>Dear Isogeochemists
>
>We are interested in analyzing DIC from groundwaters by acidification with
>H3PO4. The water samples are commonly poisoned with HgCl2 for this method.
>Would anyone know if  there is an alternative to the HgCl2?
>

Fellow Isotopists:

Just a note of warning.  We have been using a saturated solution of HgCl2
to preserve samples of oligotrophic surface water (1 mL HgCl2 in 240 mL SW)
in a study of methane cycling in the surface ocean.  Typically, we analyze
the carbon isotopic composition of CH4 within a few months of collection
(see methods in Popp et al, 1995 Anal Chem 67:405-411 and Sansone et al,
1997 Anal Chem 69:40-44).  We recently analyzed a duplicate from cruise
samples collected and analyzed about 1 year ago.  Some samples,
particularly those collected near the particle maximum, had much higher
[CH4] and lower d13C-CH4.  A colleague incubated those samples in 14C and
confirmed bacterial activity, albiet low activity!  I believe similar
examples exist in the literature.  We are testing other preservation
methods.

We suspect that HgCl2 preservation is okay for short-term storage (a few to
6 months) and is probably okay for preservation of seawater samples for DIC
analyses, where the DIC pool is large.  We have no experience with fresh
water.  These results are a mixed blessing - HgCl2 may not stop *all*
bacterial activity but it is a good excuse to urge graduate students to
analyze the samples QUICKLY.

Brian



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            Brian N. Popp, Associate Professor
University of Hawaii, Department of Geology and Geophysics
         2525 Correa Road, Honolulu, Hawaii 96822
            (808) 956-6206 Fax: (808) 956-7112
WWW Site: http://www.soest.hawaii.edu/GG/FACULTY/bpopp.html
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