Isotopic Compositions Of Individual Compounds Research Articles

Using Orbitrap mass spectrometry to assess the isotopic compositions of individual compounds in mixtures

The isotopic compositions of individual chemical species are routinely used by the geochemical, environmental, forensic, anthropological, chemical, and biomedical communities to elucidate the conditions, sources, and reaction pathways of the molecules in question. Mass spectrometric methods of measuring isotopic compositions of individual compounds generally require that analytes be pure to yield precise, accurate results, yet most applications examine materials that are mixtures of multiple components. Various methods of chemical purification, e.g., chromatography, are used to isolate analytes from mixtures prior to mass spectrometric analysis. However, these techniques take time and specialized instrumentation, both of which could potentially be obviated via the use of ultra-high-resolution mass spectrometry. Here we report on the use of Orbitrap™-based Fourier-transform mass spectrometry to perform isotope ratio measurements of single species within mixtures delivered to the mass spectrometer (MS) without prior chromatographic separation. We demonstrate that instrument biases (attributed here to space charge effects) within the Orbitrap mass analyzer can cause the measured 13C/12C ratio of a molecular ion in the presence of non-analyte-derived ‘contaminating’ species to spuriously decrease relative to the 13C/12C ratio measured for the same ion in a pure analyte. We observe that the decrease in 13C/12C is proportional to the relative concentrations of the additional ‘contaminating’ components. We then recommend several strategies by which this effect can be mediated such that accurate isotope ratios can be obtained.

Optimisation of solid-state urea clathrate formation as a chemical separation method coupled to compound-specific stable carbon isotope analysis

ABSTRACTSolid-state urea clathrate formation (SSUCF) as a chemical separation method prior to stable carbon isotope fingerprinting of diesel fuel contaminations was studied. The stable carbon isotope ratios (δ13C) of n-alkanes in diesel fuel can be used to trace the origin of a contamination. The accurate measurement of the stable isotopic composition of individual compounds requires baseline separation from any other co-eluting compounds. For this purpose silica gel column chromatography (SGCC) and SSUCF were applied. Detailed optimisation of SSUCF was performed: different activators, clathrate formation temperatures, activator volumes, clathrate formation times and sample capacity were investigated. The main benefits of the developed method are reduced clathrate formation time and increased recoveries for lower molecular weight n-alkanes. The recoveries of the developed SSUCF method ranged between 63 and 100% for C10–C24 n-alkanes with relative standard deviation no more than 7%. The precision of the gas chromatography-isotope ratio mass spectrometry measurement was acceptable with a standard deviation of the δ13C values ranging between 0.08 and 0.15‰. The absence of isotopic fractionation was also investigated.The robustness of the method was tested within a model experiment. Nine different water samples including distilled water, tap water, river water, industrial wastewaters and groundwater samples were spiked with the same diesel fuel. The water samples were extracted with n-hexane and after purification with both SGCC and SSUCF n-alkanes were measured. The δ13C values of n-alkanes were found to be similar for all samples. The importance of sample purification prior to compound-specific isotope analysis (CSIA) was also demonstrated within this model experiment by analysing samples from different stages of the sample preparation.Our results show that the proposed method can remarkably improve the precision of compound-specific stable carbon isotope analysis of n-alkanes originating from diesel contamination of the aquatic environment.

Stable carbon isotope evidence for origin of atmospheric polycyclic aromatic hydrocarbons in Zhengzhou and Urumchi, China

Origins of atmospheric polycyclic aromatic hydrocarbons (PAHs) in total suspended particulate (TSP) and particulate matter below 10 micrometers in diameter (PM10) collected in the urban areas of Urumchi and Zhengzhou, China, are discussed on the basis of carbon isotopic compositions of individual compounds. Carbon isotope ratios were measured with the system of GC-C-IRMS with an error (1σ) of less than 0.6‰. δ13C values of atmospheric PAHs in Urumchi range from -23.6‰ to -32.1‰ and from -22.5‰ to -31.0‰ in Zhengzhou. δ13C values of low-weight molecules (pyrene, fluoranthene, benzo[e]pyrene etc.) in PAHs from the two cities are concordant, and range from -23.0‰∼-25.8‰. However, the high-weight individual compounds of atmospheric PAHs in Urumchi are more depleted in 13C with increasing molecular weight of PAHs than those in Zhengzhou. δ13C mean values of benzo[a]pyrene, indeno(1,2,3-cd)pyrene and benzo(ghi)perylene in Urumchi are -28.3‰, -31.5‰ and -29.7‰, respectively. The values of those three compounds in Zhengzhou, however, are -24.5‰, -29.4‰ and -26.3‰, respectively. Our data, incorporated with the analysis of the consumption of coal and amount of motor vehicles, indicate that PAHs are mainly contributed by coal combustion in the two cities, and the contribution of automobile exhaust to PAHs in Zhengzhou is larger than that in Urumchi during the sampling period.

Polycyclic Aromatic Hydrocarbon Purification Procedures for Compound Specific Isotope Analysis

Purification and isotope analysis methods were developed to accurately measure the stable carbon isotope ratio of individual PAHs in environmental samples. Sample extracts were separated into several fractions by column chromatography and were purified by high-performance liquid chromatographic and thin-layer chromatographic techniques. The mean recovery of the purification method for all compounds was approximately 80%. The purity of isolates was verified by GC/MS in the full scan mode. The accuracy and precision of the method were verified using authentic standards of known isotopic compositions. The standard deviation for multiple analyses ranged between 0.1 and 0.4%o for standard material and between 0.1 and 0.7%o for environmental extracts after purification. The purification and isotope analysis methods were used to discern the sources of PAHs in environmental samples using variations in stable carbon isotopic compositions of individual compounds. Itwas confirmed thatthe purification method effectively purifies environmental samples while retaining original isotopic signatures so that PAH content and stable isotopic compositions can be used to infer contaminant sources in complex environmental settings.

Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks

In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in 2H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.

Isotopic order, biogeochemical processes, and earth history: Goldschmidt lecture, Davos, Switzerland, August 2002

The impetus to interpret carbon isotopic signals comes from an understanding of isotopic fractionations imposed by living organisms. That understanding rests in turn on studies of enzymatic isotope effects, on fruitful concepts of isotopic order, and on studies of the distribution of 13C both between and within biosynthetic products. In sum, these studies have shown that the isotopic compositions of biological products are governed by reaction kinetics and by pathways of carbon flow. Isotopic compositions of individual compounds can indicate specific processes or environments. Examples include biomarkers which record the isotopic compositions of primary products in aquatic communities, which indicate that certain bacteria have used methane as a carbon source, and which show that some portions of marine photic zones have been anaerobic. In such studies, the combination of structural and isotopic lines of evidence reveals relationships between compounds and leads to process-related thinking. These are large steps. Reconstruction of the sources and histories of molecular fossils redeems much of the early promise of organic geochemistry by resolving and clarifying paleoenviron-mental signals. In turn, contemplation of this new information is driving geochemists to study microbial ecology and evolution, oceanography, and sedimentology.

Understanding of reservoir gas compositions in a natural case using stepwise semi-open artificial maturation

In a study of the gas distribution and composition in different reservoirs of multilayered gas fields in South-East Asia, two types of variations with depth of the δ 13C values of gases (in particularly methane) were observed. In the normally pressured zones, the δ 13C values of methane increase steadily with depth, whilst in some overpressured zones, they first decrease with depth and then follow a regular trend. In order to explain these patterns, these data were compared to those obtained by artificial maturation of a coal using confined pyrolysis. Experiments were conducted in perfectly closed and in semi-open conditions. The free gases recovered were quantified by gas chromatography and the carbon isotopic composition of individual compounds (C 1–C 5 hydrocarbons) determined. The evolution of the δ 13C versus pyrolysis temperature obtained for the artificially generated methane is very similar to that of the natural gas. The use of the δ 13C 2– δ 13C 3 versus C 2/C 3 mol% diagram [EAGE 58th Conference and Technology Exhibition, 1996; Chem. Geol. 147 (1998) 249] allows recognition of two trends in the artificial maturation data related to the degree of opening of the experimental systems. Comparison of the data provide evidence that the closed pyrolysis results are very similar to those observed in high pressured reservoirs while those obtained in the normal pressured reservoirs follow the values of semi-open pyrolysis. The gas distributions and carbon isotope composition in the reservoirs can therefore be explained in terms of degree of opening of the system: high pressure reservoirs can be considered as fairly closed systems, with in situ gas generation, while normally pressured zones are more open and subject to important lateral migration. The composition of the gas (richness in methane) is controlled by the degree of opening of the system. Thus, our findings do not support the hypothesis that the generation of methane-rich gases in nature is controlled by transition metal catalysis.

The Use of the Isotopic Composition of Individual Compounds for Correlating Spilled Oils and Refined Products in the Environment with Suspected Sources

Correlation of crude oils, or refined products, in the environment with suspected sources is typically undertaken through the use of GC and GCMS and in certain cases bulk carbon isotope compositions. However, with crude condensates, or refined products in particular, the absence, or low concentration, of biomarkers precludes their successful use for making unique correlations. An alternative and, sometimes, complimentary technique for correlation of such products is evolving through the use of combined gas chromatography–isotope ratio mass spectrometry (GCIRMS). This approach permits determination of the carbon and hydrogen isotopic composition of individual compounds in the crude oil or refined product to produce isotopic fingerprints for use in correlation studies. In this paper, it is proposed to review applications of GCIRMS to the correlation of various spilled products with their suspected sources in different environments. Whilst not proposing that this technique will replace GC or GCMS; it is proposed that GCIRMS is a very powerful tool to be used in conjunction with GC and GCMS to make such correlations. Although isotopic fractionation has been observed in some of the lighter components such as benzene and toluene, higher carbon numbered compounds, say above C10, do not appear to undergo any significant isotopic fractionation as a result of weathering. Furthermore with refined products, isotopic fractionation of the lighter components has the potential to demonstrate the onset of natural attenuation of refined products in the environment.

The Use of the Isotopic Composition of Individual Compounds for Correlating Spilled Oils and Refined Products in the Environment with Suspected Sources

The Use of the Isotopic Composition of Individual Compounds for Correlating Spilled Oils and Refined Products in the Environment with Suspected Sources

Variation in the isotopic composition of single components in the C 4–C 20 fraction of oils and condensates

Determination of the isotopic composition of individual compounds in the C 4–C 15 range has been difficult due to the lack of suitable instrumentation. It has also been suggested that isotopic fractionation during storage might occur. With the introduction of online GC-IRMS, an instrument is now available to test whether or not isotopic fractionation occurs during storage. To test this, two condensates were analysed using the GC-IRMS method. Samples were taken for analysis at regular intervals, during storage at elevated temperatures. Only small changes in isotopic composition were observed during evaporation, with a slight enrichment in 13C in some compounds. However, the changes are so small that even prolonged storage times do not significantly affect the carbon isotopic composition of the compounds as long as there is enough material left for analysis. Two separate suites of samples, one from the Central Graben, the other from the Viking Graben, were used to demonstrate the use of the technique in correlating oils and condensates. This study shows that the oil/condensates in the Central Graben are not sourced from a single source rock, differing only in maturity, as reported earlier, but are sourced from various source facies, where especially Edda differs significantly from the others, i.e., the compounds in Edda are 1–2% 0 heavier than the rest of the analysed samples in the Central Graben. The analysed Viking Graben oils and condensates were divided into three groups based on this study.

Compound-specific δ 13C analyses of leaf lipids from plants with differing carbon dioxide metabolisms

The stable carbon isotope (δ 13C) values of (1) total leaf tissue (2), total surface lipid extracts and (3) individual n-alkanes isolated from leaves were determined for plants utilizing the C 3, C 4, and Crassulacean Acid (CAM) pathways of carbon fixation. The C 4 and CAM species had leaf tissue δ 13C values of −10 to −16‰; the total surface lipid extract δ 13C values were on average 8‰ more depleted. The C 3 plants had leaf tissue values of −25 to −30‰; total surface lipid extracts were on average 4‰ more depleted in 13C. The average n-alkane values for all plants were around 4‰ more depleted in 13C than the total surface lipid extracts and were between 1.6 and 13.8‰ depleted in 13C relative to total tissue. This depletion relative to the total tissue was on average greater for CAM (11.0‰) and C 4 plants (9.9‰) than for C 3 plants (5.9‰). Variations ranging from 0.1 to 6‰ were observed between n-alkalene homologues for the plants studied, emphasizing the need for caution when interpreting small variations in the isotopic compositions of individual compounds isolated from sedimentary environments. n-Aldehydes from C 3 plants had carbon-isotopic compositions similar to the n-alkanes from the same plants, suggesting that other biosynthetically related n-alkyl compounds (e.g. n-acids and n-alcohols) will have δ 13C values similar to the n-alkanes. Phytol isolated from C 3 plants was, on average, 1.5‰ enriched in 13C relative to the average n-alkane δ 13C values.

Compound-specific isotopic analyses: A novel tool for reconstruction of ancient biogeochemical processes

Patterns of isotopic fractionation in biogeochemical processes are reviewed and it is suggested that isotopic fractionations will be small when substrates are large. If so, isotopic compositions of biomarkers will reflect those of their biosynthetic precursors. This prediction is tested by consideration of results of analyses of geoporphyrins and geolipids from the Greenhorn Formation (Cretaceous, Western Interior Seaway of North America) and the Messel Shale (Eocene, lacustrine, southern Germany). It is shown (i) that isotopic compositions of porphyrins that are related to a common source, but which have been altered structurally, cluster tightly and (ii) that isotopic differences between geolipids and porphyrins related to a common source are equal to those observed in modern biosynthetic products. Both of these observations are consistent with preservation of biologically controlled isotopic compositions during diagenesis. Isotopic compositions of individual compounds can thus be interpreted in terms of biogeochemical processes in ancient depositional environments. In the Cretaceous samples, isotopic compositions of n-alkanes are covariant with those of total organic carbon, while δ values for pristane and phytane are covariant with those of poryphyrins. In this unit representing an open marine environment, the preserved acyclic polyisoprenoids apparently derive mainly from primary material, while the extractable n-alkanes derive mainly from lower levels of the food chain. In the Messel Shale, isotopic compositions of individual biomarkers range from −20.9 to −73.4% vs PDB. Isotopic compositions of specific compounds can be interpreted in terms of origin from methylotrophic, chemoautotrophic, and chemolithotrophic microorganisms as well as from primary producers that lived in the water column and sediments of this ancient lake.