Hello Jeremy,
Continuous flow techniques will not work if you have an oxidizing
interface. High-temperature pyrolysis in presence of excess amounts of
carbon may result in CO or CO2 that can be used for delta18O, but the
technique is not worked out.
For my Ph.D. at UCLA I developed a technique to determine oxygen
isotopes in organic matter that contains only CHON: Schimmelmann A. and
Michael J. DeNiro (1985) Determination of oxygen stable isotope ratios
in organic matter containing carbon, oxygen, hydrogen, and nitrogen.
Analytical Chemistry 57, 2644-2646. This won’t help you much, because
marine organic matter, unlike relatively pure biopolymers such as
cellulose or amino sugars, will probably contain significant amounts of
sulfur. Older organic matter may even occlude pyrite that is not easily
removed even by acid treatment. Miriam once told me that a long soaking
in dilute nitric acid at room temperature can reduce the pyrite content
(as done in Kaplan’s lab, if I remember correctly), but then you would
wonder what the nitric acid does to the organic matter in terms of
oxidation, and maybe adding to the organic oxygen pool…
A few years ago Michael J. DeNiro developed an improved method for
CHONS-organic matter: DeNiro MJ and Epstein S, 1989. Determination of
the concentration and stable isotopic composition of oxygen in organic
matter containing carbon, hydrogen, oxygen, nitrogen, and sulfur.
Analytical Chemistry 61: 1887-1889. I have no personal experience with
the method. Even if it is labor-intensive, you need to consider that
DeNiro is known for his meticulous work. He would not publish anything
that does not work, or even “cuts corners”.
Even DeNiro’s method may not be 100% foolproof for complex marine
organic matter. When you write your proposal, be careful that you
mention the possible caveat of organic phosphorus. P is a pretty strong
sink for oxygen, possibly causing fractionation of oxygen. Luckily the
Refield ratio suggests that there is comparatively little P in organic
matter, but you better have some handle on the stoichiometry of your
target compounds.
You are probably interested in the discussion on the ISOGEOCHEM list in
June 1999 about pyrolysis of organic matter with subsequent oxygen
isotopic analysis. The ISOGEOCHEM list contributions are searchable on
the web. Let me know if you have trouble finding the contributions from
June 2 by Barbara Kornexl, and from June 15 by Rolf Siegwolf. They
give further references. The following article lists some earlier
relevant work: Saurer et al., 1998, Analytical Chemistry 70: 2074-2080.
I know the following reference, but didn’t see the article: Werner RA
et al., 1996. On-line determination of d18O values of organic
substances. Analytica Chimica Acta 319: 159-164. Matthias Saurer
briefly reported his experience about this method on the ISOGEOCHEM list
on 31 October 1997. See also Alex Sessions’ contribution on 3 November
1997, and on the same day by Matthias Gehre.
Not all organic oxygen has paleoenvironmental significance, because some
is actually exchangeable with water oxygen. Malzev and Galimov (1985)
observed that oxygen in some functional groups (OH, COOH, CO) in humic
acids exchanges isotopically with water oxygen at neutral pH at 80 deg C
over 70 hours: Malzev KA and EM Galimov 1986. Oxygen isotope exchange
in a humic acid-water system. Geochemistry International 23(7):163-164.
Luckily ocean water is comparatively well mixed, so any isotopic
exchange with ocean water oxygen should not destroy a strong biogenic
isotopic signal in bulk organic oxygen. What happens in the lab when
you add aqueous media (acids, alkaline solutions, washing with DI…) is
another matter. You will have to use isotopically defined solutions, or
better deliberately equilibrate the exchangeable organic oxygen in the
final organic isolate in contact with a “standard water” just before the
isotopic analysis. Few people have thought about this so far. Maybe
you can adopt in part my approach of equilibration of organic compounds
in water vapor (as I do it with hydrogen isotopes, see Schimmelmann, A.,
Michael D. Lewan, and Robert P. Wintsch (1999) D/H isotope ratios of
kerogen, bitumen, oil, and water in hydrous pyrolysis of source rocks
containing kerogen types I, II, IIS, and III. Geochimica et Cosmochimica
Acta 63, 3751-3766).
Of course you will need to demineralize the sediment to obtain kerogen,
or alternatively you can extract soluble humic acids. If you work with
unconsolidated, organic-rich sediments, you may want to initially test
oxygen isotope applications using humic acids, because this lets you
avoid the low-pH treatment encountered during typical kerogen
preparation. Humic acids can first be extracted in alkaline solution
under nitrogen, then be reprecipitated by mild acidification,
centrifuged, washed in slightly acidified water, and freeze-dried.
Marine humic acids can be expected to have a higher abundance in 18O
than humic acids that are derived from terrestrial biomass, as long as
the terrestrial biomass was photosynthesized in an environment with
relatively 18O-depleted meteoric water.
If you still want to go with the kerogen, I will have some suggestions.
The following method, for example, avoids excessive pH and temperature:
Robl TJ and BH Davis. 1993. Comparison of the HF-HCl and HF-BF3
maceration techniques and the chemistry of resultant organic
concentrates. Organic Geochemistry 20: 249-255.
There is a huge mountain of references on deltaO in cellulose, but that
is probably not applicable to your research.
Best wishes, Arndt
Jeremy JACOB wrote:
> Hi everybody,
>
> Could anyone help me finding information about studies on molecular
> specific d18O in organic matter. Is it possible via IRMS ?
>
> Thanks for advices or references,
>
> Jeremy
> Jeremy JACOB
> Laboratoire Sédimentation et Diagenèse de La Matière Organique
> Université d'Orléans
> Bâtiment Géosciences
> 45067 Orléans Cédex 2
> Tel :(0033) (0)2.38.41.71.71 poste 79.05
> Fax :(0033) (0)2.38.41.73.08
> E-mail : [log in to unmask]
--
Arndt Schimmelmann, Ph.D.
Senior Scientist
Department of Geological Sciences
Biogeochemical Laboratories
Geology 129, Indiana University
1001 East 10th Street
Bloomington, IN 47405-1405
Ph (812) 855-7645
home (812) 339-3708
FAX (812) 855-7961
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http://www.indiana.edu/~geosci/people/faculty/schimmelmann.html
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