Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13% for S-33 are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed S-36 depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant S-36 anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.

Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13% for S-33 are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed S-36 depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant S-36 anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.

Multidimensional solid-state NMR spectroscopy is used to refine the identification and abundance determination of functional groups in insoluble organic matter (IOM) isolated from a carbonaceous chondrite (Murchison, CM2). It is shown that IOM is composed primarily of highly substituted single ring aromatics, substituted furan/pyran moieties, highly branched oxygenated aliphatics, and carbonyl groups. A pathway for producing an IOM-like molecular structure through formaldehyde polymerization is proposed and tested experimentally. Solid-state C-13 NMR analysis of aqueously altered formaldehyde polymer reveals considerable similarity with chondritic IOM. Carbon X-ray absorption near edge structure spectroscopy of formaldehyde polymer reveals the presence of similar functional groups across certain Comet 81P/Wild 2 organic solids, interplanetary dust particles, and primitive IOM. Variation in functional group concentration amongst these extraterrestrial materials is understood to be a result of various degrees of processing in the parent bodies, in space, during atmospheric entry, etc. These results support the hypothesis that chondritic IOM and cometary refractory organic solids are related chemically and likely were derived from formaldehyde polymer. The fine-scale morphology of formaldehyde polymer produced in the experiment reveals abundant nanospherules that are similar in size and shape to organic nanoglobules that are ubiquitous in primitive chondrites.

Multidimensional solid-state NMR spectroscopy is used to refine the identification and abundance determination of functional groups in insoluble organic matter (IOM) isolated from a carbonaceous chondrite (Murchison, CM2). It is shown that IOM is composed primarily of highly substituted single ring aromatics, substituted furan/pyran moieties, highly branched oxygenated aliphatics, and carbonyl groups. A pathway for producing an IOM-like molecular structure through formaldehyde polymerization is proposed and tested experimentally. Solid-state C-13 NMR analysis of aqueously altered formaldehyde polymer reveals considerable similarity with chondritic IOM. Carbon X-ray absorption near edge structure spectroscopy of formaldehyde polymer reveals the presence of similar functional groups across certain Comet 81P/Wild 2 organic solids, interplanetary dust particles, and primitive IOM. Variation in functional group concentration amongst these extraterrestrial materials is understood to be a result of various degrees of processing in the parent bodies, in space, during atmospheric entry, etc. These results support the hypothesis that chondritic IOM and cometary refractory organic solids are related chemically and likely were derived from formaldehyde polymer. The fine-scale morphology of formaldehyde polymer produced in the experiment reveals abundant nanospherules that are similar in size and shape to organic nanoglobules that are ubiquitous in primitive chondrites.

Although focused ion beam (FIB) microscopy has been used successfully for milling patterns and creating ultra-thin electron and soft X-ray transparent sections of polymers and other soft materials, little has been documented regarding FIB-induced damage of these materials beyond qualitative evaluations of microstructure. In this study, we sought to identify steps in the FIB preparation process that can cause changes in chemical composition and bonding in soft materials. The impact of various parameters in the FIB-scanning electron microscope (SEM) sample preparation process, such as final milling voltage, temperature, ion beam overlap and mechanical stability of soft samples, was evaluated using two test-case materials systems: polyacrylamide, a low melting-point polymer, and Wyodak lignite coal, a refractory organic material. We evaluated changes in carbon bonding in the samples using X-ray absorption near-edge structure spectroscopy (XANES) at the carbon K edge and compared these samples with thin sections that had been prepared mechanically using ultramicrotomy. Minor chemical changes were induced in the coal samples during FIB-SEM preparation, and little effect was observed by changing ion-beam parameters. However, polyacrylamide was particularly sensitive to irradiation by the electron beam, which drastically altered the chemistry of the sample, with the primary damage occurring as an increase in the amount of aromatic carbon bonding (C=C). Changes in temperature, final milling voltage and beam overlap led to small improvements in the quality of the specimens. We outline a series of best practices for preparing electron and soft X-ray transparent samples, with respect to preserving chemical structure and mechanical stability of soft materials using the FIB.

Although focused ion beam (FIB) microscopy has been used successfully for milling patterns and creating ultra-thin electron and soft X-ray transparent sections of polymers and other soft materials, little has been documented regarding FIB-induced damage of these materials beyond qualitative evaluations of microstructure. In this study, we sought to identify steps in the FIB preparation process that can cause changes in chemical composition and bonding in soft materials. The impact of various parameters in the FIB-scanning electron microscope (SEM) sample preparation process, such as final milling voltage, temperature, ion beam overlap and mechanical stability of soft samples, was evaluated using two test-case materials systems: polyacrylamide, a low melting-point polymer, and Wyodak lignite coal, a refractory organic material. We evaluated changes in carbon bonding in the samples using X-ray absorption near-edge structure spectroscopy (XANES) at the carbon K edge and compared these samples with thin sections that had been prepared mechanically using ultramicrotomy. Minor chemical changes were induced in the coal samples during FIB-SEM preparation, and little effect was observed by changing ion-beam parameters. However, polyacrylamide was particularly sensitive to irradiation by the electron beam, which drastically altered the chemistry of the sample, with the primary damage occurring as an increase in the amount of aromatic carbon bonding (C=C). Changes in temperature, final milling voltage and beam overlap led to small improvements in the quality of the specimens. We outline a series of best practices for preparing electron and soft X-ray transparent samples, with respect to preserving chemical structure and mechanical stability of soft materials using the FIB.

Insoluble organic matter (IOM) isolated from 22 carbonaceous and ordinary chondrites spanning a wide range of groups and petrologic types were analyzed using Fourier transform infrared spectroscopy (FTIR). Based on common IR spectral features, it is observed that IOM falls into 4 molecularly distinct groups (designated here as A through D). Spectral group A includes type 1 and 2 chondrites and exhibits intense aliphatic C-H and carboxyl vibrational peaks. Spectral group B includes the least metamorphosed type 3 chondrites and Tagish Lake, and exhibits weaker aliphatic and carboxyl vibrational intensity. Spectral groups C and D include metamorphosed type >= 3.1 chondrites and a heated CM chondrite. The carbonyl stretching features in spectral groups C and D differ from that in spectral groups A and B and from each other. In spectral group C, the carbonyl stretching is assigned to cyclic unsaturated lactones; in spectral group D carbonyl exists predominantly in the form of unsaturated ketone moieties. Both spectral groups C and D have a relatively narrow band structure around 1210 cm(-1) (assigned to aromatic skeletal modes) as compared with spectral groups A and B, which is consistent with the formation of more condensed aromatics by extensive thermal metamorphism. The differences in carbonyl structures in spectral groups C and D are not the result of different effective metamorphic temperatures, rather these differences likely result from variation in the activity of water and oxygen at different stages of parent body metamorphism. Such environmental variations must be local phenomena in the parent bodies as there is no correlation between spectral grouping and chondrite class or group. (C) 2011 Elsevier Ltd. All rights reserved.

Insoluble organic matter (IOM) isolated from 22 carbonaceous and ordinary chondrites spanning a wide range of groups and petrologic types were analyzed using Fourier transform infrared spectroscopy (FTIR). Based on common IR spectral features, it is observed that IOM falls into 4 molecularly distinct groups (designated here as A through D). Spectral group A includes type 1 and 2 chondrites and exhibits intense aliphatic C-H and carboxyl vibrational peaks. Spectral group B includes the least metamorphosed type 3 chondrites and Tagish Lake, and exhibits weaker aliphatic and carboxyl vibrational intensity. Spectral groups C and D include metamorphosed type >= 3.1 chondrites and a heated CM chondrite. The carbonyl stretching features in spectral groups C and D differ from that in spectral groups A and B and from each other. In spectral group C, the carbonyl stretching is assigned to cyclic unsaturated lactones; in spectral group D carbonyl exists predominantly in the form of unsaturated ketone moieties. Both spectral groups C and D have a relatively narrow band structure around 1210 cm(-1) (assigned to aromatic skeletal modes) as compared with spectral groups A and B, which is consistent with the formation of more condensed aromatics by extensive thermal metamorphism. The differences in carbonyl structures in spectral groups C and D are not the result of different effective metamorphic temperatures, rather these differences likely result from variation in the activity of water and oxygen at different stages of parent body metamorphism. Such environmental variations must be local phenomena in the parent bodies as there is no correlation between spectral grouping and chondrite class or group. (C) 2011 Elsevier Ltd. All rights reserved.