Hi all
We have been routinely analysing carbonates in a gas bench (Delta plus XP) 
for last 1.5 years. we obtained good precision of ~0.01 to 0.03 per mil for 
oxygen (in a single measurement) over a range of  44 signal 3000 mv to 20000 
mv (from 100 ug to several hundred ug level of pure carbonate). Over last 
one month the precision level has been bad (>0.1 per mil) particularly for 
low signal (like <7000 mv) or very high signal (say >25000 mv). The 
precision is generally good between 8000mv and 20000 mv even now. For water 
sample of ~ 12000 mv excellent precision is obtained even now.  My question 
is

1) Why precision goes bad at >25000 mv signal? Is it that the nafion tube 
cant cope up with so much of water produced n eventually affecting the 
source?
2) Why precision has gone down for <7000 mv signal when we were getting a 
good precision earlier for even 100 ug (~3000 mv) sample? In fact we were 
getting ready to push the machine for analysing even 50 ug sample without 
cryofocussing n normal configuration of gas bench (i.e. normal large vial).
Does it indicate detereorating filament condition after 1.5 yrs of usage? 
Trap/Box current though dont suggest that.
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Anindya Sarkar
Associate Professor & PI, National Stable Isotope Facility
Department of Geology and Geophysics
Indian Institute of Technology
Kharagpur 721302
West Bengal, INDIA
Tel.: 0091-3222-283392 (O) 283393 (R) 220184 (R)
        Cell: 09434043377
Fax.: 0091-3222-282268
http://anindya-sarkar.tripod.com
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