Analysis of carbon isotope composition | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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Research Departments

Stable isotope geochemistry

  Analysis of carbon isotope composition



Carbon isotope composition (ratio 13C/12C) of many samples (sediments, metasediments, rocks from altered oceanic crust, soils, particles...) are routinely measured for two different carbon species, organic carbon (reduced form) and carbonates (oxidized form). Both species are analyzed specifically, although bulk-rock analysis is also possible. We also analyze the isotope composition of dissolved inorganic carbon in terrestrial fluids (hydrothermal systems, lakes, rivers).

 

The analysis of organic carbon generally requires a sample treatment under acidic conditions (HCl) in order to remove any carbonate present in the sample. The analysis is then performed on the solid-residue of acid digestion, by a sealed-tube combustion technique or elemental analyser coupled to a mass spectrometer (Delta Plus XP, Delta V).

The carbonates (calcite, dolomite, magnésite, ankérite, sidérite) are generally analyzed by He continuous flow mass smectrometry (AP2003), after carbon extraction and transformation into CO2 by H3PO4 acidification.


For samples with low C concentration, such as those from oceanic crust (typically containing few hundred's of ppm CO2), we developped a step-heating technique for C extraction, using either pyrolisis of combustion, at temperature ranging from 250 to 1100°C. This type of analysis requires only low amount of rock powder (50 to 100mg), and is thus also suitable for small size samples such as those produced during laboratory experiments. Carbon isotopes of diamonds are also routinely analyzed by combustion on samples generally between 0.02 to 3 mg.

The analytical precision on C isotope composition is typically of ± 0.03 ‰.

 

For analysis of dissolved inorganic carbon (DIC) in fluids, a technique was developped allowing extraction of dissolved C species in the form of CO2 by sample acidification using H3PO4. The gas is analyzed on our He continuous flow mass spectromer with a precision better than ±0.1 ‰ for DIC > 1mM, and a precision of about ±0.3 ‰ for DIC < 0.5 mM.

 


For further information, please contact any people in our "carbon team" (Magali Ader, Carine Chaduteau, Vincent Busigny, Isabelle Martinez, Magali Bonifacie, Cyril Aubaud or Pierre Cartigny).