Unresolved Complex Mixture Research Articles

Characterisation of radiogenic monoalkenes in Australian oils and condensate

Thirteen Australian oils and one condensate, covering oil reservoir ages from Mesoproterozoic to Early Cretaceous, show monoalkene contents varying from 0.01 to 22.3 wt% of the whole liquid. Radiolysis of saturated hydrocarbons is the most likely process leading to oils with high alkene contents. The major radiolytic component is an unresolved complex mixture (UCM). The bulk of the resolved alkene compounds are positional isomers of n-alkenes. Methyl branched and cyclohexyl alkenes are minor components. Internal n-alkene isomers have a trans configuration dominant over the cis isomer. The oil with the longest reservoir residence time shows the highest content of internal n-alkenes relative to terminal 1-alkenes as well as the highest trans/cis ratio, suggesting the extended time has resulted in rearrangement to near thermodynamic equilibrium of the congruent monoalkenes. The radiolytic monoalkenes in the Ordovician-reservoired oil with the highest alkene content is likely influenced by a higher probability of intermolecular interactions and different product pathways in a complex mixture. Here, the relative proportion of alkene mimics the relative abundance of n-alkanes, suggesting that radiolytic C–C bond cleavage is suppressed when the alkene/alkane ratio is elevated and that the preferred pathway of n-alkane radiolysis favours the production of terminal monoalkenes. Radiolysis of the alkane UCM together with crosslinking and branching of n-alkane-derived radiolysis products contribute to the higher relative proportion of the alkene UCM. The similar carbon and hydrogen isotopic ratios of the n-alkanes and n-alkenes supports a parent–daughter relationship.

Vapor- and aerosol-phase atmospheric organic matter in urban air of the Midwest USA

Vapor- and aerosol-phase atmospheric organic matter were collected in East St. Louis, MO using the high-volume sampling method. Samples were processed by traditional analytical methods and analyzed by multidimensional gas chromatography with time-of-flight mass spectrometric detection. Levels of identified, hydrocarbon-like organic vapor and aerosol species (i.e., HOV and HOA, respectively) were 10–42 ng m−3 and 0.020–3.6 ng m−3, respectively. Concentrations of identified, oxygenated organic vapor and aerosol species (i.e., OOV and OOA, respectively) were 2–66 ng m−3 and 23–310 ng m−3, respectively. The principal feature of the HOV was an unresolved complex mixture that represented 54.5 ± 11.3% of the total HOVs during the field campaign. Levels of n- and branched alkanes, alkyl and cycloalkylbenzenes, polyaromatic hydrocarbons (PAHs), and alkyl-substituted PAHs generally declined from the morning rush hour to the 1000–1400 sampling period, which was similar to the expected trend in reactivity with respect to OH. The OOV included aliphatic mono-carboxylic acids, aliphatic and aromatic alcohols, aldehydes, and ketones, and alicyclic ketones, alcohols, and epoxides (i.e., montoterpenoids). The bulk of the OA species (∼99%) were a complex mixture of OOA, which included multifunctional n-aliphatic, alicyclic, and aromatic hydrocarbons, dicarboxylic and ketocarboxylic aliphatic and dicarboxylic monoaromatic acids, lactones, tetrols, and pentitols. The sampling and analytic techniques provided quantitative molecular information for HOVs in ambient air, which are a missing source of secondary organic aerosol precursors. Molecular characterization and quantitation of HOVs and OOA species will facilitate predictions of SOA formation using molecular-specific models.

Evolution of the sources contribution of aliphatic hydrocarbons and their fate in Sepetiba Bay, Rio De Janeiro, Brazil

Sepetiba Bay basin hosts industrial and harbors since the 1950s. Despite this impact history, hydrocarbon levels and sources were evaluated only in the 2000s and 2010s. Therefore, this study aimed to assess the evolution of aliphatic hydrocarbon sources and their fate in Sepetiaba Bay sediments after a significant increase in population, industries, and cargo handling. Surface sediment was predominantly composed of sand and showed a significant difference between TOC (0.20–3.45%) and total n-alkanes concentrations (0.01–3.31 μg g−1) among internal and external sectors of the bay (Mann-Whitney's U test, p < 0.05; n = 26). The n-alkanes concentrations indicate unpolluted sediments and remain in the same order of magnitude of two decades ago. The diagnostic ratios based on major hydrocarbon, isoprenoids, odd/even n-alkanes, and unresolved complex mixture indicate a mixture of allochthonous and autochthonous sources at almost all stations. Even if an oil fingerprint between the internal sector and the harbor area were observed, further studies are required to be sure.

Evaluation of Aliphatic Hydrocarbons in Surface Sediments of Lagoa Mirim (RS, Brazil).

In this study, the hydrocarbons (HCs) levels in sediments from Lagoa Mirim, situated in the south of Brazil, were verified. The methodology brought together stages of pre-sonification, soxhlet extraction, and determination by gas chromatography coupled with mass spectrometry (GC/MS). Ten sample points were evaluated where ∑n-alkanes varied between 1.46µg kg-1 ± 4.0% and 10.10µg kg-1 ± 17.6%. Diagnostic indexes were calculated, being: Carbon Preferential Index (CPI), terrestrial/aquatic ratio (TAR), unresolved complex mixture (UCM), UCM/∑n-alkanes ratio, and n-alkane ratio with Low molecular weight hydrocarbon and High molecular weight hydrocarbons (HMW/LMW). In general, the results of this study indicate a low anthropogenic impact in the environment.

Investigation of aliphatic hydrocarbons in core sediments of Brunei Bay, East Malaysia

The distribution of aliphatic hydrocarbons in three sediment cores from Brunei Bay was investigated in order to understand their sources and the biogeochemical processes of these hydrocarbons. The total concentrations of C15 to C37n-alkanes ranged from 0.70 to 16.5 μg g−1. Traces of hopanes with C29–C31 carbon homologs were detected in the study area. The carbon preference index (CPI15–37) ranged from 1.23 to 3.42 coupled with the natural n-alkane ratio (NAR19–32) ratios (1.52 to 5.34), and the presence of unresolved complex mixtures and hopanes, suggested slight contamination by anthropogenic hydrocarbons, presumably derived from activities along the coasts. The presence of C27 trisnorhopene and diploptene, as well as their association with long-chain and short-chain n-alkanes, revealed a depositional environment of organic matter in the sediment cores.

Characterization of sedimentary organic matter and depositional processes in the Mandovi estuary, western India: An integrated lipid biomarker, sedimentological and stable isotope approach

A multiproxy study involving bulk (TOC, δ13Corg, grain size) and molecular (n-alkane biomarkers) analyses is used to investigate surface sediments from the Mandovi estuary in Goa, west coast of India to determine the origin, distribution and composition of organic matter (OM). The δ13Corg and n-alkane based indices (terrigenous/aquatic ratio (TAR) and Paq) indicate higher terrigenous OM accumulation in the river dominated upper reaches of the estuary. The presence of unresolved complex mixture (UCM), n-alkane indices (carbon preference index (CPI), average chain length (ACL), natural n-alkanes ratio (NAR)) and diagnostic isoprenoid ratios (pristane/phytane (Pr/Ph), Pr/n-C17, Ph/n-C18) helped to characterise intense human activity in the lower estuary. This conclusion is further supported by relatively high concentration of hopanes indicating petroleum contamination in the lower estuary. Furthermore, the study also highlights the important role of grain size on the distribution of OM along coastal margins, and provides detailed understanding of the variations in OM distribution/accumulation forced by natural processes and anthropogenic activities. The results of this study have significant implications for identifying natural and anthropogenic OM sources in estuarine systems especially in the context of increasing anthropogenic activities.

Optimization of the Biodegradation of Aliphatic, Aromatic, and UCM Hydrocarbons from Light Crude Oil in Marine Sediment Using Response Surface Methodology (RSM).

This study describes the optimization of the biodegradation of total aliphatic (tAHCs), total aromatic (tPAHs), and unresolved complex mixture (UCM) hydrocarbons from light crude oil in marine sediment. The response surface methodology (RSM), with a Box-Behnken design, was used to optimize the hydrocarbon fraction degradation, reported as degradation efficiency (E (%)), using four independent variables (inoculum, dispersant, light oil concentration, and carbon/nitrogen ratio), all at three levels. Analysis of variance (ANOVA) showed R2 values of 0.976, 0.974, and 0.975 for tAHCs, tPAHs, and UCM, respectively. All fractions exhibited a statistically significant effect (P < 0.05) in the second-order quadratic regression model for degradation. According to the models, the optimal degradation prediction was: 81.03% for tAHCs, 85.96% for tPAHs, and 92.86% for UCM. This work highlights the possibility of carrying out efficient biodegradation, of more than 80%, through an optimization process using different light oil concentrations, opening up possibilities of multiple response optimization.

Natural and anthropogenic organic matter inputs to intertidal deposits of the urbanized Arctic region: A multi-proxy approach

Growing anthropogenic activity in the Arctic, especially near regional municipal and transportation hubs, threatens coastal environments with significant changes and contamination. A detailed study of a wide set of organic-geochemical proxies in surface and subsurface sediments and soils around the typical urbanized Arctic area (Tromsø, Norway) was performed to investigate natural and anthropogenic input of hydrocarbons. The sites were located close to main areas of human activity and differed in the level of anthropogenic pressure: oil terminal, boat harbor and recreation beach zone in the city center, and a remote, conditionally-clean area. The samples from each site were collected along four subtidal – intertidal – supratidal transects. GC–MS data on the distribution of more than 110 compounds were obtained including n-alkanes, isoprenoids, terpanes, parent polycyclic aromatic hydrocarbons (PAHs), and alkyl PAHs. Organic matter embedded in a mineral matrix was more thermally mature than its fresh non-bound fraction. Biomarker distribution and diagnostic ratios attested to the predominantly local origin of near-shore organic matter formation (phytoplankton, algae, high foliar plants, grass). PAH composition revealed a significant input of pyrogenic components from liquid and solid fuel combustion to the surface coarse-grained sediments. Background bacterial activity was evidenced by the high content of biohopanes and hopenes in less polluted sediments and soils. The presence of an unresolved complex mixture and even n-alkanes in sediment profiles from the most polluted sites attested to the active biodegradation that, together with the erosion of the upper layers by tidal currents, reduces the toxic compound penetration deeper into the sediments.

Hydrothermal catalytic conversion and metastable equilibrium of organic compounds in the Jinding Zn/Pb ore deposit

Hydrothermal organic reactions in the presence of mineral catalysts have been well established by laboratory experiments. However, little is documented about hydrothermal catalytic conversion (HTCC) and the metastable existence of complex molecules in continental hydrothermal systems. Five freshly mined ores containing oil and solid bitumen and associated unmineralised host sediment were sampled from the Beichang mine, Jinding Zn/Pb ore deposit, Lanping, near the Indo-Eurasian collisional margin, southeastern Tibetan Plateau. Here we show evidence for a wide range of mineral (ZnS/PbS) catalysis of polycyclic aromatic compounds in the natural subsurface.An interpretation of hydrothermal alteration (non-biodegradation) was made for the unresolved complex mixture. From unmineralised host sediment through oils to the solid bitumen, a stepwise interaction of organic compounds with metal-bearing hydrothermal fluids was demonstrated by the transformation of polycyclic aromatic hydrocarbons (PAHs) and phenyl/sulfur/oxygen-containing compounds that are thought to be generated through HTCC. Thermodynamically-unstable phenyl derivatives (ortho- or α-substituted) resulted from hydrothermal alteration (ring opening) of the middle and naphthenic (low energy) rings of peri-condensed parent PAHs, as supported by the dominance of 1-phenyldibenzothiophene, 1-phenyldibenzofuran, triphenyleno[1,12-bcd]thiophene, triphenyleno[1,12-bcd]furan, 9-phenylphenanthrene, 1,1′:2′,1′′:2′′,1′′′-quaterphenyl, and the corresponding thiophenes or furans over other isomers. Heteroatom addition to phenyl-derivatives generates abundant heterocyclic compounds, which can undergo further dehydrocyclisation. In the presence of ZnS/PbS, the abundance of three compound classes (unsubstituted PAHs, ‘aryl-aryl’ linked phenyl-derivatives, and S/O atom-containing compounds) is mainly hydrothermally controlled, rather than thermodynamically controlled. Various phenyl/sulfur/oxygen-containing compounds are the metastable equilibrium products of a series of complex redox reactions under specific hydrothermal conditions. We hypothesise that the ubiquitous triphenyleno[1,12-bcd]furan is an important marker for hydrothermal organic–inorganic interactions in the Zn/Pb deposit. The relative abundance of 9-phenylphenanthrene provides a measure of the degree of hydrothermal alteration and HTCC. Our data is consistent with a close interaction of ascending metalliferous hydrothermal fluids with indigenous organic matter/compounds. Hydrothermal fluids, in the presence of sphalerite (and/or galena), are highly effective for activation of the carbon–carbon bonds in PAHs.

Intermediate volatile organic compounds emissions from vehicles under real world conditions

Real-world vehicle emission factors (EFs) for the total intermediate volatile organic compounds (total-IVOCs) and volatile organic compounds (VOCs) from mixed fleets of vehicles were quantified in the Yangtze tunnel in Shanghai. Relationships of EFs of IVOCs with fleet compositions and vehicle speed as well as secondary organic formation potentials (SOAFPs) from IVOCs and VOCs were studied. Multiple linear regression (MLR) was used to estimate EFs of total-IVOCs for gasoline and diesel vehicles. IVOCs were classified into unresolved complex mixtures (unspeciated cyclic compounds and branched alkanes (b-alkanes)) and speciated targets (11 n-alkanes and ten polycyclic aromatic hydrocarbons (PAHs)). The results showed that the average EF of total-IVOCs was 24.9 ± 7.8 mg/(km·veh), which was comparable to that of VOCs. Unspeciated cyclic compounds and b-alkanes dominated the main composition (~77% and ~19%), followed by n-alkanes (~4%) and PAHs (~1%). EFs of IVOCs showed a significant, positive relationship with diesel vehicle fractions (p < 0.05). EFs of IVOCs dropped notably with the decrease of the diesel vehicle fractions. SOAFP produced by the total organic compounds (IVOCs + VOCs) was 8.9 ± 2.5 mg/(km·veh), in which up to 86% of SOAFP was from IVOCs. Estimated EFs of total-IVOCs for gasoline vehicles and diesel vehicles were 15.3 and 219.8 mg/(km·veh) respectively. Our results demonstrate that IVOCs emitted from diesel vehicles are the main emission sources under real world conditions and significant contributions of IVOCs emissions to SOA formation is evident, which indicates the necessity of making control policies to reduce IVOCs emissions from vehicles.

Intermediate Volatile Organic Compound Emissions from Residential Solid Fuel Combustion Based on Field Measurements in Rural China.

Residential solid fuel combustion (RSFC) is a key cause of air pollution in China. In these serial studies, field measurements of RSFC from 166 rural households in eastern China were conducted to update the database of emission factors (EFs) and chemical profiles of gaseous and particulate organic pollutants, and the present study focuses on the intermediate volatile organic compounds (IVOCs), which are precursors of secondary organic aerosol (SOA). The results show that the averaged EFs of IVOCs (EFIVOC) for crop straw, fuelwood, and coal are 550.7 ± 397.9, 416.1 ± 249.5, and 361.9 ± 308.0 mg/kg, respectively, which are among the EFIVOC of gasoline vehicle, diesel vehicle, non-road machinery, and heavy fuel oil vessel, and are significantly affected by fuel, stove, and combustion efficiency. The percentages of normal alkanes (n-alkanes), branched alkanes (b-alkanes), polycyclic aromatic hydrocarbons (PAHs), and unresolved complex mixture from RSFC are 3.5 ± 1.6, 8.0 ± 3.7, 17.6 ± 6.7, and 70.9 ± 8.1%, respectively, and the compositions are featured by lower b-alkanes and higher PAHs than those of vehicle sources. The proportions of some individual n-alkanes and PAHs (such as n-C12-n-C15, naphthalene, and its alkyl substituents) can be used as indicators to differentiate RSFC from vehicle sources, while methoxyphenols can be used to distinguish biomass burning from coal combustion. Based on China's energy statistics, the total IVOC emissions from RSFC in 2014 were 175.9 Gg. These data will help to update the IVOC emission inventory and improve the estimates of SOA production in China.

Identification of two main origins of intermediate-volatility organic compound emissions from vehicles in China through two-phase simultaneous characterization

Intermediate-volatility organic compounds (IVOCs) emitted from vehicles are generally in the gas phase but may partly partition into particle phase when measured under ambient temperature. To have a complete and accurate picture of IVOC emissions from vehicles, gas- and particle-phase IVOCs from a fleet of gasoline and diesel vehicles were simultaneously characterized by dynamometer testing in Guangzhou, China. The total IVOC emission factors of the diesel vehicles were approximately 16 times those of the gasoline vehicles, and IVOCs were mainly concentrated in the particle phase in the form of the unresolved complex mixture (UCM). The chemical compositions and volatility distributions of the gas-phase IVOCs differed much between gasoline and diesel vehicles, but were similar to those of their respective fuel content. This indicated that vehicle fuel is the main origin for the gas-phase IVOC emissions from vehicles. In comparison, the chemical compositions of the particle-phase IVOCs from gasoline and diesel vehicles were similar and close to lubricating oil content, implying that lubricating oil plays an important role in contributing to particle-phase IVOCs. The highest IVOC fraction in the particle phase occurred from B16-B18 volatility bins, overall accounting for more than half of the particle-phase IVOCs for both the gasoline and diesel vehicles. A conceptual model was developed to articulate the distributions of lubricating oil contents and their evaporation and nucleation/adsorption capabilities in the different volatility bins. The IVOCs-produced secondary organic aerosol (SOA) were 1.4–2.6 and 3.9–11.7 times POAs emitted from the gasoline and diesel vehicles, respectively. The tightening of emission standards had not effectively reduced IVOC emissions and the SOA production until the implementation of China VI emission standard. This underscores the importance of accelerating the promotion of the latest emission standard to alleviate pollution from vehicles in China.

Origin and Distribution of Total Organic Matter and Aliphatic Hydrocarbons in Sedimentary Fractions of a Highly Urbanized, Tropical Estuary

This study investigated the distribution and origin of organic carbon (OC) and aliphatic hydrocarbons (AHs) in sand (≥ 63 μm) and mud ( 100 μg/g dw) concentration of the unresolved complex mixture and a high proportion (31%) of petrogenic n-alkanes. Mud-associated pollutants can be exported to adjacent continental shelf. Geochemical analyses in size-fractionated sediments of the Pina Sound proved to be a useful procedure for better understanding estuarine OC dynamics.

Geochemical Characterization of Some Biodegraded Oils from the Niger Delta Basin, Nigeria

This study aimed to examine the compositional changes resulting from biodegradation, fathom the extent of degradation, and provide clues for interpreting the evolution and the overall effects on the quality of the oils. Ten oil samples from onshore-offshore fields were analyzed using gas chromatography (GC) and GC-mass spectrometry (GC-MS) methods. GC results indicate variable loss of low molecular weight (LMW) alkanes, besides the presence of unresolved complex mixture (UCM). Saturated, aromatic, resins, and asphaltenes (SARA) compositions and low saturate/aromatic ratio confirmed evidence of biodegradation. Biodegradation levels of the considered oils range from light to moderate, based on Peters and Moldowan scale. This influenced shifts in the ‘primary compositions’ of oils probably from paraffinic or paraffinic-naphthenic oils to aromatic-naphthenic oils. Total ion chromatograms of well 1 and 2 from Northern depobelt, show presence of LMW alkanes, co-existing side by side with UCM, suggesting multiple charges. However, biomarker fingerprints of the oils lack evidence of in-reservoir mixing of biodegraded and non-biodegraded oils. Nevertheless, the possibility of oil mixing cannot be excluded, because biodegradation is progressive and is ongoing. This study, shows the process has dramatically impacted the fluid properties, commercial worth, and economic producibility of the investigated oil accumulations in the basin.

Removal of TPH, UCM, PAHs, and Alk-PAHs in oil-contaminated soil by thermal desorption

Oil-contaminated soils from a former landfill and gas station site in Korea were treated by thermal desorption. The removal efficiencies of the different oil components such as total petroleum hydrocarbons (TPH), polyaromatic hydrocarbons (PAHs), unresolved complex mixtures (UCM), and alkylated PAHs (Alk-PAHs) by thermal desorption were determined. The effects of temperatures (200, 400, and 600 °C) and treatment times (15, 30, 45, and 60 min) on the thermal desorption efficiency were studied. The treatment efficiency increased with increasing temperature from 200 to 400 °C and with increasing treatment time. Almost complete removals of TPH, UCM, PAHs, and Alk-PAHs were observed after 15 min at 400 °C. The treatment temperatures of 400 and 600 °C did not show a significant difference (p-value > 0.05). Overall, this study shows that the different components of oil in the oil-contaminated soils can be treated effectively in a relatively short time by thermal desorption, and such high removal efficiency in a relatively short time for the oil-contaminated soils can be of advantage for the treatment of highly contaminated or weathered soils where biological treatment efficiency is low.

Elucidation of the macromolecular composition of fossil biopolymers using Py-GC×GC-TOFMS

In recent times, the limits of traditional Py-GC-MS techniques in evaluating the macromolecular compositions of biopolymers have proved to be more apparent – particularly in those samples containing complex, structurally similar organic molecules, often producing large areas of unresolved complex mixtures (UCM). In this paper, we compare the use of pyrolysis comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (Py-GC×GC-TOFMS) with conventional Py-GC-MS analyses and attempt to showcase the full potential of this technique by analyzing different fossil organic materials – algal-derived Tasmanites, Ordovician microfossil Gloeocapsomorpha prisca and terrestrial plant-derived resin. With all three fossil samples, Py-GC×GC-TOFMS was able to offer a higher degree of compound separation and detection of more complex compounds. Sulphur and oxygen-bearing polar compounds were clearly separated into different homologous series. Thus, this method bypassed the problem created by the UCMs in traditional Py-GC-MS in the case of G. prisca and Tasmanites, while resolving several biomarkers in the case of the resin which aided chemotaxonomic classification. We find that Py-GC×GC-TOFMS provides a greater variety of compounds at enhanced resolution levels than conventional Py-GC-MS methods and is more suited to analyzing samples with larger variety of chemical structures and functional groups with greater confidence, particularly in unknown samples. It can thus provide the analytical capability to better characterize the chemistry of kerogens, which will help in assessments of hydrocarbon source properties and may provide new avenues to interpretation in paleobiological studies.

Porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate-bearing system from the Taixinan Basin, the South China Sea

Abstract Anaerobic oxidation of methane (AOM) is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling. Though AOM has been extensively investigated, the variations of its microorganisms and lipid biomarkers are still not fully understood. In this study, the porewater geochemical and lipid biomarker evidence for AOM was systematically investigated in the hydrate-bearing system from the Taixinan Basin, the South China Sea, aiming to better expound the vertical variations of AOM-related biomarker signatures and causes of the weak lipid biomarker patterns. Accordingly, the sulfate-methane interface (SMI) depth of 1,000 cmbsf and onset of AOM were determined on the basis of the vertical profiles of porewater parameters. The occurrence of 13C-depleted 2,6,10,15,19-pentamethylicosane (PMI) with the δ13C ratios of −77.7‰ to −131.4‰ near the SMI are in accord with the porewater parameters, despite the absence of archaeol and sn2-hydroxyarchaeol and low abundances of PMI. Meanwhile, the strongly 13C-enriched iso-C15:0 and anteiso-C15:0 fatty acids suggest that non-methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate-reducing bacteria due to the presence of unresolved complex mixtures (UCMs) and odd–even predominance (OEP) values (about 1.0). The weak in situ signal of AOM-related biomarkers may be attributed to the low-flux methane supply, and dilution effect from the high input of allochthonous and autochthonous organic matters. The great difference in biomarkers between the Taixinan Basin and previously reported sites should be further clarified on the basis of more extensive studies in various AOM-related settings.

Unresolved complex mixtures of petroleum hydrocarbons in the environment: An overview of ecological effects and remediation approaches

Unresolved complex mixtures (UCMs) of hydrocarbons are the pollutants of serious concern commonly occurring in most of the environments contaminated with petroleum hydrocarbons. UCMs constitute a relatively unidentified group of compounds compared to the well-resolved hydrocarbons that could easily be identified by the modern chromatographic methods. UCMs that accumulate in the environment cause several toxicological effects of ecological significance, and indirectly affect the human health. Despite decades-long efforts to provide adequate information in this area of research, the fate and environmental impacts of UCMs of petroleum hydrocarbons are poorly understood. Techniques for extraction and analysis of UCMs in the environment are very important in their identification and quantification. Also, remediation of toxic UCMs of petroleum hydrocarbons is all the more essential. In fact, UCMs are often neglected in the risk assessments due to lack of proper identification methods and toxicity data. This critical review presents an overview of our current knowledge on the environmental occurrence, sources, separation, and identification methods for UCMs. The ecological toxicity of UCMs toward the biota and the strategies for remediation of the environments contaminated with UCMs have also been discussed in detail.

Utility of comprehensive GC×GC-TOFMS in elucidation of aromatic hydrocarbon biomarkers

Conventional one-dimensional gas chromatography-mass spectrometry (GC–MS) has been employed for the organic compound analysis since the 1960s. In traditional GC–MS analysis, co-elution can be intense in the chromatographic region occupied by triterpenoids and aromatic steroids and these can form an unresolved complex mixture (UCM) in the total ion chromatogram (TIC). The UCM in routine GC–MS analysis alters the baseline and adds a substantial noise to mass spectra. Here we demonstrate that comprehensive GC×GC-TOFMS with higher peak capacity helps in an improved characterisation of aromatic hydrocarbon biomarkers by resolving many individual peaks. The use of GC×GC-TOFMS achieved exceptional chromatographic separation of higher plant triterpenoids, triaromatic steroids and methylated 2-methyl-2-(4,8,12-trimethyltridecyl) chromans (MTTC) in a complex aromatic fraction of a Cenozoic crude oil from the Tarapur field of the Cambay Basin. Compounds (e.g., des-A-arbora-5,7,9-triene) having scarce abundance that could not be detected with 1D GC–MS analysis were identified by GC×GC-TOFMS. Thus, this approach offers a new avenue to palaeobiological studies, hydrocarbon exploration and evaluation of the distinct palaeodepositonal environments of organic matter.

Characterization of Hydrocarbon Groups in Complex Mixtures Using Gas Chromatography with Unit-Mass Resolution Electron Ionization Mass Spectrometry

Complex mixtures of hydrocarbons are ubiquitous as petroleum fuels and, consequently, environmental contaminants. Because they contain thousands of individual components with similar molecular structures, detailed chemical characterization of hydrocarbon mixtures relies on advanced analytical techniques that are not accessible to many researchers. Many analyses of hydrocarbon mixtures instead characterize them as "unresolved complex mixtures", with quantification limited to a small number of resolvable components and/or total observed mass within specified volatility ranges. This work develops a new analytical approach to characterize the hydrocarbon component of petroleum and environmental mixtures by "hydrocarbon group" (defined by carbon number, degree of unsaturation and, in certain cases, degree of branching) using gas chromatography coupled to a unit-mass-resolution electron ionization quadrupole mass spectrometer (GC/EI-MS), a standard and widely available instrument. Average mass spectra of hydrocarbons from a widely used spectral library are combined with chromatographic signal representing the molecular ion of each hydrocarbon group to recreate the magnitude and mass spectra of the chromatogram. Characterization of hydrocarbons in diesel fuel by this approach is in good agreement with state-of-the-art techniques relying on high-resolution and fast-response mass spectrometers. Application of this approach to subsurface soil gas samples from remediated sites of underground storage tank spills demonstrates that composition of hydrocarbons in environmental samples varies significantly and that the total signal of samples from contaminated sites may contain a substantial fraction of oxygenated components.