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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