Hydrocarbons isotopic composition

Although GC-C-IRMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study [614,677,678]. Measurement of chlorine stable isotope ratios in natural samples such as rocks and waters has become routine [626,679,680], but few measurements of chlorine isotopes in chlorinated aliphatic hydrocarbons have been reported [614]. A chlorine isotope effect was found in ferf-butyl chloride [681], demonstrating that 37Cl is more strongly bound to carbon than is 35Cl. Significant differences in the <5i7Cl values of some atmospheric chlorinated... 

Mansuy et al. [97] investigated the use of GC-C-IRMS as a complimentary correlation technique to GC and GC-MS, particularly for spilled crude oils and hydrocarbon samples that have undergone extensive weathering. In their study, a variety of oils and refined hydrocarbon products, weathered both artificially and naturally, were analyzed by GC, GC-MS, and GC-C-IRMS. The authors reported that in case of samples which have lost their more volatile n-alkanes as a result of weathering, the isotopic compositions of the individual compounds were not found to be extensively affected. Hence, GC-C-IRMS was shown to be useful for correlation of refined products dominated by n-alkanes in the C10-C20 region and containing none of the biomarkers more commonly used for source correlation purposes. For extensively weathered crude oils which have lost all of their n-alkanes,it has been demonstrated that isolation and pyrolysis of the asphaltenes followed by GC-C-IRMS of the individual pyrolysis products can be used for correlation purposes with their unaltered counterparts [97]. 

Jendrzejewski N, Eggenkamp HGM, Coleman ML (2001) Characterization of chlorinated hydrocarbons from chlorine and carbon isotopic compositions Scope of application to environmental problems. Appl Geochem 16(9-10) 1021-1031... 

Rather than using the isotopic composition of methane alone James (1983, 1990) and others have demonstrated that carbon isotope fractionations between the hydrocarbon components (particularly propane, iso-butane and normal butane) within a natural gas can be used with distinct advantages to determine maturity, gas-source rock and gas-gas correlations. With increasing molecular weight, from Ci to C4, a enrichment is observed which approaches the carbon isotope composition of the source. 

Possible inter relationships of natural substances are important. Similarities of the low molecular weight alkane isomers from crude oil and Fischer-Tropsch synthesis product have been reported. A similar composition for high temperature coal carbonization has been found. The C4 to C7 alkane isomers from these sources can be calculated quantitatively with equations developed for Fischer-Tropsch products. A reversal of the concentrations of the monomethyl isomers from CG (2 Me > 3 Me) to C7 (3 Me > 2 Me) occurs in all three products comparisons at higher carbon numbers indicate some dissimilarities. Naphthene isomers for crude oil and high temperature coal carbonization also have similar compositions. Aliphatic hydrocarbons from low temperature coal processes are considerably different. The C1 isotopic composition of pure compounds from the various sources are being compared in order to provide information on their origin. 

This example illustrates how m/e values of ions that differ only in isotopic composition can be used to determine elemental compositions. The important isotopes for this purpose in addition to those of chlorine are the stable isotopes of natural abundance, 13C (1.1%), 15N (0.37%), 170 (0.04%), lsO (0.20%). As a further example, suppose that we have isolated a hydrocarbon and have determined from its mass spectrum that M + = 86 mass units. In the absence of any combination reactions there will be an (M + 1)+ ion corresponding to the same molecular ion but with one 13C in place of 12C. The intensity ratio (M + 1 )+/M+ will depend on the number of carbon atoms present, because the more carbons there are the greater the probability will be that one of them is 13C. The greater the probability, the larger the (M + 1 )+/M+ ratio. For n carbons, we expect... 

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

Table 5 Carbon isotope compositions of CO, CO2, carboxybc acids, dicarboxylic acids, and volatile hydrocarbons in the Murchison (CM2), Tagish Lake (Cl), and Orgueil (CIl) meteorites together with hydrogen isotope...

Figure 2 Carbon stable-isotope compositions of low-molecular-weight hydrocarbons, amino acids, and monocarboxylic acids from the Murchison meteorite plotted against carhon number. Carbon number 1 denotes methane and CO2, 2 denotes ethane, ethanoic acid, glycine, etc. (source Yuen et al., 1984).

Hypotheses for the origin of meteoritic organic matter must account for its molecular and isotopic composition and be consistent with models of meteorite petrogenesis consequently, a number of potential environments have been considered (Table 9). Until the early 1990s, the favored hypothesis involved the catalytic hydrogenation of CO in the solar nebula. However, a characteristic of such catalytic reactions is their structural selectivity. FTT synthesis, in particular, produces a structurally selective suite of hydrocarbons and other compounds that, initially, were believed to... 

Gilmour I. and Pillinger C. T. (1994) Isotopic compositions of individual polycyclic aromatic hydrocarbons from the Murchison meteorite. More Not. Roy. Astron. Soc. 269, 235-240. 

Yuen G., Blair N., Des Marais D. J., and Chang S. (1984) Carbon isotope composition of low molecular weight hydrocarbons and mono carboxylic acids from Murchison meteorite. Nature 307, 252-254. 

The noble gas geochemistry of natural waters, including formation waters in sedimentary basins, has been used to determine paleotemperatures in the recharge areas, to evaluate water washing of hydrocarbons, and to identify mantle-derived volatiles (Pinti and Marty, 2000). The dissolved noble gases, helium, neon, argon, krypton, and xenon in sedimentary waters, have four principal sources the atmosphere, in situ radiogenic production, the deep crust, and the mantle. These sources have characteristic chemical and isotopic compositions (Ozima and Podosek, 1983 Kennedy et al., 1997). 

In this chapter the case of the oxygen isotopic composition of the Dales Gorge Member of the Brockman iron formation will be considered (Figure 3). This formation is underlain by shales of the Marra Mamba Formation that contain extractable hydrocarbon biomarkers generated before or during a peak regional metamorphic event 2,450-2,000 Ma (Brocks et al., 1999). It is unlikely that such labile material could have survived the pervasive fluid migration that would... 

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