Isotope Ratio Mass Spectrometer Research Articles

Stable isotope analysis of Dacryoconarid carbonate microfossils: a new tool for Devonian oxygen and carbon isotope stratigraphy.

Dacryoconarids are extinct marine zooplankton known from abundant, globally distributed calcite microfossils in the Devonian, but their shell stable isotope composition has not been previously explored. Devonian stable isotope stratigraphy is currently limited to less common invertebrates or bulk rock analyses of uncertain provenance. As with Cenozoic planktonic foraminifera, isotopic analysis of dacryoconarid shells could facilitate higher-resolution, geographically widespread stable isotope records of paleoenvironmental change, including marine hypoxia events, climate changes, and biocrises. We explored the use of Dacryoconarid isotope stratigraphy as a viable method in interpreting paleoenvironments. We applied an established method for determining stable isotope ratios (δ(13) C, δ(18) O values) of small carbonate microfossils to very well-preserved dacryoconarid shells. We analyzed individual calcite shells representing five common genera using a Kiel carbonate device coupled to a MAT 253 isotope ratio mass spectrometer. Calcite shell δ(13) C and δ(18) O values were compared by taxonomic group, rock unit, and locality. Single dacryoconarid calcite shells are suitable for stable isotope analysis using a Kiel-IRMS setup. The dacryoconarid shell δ(13) C values (-4.7 to 2.3‰) and δ(18) O values (-10.3 to -4.8‰) were consistent across taxa, independent of shell size or part, but varied systematically through time. Lower fossil δ(18) O values were associated with warmer water temperature and more variable δ(13) C values were associated with major bioevents. Dacryoconarid δ(13) C and δ(18) O values differed from bulk rock carbonate values. Dacryoconarid individual microfossil δ(13) C and δ(18) O values are highly sensitive to paleoenvironmental changes, thus providing a promising avenue for stable isotope chemostratigraphy to better resolve regional to global paleoceanographic changes throughout the upper Silurian to the upper Devonian. Our results warrant further exploration of dacryoconarid stable isotope proxy sensitivity, the isotopic contrast among dacryoconarids, other taxa, and bulk rock, as well as other potential dacryoconarid proxies (Mg/Ca, Sr/Ca, (87) Sr/(86) Sr, microlaser and ion microprobe isotope techniques, and clumped isotopes) for stratigraphic research.

Stable carbon isotope ratios of CH4-rich gas inclusions in shale-hosted fracture-fill mineralization: A tool for tracing hydrocarbon generation and migration in shale plays for oil and gas

Fluid inclusion gases in minerals from shale hosted fracture-fill mineralization have been analyzed for stable carbon isotopic ratios of CH4 using a crushing device interfaced to an isotope ratio mass spectrometer (IRMS). The samples of Paleozoic strata under study originate from outcrops and wells in the Rhenish Massif and Campine Basin, Harz Mountains, and the upper slope of the Southern Permian Basin. Fracture-fill mineralization hosted by Mesozoic strata was sampled from drill cores in the Lower Saxony Basin. Some studied sites are candidates for shale gas exploration in Germany. Samples of Mesozoic strata are characterized by abundant calcite-filled horizontal fractures which preferentially occur in TOC-rich sections of the drilled sediments. Only rarely are vertical fractures filled with carbonates and/or quartz in drill cores from Mesozoic strata but in Paleozoic shale they occur frequently. The δ13C(CH4) values of fluid inclusions in calcite from horizontal fractures hosted by Mesozoic strata suggest that gaseous hydrocarbons were generated during the oil/early gas window and that the formation of horizontal fractures seems to be related to hydraulic expulsion fracturing. The calculated maturity of the source rocks at the time of gas generation lies below the maturity derived from measured vitrinite reflectance. Thus, the formation of horizontal fractures and trapping of gas that was generated in the oil and/or early gas window obviously occurred prior to maximal burial. Rapidly increasing vitrinite reflectance data seen locally can be explained by hydrothermal alteration, as indicated by increasing δ13C (CH4–CO2) values in fluid inclusions. The formation of vertical fractures in studied Mesozoic sediments is related to stages of post-burial inversion; gas-rich inclusions in fracture filling minerals recorded the migration of gas that had probably been generated instantaneously, rather than cumulatively, from high to overmature source rocks. Since no evidence is given for the presence of early generated gas in studied Paleozoic shale, it appears likely that major gas loss from shales occurred due to deformation and uplift of these sediments in response to the Variscan Orogeny.

Development and validation of an universal interface for compound-specific stable isotope analysis of chlorine (37Cl/35Cl) by GC-high-temperature conversion (HTC)-MS/IRMS.

A universal application of compound-specific isotope analysis of chlorine was thus far limited by the availability of suitable analysis techniques. In this study, gas chromatography in combination with a high-temperature conversion interface (GC-HTC), converting organic chlorine in the presence of H2 to gaseous HCl, was coupled to a dual-detection system, combining an ion trap mass spectrometer (MS) and isotope-ratio mass spectrometer (IRMS). The combination of the MS/IRMS detection enabled a detailed characterization, optimization, and online monitoring of the high-temperature conversion process via ion trap MS as well as a simultaneous chlorine isotope analysis by the IRMS. Using GC-HTC-MS/IRMS, chlorine isotope analysis at optimized conversion conditions resulted in very accurate isotope values (δ(37)Cl(SMOC)) for measured reference material with known isotope composition, including chlorinated ethylene, chloromethane, hexachlorocyclohexane, and trichloroacetic acids methyl ester. Respective detection limits were determined to be <15 nmol Cl on column with achieved precision of <0.3‰.

First continuous measurements of δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O-CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in air with a Fourier transform infrared spectrometer

Abstract. The continuous in situ measurement of δ18O in atmospheric CO2 opens a new door to differentiating between CO2 source and sink components with high temporal resolution. Continuous 13C–CO2 measurement systems have already been commercially available for some time, but until now, only few instruments have been able to provide a continuous measurement of the oxygen isotope ratio in CO2. Besides precise 13C/12C observations, the Fourier transform infrared (FTIR) spectrometer is also able to measure the 18O / 16O ratio in CO2, but the precision and accuracy of the measurements have not yet been evaluated. Here we present a first analysis of δ18O-CO2 (and δ13C-CO2) measurements with the FTIR analyser in Heidelberg. We used Allan deviation to determine the repeatability of δ18O-CO2 measurements and found that it decreases from 0.25‰ for 10 min averages to about 0.1‰ after 2 h and remains at that value up to 24 h. We evaluated the measurement precision over a 10-month period (intermediate measurement precision) using daily working gas measurements and found that our spectrometer measured δ18O-CO2 to better than 0.3‰ at a temporal resolution of less than 10 min. The compatibility of our FTIR-spectrometric measurements to isotope-ratio mass-spectrometric (IRMS) measurements was determined by comparing FTIR measurements of cylinder gases and ambient air with IRMS measurements of flask samples, filled with gases of the same cylinders or collected from the same ambient air intake. Two-sample t tests revealed that, at the 0.01 significance level, the FTIR and the IRMS measurements do not differ significantly from each other and are thus compatible. We describe two weekly episodes of ambient air measurements, one in winter and one in summer, and discuss what potential insights and new challenges combined highly resolved CO2, δ13C-CO2 and δ18O-CO2 records may provide in terms of better understanding regional scale continental carbon exchange processes.

Large-scale plant growth chamber design for elevated pCO2 and δ13C studies.

Throughout at least the next century, CO(2) fertilization and environmental stresses (e.g. nutrient, moisture, insect herbivory) are predicted to affect yields of economically important crop species. Stable isotopes of carbon are used to study plant stresses, which affect yields, but a growth chamber design that can be used to isolate the effects of environmental stresses on crop-sized species through precise maintenance of pCO(2) levels and the δ(13)C values of atmospheric CO(2) (δ(13) C(CO2)) is lacking. We designed and built low-cost plant growth chambers for growing staple crop species under precise pCO(2) and δ(13) C(CO2) conditions. Over the course of 14 hours, we assessed for pCO(2) stability at two targeted levels (ambient, ~400 ppm; and 2×, ~800 ppm) and measured the δ(13) C(CO2) value within the two chambers using a stable isotope ratio mass spectrometer. We also compared the temperature and relative humidity conditions within the two growth chambers, and in the ambient, outside air. Across our experimental period, we achieved δ(13) C(CO2) stability (ambient: -8.05 ± 0.17‰; elevated: -12.99 ± 0.29‰) that showed nearly half the variability of any previously reported values for other chamber designs. The stability of the pCO(2) conditions (ambient: 406 ± 3 ppm; elevated: 793 ± 54 ppm) was comparable with that in previous studies, but our design provided ~8 times more growing space than previous chamber designs. We also measured nearly identical temperature and relative humidity conditions for the two chambers throughout the experiment. Our growth chamber design marks a significant improvement in our ability to test for plant stress across a range of future pCO(2) scenarios. Through significant improvement in δ(13) C(CO2) stability and increased chamber size, small changes in carbon isotope fractionation can be used to assess stress in crop species under specific environmental (temperature, relative humidity, pCO(2)) conditions.

An online temperature‐controlled vacuum‐equilibration preparation system for the measurement of δ2H values of non‐exchangeable‐H and of δ18O values in organic materials by isotope‐ratio mass spectrometry

Measurement of δ(2) H values in non-exchangeable-H (δ(2) H(n)) and δ(18)O values in organic environmental samples are inconsistent among laboratories worldwide due to varied and lengthy approaches in controlling for H isotope exchange (for δ(2)H(n) values) and removal of trace moisture (δ(2)H(n) and δ(18)O values), which undermines the comparability of organic δ(2)H and δ(18)O data produced among different laboratories. An online preparation system was developed for the measurement of the δ(2)H(n) and δ(18)O values of organic samples, coupled to isotope-ratio mass spectrometers. The system features a 50-position autosampler and isolation valve where (1) the samples are held isothermal between ambient to 40-120 ± 0.1 °C for H isotopic exchange experiments (δ(2) H(n)) and drying of hygroscopic samples (δ(2) H(n) and δ(18)O), (2) the samples are evacuated to <5 mbar and flushed with helium for moisture and N(2) removal, and (3) injection of up to 500 μL of H(2)O is possible for controlled vapour exchangeable-H experiments. The system provides highly reproducible and precise δ(2)H(n) isotope estimates for a range of organic keratinous standard powders over a wide range of experimental temperatures. A reproducible sample processing regimen can now be applied to a wider range of organics and hygroscopic samples that are currently hampered by poorly controlled preparative methods amongst laboratories. Rapid and reproducible online vacuum equilibration of samples and standards for the routine measurement of δ(2)H(n) and δ(18)O values is now possible using the online equilibration system, with the added benefit that sample processing times for organic δ(2)H values are reduced from weeks to hours.

Causes for the unimodal pattern of leaf carbon isotope composition in Abies faxoniana trees growing in a natural forest along an altitudinal gradient

Leaf morphological and physiological traits of Abies faxoniana growing in a natural forest along an altitudinal gradient were measured with the aim to identify the central mechanism for the marked variation in foliar δ13C determined by an isotope ratio mass spectrometer. There is a unimodal pattern of plant functional traits in these temperate and semi-humid areas. Stomatal parameters, specific leaf area, and C/N ratio increased, whereas C, N and δ13C values decreased with increasing altitude below 3000 m a.s.l.. In contrast, they exhibited opposite trends above 3000 m a.s.l.. Our results demonstrated that high-altitude plants achieve higher water use efficiency (WUE) at the expense of decreasing nitrogen use efficiency (NUE), whereas plants at 3000 m can maintain a relatively higher NUE but a lower WUE. Such intra-specific differences in the trade-off between NUE and WUE may partially explain the altitudinal distribution of the plants in relation to moisture and nutrient availability. Our results clearly indicate that the functional relations between nutritional status and the structure of leaves are responsible for the altitudinal variations associated with δ13C. The pivotal role of specific leaf area in regulating plant adaptive responses provides a potential physiological mechanism for the observed growth advantage of populations occupying the medium altitude. These adaptive responses to altitudinal gradients showed that an altitude of approximately 3000 m a.s.l. is the optimum distribution zone for A. faxoniana, allowing the most vigorous growth and metabolism. These results improve our understanding of the various roles of environmental and biotic variables upon δ13C dynamics and provide useful information for subalpine coniferous forest management.

New Frontiers for Sulfur Isotopic Analyses

Measurements of sulfur stable isotope ratios (34S/32S) have suffered in the past from technical difficulties in analyzing low-S materials reducing their use despite their undeniable scientific interest. The measurement of 34S/32S ratios is a powerful tool for deciphering problems such as determining the sources of environmental pollutants, to detect adulteration, tracking the evolution of the redox state of the oceans, and quantifying the role of the bacterial activity in sulfide minerals genesis. We have used an innovative high–precision, easy and rapid method of simultaneous analysis of carbon (13C/12C), nitrogen (15N/14N) and sulfur (34S/32S) isotope ratios as well as elemental concentrations using a new combination of elemental analyzer and isotopic ratio mass spectrometer. This new technique provides multiple isotopic signatures on smaller sample aliquots with high precisions especially for sulfur determinations. The technique described here is Isotopic Ratio Mass Spectrometry (IRMS) on line in continuous flow mode with an Elemental Analyzer (EA) based on “purge and trap” technology rather than conventional packed-GC gas separation. Emphasis is put on the efficiency of the system to reliably combust sulfur–bearing compounds of both organic and inorganic origin with high conversion yields. This new technique demonstrates the high quality of 34S/32S measurements for samples with S concentrations lower than 1 wt%. International calibrated references of diverse sulfur-bearing materials were used to calibrate two low (<1%) S -bearing phosphorites used as compositional reference material for future use as isotopic references: BCR32 and NBS120c. High quality of 34S/32S determinations is illustrated using various international reference materials. Working calibrated material has been selected and characterized for all three isotopic signatures as well in order to fully use the capacities of the system in future work. The possibilities of such a system to reliably measure S isotopes as well as N and C within the same aliquot of sample opens up new fields of investigation in many domains where multi-isotopic approaches are required.

Usage of Carbon Isotopes in Characterizing Groundwater Age, Flow Direction, Flow Velocity and Recharge Area

Carbon-13 and Carbon-14 isotopes were utilized in this study to determine the age, flow direction, flow velocity and recharge area of groundwater in the two main aquifers of Kuwait. For this purpose 20 groundwater samples were collected from the Kuwait group aquifer and lower Dammam formation aquifer. The collected groundwater samples were analyzed for 14C activity and 13C activity. 14C activity was measured using accelerator mass spectrometry (AMS) and expressed as percentage modern carbon (pmc) while 13C was measured using an isotope ratio mass spectrometer (IRMS) and expressed as permil (‰) in the form of CO2 gas. The interpretation of the analysis results of carbon isotopes data indicated that the general groundwater flow direction is towards north east and the estimated groundwater velocities are 11 to 30 m/a for Kuwait group aquifer and 12 to 21 m/a for Dammam formation aquifer. Based on the estimated groundwater velocities, the approximate location of the recharge areas of these aquifers was identified. The groundwater ages calculated for Kuwait group aquifer varied from 800 a B.P for modern ground water in the northern part to 22500 a B.P for very old groundwater in the northeastern part, while the age of Dammam formation groundwater is 26000 to 36000 a B.P.

Improving Precision and Signal/Noise Ratios for MC-ICP-MS

High precision measurements of isotope ratios are improved by continuous innovations in multi-collector isotope ratio mass spectrometers. Recently, the Thermo Scientific TRITON Plus TIMS and NEPTUNE Plus MC-ICP-MS benefitted from the implementation of Faraday cup current amplifiers with 1013 ohm feedback resistors. This significantly improves the signal/noise ratio of ion beam measurements and the reproducibility of isotope ratios derived from low ion beam intensities (fA range). On the NEPTUNE Plus, these innovations follow the development of the Jet Interface option, itself enabling significant increases in ion beam intensities for both solution and laser ablation measurements. This paper presents data for uranium isotope ratios obtained with a NEPTUNE Plus equipped with both 1011 and 1013 ohm Faraday cup current amplifiers. The data presented here illustrate the benefits of such technology developments, which will have applications in the geosciences, environmental, nuclear and biomedical fields.

Authenticity of sugar – Identification of plant source and determination of geographical origin of sucrose

The isotope ratios of water, organic matter and micronutrients from food are dependent on the circumstances and sites of their origin and production. Analytical methods, based on mass spectrometry, are established for routine determination of isotopes. Differentiation between metabolic pathways of C3 and C4 plants is realizable by determination 13C/12C ratios which can distinguish and identify sucrose from pure beet (Beta vulgaris) and pure cane (Saccharum officinarum). Influenced by the worldwide hydrological cycle the isotope ratios of 2H/1H and 18O/16O vary systematically, the variations give information about geographical origin. The exemplarily determination of authenticity is demonstrated by using mass spectrometric isotope ratio evaluation for identification of plant source and geographical origin with the help of selected sugar samples with known origin.

Laboratory studies of carbon kinetic isotope effects on the production mechanism of particulate phenolic compounds formed by toluene photooxidation: a tool to constrain reaction pathways.

In this study, we examined compound-specific stable carbon isotope ratios for phenolic compounds in secondary organic aerosol (SOA) formed by photooxidation of isotope-label-free toluene. SOA generated by photooxidation of toluene using a continuous-flow reactor and an 8 m(3) indoor smog chamber was collected on filters, which were extracted with acetonitrile for compound-specific analysis. Eight phenolic compounds were identified in the extracts using a gas chromatograph coupled with a mass spectrometer, and their compound-specific stable carbon isotope ratios were determined using a gas chromatograph coupled with a combustion furnace followed by an isotope ratio mass spectrometer. The majority of products, including methylnitrophenols and methylnitrocatechols, were isotopically depleted by 5-6‰ compared to the initial isotope ratio of toluene, whereas the isotope ratio for 4-nitrophenol remained identical to that of toluene. On the basis of the reaction mechanisms proposed in previous reports, stable carbon isotope ratios of these products were calculated. By comparing the observed isotope ratios with the predicted isotope ratios, we explored possible production pathways for the particulate phenolic compounds.

Experimental assessment of the bioconcentration of 15N-tamoxifen in Pseudokirchneriella subcapitata

Nowadays, pharmaceutical compounds (PC) are ubiquitous in aquatic ecosystems. In addition to direct ecotoxicity, the bioconcentration of PC in organisms is a phenomenon which could have an impact on the whole ecosystem. In order to study this phenomenon, we exposed unicellular algae (Pseudokirchneriella subcapitata) to 15N-tamoxifen, an anticancer drug labelled with a stable nitrogen isotope used as a tracer. By measuring 15N enrichment over time, we were able to measure the increase of tamoxifen content in algae. This enrichment was measured by an elemental analyser coupled with an isotopic ratio mass spectrometer (EA–IRMS). Algal cells were exposed for 7d to 3 concentrations of tamoxifen: 1, 10 and 100μgL−1. Our result shows a high bioconcentration in algae from the first minutes of contact. The highest bioconcentration factor measured is around 26500. We also observe that bioconcentration is not linked to the exposure concentration. This study is the first to use stable isotopes in order to monitor PCs in aquatic organisms such as algae. The use of stable isotopes in ecotoxicology offers interesting perspectives in the field of contaminant transfer in organisms and along the trophic web.

Stable isotopic differences between summer and winter monsoon rains over southern India

Rain drop size and isotopic composition of rain, important parameters that shed light on rain formation processes, are highly sensitive to the ambient weather. We reported earlier a significant correlation between them in individual rain events (with limited sampling), but this is yet to be tested with better, longer term sampling. Here we attempt to do so over a tropical region (i.e. Tirupati, India). Rain samples were collected at short time intervals (<1 h) to capture even small variations in their stable oxygen (δ18O) and hydrogen (δD) isotopic compositions. Isotopic analyses were made using an isotope ratio mass spectrometer, and a disdrometer measured the drop size distribution. Summer rains show a progressive 18O & D depletion with time, while the winter rains fluctuate about a mean value. We find no definite correlation between the drop size and stable isotope ratios as was reported earlier, based on a smaller number of samples: the complexity of rain formation process and varying ambient weather conditions for individual rain events could be the reason. Further, there is no significant difference between the local meteoric water lines (δ18O- δD line) of summer and winter monsoon rains, though the intercepts in both the cases were significantly smaller than global meteoric waterline, suggesting significant strong influence of secondary evaporation. However, the winter rains are more depleted in D & 18O. Paleoclimate proxies such as δ18O of cave calcite or teak cellulose form this region need to be interpreted in terms of the relative seasonality of the rainfall rather than the total annual rain.

Existing and Emerging Technologies for Measuring Stable Isotope Labelled Retinol in Biological Samples: Isotope Dilution Analysis of Body Retinol Stores

This paper discusses some of the recent improvements in instrumentation used for stable isotope tracer measurements in the context of measuring retinol stores, in vivo. Tracer costs, together with concerns that larger tracer doses may perturb the parameter under study, demand that ever more sensitive mass spectrometric techniques are developed. GCMS is the most widely used technique. It has high sensitivity in terms of sample amount and uses high resolution GC, yet its ability to detect low isotope ratios is limited by background noise. LCMSMS may become more accessible for tracer studies. Its ability to measure low level stable isotope tracers may prove superior to GCMS, but it is isotope ratio MS (IRMS) that has been designed specifically for low level stable isotope analysis through accurate analysis of tracer:tracee ratios (the tracee being the unlabelled species). Compound-specific isotope analysis, where GC is interfaced to IRMS, is gaining popularity. Here, individual 13C-labelled compounds are separated by GC, combusted to CO2 and transferred on-line for ratiometric analysis by IRMS at the ppm level. However, commercially-available 13C-labelled retinol tracers are 2 - 4 times more expensive than deuterated tracers. For 2H-labelled compounds, GC-pyrolysis-IRMS has now become more generally available as an operating mode on the same IRMS instrument. Here, individual compounds are separated by GC and pyrolysed to H2 at high temperature for analysis by IRMS. It is predicted that GC-pyrolysis-IRMS will facilitate low level tracer procedures to measure body retinol stores, as has been accomplished in the case of fatty acids and amino acids. Sample size requirements for GC-P-IRMS may exceed those of GCMS, but this paper discusses sample preparation procedures and predicts improvements, particularly in the efficiency of sample introduction.

Bioaccumulation and trophic dilution of human pharmaceuticals across trophic positions of an effluent-dependent wadeable stream.

Though pharmaceuticals are increasingly observed in a variety of organisms from coastal and inland aquatic systems, trophic transfer of pharmaceuticals in aquatic food webs have not been reported. In this study, bioaccumulation of select pharmaceuticals was investigated in a lower order effluent-dependent stream in central Texas, USA, using isotope dilution liquid chromatography-tandem mass spectrometry (MS). A fish plasma model, initially developed from laboratory studies, was tested to examine observed versus predicted internal dose of select pharmaceuticals. Pharmaceuticals accumulated to higher concentrations in invertebrates relative to fish; elevated concentrations of the antidepressant sertraline and its primary metabolite desmethylsertraline were observed in the Asian clam, Corbicula fluminea, and two unionid mussel species. Trophic positions were determined from stable isotopes (δ(15)N and δ(13)C) collected by isotope ratio-MS; a Bayesian mixing model was then used to estimate diet contributions towards top fish predators. Because diphenhydramine and carbamazepine were the only target compounds detected in all species examined, trophic magnification factors (TMFs) were derived to evaluate potential trophic transfer of both compounds. TMFs for diphenhydramine (0.38) and carbamazepine (1.17) indicated neither compound experienced trophic magnification, which suggests that inhalational and not dietary exposure represented the primary route of uptake by fish in this effluent-dependent stream.

Simultaneous N, C, S stable isotope analyses using a new purge and trap elemental analyzer and an isotope ratio mass spectrometer.

The evolution of stable isotope applications has demonstrated the increasing need for the determination of more than one isotopic signature from the same sample. Simultaneous determinations of (13)C/(12)C and (15)N/(14)N have become a widespread technique but up to now very few fully automated systems have offered the possibility of also measuring (34)S/(32)S from the same sample aliquot. This could be critical when sample amounts are limited, but it could also represent a significant gain of analytical time or cost. The technique that we are presenting provides these multiple isotopic signatures on small sample aliquots with high precisions, especially for sulfur determinations. A high-precision, easy and rapid method for the simultaneous measurement of carbon ((13)C/(12)C), nitrogen ((15)N/(14)N) and sulfur ((34)S/(32)S) ratios as well as elemental concentrations was employed, using a new combination of an elemental analyzer and an isotope ratio mass spectrometer. The elemental analyzer was based on 'purge and trap' technology rather than conventional packed-gas chromatography (GC) gas separation. Emphasis was put on the efficiency of the system to reliably combust sulfur-bearing compounds of both organic and inorganic origin with high conversion yields. High-quality measurement of (34)S/(32)S ratios was obtained using various international reference materials. A working calibrated material was also selected and characterized for all three isotopic signatures in order to fully use the capacities of the system in future work. The possibilities of such a system for the reliable measurement of S isotope ratios as well as N and C isotope ratios within the same aliquot of sample opens up new fields of investigation in many domains where multi-isotopic approaches are required.

A novel high-temperature combustion based system for stable isotope analysis of dissolved organic carbon in aqueous samples. I: development and validation.

Traditionally, dissolved organic carbon (DOC) stable isotope analysis (SIA) is performed using either offline sample preparation followed by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) or a wet chemical oxidation (WCO)-based device coupled to an isotope ratio mass spectrometer. The first method is time-consuming and laborious. The second involves the risks of underestimation of DOC concentration and isotopic fractionation due to incomplete oxidation. The development of an analytical method for accurate and sensitive DOC SIA is described in this study. A high-temperature combustion (HTC) system improves upon traditional methods. A novel total organic carbon (TOC) system, specially designed for SIA, was coupled to an isotope ratio mass spectrometer. An integrated purge and trap technique (peak focusing), flexible injection volume (0.05-3 mL), favorable carrier gas flow, modified ash crucible, new design of combustion tube and optimized drying system were used to achieve the necessary performance. The system can reliably measure concentrations up to 1000 mgC/L. Compounds resistant to oxidation, such as barbituric acid, melamine and humic acid, were analyzed with recovery rates of 100 ± 1% proving complete oxidation. In this initial testing, the δ(13) C values of these compounds were determined with precision and trueness of ≤ 0.2‰ even with 3.5% salinity. Further tests with samples with low DOC concentrations resulted in LOQSIA method values of 0.5 mgC/L and 0.2 mgC/L for LOQSIA instr , considering an accuracy of ± 0.5‰ as acceptable. The novel HTC system coupled to an isotope ratio mass spectrometer resulted in significantly improved sensitivity. The system is suitable for salt-containing liquids and compounds that are resistant to oxidation, and it offers a large concentration range. A second paper (which follows this one in this issue) will present a more comprehensive assessment of the analytical performance with a broad set of solutions and real samples. This highly efficient TOC stable isotopic analyzer will probably open up new possibilities in biogeochemical carbon cycle research.

Biomarker and stable carbon isotopic study of Eocene sediments of North-Western Potwar Basin, Pakistan

Abstract Eocene sediments from the Potwar Basin Pakistan have been characterized geochemically using liquid chromatography, gas chromatograph–mass spectrometry (GC–MS), GC–isotope ratio MS and metastable reaction monitoring (MRM) GC–MS. Biomarker and non-biomarker parameters have been used to evaluate the origin, paleodepositional conditions, lithology and thermal maturation of organic matter. The n -alkanes distribution profiles observed for the studied samples suggest an abundance of low molecular weight compounds in the Upper Eocene sequence, which is consistent with higher marine input. On the other hand, the bimodal n -alkanes distribution observed with depth may suggest terrigenous contribution. An enrichment of the δ 13 C of low molecular weight n -alkanes ( n -C 18 ) reveals marine origin (algal and bacterial) while depleted 13 C values observed for n -alkanes greater than n -C 22 indicate terrigenous origin of these compounds. Despite ample biomarker parameters as to the noticeable marine origin of the organic matter, presence of oleanane and retene as well as the enrichment of the δ 13 C values of the long chain n -alkanes suggest significant terrestrial contribution in the studied organic matter. An interesting feature of the δ 13 C isotopic profiles of n -alkanes higher than C 25 is so called saw tooth profile, in which the odd carbon number compounds were more depleted in 13 C than the adjacent even numbered compounds. This feature may further support the assumption that the source of these n -alkanes lay in plant waxes. Depositional environment of these sediments were typical marine with oxic to suboxic conditions and showed variation in oxicity with depth. Our results indicate that the Upper Eocene sediments have been deposited under oxic conditions while the lower Eocene strata are more likely to be deposited under restricted suboxic reducing marine conditions. Significantly low concentration of rearranged steranes and hopanes suggested carbonate lithology for these sediments. Aliphatic and aromatic hydrocarbons geochemical data indicate level of maturity corresponding to the onset of oil generation. Therefore, it is suggested that these sediments are more likely to be considered as possible hydrocarbon source rocks within the Potwar Basin.

Sample matrix effects on measured carbon and oxygen isotope ratios during continuous-flow isotope-ratio mass spectrometry.

Continuous-flow isotope-ratio mass spectrometry (CF-IRMS) is frequently used to analyze CO2 found in media such as air, breath, and soil pore space gas with the aid of a sample preparation and transfer device such as a Gasbench II. This study investigated the effect that matrices other than helium (He) have on the measured δ(13)C and δ(18)O isotope ratios of CO2. Identical CO2 was added to sample vials with matrices of pure He, pure N2, or a 21:79 mixture of O2/N2 and analyzed by a ThermoFinnigan Delta(Plus) XP isotope-ratio mass spectrometer coupled to a ThermoFinnigan Gasbench II. Variables such as CO2 concentration, sample analysis sequence, and sample matrix removal ('blanking') through manipulation of an injection and dilution open split were tested to identify systematic isotope ratio offsets between the different matrix types. The process of blanking induced a δ(13)C and δ(18)O offset of ≤0.2‰ between otherwise identical populations of CO2 samples in He. The (13)C/(12)C and (18)O/(16)O isotope ratios of CO2 sampled from pure N2 or a mixture of O2/N2 were found to be within 0.1 to 0.2‰ of those of an identical CO2 sampled from a He matrix when N2 or O2/N2 was removed prior to transport to the mass spectrometer. The measured oxygen isotope ratios of CO2 sampled from N2 and O2/N2 varied by as much as 0.6‰ and 4‰, respectively, if matrix gas was not removed prior to ionization. Sampling CO2 from matrices similar to air does not significantly affect the measured (13)C/(12)C and (18)O/(16)O isotope ratios of CO2 when a gas-handling procedure that includes the removal of matrix gas is utilized. This procedure is much preferable to introducing matrix gas and potentially isobaric interference to the ion source.