Two new organic reference materials for delta13C and delta15N measurements and a new value for the delta13C of NBS 22 oil

Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different 13C and 15N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a delta13C value of -26.24 per thousand relative to VPDB and a delta15N value of -4.52 per thousand relative to N2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in 13C and 15N. USGS41 has a delta13C value of +37.76 per thousand and a delta15N value of +47.57 per thousand. The delta13C and delta15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta13C=+1.95 per thousand ), L-SVEC lithium carbonate (delta13C=-46.48 per thousand ), IAEA-N-1 ammonium sulfate (delta15N=0.43 per thousand ), and USGS32 potassium nitrate (delta15N=180 per thousand ) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta13C is better than 0.13 per thousand, and that of delta15N is better than 0.13 per thousand in 100-microg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a delta13C value for NBS 22 oil of -29.91 per thousand, in contrast to the commonly accepted value of -29.78 per thousand for which off-line blank corrections probably have not been quantified satisfactorily.     

Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry delta18O measurements of nitrogen-containing compounds

On-line determination of the oxygen isotopic composition (delta(18)O value) in organic and inorganic samples is commonly performed using a thermal conversion elemental analyzer (TC-EA) linked to a continuous flow isotope ratio mass spectrometry (IRMS) system. Accurate delta(18)O analysis of N-containing compounds (like nitrates) by TC-EA-IRMS may be complicated because of interference of the N(2) peak on the m/z 30 signal of the CO peak. In this study we evaluated the effectiveness of two methods to overcome this interference which do not require any hardware modifications of standard TC-EA-IRMS systems. These methods were (1) reducing the amount of N(2) introduced into the ion source through He dilution of the N(2) peak and (2) an improved background correction on the CO m/z 30 sample peak integration.Our results show that He dilution is as effective as diverting the N(2) peak in order to eliminate this interference. We conclude that the He-dilution technique is a viable method for the delta(18)O analysis of nitrates and other N-containing samples (which are not routinely measured using He dilution) using TC-EA-IRMS, since it can easily be programmed in the standard software of IRMS systems. With the He-dilution technique delta(18)O values of the nitrate isotope standards USGS34, IAEA-N3 and USGS35 were measured using the shortest possible traceability chain to the VSMOW-SLAP scale, and the results were -28.1 +/- 0.1 per thousand, +25.5 +/- 0.1 per thousand and +57.5 +/- 0.2 per thousand, respectively. An improved background correction was also an effective method, but required manual correction of the raw data.     

Determination of the 15N/14N, 17O/16O, and 18O/16O ratios of nitrous oxide by using continuous-flow isotope-ratio mass spectrometry

We developed a rapid, sensitive, and automated analytical system to determine the delta15N, delta18O, and Delta17O values of nitrous oxide (N2O) simultaneously in nanomolar quantities for a single batch of samples by continuous-flow isotope-ratio mass spectrometry (CF-IRMS) without any cumbersome and time-consuming pretreatments. The analytical system consisted of a vacuum line to extract and purify N2O, a gas chromatograph for further purification of N2O, an optional thermal furnace to decompose N2O to O2, and a CF-IRMS system. We also used pneumatic valves and pneumatic actuators in the system so that we could operate it automatically with timing software on a personal computer. The analytical precision was better than 0.12 per thousand for delta15N with >4 nmol N2O injections, 0.25 per thousand for delta18O with >4 nmol N2O injections, and 0.20 per thousand for Delta17O with >20 nmol N2O injections for a single measurement. We were also easily able to improve the precision (standard errors) to better than 0.05 per thousand for delta15N, 0.10 per thousand for delta18O, and 0.10 per thousand for Delta17O through multiple analyses with more than four repetitions with 190 nmol samples using the automated analytical system. Using the system, the delta15N, delta18O, and Delta17O values of N2O can be quantified not only for atmospheric samples, but also for other gas or liquid samples with low N2O content, such as soil gas or natural water. Here, we showed the first ever Delta17O measurements of soil N2O.     

Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid delta13C values determined by gas chromatography-combustion-isotope ratio mass spectrometry

Litter-dwelling (epigeic) Lumbricus rubellus and soil-dwelling (endogeic) Allolobophora chlorotica earthworms were observed aggregating under C(3) (delta(13)C = -31.3 per thousand; delta(15)N = 10.7 per thousand) and C(4) (delta(13)C = -12.6 per thousand; delta(15)N = 7.5 per thousand) synthetic dung pats applied to a temperate grassland (delta(13)C = -30.3 per thousand; delta(15)N = 5.7 per thousand) in an experiment carried out for 372 days. Bulk delta(13)C values of earthworms collected from beneath either C(3) or C(4) dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C(4) dung were significantly (13)C-enriched after 56 days (delta(13)C = -23.8 per thousand) and 112 days (delta(13)C = -22.4 per thousand) compared with those from C(3) dung treatments (56 days, delta(13)C = -26.5 per thousand; 112 days, delta(13)C = -27.0 per thousand), and (ii) A. chlorotica were 2.1 per thousand (13)C-enriched (delta(13)C = -24.2 per thousand) relative to those from C(3) dung (delta(13)C = -26.3 per thousand) treatments after 372 days. Bulk delta(15)N values did not suggest significant uptake of dung N by either species beneath C(3) or C(4) dung, but showed that the endogeic species (total mean delta(15)N = 3.3 per thousand) had higher delta(15)N values than the epigeic species (total mean delta(15)N = 5.4 per thousand). Although the two species exhibited similar fatty acid profiles, individual fatty acid delta(13)C values revealed extensive routing of dietary C into body tissue of L. rubellus, but minor incorporation into A. chlorotica. In particular, the direct incorporation of microbial biomarker fatty acids (iC(17:0), aC(17:0)) from (13)C-labelled dung in situ, the routing of dung C into de novo synthesised compounds (iC(20:4)(omega)(6),C(20:5)(omega)(3), and the assimilation of essential fatty acids ((C(18:1)(omega)(9), C(18:1)(omega(7), C(18:2)(omega(6), C(18:3)(omega)(3)) derived from dung, were determined.     

Oxygen isotope analysis of carbonates in the calcite-dolomite-magnesite solid-solution by high-temperature pyrolysis: initial results

Accurate and efficient measurement of the oxygen isotope composition of carbonates (delta(C) (18)O) based on the mass spectrometric analysis of CO(2) produced by reacting carbonate samples with H(3)PO(4) is compromised by: (1) uncertainties associated with fractionation factors (alpha(CO)(2)C) used to correct measured oxygen isotope values of CO(2)(delta(CO(2)(18)O) to delta(C) (18)O; and (2) the slow reaction rates of many carbonates of geological and environmental interest with H(3)PO(4). In contrast, determination of delta(C) (18)O from analysis of CO produced by high-temperature (>1400 degrees C) pyrolytic reduction, using an elemental analyser coupled to continuous-flow isotope-ratio mass spectrometry (TC/EA CF-IRMS), offers a potentially efficient alternative that measures the isotopic composition of total carbonate oxygen and should, therefore, theoretically be free of fractionation effects. The utility of the TC/EA CF-IRMS technique was tested by analysis of carbonates in the calcite-dolomite-magnesite solid-solution and comparing the results with delta(C) (18)O measured by conventional thermal decomposition/fluorination (TDF) on the same materials. Initial results show that CO yields are dependent on both the chemical composition of the carbonate and the specific pyrolysis conditions. Low gas yields (<100% of predicted yield) are associated with positive (>+0.2 per thousand) deviations in delta(C(TC/EA) (18)O compared with delta(C(TDF) (18)O. At a pyrolysis temperature of 1420 degrees C the difference between delta(C) (18)O measured by TC/EA CF-IRMS and TDF (Delta(C(TC/EA,TDF) (18)O) was found to be negatively correlated with gas yield (r = -0.785) and this suggests that delta(C) (18)O values (with an estimated combined standard uncertainty of +/-0.38 per thousand) could be derived by applying a yield-dependent correction. Increasing the pyrolysis temperature to 1500 degrees C also resulted in a statistically significant correlation with gas yield (r = -0.601), indicating that delta(C) (18)O values (with an estimated uncertainty of +/-0.43 per thousand) could again be corrected using a yield-dependent procedure. Despite significant uncertainty associated with TC/EA CF-IRMS analysis, the magnitude of the uncertainty is similar to that associated with the application of poorly defined values of alpha(CO)(2), (C) used to derive delta(C) (18)O from delta(CO(2) (18)O measured by the H(3)PO(4) method for most common carbonate phases. Consequently, TC/EA CF-IRMS could provide a rapid alternative for the analysis of these phases without any effective deterioration in relative accuracy, while analytical precision could be improved by increasing the number of replicate analyses for both calibration standards and samples. Although automated gas preparation techniques based on the H(3)PO(4) method (ISOCARB, Kiel device, Gas-Bench systems) have the potential to measure delta(CO)(2) (18)O efficiently for specific, slowly reacting phases (e.g. dolomite), problems associated with poorly defined alpha(CO)(2), (C) remain. The application of the Principle of Identical Treatment is not a solution to the analysis of these phases because it assumes that a single fractionation factor may be defined for each phase within a solid-solution regardless of its precise chemical composition. This assumption has yet to be tested adequately.     

Multiple stable isotope (18O, 13C, 15N and 34S) analysis of human hair to identify the recent migrants in a rural community in SW England

Relationships between recent migration and hair delta(18)O values were examined for 40 people living in a rural community in SW England. The isotopic contents of 35 'local' hair samples were compared with those of 5 recently arrived individuals (from Australia, Canada, Chile, Germany and the USA). The hair delta(18)O values of these 'visitors' were +7.9 (Omaha, USA), +11.2 (Jena, Germany), +12.1 (Osorno, Chile), +12.6 (Montreal, Canada) and +14.3 per thousand (Adelaide, Australia). The hair value for the USA visitor (+7.9 per thousand) fell outside the range for the 33 local adult residents, +10.5 to +14.3 per thousand (+12.7 +/- 0.8 per thousand). Hair delta(18)O values did not identify the individuals from Adelaide, Montreal and Osorno as 'visitors', but hair delta(13)C or delta(34)S data did. Combining the hair delta(18)O, delta(13)C and delta(34)S values using principal components analysis (two components explained 89% of the overall variation among the 40 subjects) helped to more clearly distinguish European from non-European individuals, indicating the existence of global overall isotope (geo-origin) relationships.     

A new method to determine the 17O isotopic abundance in CO2 using oxygen isotope exchange with a solid oxide.

This paper discusses a simple method to determine 17O isotope excess or deficiency ('mass-independent isotopic composition') in CO2 gas. When applying conventional mass spectrometry of CO2 (m/z 44, 45 and 46) to determine the 17O/16O ratio, the 13C/12C ratio has to be established separately. This can be achieved by analysing an aliquot of sample CO2 before and after subjecting it to oxygen isotope exchange with a pool of oxygen with 'normal' 17O/16O ratio, i.e. with Delta17O approximately equal to delta17O-0.516 x delta18O = 0. Cerium oxide has been shown to be practically well suited for the exchange of CO2 oxygen; the reagent is safe and does not produce any contamination. The CO2-CeO2 exchange reaction has 99.8 +/- 0.7% recovery yield. At 650 degrees C this reaction reaches equilibrium in 30 min and, as tested, results in complete oxygen replacement. Delta17O determinations depend on accuracy of CO2 delta measurements: the repeatability of +/-0.015 per thousand (1sigma) in delta(45)R and delta(46)R determination relative to the working reference results in an error of Delta17O as small as +/-0.33 per thousand. Such a precision is sufficient for Delta17O determination in stratospheric CO2. The calculated Delta17O value systematically depends on absolute 17R and 13R ratios in isotopic reference materials, which are presently not yet known with certainty (the 17R value is most important), and may be inadequate for 17O-correction with a = 0.516. Within the present uncertainty, Delta17O determined in 17O-enriched CO2 agrees with the value directly measured in the enriched O2 from which this CO2 was produced. Besides Delta17O determination, investigated CO2-CeO2 equilibration may have several other implications. Fast, complete isotopic exchange of CO2 by reaction with CeO2 may also be employed to get reproducible 17O-correction and, hence, to better monitor small delta13C shifts and to isotopically equilibrate mixtures of CO2 gases.     

Using dual-bacterial denitrification to improve delta15N determinations of nitrates containing mass-independent 17O

The bacterial denitrification method for isotopic analysis of nitrate using N(2)O generated from Pseudomonas aureofaciens may overestimate delta(15)N values by as much as 1-2 per thousand for samples containing atmospheric nitrate because of mass-independent (17)O variations in such samples. By analyzing such samples for delta(15)N and delta(18)O using the denitrifier Pseudomonas chlororaphis, one obtains nearly correct delta(15)N values because oxygen in N(2)O generated by P. chlororaphis is primarily derived from H(2)O. The difference between the apparent delta(15)N value determined with P. aureofaciens and that determined with P. chlororaphis, assuming mass-dependent oxygen isotopic fractionation, reflects the amount of mass-independent (17)O in a nitrate sample. By interspersing nitrate isotopic reference materials having substantially different delta(18)O values with samples, one can normalize oxygen isotope ratios and determine the fractions of oxygen in N(2)O derived from the nitrate and from water with each denitrifier. This information can be used to improve delta(15)N values of nitrates having excess (17)O. The same analyses also yield estimates of the magnitude of (17)O excess in the nitrate (expressed as Delta(17)O) that may be useful in some environmental studies. The 1-sigma uncertainties of delta(15)N, delta(18)O and Delta(17)O measurements are +/-0.2, +/-0.3 and +/-5 per thousand, respectively.     

A simple method for oxygen-18 determination of milligram quantities of water using NaHCO3 reagent

This paper presents a modified H(2)O-CO(2) equilibration method for stable oxygen isotopic composition (delta(18)O) analysis of water. This method enables rapid and simple delta(18)O analysis of milligram quantities of water, by employing solid reagent NaHCO(3) as the CO(2) source, a small (0.6 mL) glass vial for the equilibration chamber, and an isotope-monitoring gas chromatography/mass spectrometry (irm-GC/MS) system for delta 18O(CO2) analysis. This method has several advantages, including simple handling for the H(2)O-CO(2) equilibration (without purging and/or evacuation treatments), rapid and easy delta(18)O analysis of equilibrated CO(2), and highly sensitive and highly precise delta(18)O analysis of H(2)O, using samples as small as 10 mg and with a precision of less than +/-0.12 per thousand. The time needed to attain oxygen isotopic equilibration between CO(2) and water is also comparable (17 h for 10 mg H(2)O and 10 h for 100 mg H(2)O) to other previous methods using CO(2) gas for the CO(2) source. The extent of delta(18)O variation of sample water from its initial delta(18)O value due to isotope exchange with added NaHCO(3) is also discussed. It is concluded that the correction needed is negligible (less than 0.1 per thousand ) as long as the oxygen atom ratio (O(NaHCO3)/O(H2O)) is less than 3.3 +/- 10(-3) and provided the delta18O(H2O) determination is made by comparing delta(18)O of CO(2) equilibrated with sample water and that equilibrated with standard water of a moderately close delta(18)O value, less than 30 per thousand difference.     

Delta13C and delta18O isotopic composition of CaCO3 measured by continuous flow isotope ratio mass spectrometry: statistical evaluation and verification by application to Devils Hole core DH-11 calcite

A new method was developed to analyze the stable carbon and oxygen isotope ratios of small samples (400 +/- 20 micro g) of calcium carbonate. This new method streamlines the classical phosphoric acid/calcium carbonate (H(3)PO(4)/CaCO(3)) reaction method by making use of a recently available Thermoquest-Finnigan GasBench II preparation device and a Delta Plus XL continuous flow isotope ratio mass spectrometer. Conditions for which the H(3)PO(4)/CaCO(3) reaction produced reproducible and accurate results with minimal error had to be determined. When the acid/carbonate reaction temperature was kept at 26 degrees C and the reaction time was between 24 and 54 h, the precision of the carbon and oxygen isotope ratios for pooled samples from three reference standard materials was 相似文献   

Reporting small Delta 17O values: existing definitions and concepts

The three-isotope tracer Delta(17)O is increasingly used in atmospheric chemistry and other research areas. Thanks to the development of isotope-ratio mass spectrometry (IRMS), delta(17)O and delta(18)O can be determined with a precision of a few 0.01 per thousand, and values for Delta(17)O may be calculated with similar precision. However, interpreting small and precisely determined Delta(17)O values as a deviation from an expected mass-dependent fractionation process is not straightforward. Several aspects are of high importance. In the present paper we review existing definitions, formulas and some other aspects of Delta(17)O reporting. One of the most confusing aspects is a variance of definitions and corresponding formulas. While Delta(17)O is traditionally defined to characterise a data point, i.e. Delta(17)O is considered as a deviation from an expected mass-fractionation line, the recently introduced definition (Miller MF. Geochim. Cosmochim. Acta 2002; 66: 188) characterises a fractionation line itself, in terms of its ordinate intercept. The formulas corresponding to this definition gives a characteristic for a specific process. When the 'traditionally defined' Delta(17)O is in use, an expected fractionation processes--the key point for Delta(17)O reporting--should be defined and parameterised with the same accuracy as intended for reporting Delta(17)O. When Delta(17)O is reported for a data point, not only a value for lambda but an ordinate intercept of a reference fractionation line should be given with high accuracy. We note that defining a single fractionation process is hardly possible for many natural compounds. For such compounds we propose to use a phenomenological reference line, namely an isotope composition range of natural sources. Next, aspects of Delta(17)O comparison and mass-balance calculations are considered. All the aspects considered for Delta(17)O may be relevant for others three-isotope tracers, e.g. Delta(33)S.     

High precision measurements of 17O/16O and 18O/16O ratios in H2O

We have optimized the method of water fluorination using the solid reagent CoF3 to produce O2. This allows isotope ratio measurements by dual-inlet mass spectrometry with very high precision of 0.01 to 0.03/1000 for both delta17O and delta18O. Using this method, delta17O and delta18O of atmospheric O2 were determined as 12.08 and 23.88/1000 vs. VSMOW, respectively. Likewise, delta17O and delta18O of GISP were -13.12 and -24.73/1000, and for SLAP they were -29.48 and -55.11/1000 vs. VSMOW, respectively. Analysis of these data in a ln(delta17O + 1) vs. ln(delta18O + 1) plot yields a line with a regression coefficient (lambda) of 0.5279 +/- 0.0001 (R2 = 0.999999). We also determined the fractionation factors 17alpha and 18alpha in liquid-vapor equilibrium, and found that the ratio ln 17alpha/ln 18alpha is constant (0.529 +/- 0.001) over the temperature range 11.4 to 41.5 degrees C.     

Uncertainties in the oxygen isotopic composition of barium sulfate induced by coprecipitation of nitrate

Coprecipitation of nitrate and sulfate by barium has probably resulted in significant error in numerous studies dealing with the oxygen isotopic composition of natural sulfates using chemical/thermal conversion of BaSO(4) and analysis by isotope ratio mass spectrometry. In solutions where NO(3) (-)/SO(4) (2-) molar ratios are above 2 the amount of nitrate coprecipitated with BaSO(4) reaches a maximum of approximately 7% and decreases roughly linearly as the molar ratio decreases. The fraction of coprecipitated nitrate appears to increase with decreasing pH and is also affected by the nature of the cations in the precipitating solution. The size of the oxygen isotope artifact in sulfate depends both on the amount of coprecipitated nitrate and the delta(18)O and Delta(17)O values of the nitrate, both of which can be highly variable. The oxygen isotopic composition of sulfate extracted from atmospheric aerosols or rain waters are probably severely biased because photochemical nitrate is usually also present and it is highly enriched in (18)O (delta(18)O approximately 50-90 per thousand) and has a large mass-independent isotopic composition (Delta(17)O approximately 20-32 per thousand). The sulfate delta(18)O error can be 2-5 per thousand with Delta(17)O artifacts reaching as high as 4.0 per thousand. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Using delta15N and delta18O to evaluate the sources and pathways of NO3- in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions

The origin of NO(3) (-) yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) was measured from the 'inter-flow' and 'drain-flow' of two 1 ha drained lysimeter plots, one of which had received an application of 21 m(3) of NH(4) (+)-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO(3) (-)-N from the two lysimeters were comparable; an initial flush of NO(3) (-)-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO(3) (-)-N L(-1). The isotopic signature of the delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of delta(15)N-NO(3) (-) during rainfall events ranged between -1.6 to +5.2 per thousand and +0.4 to +11.1 per thousand from the inter-flow and drain-flow, respectively. Variation in the delta(18)O-NO(3) (-) ranged from +2.0 to +7.8 per thousand and from +3.3 to +8.4 per thousand. No significant relationships between the delta(15)N-NO(3) (-) or delta(18)O-NO(3) (-) and flow rate were observed in most cases although delta(18)O-NO(3) (-) values indicated a positive relationship and delta(15)N-NO(3) (-) values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical delta(18)O-NO(3) (-) for soil microbial NO(3) (-) indicated that the contribution of rainfall NO(3) (-) accounted for 8% of the NO(3) (-) in the lysimeter drainage at most. The calculated contribution of rainfall NO(3) (-) was not enough to account for the depletion in delta(15)N-NO(3) (-) values observed during the duration of the rainfall event 2. The relationship between delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) from the drain-flow indicated that denitrification was causing enrichment in the isotopes from this pathway. The presence of slurry seemed to cause a relative depletion in delta(18)O-NO(3) (-) in the inter-flow and delta(15)N-NO(3) (-) in the drain-flow compared with the zero-slurry lysimeter. This may have been caused by increased microbial nitrification stimulated by the presence of increased NH(4) (+)-N.     

Extraction of CO2 from air samples for isotopic analysis and limits to ultra high precision delta18O determination in CO2 gas.

The determination of delta18O values in CO2 at a precision level of +/-0.02 per thousand (delta-notation) has always been a challenging, if not impossible, analytical task. Here, we demonstrate that beyond the usually assumed major cause of uncertainty - water contamination - there are other, hitherto underestimated sources of contamination and processes which can alter the oxygen isotope composition of CO2. Active surfaces in the preparation line with which CO2 comes into contact, as well as traces of air in the sample, can alter the apparent delta18O value both temporarily and permanently. We investigated the effects of different surface materials including electropolished stainless steel, Duran glass, gold and quartz, the latter both untreated and silanized. CO2 frozen with liquid nitrogen showed a transient alteration of the 18O/16O ratio on all surfaces tested. The time to recover from the alteration as well as the size of the alteration varied with surface type. Quartz that had been ultrasonically cleaned for several hours with high purity water (0.05 microS) exhibited the smallest effect on the measured oxygen isotopic composition of CO2 before and after freezing. However, quartz proved to be mechanically unstable with time when subjected to repeated large temperature changes during operation. After several days of operation the gas released from the freezing step contained progressively increasing trace amounts of O2 probably originating from inclusions within the quartz, which precludes the use of quartz for cryogenically trapping CO2. Stainless steel or gold proved to be suitable materials after proper pre-treatment. To ensure a high trapping efficiency of CO2 from a flow of gas, a cold trap design was chosen comprising a thin wall 1/4" outer tube and a 1/8" inner tube, made respectively from electropolished stainless steel and gold. Due to a considerable 18O specific isotope effect during the release of CO2 from the cold surface, the thawing time had to be as long as 20 min for high precision delta18O measurements. The presence of traces of air in almost all CO2 gases that we analyzed was another major source of error. Nitrogen and oxygen in the ion source of our mass spectrometer (MAT 252, Finnigan MAT, Bremen, Germany) give rise to the production of NO2 at the hot tungsten filament. NO2+ is isobaric with C16O18O+ (m/z 46) and interferes with the delta18O measurement. Trace amounts of air are present in CO2 extracted cryogenically from air at -196 degrees C. This air, trapped at the cold surface, cannot be pumped away quantitatively. The amount of air present depends on the surface structure and, hence, the alteration of the measured delta18O value varies with the surface conditions. For automated high precision measurement of the isotopic composition of CO2 of air samples stored in glass flasks an extraction interface ('BGC-AirTrap') was developed which allows 18 analyses (including standards) per day to be made. For our reference CO2-in-air, stored in high pressure cylinders, the long term (>9 months) single sample precision was 0.012 per thousand for delta13C and 0.019 per thousand for delta18O.     

High-precision determination of 18O/16O ratios of silver phosphate by EA-pyrolysis-IRMS continuous flow technique

A high-precision, and rapid on-line method for oxygen isotope analysis of silver phosphate is presented. The technique uses high-temperature elemental analyzer (EA)-pyrolysis interfaced in continuous flow (CF) mode to an isotopic ratio mass spectrometer (IRMS). Calibration curves were generated by synthesizing silver phosphate with a 13 per thousand spread in delta(18)O values. Calibration materials were obtained by reacting dissolved potassium dihydrogen phosphate (KH(2)PO(4)) with water samples of various oxygen isotope compositions at 373 K. Validity of the method was tested by comparing the on-line results with those obtained by classical off-line sample preparation and dual inlet isotope measurement. In addition, silver phosphate precipitates were prepared from a collection of biogenic apatites with known delta(18)O values ranging from 12.8 to 29.9 per thousand (V-SMOW). Reproducibility of +/- 0.2 per thousand was obtained by the EA-Py-CF-IRMS method for sample sizes in the range 400-500 microg. Both natural and synthetic samples are remarkably well correlated with conventional (18)O/(16)O determinations. Silver phosphate is a very stable material and easy to degas and, thus, could be considered as a good candidate to become a reference material for the determination of (18)O/(16)O ratios of phosphate by high-temperature pyrolysis.     

Investigation of preparation techniques for δ2H analysis of keratin materials and a proposed analytical protocol

Accurate hydrogen isotopic measurements of keratin materials have been a challenge due to exchangeable hydrogen in the sample matrix and the paucity of appropriate isotopic reference materials for calibration. We found that the most reproducible δ(2)H(VSMOW-SLAP) and mole fraction of exchangeable hydrogen, x(H)(ex), of keratin materials were measured with equilibration at ambient temperature using two desiccators and two different equilibration waters with two sets of the keratin materials for 6 days. Following equilibration, drying the keratin materials in a vacuum oven for 4 days at 60 °C was most critical. The δ(2)H analysis protocol also includes interspersing isotopic reference waters in silver tubes among samples in the carousel of a thermal conversion elemental analyzer (TC/EA) reduction unit. Using this analytical protocol, δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of USGS42 and USGS43 human-hair isotopic reference materials were determined to be -78.5 ± 2.3 ‰ and -50.3 ± 2.8 ‰, respectively. The measured x(H)(ex) values of keratin materials analyzed with steam equilibration and N(2) drying were substantially higher than those previously published, and dry N(2) purging was unable to remove absorbed moisture completely, even with overnight purging. The δ(2)H values of keratin materials measured with steam equilibration were about 10 ‰ lower than values determined with equilibration in desiccators at ambient temperatures when on-line evacuation was used to dry samples. With steam equilibrations the x(H)(ex) of commercial keratin powder was as high as 28%. Using human-hair isotopic reference materials to calibrate other keratin materials, such as hoof or horn, can introduce bias in δ(2)H measurements because the amount of absorbed water and the x(H)(ex) values may differ from those of unknown samples. Correct δ(2)H(VSMOW-SLAP) values of the non-exchangeable fractions of unknown human-hair samples can be determined with atmospheric moisture equilibration by normalizing with USGS42 and USGS43 human-hair reference materials when all materials have the same powder size.     

Deuterium and oxygen-18 determination of microliter quantities of a water sample using an automated equilibrator

We describe a modified version of the equilibration method and a correction algorithm for isotope ratio measurements of small quantities of water samples. The deltaD and the delta(18)O of the same water sample can both be analyzed using an automated equilibrator with sample sizes as small as 50 microL. Conventional equilibration techniques generally require water samples of several microL. That limitation is attributable mainly to changes in the isotope ratio ((18)O/(16)O or D/H) of water samples during isotopic exchange between the equilibration gas (CO(2) or H(2)) and water, and therefore the technique for microL quantities of water requires mass-balance correction using the water/gas (CO(2) or H(2)) mole ratio to correct this isotopic effect. We quantitatively evaluate factors controlling the variability of the isotopic effect due to sample size. Theoretical consideration shows that a simple linear equation corrects for the effects without determining parameters such as isotope fractionation factors and water/gas mole ratios. Precisions (1-sigma) of 50-microL meteoric water samples whose isotopic compositions of -1.4 to -396.2 per thousand for deltaD are +/-0.5 to +/-0.6 per thousand, and of -0.37 to -51.37 per thousand for delta(18)O are +/-0.01 to +/-0.11 per thousand.     

One-electron reduction of aqueous nitric oxide: a mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.     

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.