Measured Isotope Values Research Articles

δ13 C values of urinary 19-norandrosterone in antidoping samples and potential for adverse findings from boar offal consumption.

19-Norandrosterone (19NA) is the preferred urinary target compound to identify doping with nandrolone or related 19-norsteroids. At concentrations between 2.5 and 15 ng/mL, isotope ratio mass spectrometry (IRMS) is required to establish exogenous origin of urinary 19NA. An absolute difference of 3‰ between urinary 19NA and an endogenous reference compound (ERC) constitutes a finding for exogenous origin of 19NA. Over the last 3years, 77 samples containing urinary 19NA between 2.5 and 15 ng/mL were analyzed at our laboratory. The measured δ13 C values for 19NA ranged from -29.5‰ to -16.8‰. In comparison, the δ13 C values for the corresponding urinary ERCs ranged from -22.4‰ to -16.2‰. Due to the considerable overlap in values between the target compound and the natural range of urinary ERCs, it can be challenging to distinguish between endogenous and exogenous origins of urinary 19NA. In addition, it is well known that consumption of offal from non-castrated pigs can produce 19NA in urine. To determine whether this could cause a positive IRMS finding under the current IRMS positivity criteria, meat from non-castrated boars fed a mixture of corn and soy was consumed by 13 volunteers. Two volunteers produced 19NA findings above 2.5ng/mL, and the measured isotope values, while inconsistent with documented 19-norsteroid preparations, did meet IRMS positivity criteria. However, these increases in 19NA urinary concentrations were short-lived due to rapid elimination. Timely follow-up collections may help support a claim for dietary exposure when low urinary concentrations of 19NA with pseudo-endogenous isotope values are observed.

Stable isotope geochemistry of the modern Shinfa River, northwestern Ethiopian lowlands: a potential model for interpreting ancient environments of the Middle Stone Age

Abstract Several years of weekly sampling of waters from the Shinfa River watershed in the lowlands of northwestern Ethiopia yielded 275 samples with δ D vsmow and δ 18 O vsmow values ranging from c. −10 to +100‰ and from c. −2 to +20‰, respectively. Wet season (summertime) Shinfa River water stable hydrogen and oxygen isotope values are among the lowest reported in this study, whereas the dry season (winter/spring) usually records a progressive trend towards +100 and +20‰, respectively. Overlapping with this interval of Shinfa River water sampling, air temperatures ( n = 155) also were recorded at the same time; temperatures range from c. 18 to 47°C. The coolest temperatures occur during the summer wet season, associated with the arrival of the Kiremt rains in the region, whereas the warmest temperatures occur towards the end of the dry season. In order to evaluate the extent to which this rather extreme isotope hydrology is recorded in the sediments and biota of the Shinfa River system, both hardwater calcareous deposits precipitated on basalt cobbles by evaporation in the Shinfa River channel during the dry season and aragonite from three different modern bivalve mollusc species were collected and analysed for their stable oxygen and carbon isotope compositions. Hardwater calcareous deposit δ 18 O vpdb and δ 13 C vpdb values range from c. −2 to +5‰ and c. −9 to +7‰, respectively, and preserve a trend towards progressively more positive δ 18 O vpdb and δ 13 C vpdb values through the course of the dry season. Shinfa River mollusc aragonite powders ( n = 51) were serially sampled from cf. Coelutura aegyptica , cf. Chambardia rubens and Etheria elliptica species. All species record oxygen and carbon isotopes between c. −2 and +7‰ and between c. −18 and −8‰, and each species records coherent trends between those extremes as well as a positive parametric correlation between measured oxygen and carbon isotope values. However, there does appear to be some variability of measured isotope values by species, suggesting that species-specific metabolic differences may impact the resulting range of aragonite stable carbon and oxygen values. Based upon the measured Shinfa River water δ 18 O vsmow and corresponding water temperatures at the time of sampling, a possible range of Shinfa River calcite and aragonite δ 18 O vpdb values were calculated in conjunction with well-established calcite–water and aragonite–water oxygen isotope fractionation equations. These ‘fictive’ calcite and aragonite δ 18 O vpdb values range from c. −5 to +15‰, which is a much larger range than previously documented from analyses of the hardwater calcareous deposits and mollusc aragonite samples. The narrower range of values in the natural calcite and aragonite samples may be attributed to several mechanisms, including time averaging and environmental stress. Nevertheless, the stable oxygen isotopic compositions of these natural samples offer a minimum assessment of the environmental extremes which occur in this region today, and provide a model for reconstructing the environments of the past.

Using oxygen and hydrogen stable isotopes to track the migratory movement of Sharp-shinned Hawks (Accipiter striatus) along Western Flyways of North America.

The large-scale patterns of movement for the Sharp-shinned Hawk (Accipiter striatus), a small forest hawk found throughout western North America, are largely unknown. However, based on field observations we set out to test the hypothesis that juvenile migratory A. striatus caught along two distinct migration routes on opposite sides of the Sierra Nevada Mountains of North America (Pacific Coast and Intermountain Migratory Flyways) come from geographically different natal populations. We applied stable isotope analysis of hydrogen (H) and oxygen (O) of feathers, and large scale models of spatial isotopic variation (isoscapes) to formulate spatially explicit predictions of the origin of the migrant birds. Novel relationships were assessed between the measured hydrogen and oxygen isotope values of feathers from A. striatus museum specimens of known origin and the isoscape modeled hydrogen and oxygen isotope values of precipitation at those known locations. We used these relationships to predict the origin regions for birds migrating along the two flyways from the measured isotope values of migrant’s feathers and the associated hydrogen and oxygen isotopic composition of precipitation where these feathers were formed. The birds from the two migration routes had overlap in their natal/breeding origins and did not differentiate into fully separate migratory populations, with birds from the Pacific Coast Migratory Flyway showing broader natal geographic origins than those from the Intermountain Flyway. The methodology based on oxygen isotopes had, in general, less predictive power than the one based on hydrogen. There was broad agreement between the two isotope approaches in the geographic assignment of the origins of birds migrating along the Pacific Coast Flyway, but not for those migrating along the Intermountain Migratory Flyway. These results are discussed in terms of their implications for conservation efforts of A. striatus in western North America, and the use of combined hydrogen and oxygen stable isotope analysis to track the movement of birds of prey on continental scales.

Isotope fractionation of micropollutants during large-volume extraction: heads-up from a critical method evaluation for atrazine, desethylatrazine and 2,6-dichlorobenzamide at low ng/L concentrations in groundwater

ABSTRACT Micropollutants are frequently detected in groundwater. Thus, the question arises whether they are eliminated by natural attenuation so that pesticide degradation would be observed with increasing residence time in groundwater. Conventional analytical approaches rely on parent compound/metabolite ratios. These are difficult to interpret if metabolites are sorbed or further transformed. Compound-specific stable isotope analysis (CSIA) presents an alternative for identifying degradation based on the analysis of natural isotope abundances in pesticides and their changes during degradation. However, CSIA by gas chromatography–isotope ratio mass spectrometry is challenged by the low concentrations (ng/L) of micropollutants in groundwater. Consequently, large amounts of water need to be sampled requiring enrichment and clean-up steps from interfering matrix effects that must not introduce artefacts in measured isotope values. The aim of this study was to evaluate the accuracy of isotope ratio measurements of the frequently detected micropollutants atrazine, desethylatrazine and 2,6-dichlorobenzamide after enrichment from large water volumes (up to 100 L) by solid-phase extraction with consecutive clean-up by HPLC. Associated artefacts of isotope discrimination were found to depend on numerous factors including organic matter content and extraction volume. This emphasizes the necessity to perform a careful method evaluation of sample preparation and sample pre-treatment prior reliable CSIA.

The Small Spring Method (SSM) for the definition of stable isotope–elevation relationships in Northern Calabria (Southern Italy)

A total of 141 samples mostly located in the Coastal Chain and the Sila Massif of Northern Calabria have been collected from 125 different springs and analyzed chemically and for the isotopic ratios 2H/1H and 18O/16O of water, with the ultimate aim to investigate isotope–elevation relationships for local meteoric waters. The Small Springs Method (SSM) has been adopted for this purpose. The hydrogeological characteristics of the watersheds of interest indicate that the sampled springs are discharged from water circuits hosted in alluvial and colluvial deposits rather than in the underlying rock substrata. The chemical characteristics of the sampled waters suggest that they are immature and related to shallow and short water circuits, in spite of the variable degree of chemical evolution. The spring waters of relatively high Total Ionic Salinity (TIS >12.5 meq/L) and those from watersheds of comparatively large elevation range (>650 m) and relatively large areas (>17 km2) have been excluded from the analysis of elevation–isotope relationships. An oxygen isotope lapse rate of −0.194 ‰/100 m has been found for the spring water samples of Northern Calabria as a whole, whereas the obtained hydrogen isotope lapse rates are −1.23 ‰/100 m for the Coastal Chain and −1.45 ‰/100 m for the Sila Massif based on measured isotope values and optimized elevation. The following O-isotopes equation is recommended to estimate the average elevation (Hi in m) of the recharge areas of Northern Calabria aquifer systems:δO18=−0.00194·Hi−5.91 (1)The maximum estimated uncertainty due to variations in the δ18O value over time is 202 m. Use of H isotopes is not recommended for constraining average recharge elevations due to important local variations, reflecting in different relations for the Coastal Chain and the Sila Massif. Assuming that Equation (1) holds true throughout Northern Calabria, an iso-distribution map of the δ18O value in rainfall has been drawn for this area.

Concentration effects on laser‐based δ18O and δ2H measurements and implications for the calibration of vapour measurements with liquid standards

Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ(18) O, δ(2) H), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in δ(18) O and δ(2) H with water vapour concentration. For δ(18) O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ(2) H, we found diverging patterns for liquid and vapour samples, with no dependence on δ-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for δ(2) H than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the δ-values into account. Copyright © 2010 John Wiley & Sons, Ltd.

Using Strontium Isotopes to Study Site Accumulation Processes

Strontium isotopes (87Sr/86) in tooth enamel reflect the geological subtrate on which an animal lived during tooth development. Therefore, strontium isotopes of tecth in fosil cave accumulations are potentially useful in determining whether an animal was native to the vicinity of the site or was brought in by other agents such as predators from farther afield. In this study, we tested the ability of strontium isotopes to help determine the origins of fossil rodents in Gladysvale Cave, South Africa. First, biologically available 87 Sr/86) ratios were established using modern plants recovered from three geologically distintct areas, the Malmani dolomite, the Hekpoort andesite/basalt, and the Timeball Hill shale, all of which were found to be significantly different. Strontium isotope values were then measured on tooth enamel of rodents from a moder barn ow (Tyto alba) roost in Gladysvale (67%) versus those from other geological zones. We then measured strontium isotope values of enamel from 14 fossil rodent teecth from Gladysvale Cave. The average and range of values for the fossil rodents is similar to that of the modern owl roost rodents. Fifty-seven percent of the fossil rodent probably derived from the local dolomite, while others were brought in from at least 0.8 km away. A pilot study of 87Sr/86Sr ratios of fossil rodent teeth from Swartkrans Member 1 and Sterkfontein Member 4 indicates that 81% and 55% of those rodents, respectively, are from the local dolomite subtrate. Overall, this study shows that strontium isotopes can be a useful tool in taphonomy analyses by identifying non. Local individuals, and has great potential for elucidating more of the taphonomic history of fossil accumulations in the dolomitic cave sites of South Africa.

Variations in expression of carbon isotope fractionation of chlorinated ethenes during biologically enhanced PCE dissolution close to a source zone

The stable carbon isotope values of tetrachloroethene (PCE) and its degradation products were monitored during studies of biologically enhanced dissolution of PCE dense nonaqueous phase liquid (DNAPL) to determine the effect of PCE dissolution on observed isotope values. The degradation of PCE was monitored in a 2-dimensional model aquifer and in a pilot test cell (PTC) at Dover Air Force Base, both with emplaced PCE DNAPL sources. Within the plume down gradient from the source, the isotopic fractionation of dissolved PCE and its degradation products were consistent with those observed in biodegradation laboratory studies. However, close to the source zone significant shifts in the isotope values of dissolved PCE were not observed in either the model aquifer or PTC due to the constant input of newly dissolved, non fractionated PCE, and the small isotopic fractionation associated with PCE reductive dechlorination by the mixed microbial culture used. Therefore the identification of reductive dechlorination in the presence of PCE DNAPL was based upon the appearance of daughter products and the isotope values of those daughter products. An isotope model was developed to simulate isotope values of PCE during the dissolution and degradation of PCE adjacent to a DNAPL source zone. With the exception of very high degradation rate constants (>1/day) stable carbon isotope values of PCE estimated by the model remained within error of the isotope value of the PCE DNAPL, consistent with measured isotope values in the model aquifer and in the PTC.

Estimation of lake water—groundwater interactions in meromictic mining lakes by modelling isotope signatures of lake water

A method is presented to assess lake water–groundwater interactions by modelling isotope signatures of lake water using meteorological parameters and field data. The modelling of δ18O and δD variations offers information about the groundwater influx into a meromictic Lusatian mining lake. Therefore, a water balance model is combined with an isotope water balance model to estimate analogies between simulated and measured isotope signatures within the lake water body. The model is operated with different evaporation rates to predict δ18O and δD values in a lake that is only controlled by weather conditions with neither groundwater inflow nor outflow. Comparisons between modelled and measured isotope values show whether the lake is fed by the groundwater or not. Furthermore, our investigations show that an adaptation of the Craig and Gordon model [H. Craig, L.I. Gordon. Deuterium and oxygen-18 variations in the ocean and the marine atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, E. Tongiorgi (Ed.), pp. 9–130, Consiglio Nazionale delle Ricerche, Laboratorio di Geologia Nucleare, Pisa (1965).] to specific conditions in temperate regions seems necessary.

Stable isotope composition of Late Cretaceous benthic foraminifera from the southern South Atlantic: Biological and environmental effects

The stable carbon and oxygen isotope composition of different benthic foraminiferal species of the latest Campanian and earliest Maastrichtian from Ocean Drilling Project Hole 690C (Weddell Sea, southern South Atlantic, ∼1800 m paleowater depth) have been investigated. The total range of measured isotope values of all samples exceeds ∼4‰ for δ 13C and 1.1‰ for δ 18O. Carbon isotope values of proposed deep infaunal species are generally similar or only slightly lower when compared to proposed epifaunal to shallow infaunal species. Interspecific differences vary between samples probably reflecting temporal changes in organic carbon fluxes to the sea floor. Constantly lower δ 13C values for Pullenia marssoni and Pullenia reussi suggest the deepest habitat for these species. The strong depletion of δ 13C values by up to 3‰ within lenticulinids may be attributed to a deep infaunal microhabitat, strong vital effects, or different feeding strategy when compared to other species or modern lenticulinids. The mean δ 18O values reveal a strong separation of epifaunal to shallow infaunal and deep infaunal species. Epifaunal to shallow infaunal species are characterized by low δ 18O values, deep infaunal species by higher values. This result possibly reflects lower metabolic rates and longer life cycles of deep infaunal species or the operating of a pore water [CO 3 2−] effect on the benthic foraminiferal stable isotopes. Pyramidina szajnochae shows an enrichment of oxygen isotopes with test size comprising a total of 0.6‰ between 250 and 1250 μm shell size. Although δ 13C lacks a corresponding trend these data likely represent the presence of changes in metabolic rates during ontogenesis. These results demonstrate the general applicability of multi-species stable isotope measurements of pristine Cretaceous benthic foraminifera to reconstruct past microhabitats and to evaluate biological and environmental effects on the stable isotope composition.

Stable isotope fractionation analysis as a tool to monitor biodegradation in contaminated acquifers

The assessment of biodegradation in contaminated aquifers has become an issue of increasing importance in the recent years. To some extent, this can be related to the acceptance of intrinsic bioremediation or monitored natural attenuation as a means to manage contaminated sites. Among the few existing methods to detect biodegradation in the subsurface, stable isotope fractionation analysis (SIFA) is one of the most promising approaches which is pronounced by the drastically increasing number of applications. This review covers the recent laboratory and field studies assessing biodegradation of contaminants via stable isotope analysis. Stable isotope enrichment factors have been found that vary from no fractionation for dioxygenase reactions converting aromatic hydrocarbons over moderate fractionation by monooxygenase reactions ( ɛ=−3‰) and some anaerobic studies on microbial degradation of aromatic hydrocarbons ( ɛ=−1.7‰) to larger fractionations by anaerobic dehalogenation reactions of chlorinated solvents ( ɛ=between −5‰ and −30‰). The different isotope enrichment factors can be related to the respective biochemical reactions. Based on that knowledge, we discuss under what circumstances SIFA can be used for a qualitative or even a quantitative assessment of biodegradation in the environment. In a steadily increasing number of cases, it was possible to explain biodegradation processes in the field based on isotope enrichment factors obtained from laboratory experiments with pure cultures and measured isotope values from the field. The review will focus on the aerobic and anaerobic degradation of aromatic hydrocarbons and chlorinated solvents as the major contaminants of groundwater. Advances in the instrumental development for stable isotope analysis are only mentioned if it is important for the understanding of the application.

Sulfur isotopic measurements from a West Antarctic ice core: implications for sulfate source and transport

AbstractMeasurements of δ34S covering the years 1935–76 and including the 1963 Agung (Indonesia) eruption were made on a West Antarctic firn core, RIDSA (78.73˚ S, 116.33˚ W; 1740ma.s.l.), and results are used to unravel potential source functions in the sulfur cycle over West Antarctica. The δ34S values of SO42– range from 3.1‰ to 9.9‰. These values are lower than those reported for central Antarctica, from near South Pole station, of 9.3–18.1‰ (Patris and others, 2000). While the Agung period is isotopically distinct at South Pole, it is not in the RIDSA dataset, suggesting differences in the source associations for the sulfur cycle between these two regions. Given the relatively large input of marine aerosols at RIDSA (determined from Na+ data and the seasonal SO42– cycle), there is likely a large marine biogenic SO42– influence. The δ34S values indicate, however, that this marine biogenic SO42–, with a well-established δ34S of 18‰, is mixing with SO42– that has extremely negative δ34S values to produce the measured isotope values in the RIDSA core. We suggest that the transport and deposition of stratospheric SO42– in West Antarctica, combined with local volcanic input, accounts for the observed variance in δ34S values.

Reduced sulfur in euxinic sediments of the Cariaco Basin: sulfur isotope constraints on organic sulfur formation

Reduced sulfur accumulation in Holocene and latest Pleistocene euxinic marine sediments from the Cariaco Basin, Venezuela, was investigated to constrain the timing and possible pathways of organic matter (OM) sulfurization. Data were collected for a diverse suite of sulfur species, including concentrations and sulfur isotope compositions of pore-water sulfide, pore-water sulfate, pyrite sulfur, total organic sulfur (TOS), kerogen sulfur (KS), and polar bitumen sulfur (PBS). Results suggest that there was a period during which almost no diagenetic pyrite formed in the sediments of the Cariaco, coincident with a shift from high to lower sedimentation rates and a concomitant change in the delivery of organic matter to the sediments. The sulfur isotope composition of organic matter was predicted based on assumed pathways using weighted isotopic mass balance calculations and compared to measured isotope values for organic sulfur. These results indicate that organic sulfur is derived primarily from pore-water sulfide, with minor contributions from primary bio-sulfur (e.g., in proteins derived from algae and bacteria). The predicted sulfur isotope values of organic sulfur compounds (OSC) suggest that pore-water sulfide is the ultimate source of reduced sulfur for incorporation into organic matter. It is possible, however, that reactive sulfur intermediates such as elemental sulfur or polysulfides react directly with organic matter. These intermediate sulfur species are likely formed through partial oxidation of sulfide by anaerobic sulfide-oxidizing microbes living in the sediments.

Assessment of Bacterial Degradation of Aromatic Hydrocarbons in the Environment by Analysis of Stable Carbon Isotope Fractionation

13C/12C stable carbon isotope fractionation was used to assess biodegradation in contaminated aquifers with toluene as a model compound. Different strains of anaerobic bacteria (Thauera aromatica, Geobacter metallireducens, and the sulfate-reducing strain TRM1) showed consistent 13C/12C carbon isotope fractionation with fractionation factors between αC = 1.0017 and 1.0018. In contrast, three cultures of aerobic organisms, using different mono- and dioxygenase enzyme systems to initiate toluene degradation, showed variable isotope fractionation factors of αC = 1.0027 (Pseudomonasputida strain mt-2), αC = 1.0011 (Ralstonia picketii), andαC = 1.0004 (Pseudomonas putida strain F1). The great variability of isotope fractionation between different aerobic bacterial strains suggests that interpretation of isotope data in oxic habitats can only be qualitative. A soil column was run as a model system for contaminated aquifers with toluene as the carbon source and sulfate as the electron acceptor and samples were taken at different ports along the column. Microbial toluene degradation was calculated based on the 13C/12C isotope fractionation factors of the batch culture experiments together with the observed 13C/12C isotope shifts of the residual toluene fractions. The calculated percentage of biodegradation, B, correlated well with the decreasing toluene concentrations at the sampling ports and indicated the increasing extent of biodegradation along the column. The theoretical toluene concentrations as calculated based on the isotope values matched the measured concentrations at the different sampling ports indicating that the Rayleigh equation can be used to calculate biodegradation in quasi closed systems based on measured isotope shifts. A similar attempt was performed to assess toluene degradation in a contaminated, anoxic aquifer. A transect of groundwater wells was monitored along the main direction of the groundwater flow and revealed decreasing concentrations accompanied with an increase in the 13C/12C stable carbon isotope ratio of the residual toluene. Calculation of the extent of biodegradation based on the isotope values and laboratory derived isotope fractionation factors showed that the residual toluene was degraded to more than 99% by microbial activity. Calculation of the theoretical residual toluene concentrations based on the measured isotope values described the strongly decreasing concentrations along the plume. Other aromatic hydrocarbons like benzene and naphthalene which were analysed in the same course also showed decreasing concentrations along the groundwater flow path accompanied by increasing δ13C values indicating biodegradation.

Differences in CH4 oxidation and pathways of production between rice cultivars deduced from measurements of CH4 flux and δ13C of CH4 and CO2

We report measurements of CH4 flux and δ13C and δD values of emitted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Beaumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for the differences in CH4 emissions between cultivars. Using the measured isotope values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxidation and production throughout the growing season. For instance, rhizospheric CH4 oxidation increased as the season progressed in both Mars and Lemont cultivars. Late in the season, however, 71±10% of CH4 produced in the Mars plot was oxidized compared to only 39±10% in the Lemont plot. The contribution of acetate fermentation to methanogenesis at specific times during the season was calculated using measured isotopic values and assuming identical isotopic fractionation factors in methanogenic pathways for the cultivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H2 was estimated by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 production from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemont cultivar, while an increase followed by a decrease near the end of the season was observed for the Mars cultivar.

Analytical improvements in irm-GC/MS analyses: Advanced techniques in tube furnace design and sample preparation

Peak resolution is an essential requirement for achieving accurate irm-GC/MS compound-specific isotope measurements. Obtaining accurate isotope data from petroleum extracts, in particular, has proven especially difficult due to co-elutions that typically comprise crude oil mixtures. New enhancements in peak resolution in irm-GC/MS analyses have been achieved by substituting a polyimide-coated glass capillary as the furnace reactor. The advantages of the design are significant, and include: (1) incorporation of a zero dead-volume combustion reactor, resulting in almost no loss in gas chromatographic peak resolution, equating to less compound interferences and more accurately measured isotope values and (2) use of direct capillary-to-capillary leak tight, zero dead-volume connections that reduce the incidence of leak problems generally encountered as a result of using fittings of substantially different sizes. In addition, improvements in analytical chromatography using size-exclusion techniques such as urea adduction and molecular sieving reduces the number of components in a sample, and have resulted in the acquisition of more accurate isotope data. Experiments comparing paraffin isotope measurements for whole oil samples to those of urea adducted samples of the same oil have shown that differences of 2‰ are not uncommon. Data from experiments with isotope standards indicate that urea adduction and molecular sieving techniques have no observable fractionation effect on the measurement of isotope values for paraffin components in these mixtures. Considering the intensive effort and expense in generating irm-GC/MS data, the use of relatively inexpensive chromatography techniques such as urea adduction and molecular sieving in isolating components of interest for accurate isotope analysis is justified.

Isotopic determination of snow-water contribution to soil water and groundwater

The stable isotopes oxygen-18 and deuterium were used to determine the seasonal contributions of snow and rain to the soil and groundwater systems of a prairie agricultural site during the period, 1985–1987. Using measured isotope values and a temperature-isotope relationship the isotope values of precipitation during the previous 10 years at Edmonton, Canada, were determined. Winter precipitation had average oxygen-18 and deuterium values of −25.6%. and −194%., respectively; spring through autumn rains had weighted values of −16.2%. and −125%., respectively. Soil samples taken in the top 4m indicated that mixing of infiltrating waters was generally complete by a depth of 30 cm. With algebraic manipulation of measured isotope concentrations of soil and precipitation waters with known physical processes, it was possible to estimate the seasonal composition of the waters in the top 4 m. After correction for evaporation, both soil water (0–0.9 m) and shallow groundwater (3–4 m) had lower oxygen-18 values (−18.7%.; and −20.3%., respectively) than the weighted average annual precipitation concentration (−18.1%.). Using isotopes the snow-water composition of soil water was estimated to be 27% and of groundwater to be 44%. These values are higher than the proportion of winter precipitation of total annual precipitation (21% with rains ⩽5 mm day −1 excluded). The higher snow-water content of groundwater, along with the fact that the groundwater underwent less fractionation owing to evaporation than soil waters, could be the result of lateral flow from nearby small depressions where snowmelt runoff waters accumulated during spring. These findings emphasize the importance of the contribution of winter precipitation to groundwater recharge.