Hi Patrick,
there was little response to my e-mail sent to the list on 16. March 2006,
in which I described our mysterious nitrogen experience. So I'd like to add
some more facts, perhaps pointing out parallels with your problem.
In our laboratory a poor standard deviation of d15N values occurred with
organic samples, while standards N1 and N2 were OK. The problem first
occurred by the end of 2002, after several months without this effect (we
installed the equipment during summer 2002).
The poor N repeatability of organic samples was accompanied by a peak
tailing and a decrease in d15N values, while d15N was positively correlated
with the total peak area. The d13C accuracy and precision were not affected
at any time.
After a long time of troubleshooting we found out that the N-peak tailing
increased when the combustion was improved, i.e. a brighter flash produced
a broader tailing. Also addition of V2O5 worsened the peak form. The use of
Ag instead on Sn capsules reduced the tailing but did not completely solve
the problem.
Finally we ended up using a sampling delay of 0 seconds which consequently
decreased the effective combustion temperature. For our organic samples
this works fine (SD = 0.03 permil for nitrogen) and was confirmed with
several standards.
So, I suggest to test this setting in your lab and to post the results to
the list.
Though we found this indirect solution, I am highly interested in getting
an explanation for the observed effect. Moreover, any advice that would
allow to use normal EA settings for organic N analysis in our lab would be
appreciated.
Good luck,
Joachim
At 17:29 28.03.2006 -0500, you wrote:
>Greetings,
>
>We are having trouble with N analysis using a Costech EA interfaced to a
>Delta Plus XL via a Conflo III. This problem may be similar to a listserve
>thread back in 2004 (see Item #009153) to which I could not determine if a
>solution was presented, so here's the problem today...
>
>Salt standards (N1, N11) run very well, but organic N standards (e.g., wheat
>flour, peach leaves) return higher than expected 15N. The deviation worsens
>with increased weight of the organic standard with up to 10 permil 15N
>enrichment at mass 28 peaks of 4 volts. Throughout this problem 13C has
>remained nearly perfect with all standards.
>
>In my own tests, I have added C sources (sucrose, cellulose) to N11 salt and
>identified that the problem seems to be related to the amount of organic C
>present (linear increase in 15N from mass 44 peaks of 1 to 8 volts).
>Moreover, there are different effects from different C sources. Cellulose
>caused the greatest effect (10 permil), while sucrose only had a slight
>effect (1 permil).
>
>The system has worked reasonably well for more than 5 years, but this
>problem just appeared about a month ago. We've tried changing all columns
>(numerous times), water trap, baking out the EA column, retuning, checking
>linearity, etc.. Also, increasing the O2 injection volume (from semimicro
>to macro setting) actually worsened the problem by an additional 1-2 permil.
>
>In light of the sudden nature of the change, and the fact that C type is
>important in the effect, we're running out of answers. Does anyone have any
>suggestions as to what might be happening? We would be most grateful for
>any advice...
>
>Thanks in advance,
>
>Patrick Inglett
>University of Florida
>Department of Soil and Water Science
************************************************************************************
Dr. Joachim Molkentin
Bundesforschungsanstalt für Ernährung und Lebensmittel (Standort Kiel)
Institut für Chemie und Technologie der Milch
Hermann-Weigmann-Str. 1
D-24103 Kiel
Germany
Tel: +49 (0) 431 609 2224
Fax: +49 (0) 431 609 2300
Email: [log in to unmask]
Web: http://www.bfel.de
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