Isotope Geochemists,

A few weeks ago I asked the isotope listserver about the consequences of
running labeled N-15 plant and soil samples on my Delta + XLS system that
up to this point had only run natural abundance samples.  No one seemed to
have a clear answer on this issue, several of you thought it would be OK
with continuous flow.

We ran over 300 labeled N-15 samples, some as high as 10-12 wt % N-15
(+40,000 per mil).  I'm not sure of the accuracy of the IRMS at these N-15
concentrations, but our internal lab standard (NO3 +4.2 per mil) drifted up
to about 10 per mil more positive during the 3 days.

We then changed the oxidation and reduction tubes, water trap and let the
IRMS pump down over the weekend.  On the following monday the mass 29
backgound was high, but this went down to normal levels after about 3 days
and now the natural abundance std's are running normally.

We had run some labeled samples on a MAT 251 (static dual inlet instrument)
a few years ago and it took several weeks and a source cleaning to get the
backgrounds back down to normal levels.  This does not appear to be the
case with CF-IRMS, although we have only run 300  labeled samples so far.

Hope you find this useful,

Bill
----------------------------------------------------------
William J. Showers
Associate Professor
Dept. of Marine, Earth, & Atm Sciences
North Carolina State University

(919) 515-7143 - voice
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