Colin T. Pillinger

The carbon isotopic composition of individual petroleum lipids

Abstract A combination of gas chromatography and stable isotope mass spectrometry has been used to investigate the carbon isotopic composition of individual petroleum lipids. The technique, known as gas chromatography isotope ratio mass spectrometry (GC-IRMS) is capable of separating a few micrograms of specific heavy hydrocarbons to allow measurement of their carbon isotopic composition. Stable isotope compositions of whole oils are already used as a tool in the petroleum industry, but as is well known, the bulk value is only an average of the components and may, of course, be misleading if considered in isolation. It is envisaged that a knowledge of intermolecular isotopic ratios of simple alkanes and aromatics will give such molecules, for correlation purposes, the specificity of more complicated molecules. A suite of three oils from a Central European Basin have been used to investigate the value of GC-IRMS for correlation purposes. Two of the oils are known to come from the same family while the third is unrelated on the basis of conventional geochemical methods of correlation.

Stable carbon isotopic analysis of sedimentary organic matter by stepped combustion

Abstract A stepped combustion technique, incremental heating in oxygen, has been used to study the carbon isotopic composition of carbonaceous sediments. This technique can afford data on the organic matter of the sediment and can give information on the structure, namely the variations in the isotopic composition of different portions, of the kerogen. Green River shale has been used as a model material and results indicate that it contains two distinct carbonaceous components, a carbonate and kerogen. Isotopic compositions for the kerogen measured by stepped combustion closely match those obtained by normal methods following preconcentration of the kerogen by mineral removal and solvent extraction. The variation in isotopic composition with combustion temperature has been used to investigate relationships with maturity and to look at its use as a possible correlation tool.

An interstellar dust component rich in 12C

Chondritic meteorites contain grains which condensed in circumstellar environments and carry unusual isotope signatures1 providing clues to the location in which grain formation took place. Isotope measurements on grains that are composed entirely of carbon or carbon compounds have, up to now, given compositions either close to typical Solar System values or enriched in 13C (refs 2–5). Despite the existence of astrophysical environments where 12C production predominates, highly 12C-enriched carbon com-ponents in meteorites are uncommon. Here we report the results of an analysis of an acid-resistant residue from the Allende (CV3) meteorite1,6, extensively combusted in oxygen at low temperatures (<500 °C) to remove isotopically normal carbon, which revealed the presence of a small amount (<0.1 p.p.m.) of carbon with a 12C/13C ratio close to 120, well above the value of 89 ± 3 found in terrestrial samples or bulk meteorites. This component is denoted Cλ. Stellar evolutionary processes favour lowering of the 12C/13C ratio; hence Cλ, which has a combustion temperature consistent with graphite, might be a primitive type of interstellar material.

COMETS AND THE ORIGIN OF LIFE - THE STABLE ISOTOPE APPROACH

To establish links between comets and the origin of life, a greatly improved knowledge of the carbon isotopic composition of cometary bodies will be necessary. Some suggestions are presented as to how the required data might be obtained from presumed terrestrial cometary debris or from in situ measurements by a cometary probe.