Hi Andrea:

I analyzed a fair number of spiked N samples on filters with the sealed tube
method, and had good luck with putting the folded filter inside a little
quartz boat (2 cm long, 6 mm od) that fit in the larger quartz/vycor
combustion tube.  This prevented cracking of the outer tube, most of the
time.  I also experimented with wrapping the filter in foils of various
types but was less happy with the results.

I know several folks who routinely run spiked N samples with their EA-CF
systems, with little problem of cross-contamination between other spiked
samples.  I suppose you could run blanks between samples if you were very
concerned about contamination. However, they were VERY careful about
switching back and forth between spiked and non-spiked samples.  You might
want to get a spare carousel for the spiked samples so you don't have to
worry about any cross-contamination via escaped powder from the boats.  We
have had no problem with combusting filters with the EA -- just that the
glass clogs the tubes and requires more frequent replacement or cleaning.

--Carol


At 09:44 AM 7/9/99 -0400, you wrote:
>Dear all,
>        I need to analyze 15N enriched samples from a N-uptake experiment
>(phytoplankton on GF/C filters) with a VG SIRA II mass spec. The samples
>were incubated with 99% 15-N in form of NH4Cl and KNO3. Tracer addition was
>about 10% of ambient dissolved N concentration (i.e, 0.3ug/L 15NO3-N and
>3ug/L 15NH4-N).
>As I have no experience with analyzing enriched samples and my SIRA manual
>is essentially useless in this regard, I would appreciate warnings/hints
>from SIRA (or similar intrument) users. In particular, is there anything I
>need to modify in the N2 parameter file or tuning of the instrument? I
>assume (or hope...) that the instrument should be able to handle such
>samples without hardware modifications.
>In order to avoid memory problems with our EA-CF setup the samples will be
>combusted in Vycor tubes following the procedure of Kendall and Grim (1990)
>and cracked on the manifold.
>
>Thanks for your help!
>
>Cheers,
>Andrea
>
>
>=================================================================
>|    Dr. Andrea Lini           |  Stable Isotope Laboratory     |
>|    Department of Geology     |  Phone: (802) 656 02 45        |
>|    University of Vermont     |  Fax:   (802) 656 00 45        |
>|    Burlington, VT 05405      |  E-mail:[log in to unmask]      |
>|    U.S.A.                    |  ISOGEOCHEM  list-owner        |
>=================================================================
>| ISOGEOCHEM URL: http://geology.uvm.edu/geowww/isogeochem.html |
>=================================================================
>
***************************************************************************
Carol Kendall                                           Phone:  +1 650 329 4576
U.S. Geological Survey                         Fax:     +1 650 329 5590
345 Middlefield Road, MS 434
Menlo Park, CA, 94025 USA                Internet:   [log in to unmask]