I have a MAT 251 and Delta+ XL, each instrument has different tuning
characteristics and isotopic / sample size linearities.  You should tune
each instrument for maximum linearity, or know what the voltage range is
for an isotopic linearity that is acceptable for you.  The new Isodat
software versions have linearity measuring utility programs, we generally
can get fairly good linearity from 0.5 to 7 volts in continuous flow on the
Delta, and even lower voltage ranges on the 251 with a dual inlet
system.  Linearities will change over time as the source gets dirty and the
filament wears, you may want to check this periodically, especially if your
calibration standards shift or get more variable over time.

It was not clear from your message, are you measuring voltages from 10 to
40 volts?  If so this is well above any voltages we have tried to
measure.  You might repeat the measurements in the 1-7 volt range and see
if you get better results.  You are doing it the right way with tank CO2,
but you might want to make sure you are not contaminating the CO2 by what
ever method you get it from the tank to your IRMS.  You also have to be
careful of the isotopic difference between your machine standard and
samples.  If you are using tank CO2 which is generally fairly negative, and
are trying to measure carbonate samples which are near 0 per mil, the
isotopic difference between the two gases may be large and this will
decrease your precision.

Hope this is useful,

At 03:10 PM 5/14/2003 -0700, you wrote:

>We recently installed a new Finnigan MAT253. After the install, we
>discovered that the isotopic results being reported by ISODAT were varying
>as a function of sample size. I would normally chalk this up to incomplete
>sample combustion or acidification, but these samples were coming directly
>from our CO2 standard tank. Finnigan has been totally unable to even
>suggest what the problem might be or how to pursue it. I think the problem
>is in how ISODAT (ver. 1.5) is processing the output from the 253, but its
>not at all clear from the manual how to troubleshoot such a problem. There
>is a -0.6 per mil reduction in d18O and a +0.3 per mil increase in d13C on
>CO2 samples from the same tank ranging between 10K mV and 40K mV in size.
>Any advise or tales of similar experiences would be greatly appreciated.
>Geoffrey Garrison, Associate Researcher
>Department of Earth and Space Sciences University of Washington
>63 Johnson Hall, Box 351310
>Seattle, WA  98195
>(206)616-8763, office
>(206)543-6327, lab
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Dr. William J. Showers
Dept of Marine, Earth & Atm Sciences
Box 8208
North Carolina State University
Raleigh NC 27695

(919) 515 - 7143 Office
(919) 515 - 7802 Fax
(919) 515 - 3689 Lab
(919) 515 - 7911 Field Lab
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