Bruce and others,

     We also work with lots of freshwater samples that contain carbonate.
We have analyzed many algae, macrophyte, fish, sediment, POM, and
invertebrate samples for C and N isotopes (and elemental %), both acidified
and unacidified, using the vapor acidification method of Yamamuro and
Kayanne (1995), and will be preparing a small paper on the results.  We are
generally happy with the method but find that it does affect the N content
and d15N of many sample types.  We found that vapor acidification caused
lower values on average for both d15N and %N while organic carbon didn't
seem to be significantly affected.  The magnitude of the shifts are species
specific, with macrophytes and snails showing the largest d15N shifts (about
1 per mil).  Sediments and POM were the most erratic (meaning that the
shifts were variable) and fish showed almost no shift.  For the present, we
run most materials containing inorganic carbon unacidified for d15N (and %N)
and acidified for d13C (and %C).  This is twice as much work but gives us
the best data.  We presented the data at the CF-IRMS workshop in BC this
year and found that people had a variety of experiences with acidification
using both acid vaporization and acid washing techniques. The existing
methods can probably be improved or adjusted for a specific application but
otherwise it is best to run some tests on the types of samples you'll be
analyzing to see the effects of your acidification method.  Best of luck.

                                                             Steve Silva

Yamamuro, M., and Kayanne, H., 1995, Rapid direct determination of organic
carbon and nitrogen in carbonate-bearing sediments with a Yanaco MT-5 CHN
analyzer. Limnol. Oceanogr, 40(5), pp. 1001-1005.

Steven Silva
345 Middlefield Rd.
Menlo Park, CA 94025
ph:  (650)329-4558
fax: (650)329-5590
email: [log in to unmask]

At 01:20 PM 11/23/99 -0700, you wrote:
>Dear List,
>We run many samples of plant and animal tissue from freshwaters that are
>high in carbonates.  We're interested in finding a way to remove carbonates
>before analysis so that we just get the organic carbon for d13C
>measurements.  I've heard of washing with dilute acid or even just water,
>but have also heard of shifts in d15N and d13C (of the organic matter) that
>can result from such treatment.
>Is there any consensus on the best way to remove carbonates from such
>samples without biasing either the d13C or d15N readings of the organic
>matter in the sample?  Any experiences out there with good or bad
>Thanks for your input.
>Bruce Hungate
>Bruce Hungate
>Assistant Professor
>Dept. of Biological Sciences
>Box 5640
>Northern Arizona University
>Flagstaff AZ 86011-5640
>phone:  520 523 0925
>fax:    520 523 7500
>email:  [log in to unmask]
>Stable Isotope Lab