Relative Abundance Of Isomers Research Articles

Environmental Drivers of Dissolved Organic Matter Molecular Composition in the Delaware Estuary

Estuaries as connectors of freshwater and marine aquatic systems are hotspots of biogeochemical element cycling. In one of the best studied temperate estuaries, the Delaware Estuary (USA), we investigated the variability of dissolved organic matter (DOM) over five sampling cruises along the salinity gradient in August and November of 3 consecutive years. Dissolved organic carbon (DOC) concentrations were more variable in the upper reaches of the estuary (245±49 µmol L-1) than at the mouth of the estuary (129±14 µmol L-1). Bulk DOC decreased conservatively along the transect in November but was non-conservative with increased DOC concentrations mid-estuary in August. Detailed analysis of the solid-phase extractable DOM pool via ultrahigh resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) revealed compositional differences at the molecular level that were not reflected in changes in concentration. Besides the mixing of terrestrial and marine endmember signatures, river discharge levels and biological activity were found to impact DOM molecular composition. DOM composition changed less between August and November than along the salinity gradient. Relative contributions of presumed photolabile DOM compounds did not reveal non-conservative behavior indicative of photochemical processing; suggesting that on the timescales of estuarine mixing photochemical removal of molecules plays a minor role in the turbid Delaware Bay. Overall, a large portion of molecular formulae overlapped between sampling campaigns and persisted during estuarine passage. Extending the analysis to the structural level via the fragmentation of molecular masses in the FT-ICR-MS cell, we found that the relative abundance of isomers along the salinity gradient did not change, indicating a high structural similarity of aquatic DOM independent of the origin. These results point towards a recalcitrant character of the DOM supplied by the Delaware River. We demonstrate that in addition to bulk DOC quantification, detailed information on molecular composition is essential for constraining sources of DOM and to identify the processes that impact estuarine DOM, thereby controlling amount and composition of DOM eventually discharged to the ocean through estuaries.

Optical to ultraviolet spectra of sandwiches of benzene and transition metal atoms: Time dependent density functional theory and many-body calculations

The optical spectra of sandwich clusters formed by transition metal atoms (titanium, vanadium, and chromium) intercalated between parallel benzene molecules have been studied by time-dependent density functional theory (TDDFT) and many-body perturbation theory. Sandwiches with different number of layers, including infinite chains, are considered. The lowest excitation energy peaks in the spectra are characteristic of the robust bonding in these complexes. The excitation energies vary in a systematic way with the metal atoms and with the cluster size, and so these materials could be used to tune the optical properties according to specific functionality targets. The differences in the spectra could be used to identify relative abundances of isomers with different spins in experimental studies. As a salient feature, this theoretical spectroscopic analysis predicts the metallization of the infinite (TiBz)(infinity) chain, which is not the case of (CrBz)(infinity).

Moss: A powerful tool for dioxin monitoring

Plants are used as target organisms in most biomonitoring studies of atmospheric deposition of dioxins and furans (PCDD/F) because the most common route of entry of these pollutants into ecosystems is through soils and plants. However, comparison of the results of such studies is difficult because different plant species are used. In the present study we propose the use of mosses as biomonitors of PCDD/F deposition because of their suitable characteristics (absorption of substances from the atmosphere only, large absorbing surface, wide distribution of some species, etc.). We measured the PCDD/F concentrations in tissues of Pseudoscleropodium purum collected in areas far from known sources of pollution and in the surroundings of potential sources of these pollutants (several types of industries, incinerators and burning rubbish dumps). Part of the samples were collected with the aim of monitoring changes in the pollution intensity around some selected sources over time, others were collected to study the spatial gradient of pollution around other sources. Our results indicate that the measurements of the concentration of PCDD/F in the moss tissue allow the detection of strong and weak pollution sources and that these measurements are sensible enough to monitorize changes in the pollution intensity along time, as well as to describe the spatial gradients of pollution created around point sources, and the differences in the relative abundance of isomers (homologue profiles) among different sources. We concluded that mosses are as good biomonitors for PCDD/F as they are for other types of pollutants.

Naphthalenes in Neogene sediments of the Shinjo basin, Japan.

Naphthalene and its methyl homologs were found in Miocene to Pliocene sediments of the Shinjo basin. The total concentrations of naphthalenes ranged from 26 to 5, 700 ng g-1 from the top to the bottom sediments of an about 1, 600 m thick sequence. Naphthalenes were mainly distributed in the Kusanagi Formation and their depth profile showed a characteristic feature, indicating a concentration peak at the onset of oil-generation layer. The ratio of 2- to 1-methylnaphthalene as well as the relative abundance of dimethylnaphthalene isomers, except for 1, 8-isomer, showed no depth dependence. Therefore, it seems that no significant isomerizations of the naphthalene homologs had occurred during the burial diagenesis around and above the oil-generating zone in the sediments. However, as to the least stable 1, 8-dimethylnaphthalene isomer, the negative logarithmic change of the relative abundance of the isomer (Alexander et al., 1984) was observed showing a trend of increasing with depth.

Molecular structures of sedimentary 8,14-secohopanes inferred from their gas chromatographic retention behaviour

The branched and cyclic alkane fractions from a biodegraded crude oil and the solvent extract of a mature sediment were analysed for extended 17α-hopanes/moretanes and extended 8,14-secohopanes using gas chromatography-mass spectrometry techniques. The biodegraded crude oil contained abundant 8,14-secohopanes in the form of pseudohomologous series with carbon numbers ranging from 31 to 35. Comparison of the chromatograms of the extended 8,14-secohopanes with those of the extended 17α-hopanes and moretanes from the sediment extract revealed some interesting similarities. Firstly, the C 31 moretanes (22S, 22R) were not resolved but the corresponding C 31 17α-hopanes were well resolved. In the case of the 8,14-secohopanes three of the six groups of C 31 compounds were either poorly resolved into doublets or appeared as single peaks while the other three compound groups were well-resolved into doublets. This suggests that one group of 8,14-secohopanes behave in a similar manner to the moretanes and another group have a behaviour similar to that of the 17α-hopanes. Another feature of the distribution of extended moretanes, namely the greater relative abundance of the second eluting isomer of each pair (22S and 22R diasteromers), was also observed in the “moretane-like” 8,14-secohopanes. These similarities in GC behaviour and in the relative abundances of isomers has been interpreted as evidence for “hopane-type” and “moretane-type” extended 8,14-secohopanes.

Identification and origins of isopropylmethylphenols in crude oils

Crude oils have been analysed for isopropylmethylphenols using GC-FID, GC-MS and GC-FTIR techniques. Six of the 10 isomeric isopropylmethylphenols have been identified in crude oils by comparison of their chromatographic behaviour and infrared and mass spectral properties with authentic standards. The relative abundances of isomers vary among the samples, however three different types of isomer distribution patterns have been recognized. The relative abundances of isomers in the first type are at or near equilibrium values. In the second type, isomers with isopropyl groups in the ortho and para positions are abundant and may represent the products from isopropylation of cresols. A third type of crude oils has high relative concentrations of the natural products carvacrol and thymol. Because these compounds occur widely in extant conifers and angiosperms, their presence in crude oils whose source rocks were deposited when these plant types were widespread suggests that they also originate from these plants.

Methylbiphenyl, ethylbiphenyl and dimethylbiphenyl isomer distributions in some sediments and crude oils

Petroleum samples and organic extracts from two sedimentary sequences have been analysed using capillary gas chromatography and GC-MS techniques for isomeric methylbiphenyls, ethylbiphenyls and dimethylbiphenyls. The relative abundances of isomers with substituents in the ortho positions decrease with increasing depth (maturity) in both sedimentary sequences. Accordingly, these compounds appear to offer potential as indicators of thermal maturity of sediments.

Theoretical Studies of Interstellar Isomers

Both observational and theoretical studies of molecular cloud chemistry are hindered by many difficulties which are partially circumvented by studying the relative abundances of isomers synthesized from a common set of chemical precursors. Theoretical studies aimed at understanding interstellar abundance ratios for the isomers HCN/HNC, HCO+/HOC+, CH3CN/CH3NC, and C3H2 have been performed.

Determination of hydrocarbon types in low olefinic naphthas by field ionization mass spectrometry.

A method for the determination of hydrocarbon types in low olefinic naphthas by field ionization (FI) mass spectrometry was developed. Contrary to the statements in some of the published articles, we found that the ion sensitivities of saturated hydrocarbons in FI mass spectra were not affected by the presence of aromatic hydrocarbons. Based on the sensitivity data of 154 pure hydrocarbons and the relative abundances of isomers in petroleum naphthas, the relative sensitivities of individual hydrocarbon types for each carbon number were established. The results of analysis by this method on 15 straight run naphthas agreed very closely with those obtained by ASTM D 2789 and by GC-MS. Repeatability standard deviations of this method ranged from 0.1 to 1.4 vol.% for individual hydrocarbon types. Carbon number distribution, octane number, and specific gravity could be calculated by this method together with hydrocarbon-type composition, and these calculated values were also in fair agreement with the observed values.