The International Measurement Evaluation Programme, IMEP-8: carbon and oxygen isotope ratios in CO(2).

In IMEP-8, two samples of high purity CO(2)(g), with different carbon and oxygen isotope ratios were distributed to 27 participants, originating from 14 countries and from various isotopic measurement domains (geochemistry, atmospheric and food chemistry), but particularly set up for food laboratories. In total 19 laboratories reported results. The outcome of this comparison exercise shows that the laboratories reported carbon and oxygen isotope results in good agreement with the reference values across the domains. The reported results for delta(13)C(VPDB) (carbon) for both materials are within 1 per thousand. However, for the reported results of delta(18)O(VPDB) (oxygen) for both materials the overall spread of the reported results is about 11 per thousand. Within this spread two distinct groups of participants can be identified, where the results within each group vary about 2 per thousand. The latter seems to be caused by calculation errors by participants of the reporting delta(18)O(VPDB) values. As requested, participants also reported the isotope amount ratio for carbon and oxygen in the CO(2) samples. For carbon, all reported results for both materials agree with the isotope ratio value, which can be traced back to the value reported by Craig. For oxygen, all results are in good agreement and deviate by a maximum of 0.5% from the reference values measured at IRMM. Work carried out indicates the carbon isotope ratio, for both samples IMEP-8A and IMEP-8B, differ from those reported by Craig by as much as 1.2%. In the case of oxygen, this deviation is far smaller. Both data sets, i.e. the one realised by Craig and the one realised at IRMM, demonstrate traceability to SI. It is clear that both values significantly disagree.

The global methane cycle: isotopes and mixing ratios, sources and sinks.

A review of the global cycle of methane is presented with emphasis on its isotopic composition. The history of methane mixing ratios, reconstructed from measurements of air trapped in ice-cores is described. The methane record now extends back to 420 kyr ago in the case of the Vostok ice cores from Antarctica. The trends in mixing ratios and in delta13C values are reported for the two Hemispheres. The increase of the atmospheric methane concentration over the past 200 years, and by 1% per year since 1978, reaching 1.7 ppmv in 1990 is underlined. The various methane sources are presented. Indeed the authors describe the methane emissions by bacterial activity under anaerobic conditions in wet environments (wetlands, bogs, tundra, rice paddies), in ruminant stomachs and termite guts, and that originating from fossil carbon sources, such as biomass burning, coal mining, industrial losses, automobile exhaust, sea floor vent, and volcanic emissions. Furthermore, the main sinks of methane in the troposphere, soils or waters via oxidation are also reported, and the corresponding kinetic isotope effects.

Application of stable isotope ratio analysis to the characterization of the geographical origin of olive oils.

To gain information about the geographical origin of oil samples, measurements of delta(13)C and delta(18)O of the whole oil and some of its fractions have been performed on samples coming from fruits of Olea europaea L. produced in Greece, Morocco, Spain, Italy, Tunisia, and Turkey. The results obtained by applying statistical procedures have given pieces of evidence that oil samples have shown the trend to cluster according to the different climatic areas of growing environment of fruits. Some confusion has been observed for samples coming from neighboring countries having similar climates.

Determination of the (13)C Content of Glycerol Samples of Different Origin.

The average carbon isotope value (delta(13)C) of 63 samples of glycerol from over 30 different sources has been determined. The results indicate that it is possible to distinguish the glycerol obtained from the glycerides produced in plants following C-3 and C-4 carbon fixation pathways. The samples obtained from animal sources seem to reflect the composition of the material consumed, as well as that produced by sugar fermentation.