeing with
IRMS), and less than 100 mV on M28, 29, and 30. My water signal is
fine. I was using viton O-rings from Thermo a couple of weeks ago
with much better M40 stability, yet still a very very slight
increase. I had to change the column this week, and I'm using O-rings
from Costech (part no. 061106). Any difference between the two?!?
Does it take a while for these O-rings to create a seal at high
temperature? Another question: At its tightest, the top fitting still
displays its threads and I'm wondering if this is normal? Other
potentially noteworthy info. I'm using a brand new glassy carbon tube
and alumina tube. The helium carrier is with the new setup of bottom
fed supply. I have also been able to tighten the fittings somewhat
with the oven at temperature, yet still a leak at the top persists. I
also have fans blowing on both the bottom and top fittings.
Unfortunately, I do not have an extensive amount of experience TC/EA
but this does not seem like rocket science at the same time. I know
where the leak is after all. Any recommendations will be extensively
appreciated.
Greg
P.S. Mike, thanks for all your help so far, but I had to throw this
one out to a wider audience. Frustration is getting the better of me
right now.
--
Greg Cane
Associate Specialist
Department of Earth System Science
University of California, Irvine
2224 Croul Hall
Irvine, CA 92697-3100
phone: (949) 824-6446
fax: (949) 824-3874
e-mail:[log in to unmask]
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