Patrick, 

We had a similar experience when running dual isotope analysis on
organic materials. This appeared to be inteference from CO. Have you
tried tried to do the same tests with a CO2-trap installed?

--Per

> -----Original Message-----
> From: Stable Isotope Geochemistry 
> [mailto:[log in to unmask]] On Behalf Of Patrick Inglett
> Sent: Wednesday, March 29, 2006 12:30 AM
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] EA nitrogen mystery
> 
> 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
> 
>