Hi Linda,

Beam 1 of 5e-10 sounds quite high for normal background, my machine is
usually around 1 to 2e-10 and if it gets up to 3e-10 I am looking to find
why the background is higher than normal. What seals are you using at the
top of each column (combustion and oxidatation). Are you using O-rings or
flat seals. I use flat seals, available from Europa or other consumable
people, and these seem to be more reliable than o-rings (most people not
using flat seals use 2 O-rings to ensure a good seal). Anyway I sometimes
find the top fitting, especailly on the combustion tube is not always done
up tight enough, and just a 1/4 or 1/2 turn can bring the background down
(my hands aren't strong enough to do this up properly, so I tend to just
use a pair of large mouthed pliers or vice grips, but be warned about
over-tightening or you'll crack the top of your tube!).

Otherwise what were the last samples you ran before Xmas? It is possible if
they had halides or sulphur in the samples that your combustion tube is now
no good. I sometimes find when running C&N samples that it is best to
change the combusiton tube straight after running a nasty batch, so the
residues don't sit there and react with the Ag wool. Could be your column
is now exhausted. When my columns become quickly exhausted due to the Ag
wool problem, I will tend to reuse the CrO3, as it is so expensive and I
have found no problems with reuse. Generally I throw the CrO3 away, as most
columns last around 2 months for me, but if I have to change a column
within 1-5 weeks, I may reconsider using the CrO3, based on the no of
samples put through etc.

I tend to agree with David that you have probably developed a hairline
crack in your column, usually around the Ag wool/CuO level. But once you've
found your problem, I would also try and see if you can't bring the
background down even further from 5e-10. It is not a huge amount, but for
small sample N, every little bit helps.

Cheers

Karyne


We are using the Europa ANCA-SL 20/20 in our lab, and I am seeing a higher
than normal signal for nitrogen in beam one.   It is normally around 5e-10,
but now it is 1.8e-9.

It was fine two weeks ago, and I ran several samples on the SL with very
good results.   The instrument was placed into standby mode on December 22,
and our lab was closed until January 2.   I replaced the quartz liner in
the combustion tube and allowed the system to purge before opening the
isolation valve.    I also baked out the GC column for 4 hours at 120C.
Today I am still receiving a high signal.   I could not detect any leaks,
and the tuning parameters are good.

Any suggestions?


Linda Osborne
Arizona State University

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Dr Karyne Rogers
Stable Isotope Laboratory Team Leader
Institute of Geological and Nuclear Sciences
PO Box 31-312
Lower Hutt
New Zealand

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