Dear Tim, Paul and others,
Although the story is not incorrect as given by Paul, things are more
complicated then that. If determining 12C and 13C mole fractions (by
absolute measurement..!) one has to consider the 16O, 17O and 18O in the CO2
produced by acid digestion from the carbonate material. This acid digestion
normally is done with phosphoric acid buffered with P2O5, but for just
carbon isotope determination can be done with other acids too.
For absolute measurement, which only is done in a few specialized
laboratories, a combination of mass spectrometry and preparing/measuring
gravimetrically very pure gas mixtures is applied. It includes a large
amount of precise calculations and it is a very labourious job to do.
Values taken from the table as given in the chapter on stable isotope ref
materials by Manfred Groening for my books:
PDB carbonate 13C/12C (11237.2±30) x 10-6 Craig (1957)
VPDB - 13C/12C (11224±28) x 10-6 (calculated from NBS 19)
NBS 19 calcite 13C/12C (11202±28) x 10-6 Zhang & Li (1990)
Coplen et al. (2002a,b) gave for VPDB:
12C mole fraction: 0.988944(28)
13C - - : 0.011056(28)
This results in 13C/12C of 0.011179 if neglecting the 'uncertainty'.
Refs:
Coplen T. B., Hopple J. A., Böhlke J. K., Peiser H. S., Rieder S. E., Krouse
H. R., Rosman K. J. R., Ding T., Vocke R. D. J., Révész K. M., Lamberty A.,
Taylor P. D. P., and De Bièvre P. (2002) Compilation of minimum and maximum
isotope ratios of selected elements in naturally occurring terrestrial
materials and reagents. U.S. Department of the Interior - U.S. Geological
Survey. Reston, Virginia. Water-Resources Investigations Report 01-4222: 98
p.
Coplen T. B., Böhlke J. K., De Bièvre P., Ding T., Holden N. E., Hopple J.
A., Krouse H. R., Lamberty A., Peiser H. S., Révész K., Rieder S. E., Rosman
K. J. R., Roth E., Taylor P. D. P., Vocke R. D. J., and Xiao Y. K. (2002)
Isotope-abundance variations of selected elements. Pure Appl. Chem. 74(10),
1987-2017.
Craig H. (1957) Isotopic standards for carbon and oxygen and correction
factors for mass-spectrometric analysis of carbon dioxide. Geochim.
Cosmochim. Acta 12, 133-149.
Zhang Q.-L. and Li W.-J. (1990) A calibrated measurement of the atomic
weight of carbon. Chinese Sci. Bull. 35(4), 290-296.
Best wishes,
Pier.
**********************************************************************
Dr. Pier A. de Groot
Pastoor Moorkensstraat 16
2400 Mol - Achterbos
Belgium
Tel. +32 (0)14 326 205
e-mail: [log in to unmask]
Visit my WEB-site about my ³Handbook of Stable Isotope Analytical
Techniques² at:
http://users.pandora.be/handbook/index.html
last update: 17 Maart, 2004.
**********************************************************************
> "Beginning in the 1950's, isotope-abundance measurements of carbon isotopes
> were expressed relative to Peedee belemnite (PDB), which was assigned a
> d13C value of 0 permil. Because the supply of PDB is exausted, IUPAC
> recommended in 1993 (Coplen, 1994; IUPAC, 1994) that carbon isotope
> abundances be reported relative to VPRB (Vienne PDB, the new primary
> reference for carbon isotope ratios having a d13C value of 0 permil)by
> assigning an exact d13C value of +1.95 on the VPDB scale to the IAEA
> reference material NBS 19 calcium carbonate."
>
> Therefore, the carbon isotopic composition of a material with d13C = 0
> permil relative to VPRD will be the following:
>
> 12C: mole fraction 0.98894428
> 13C: mole fraction 0.01105628
>
> (Chang and Li, 1990; Rosman amd Taylor, 1998)
>
> The answer to your first question is: 13C/12C ~0.00111798
>
> The answer to your second question is: It was not measured, it was assigned.
>
> P.
>
> _______________________________________
>
> Dr Paul ALEXANDRE
>
> Department of Geological Sciences
> Queen's University
> Kingston, Ontario
> CANADA K7L 3N6
>
> phone: (613) 533-2908
> fax:(613) 533-6592
> email: [log in to unmask]
>
> http://geol.queensu.ca/
> _______________________________________
>
>
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