Dear carbonate analysts,

Please find a copy of the text I prepared for my books on SI analytical
techniques below. Basically there are 3 methods: roasting (vacuum or under
inert gas flow), chemical treatment, and plasma ashing.

The English of my text needs quite some polishing/correction, and also there
might be some small errors or inconsistencies, and I have not completed my
text yet - but I think these preliminary chapters give a reasonable overview
of the problem to remove organic matter from carbonates.
I have seen a statement saying (I cannot find back the source actually!!):
"no method will remove organic matter completely from the samples - only
complete dissolution of the sample will work....". I have confirmation by
personal discussions about this view.
I hope this will help (or confuse things a bit more... removal of organic
matter from rock samples is not a solved problem).

Best wishes,
Pier.

********************************************************************
Dr. Pier A. de Groot
Economic Geology Research Unit
University of the Witwatersrand
Private Bag 3
2050 Johannesburg
South Africa
Tel. +27 11 717 6582  Fax. +27 11 339 1697
E-mail: [log in to unmask]

WEB-site on "Handbook of Stable Isotope Analytical Techniques (2 volumes)":
http://www.icon.co.za/~pagroot/index_1.html
last update: November 8, 2000
********************************************************************
[From my Chapter 4-5: C- and O-isotopes on carbonate rock and minerals]

***

4-5.1.3.1 - Removal of organic compounds from carbonate samples
 Carbonate minerals, fossil- or soil samples containing organic compounds
are treated by most of the analysts before digestion with phosphoric acid.
There are several ways to remove organic compounds reviewed below.
Emiliani (1966) state that different methods used to remove organic matter
from carbonate samples may lead to somewhat different isotopic results,
depending on the method used.

Roasting
The sample may be roasted under a flow of inert gas, e.g. dry nitrogen,
helium, argon gas, to prevent oxidation (fractionation) of the carbonate
minerals at a temperature between 375 and 500°C for a period of 15 to 60
minutes (Epstein et al., 1953, 1961;  Craig, 1953; Keith & Weber, 1964;
Bowen, 1966; Shackleton & Kennett, 1975; Duplessy, 1978; Gaffey et al, 1991;
Quade et al., 1995; Quade & Cerling, 1995; Robert & Kennett, 1997)(Figure
4-5.11). Aragonite will be changed into calcite by this process, important
for the choice of the acid digestion fractionation factor (Epstein et al.,
1961; Gaffey et al., 1991). Samples are loaded into the roasting device
while < 200°C, and temperature is raised first after air is flushed out of
the system (20 minutes flushing).
Helium gas, eventually used as inert gas, is cleaned by passing it over hot
(750°C) copper oxide and then through a liq-N2 trap (Epstein et al, 1961),
or over hot copper (500°C) and through an activated charcoal trap cooled by
liq-N2 (He flow rate: 0.4 cm3/second)(Epstein et al., 1953).

 Roasting under vacuum is another possibility for removal of organic matter
from carbonate samples (e.g. Naydin et al., Russian, in Geochemistry 1956;
Duplessy, 1978; Matthews et al., 1980; Lini et al., 1992; Sarkar et al.,
1990; Zheng et al., 1993; S. Carpenter, ID).

 Winter and Knauth (1992) reason that Precambrian organic material generally
is mature kerogen, which no longer contains volatile components which might
interfere with the phosphoric reaction with the carbonate and fractionate
the resulting isotopic value. Roasting of sample may be omitted in those
cases.
 Lécuyer (1996) and Gaffey et al. (1991) report the effects of heating on
the structure of calcite and aragonite. CO2 is produced in small quantities
at more elevated temperature - the CO2 volume increases with temperature.
 Heating affects skeletals of different species in different ways. Dolomite
can be formed from high Mg-calcites (Gaffey et al., 1991).

 Organic material breaks down rapidly when heated to temperatures „ 100°C.
Thermal degradation of organic material involves depolymerization, bond
scission, loss of functional groups, formation of free radicals and
evolution of H2O, CO and CO2 (Gaffey et al., 1991). Alteration of the
organic compounds may leave behind a residue which coats the mineral phases
and reduces their interaction in reactions (Gaffey et al., 1991). Roasting
alters rather than removes organic matter from the carbonates (Gaffey et
al., 1991).

 During heating of the carbonate, interstitial and inclusion fluids are also
released from the carbonate (Gaffey et al., 1991), and may cause oxygen
isotopic fractionation in the carbonate material.

Chemical treatment
 Carbonate samples can be washed with a 5.0 - 5.25% sodium hypochlorite
solution (= ³Clorox²) for one or two days at room temperature to remove
possible organic matter (Emiliani, 1966; Clayton et al., 1968a; Love &
Woronow, 1989), followed by rinsing in distilled water several times
(S.Carpenter, ID). Hypochlorite acid is used in combination with air
abrasive methods and ultrasonic cleaning for removal of organic matter from
samples by Yin et al. (1995).
 Brasier et al. (1993) reports cleaning of carbonate fossils, containing
organic matter, by washing with H2O2 and (CH3)2CO (acetone), followed by
drying for 30 minutes at 60°C. H2O2 (10%) is used by D¹Hondt & Lindinger
(1994) to treat foraminifera or powders of organic material containing
carbonate before reaction with phosphoric acid, while Love & Woronow (1989)
use a 30% solution during 24 hours to remove organic material from
aragonite.
 Love & Woronow (1989) use 5 N NaOH solution to boil a sample for 3 minutes
to remove organic material.
 Bleaching effects for removal of organic material is studied by Gaffey et
al. (1991) and Gaffey & Bronnimann (1993). They studied effects by H2O2,
NaOCl (= Chlorox), and NaOH reaction on organic and carbonate materials.
Their findings are that full-strength (5%) NaOCl is most effective for
organic matter removal, while H2O2 can cause dissolution and etching of the
carbonate material and NaOH alone hardly removed organic matter. Complete
removal of organic material is only possible with complete dissolution of
the carbonate material (Gaffey et al., 1991; Gaffey & Bronnimann, 1993).
 Sarkar et al. (1990) applied H2O2 and methanol treatment to remove organic
matter from carbonate and foraminifera material - see comment below on the
efficiency of this treatment.
 D¹Hondt & Lindinger (1994) soak bulk sediment samples in a solution of 40 g
hexametaphosphate and 20 l distilled deionized water, buffered to a pH of 7
by 58% ammonium hydroxide to sample foraminifera. Disaggregated samples are
washed in  water in 38 and 63 mm sieves and oven-dried (< 50°C) overnight.
This procedure is repeated two or three times to fully disaggregate and
clean the foraminiferal samples.

Plasma ashing
 Plasma ashers are used (Goreau, 1977; Bernius et al. (1985); Franchi et al.
(1986); Jones et al., 1987; R. McEwan, ID; M. Coleman, ID; S. Carpenter, ID;
J. Cali, ID) to transfer organic matter in carbonates (or other rock types)
into carbon.
Goreau (1977) found no isotopic exchange between carbonate and oxygen caused
by low temperature oxygen plasma treatment (1 hour) of sample. Standard
plasma ashing treatment takes 15 minutes.
Jones et al. (1987) use a plasma unit at 150°C for hydrocarbon-rich
carbonates before isotopic analysis.
M. Coleman (ID) report the study by a PhD student showing plasma ashers lead
to good results while bleaching or H2O2 methods do not. This is opposed by
S. Carpenter (ID), who state that, based on Gaffey & Bronnimann (1993),
diluted hypochloric acid is the best method for removal of organic matter
from carbonate rock.
J. Cali (ID) mention the need for use of dry samples (dried at low
temperature in a stove) before organic matter can be converted into carbon
in a plasma asher. The plasma asher needs a pressure of < 0.1 - 1 mm Hg, and
water containing samples prevent reaching this pressure when pumped.
Franchi et al. (1986) pretreated rocks by sonication in methanol before
using a plasma asher.

4-5.1.3.3 - General comments on pretreatment of carbonates
 Sarkar et al. (1990) tested different combinations of pretreatment of
carbonates containing organic matter, including powdering (homogenization;
larger reaction surface), treatment with H2O2 and methanol, vacuum roasting
(400°C, 1 - 2 hours), and ultrasonic treatment (removal of extraneous
carbonate or other particles). Phosphoric acid digestion on these treated
samples was compared with acid digestion on non-treated samples - no
difference was found in isotopic compositions of carbon or oxygen between
samples treated with different combination of treatments and untreated
samples. Pure phosphoric acid was used instead of the ³green acid²
containing CrO3 as in the original prescription by McCrea (1950). This
oxidator (CrO3) causes addition of organic carbon to the CO2 during the
³green  acid² attack, while pure phosphoric acid shows no isotopic shift -
no reaction between the acid and the organic matter occurs (Duplessy, 1970;
Sarkar et al., 1990).
 Complete removal of organic matter from carbonates (or sediments) is very
difficult to establish without effecting the isotopic composition of the
material (Sheppard, pers. com.). This is strongly dependent on the
distribution and structure how the organic matter is fixed into the
carbonate (sediment). If strongly Œintergrown¹ with the mineral or grain
structures, then complete dissolution of the material seems the only way to
reach complete removal of the organic matter, obviously rendering the
material invalid for isotopic determination.


NB (ID) stands for Isogeochem list discussion.

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