Dear Bruno and interested Please take note of new phone numbers :

> >enrichment become seriously problematic, if any. I'm suspicious
> that these
> >problems are smaller than they are often rumored to be, and that use of a
> >single instrument for both natural abundance and enriched applications
> >(even beyond 1000 per mil) is possible if sufficient care is taken to
> >monitor carryover, making sure that, after running a batch of enriched
> >samples, carryover is no longer occurring before switching to natural
> >abundance samples. Others with experience here care to comment?

For the past 5 years we have been analysing N-15 enriched samples (up to
5000 permill) on the same system (Carlo Erba NA-1500 NC interfaced with a
Micromass Optima mass spec) that we also use to determine natural abundance.
I agree with Willi's comment : memory effect exclusively occurs in the EA
and our experience leads me to believe that it's almost occurs in the top of
the combustion furnace. In test batches we have notice carryover, only on
the order of 3-5 permill, on natural abundance standards immediately after
enriched (3000 permill) samples when we use a new combustion furnace ; this
carryover had disappeared with the 2nd or 3rd thin capsule after enriched
samples.
On the other hand, the same type of test batch had given a carryover on the
order of 20-25 permill when the EA system was nearing exhaustion (with a lot
of accumulated ashes in particular) and this memory effect only disappeared
for the 5th capsule of natural abundance standard.

My conclusion is that it's possible to have N-15 measurements of natural
abundance and enriched samples with a good precision on the same system with
a minimum of precautions :
- In a the same batch, you have to classify your samples in order of
expected enrichment
- Immediately after enriched samples, blank or natural abundance standard
(at least 3 capsules) have to be burned to "purge" the system
If you're too afraid by the rumours, you only have to change the chemicals
of EA...


Hope this helps.
Regards.
Jean-Noël

Please take note of new phone numbers :
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Dr. J-N Thibault           Mass Spec. Lab.
INRA St-Gilles             UMRVP
Domaine de la prise        Tel:(33)2 23 48 5067
35590 SAINT-GILLES         Fax:(33)2 23 48 5080
  FRANCE
  Email : [log in to unmask]
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