Hi Rick,

I am not sure if this directly answers your question about the mechanisms
that cause carryover but here are some trends I have noticed that I
thought relate to carryover of enriched 15N.

I think carryover is mainly affected by the sample matrix run through the
EA-IRMS system, as well as the IRMS being utilized.  Our lab has spent the
last year developing a set of 15N lettuce standards at baseline, 100, 300,
500, 1000, and 5000 per mil.  This was done to accommodate a range of
enriched 15N samples that were being submitted to our facility.  I can
usually run replicates of the 100 per mil standard with little to no
carryover (results have ranged from 0.2-0.5 per mil precision) while
carryover at the top end can range from 5-10 per mil (not bad considering
we are dealing with a 5000 per mil in-house standard).  I think this is
directly affected by conditions within the EA itself (i.e. number of
samples run through the both the oxidation and reduction columns,
depletion level of the reduction column).  I plan on running more of these
standards in the coming months and hope to have an updated presentation
for the CF-IRMS Workshop in Fredericton in June 2007.  In the meantime, I
would like to propose a ring test of enriched 15N materials that isotope
facilities are using as in-house standards in order to establish ranges of
carryover used in various IRMS systems for varying sample types. Currently
our lab is equipped with a Thermo Delta Plus system that is capable of
evaluating this effect, as well as a Geo 20-20 interfaced to an ANCA-SL
EA, so I can't tell you anything about GV systems in this capacity.

I mentioned that various sample matrices can impact the overall carryover
within a sample run.  I notice that enriched 15N soils and sediments have
lower precision on the in-house working standards than do animal or plant
tissues; however, this is normally the case even at natural abundance
levels (depending on number of samples analyzed before deashing, size of
sample, etc): we can see variations ranging from 0.1-0.25 per mil.  We
have developed natural abundance soil in-house standards to measure this
trend.  When combusting soils or sediments there is more ash buildup
perhaps contributing to this effect, whereas with plant and animal tissues
there is minimal carryover.

One thing you might want to check is the level of carryover at certain
points in column depletion, particularly the reduction column.  The actual
cause of the carryover effect I think is tied to the state of 1 or both of
these columns.  I am interested to see other responses to your question. 
One thing that would be interesting is to see what kind of carryover
effect is taking place in systems that do not utilize a continuous flow
system.  Are labs seeing carryover in measurements made on a dual inlet
system.  If so, to what degree?  This could possibly give an indication to
how much the IRMS is involved in the carryover.

Take care,

Art
--------------------------------------------
Cornell University Stable Isotope Laboratory
E440 Corson Hall
Tower Road; Cornell University
Ithaca, NY 14853
(607) 254-1382
--------------------------------------------

> Hi All,
>
> I've been running mass specs for over 7 years now, and I have to admit
> (somewhat embarrassingly) that I still cannot explain to students what
> exactly causes carry-over inside my EA-IRMS system when running
> 15N-enriched samples.  If the samples are 100% combusted, how can there be
> any 15N leftover from the previous sample?  I know that, after running
> enriched samples, when I replace the reactors with fresh ones and start
> analyzing natural abundance samples again, all is ok.  So I'm guessing
> that
> the carry-over problem has to do with either the oxidation or reduction
> reactor.
>
> Any insight on this would be very much appreciated.
>
> Cheers,
> Rick
>
> Richard (Rick) Doucett, Ph.D., Associate Director
> Colorado Plateau Analytical Laboratory
> Northern Arizona University
> Tel: 928.523.0967 (lab)
> Tel: 928.523.7265 (office)
> Fax: 928.523.7500
>
> Stable Isotope Lab Website: http://www.isotope.nau.edu
>
> Mailing address for packages:
> Dept. Biological Sciences
> Northern Arizona University
> Building 21, South Beaver Street
> Flagstaff, AZ, 86011-5640
>


-- 
Art Kasson
Cornell Isotope Laboratory
Dept Ecology and Evolutionary Biology
E440 Corson Hall
Tower Road
Ithaca NY 14853 USA
Phone: 607.254.1382
Fax: 607.255.8088
COIL website
http://www.cobsil.com
Isogeochem website
http://www.isogeochem.com