Harry,
we recently ran sediments from Florida Bay for C/N on our Carlo Erba/Optima
setup.  the following is from an email sent to me by the person who prepped
these samples.

"FB seds are approximately 90-95% carbonate.  I added 40mL of 2N HCl to 1
gram of sed., sonicated for 5min., sit overnight, and rinse until  the pH
is close to that of D.I. water , then dry in oven
overnight, grind, and analyze.  Apparently this "wash out" method (as it's
called in the literature) results in loss of up to 50% of organic carbon
due to loss of acid soluble fraction.  However, the %C and N ratio
generally stays the same."

we also tried the "fuming acid" technique (adding acid to the boat itself),
and seemed to get d13C numbers that were lighter by about 1.5 per mil as
compared to the "wash out" method.  Perhaps the lighter acid-soluble
organic C is remaining in the silver cup versus being washed away in the
other method...  We haven't explored this much further yet.  For what it's
worth,  the wash out method is more labor intensive--but the fuming acid
took over a week to completely remove carbonate material from our sediments.

good luck,
Cyndi

Cyndi Brock
U.S. Geological Survey
DFC, Bldg. 21, MS 963
Denver, CO  80225

office: 303-236-0278
lab:    303-236-7694
fax:    303-236-4930
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