Dear Rick,

I recall hearing of an experiment someone once did (and apologies to
whoever - I can't remember).  A series of enriched N-15 samples were run
then the reduction reactor was changed. On running natural abundance
sample straight after, no memory was observed. When the initial
reduction tube was replaced and further natural abundance sample were
run the memory effect reappeared.

Some Cu/N2 or Cu/NxOy species appears to be formed. When the oxygen
pulse from the next sample hits the copper and oxidises it, this bound
nitrogen is released and so appears as a "memory" effect.

Best Regards

Jon Davies
Technical Manager
Elemental Microanalysis Ltd.



-----Original Message-----
From: Richard Doucett [mailto:[log in to unmask]] 
Sent: 26 February 2007 21:03
To: [log in to unmask]
Subject: [ISOGEOCHEM] what causes 15N carry-over in EA-IRMS?


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