Patrick,
Do you have the same chromatogramm as months before. That seems your GC
does not separate well N2 and CO2. And in the ion source CO ions make
input to the 28 mass.
Sergey.



On Tue, March 28, 2006 4:29 pm, Patrick Inglett 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
>


-- 
Sergey OLEYNIK, PhD
Ecology and Evolution
Dept Biological Sciences
University of Illinois, Chicago
845 W. Taylor st.
UIC, 3270 SES (m/c 066)
Chicago, IL 60607
Phone: office 312-3550579
lab 312-3550600
Fax: 312-4132435
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