Isotope Ratio Mass Spectrometer Research Articles

Stable carbon isotopic ratios of CH 4–CO 2-bearing fluid inclusions in fracture-fill mineralization from the Lower Saxony Basin (Germany) – A tool for tracing gas sources and maturity

The stable carbon isotopic ratios (δ 13C) of methane (CH 4) and carbon dioxide (CO 2) of gas-rich fluid inclusions hosted in fracture-fill mineralization from the southern part of the Lower Saxony Basin, Germany have been measured online using a crushing device interfaced to an isotopic ratio mass spectrometer (IRMS). The data reveal that CH 4 trapped in inclusions seems to be derived from different source rocks with different organic matter types. The δ 13C values of CH 4 in inclusions in quartz hosted by Carboniferous rocks range between −25 and −19‰, suggesting high-maturity coals as the source of methane. Methane in fluid inclusions in minerals hosted by Mesozoic strata has more negative carbon isotope ratios (−45 to −31‰) and appears to represent primary cracking products from type II kerogens, i.e., marine shales. There is a positive correlation between increasing homogenization temperatures of aqueous fluid inclusions and less negative δ 13C(CH 4) values of in co-genetic gas inclusions probably indicating different mtaturity of the potential source rocks at the time the fluids were released. The CO 2 isotopic composition of CH 4–CO 2-bearing inclusions shows slight negative or even positive δ 13C values indicating an inorganic source (e.g., water–rock interaction and dissolution of detrital, marine calcite) for CO 2 in inclusions. We conclude that the δ 13C isotopic ratios of CH 4–CO 2-bearing fluid inclusions can be used to trace migration pathways, sources of gases, and alteration processes. Furthermore, the δ 13C values of methane can be used to estimate the maturity of the rocks from which it was sourced. Results presented here are further supported by organic geochemical analysis of surface bitumens which coexist with the gas inclusion-rich fracture-fill mineralization and confirm the isotopic interpretations with respect to fluid source, type and maturity.

Multiple-sulfur isotope effects during photolysis of carbonyl sulfide

Abstract. Laboratory experiments were carried out to determine sulfur isotope effects during ultraviolet photolysis of carbonyl sulfide (OCS) to carbon monoxide (CO) and elemental sulfur (S0). The OCS gas at 3.7 to 501 mbar was irradiated with or without a N2 bath gas using a 150 W Xe arc lamp. Sulfur isotope ratios for the product S0 and residual OCS were analyzed by an isotope ratio mass-spectrometer with SF6 as the analyte gas. The isotope fractionation after correction for the reservoir effects is −6.8‰ for the ratio 34S/32S, where product S0 is depleted in heavy isotopes. The magnitude of the overall isotope effect is not sensitive to the addition of N2 but increases to −9.5‰ when radiation of λ > 285 nm is used. The measured isotope effect reflects that of photolysis as well as the subsequent sulfur abstraction (from OCS) reaction. The magnitude of isotope effects for the abstraction reaction is estimated by transition state theory to be between −18.9 and −3.1‰ for 34S which gives the photolysis isotope effect as −10.5 to +5.3‰. The observed triple isotope coefficients are ln(δ34S + 1)/ln(δ34S + 1) = 0.534 ± 0.005 and ln(δ36S + 1)/ln(δ34S + 1) = 1.980 ± 0.021. These values differ from canonical values for mass-dependent fractionation of 0.515 and 1.90, respectively. The result demonstrates that the OCS photolysis does not produce large isotope effects of more than about 10‰ for 34S/32S, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.

Ptolemy operations and results during the Lutetia flyby

The September 5th 2008 Rosetta flyby of asteroid 2867 Šteins raised tantalising questions concerning the possibility of outgassing and extant exospheres surrounding small bodies within the asteroid belt. With this in mind and recognising the size and composition of asteroid 21 Lutetia it was decided to attempt a detection of such possible outgassing using a number of Rosetta instruments, including Ptolemy, a miniature gas chromatograph isotope ratio mass spectrometer aboard the lander Philae. Ptolemy collected 5 sets of mass spectra at hourly intervals during the Lutetia encounter; 2 background measurements either side of close approach and an exosphere detection measurement as the spacecraft passed over the subsolar point. An increase in pressure was detected at the subsolar point but the cause (exosphere or spacecraft) was unresolved with the realisation that any such detection is hindered by outgassing from the spacecraft and instrument itself. Constraints are placed on the properties of Lutetia exosphere, structure and production rates. The experience gained is used to provide suggestions for future missions or spacecraft events aimed at the detection and characterisation of the exospheres of Solar System bodies.

Liquid chromatography/mass spectrometry stable isotope analysis of dissolved organic carbon in stream and soil waters

A commercial interface coupling liquid chromatography (LC) to a continuous-flow isotope ratio mass spectrometry (CF-IRMS) instrument was used to determine the δ(13) C of dissolved organic carbon (DOC) in natural waters. Stream and soil waters from a farmland plot in a hedgerow landscape were studied. Based on wet chemical oxidation of dissolved organics the LC/IRMS interface allows the on-line injection of small volumes of water samples, an oxidation reaction to produce CO(2) and gas transfer to the isotope ratio mass spectrometer. In flow injection analysis (FIA) mode, bulk DOC δ(13)C analysis was performed on aqueous samples of up to 100 μL in volume in the range of DOC concentration in fresh waters (1-10 mg C.L(-1)). Mapping the DOC δ(13)C spatial distribution at the plot scale was made possible by this fairly quick method (10 min for triplicate analyses) with little sample manipulation. The relative contributions of different plot sectors to the DOC pool in the stream draining the plot were tentatively inferred on the basis of δ(13)C differences between the hydrophilic and hydrophobic components.

Urinary excretion ofin vivo13C-labelled milk oligosaccharides in breastfed infants

Recent observations indicate that human milk oligosaccharides (HMO) are involved in a variety of physiological processes in infants. Their metabolic fate, however, is virtually unknown. We investigated metabolic aspects in infants after endogenous 13C-labelling of HMO. An oral bolus of natural and 13C-labelled galactose (Gal; 23 g Gal+4 g 13C-Gal) was given to ten lactating women. Aliquots of milk at each nursing as well as breath samples from the mothers and urine from their infants were collected over 36 h. The 13C-enrichment of HMO and their renal excretion was determined by isotope ratio-MS; characterisation was achieved by fast atom bombardment-MS. After the Gal bolus was given, an immediate 13C-enrichment in milk and in infants' urine was observed which lasted 36 h. Mass spectrometric analysis of 13C-enriched urinary fractions confirmed the excretion of a variety of neutral and acidic HMO without metabolic modification of their structures. Components with glucose split off at the reducing end were also detectable. Quantitative data regarding the infants' intake of lacto-N-tetraose and its monofucosylated derivative lacto-N-fucopentaose II ranged from 50 to 160 mg with each suckling, respectively; renal excretion of both components varied between 1 and 3 mg/d. Since the intake of individual HMO by the infants was in the range of several hundred mg per suckling, i.e. several g/d, and some of these components were excreted in mg amounts as intact HMO with the infants' urine, not only local but also systemic effects might be expected.

Can gas chromatography combustion isotope ratio mass spectrometry be used to quantify organic compound abundance?

Quantifying the concentrations of organics such as phospholipid fatty acids (PLFAs) and n-alkanes and measuring their corresponding (13)C/(12)C isotope ratios often involves two separate analyses; (1) quantification by gas chromatography flame ionisation detection (GC-FID) or gas chromatography/mass spectrometry (GC/MS), and (2) (13) C-isotope abundance analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC-C-IRMS). This requirement for two separate analyses has obvious disadvantages in terms of cost and time. However, there is a history of using the data output of isotope ratio mass spectrometers to quantify various components; including the N and C concentrations of solid materials and CO(2) concentrations in gaseous samples. Here we explore the possibility of quantifying n-alkanes extracted from sheeps' faeces and fatty acid methyl esters (FAMEs) derivatised from PLFAs extracted from grassland soil, using GC-C-IRMS. The results were compared with those from GC-FID analysis of the same extracts. For GC-C-IRMS the combined area of the masses for all the ions (m/z 44, 45 and 46) was collected, referred to as 'area all', while for the GC-FID analysis the peak area data were collected. Following normalisation to a common value for added internal standards, the GC-C-IRMS 'area all' values and the GC-FID peak area data were directly compared. Strong linear relationships were found for both n-alkanes and FAMEs. For the n-alkanes the relationships were 1:1 while, for the FAMEs, GC-C-IRMS overestimated the areas relative to the GC-FID results. However, with suitable reference material 1:1 relationships were established. The output of a GC-C-IRMS system can form the basis for the quantification of certain organics including FAMEs and n-alkanes.

Determination of the concentration of nitrogenous bio-organic compounds using an isotope ratio mass spectrometer operating in continuous flow mode

The quality of the determination of compound-specific isotopic content at natural abundance by gas chromatography-isotope ratio measurement-mass spectrometry (GC-irm-MS) relies on the stability of the voltage generated by the ion detector Faraday cages. The application of GC-irm-MS to the determination of δ(13)C (‰) and δ(15)N (‰) is now routine. However, for numerous applications, it is necessary to determine both the isotope content (δ(15)N) and the quantity (in micromoles) of analyte present. We now show that it is possible for nitrogen-containing compounds to measure how much analyte is present with an irm mass spectrometer linked to a GC by exploiting the integrated N(2) total ion current intensity (Vs) generated by measuring the (15)N/(14)N isotope ratio. The method is validated over a range of concentration (2-70 mmol/L) and δ(15)N (-70 to +50‰) values for six molecules of diverse chemical nature and functionality (nortropine, norpseudotropine, nortropinone, cysteine, taurine, glutathione). It is shown that once the ion current is calibrated, the quantitative values are of a comparable quality to those obtained from GC with flame ionization detection (GC-FID). In addition, it is demonstrated that over a definable range, the δ(15)N (‰) value is independent of the quantity of analyte introduced, confirming the validity of this method.

δD and δ 13C analyses of atmospheric volatile organic compounds by thermal desorption gas chromatography isotope ratio mass spectrometry

This paper describes the establishment of a robust method to determine compound specific δD and δ 13C values of volatile organic compounds (VOCs) in a standard mixture ranging between C 6 and C 10 and was applied to various complex emission samples, e.g. from biomass combustion and car exhaust. A thermal desorption (TD) unit was linked to a gas chromatography isotope ratio mass spectrometer (GC–irMS) to enable compound specific isotope analysis (CSIA) of gaseous samples. TenaxTA was used as an adsorbent material in stainless steel TD tubes. We determined instrument settings to achieve a minimal water background level for reliable δD analysis and investigated the impact of storage time on δD and δ 13C values of collected VOCs (176 days and 40 days of storage, respectively). Most of the standard compounds investigated showed standard deviations (SD) < 6‰ (δD) when stored for 148 days at 4 °C. However, benzene revealed occasionally D depleted values (21‰ SD) for unknown reasons. δ 13C analysis demonstrated that storage of 40 days had no effect on VOCs investigated. We also showed that breakthrough (benzene and toluene, 37% and 7%, respectively) had only a negligible effect (0.7‰ and 0.4‰, respectively) on δ 13C values of VOCs on the sample tube. We established that the sample portion collected at the split flow effluent of the TD unit can be used as a replicate sample for isotope analysis saving valuable sampling time and resources. We also applied TD-GC–irMS to different emission samples (biomass combustion, petrol and diesel car engines exhaust) and for the first time δD values of atmospheric VOCs in the above range are reported. Significant differences in δD of up to 130‰ were observed between VOCs in emissions from petrol car engine exhaust and biomass combustion (Karri tree). However, diesel car emissions showed a high content of highly complex unresolved mixtures thus a baseline separation of VOCs was not achieved for stable hydrogen isotope analysis. The ability to analyse δD by TD-GC–irMS complements the characterisation of atmospheric VOCs and is maybe used for establishing further source(s).

Stable isotope ratio measurements of royal jelly samples for controlling production procedures: impact of sugar feeding

The carbon and nitrogen stable ratios of royal jelly (RJ) samples from various origins are determined using an elemental analyser linked online to an isotope ratio mass spectrometer to evaluate authenticity and adulteration. The (13)C/(12)C and (15)N/(14)N stable isotope ratios are measured in more than 500 RJs (domestic, imported and derived from feeding experiments) in order to obtain isotopic measurements that take into account seasonal, botanical and geographical effects. Authenticity intervals are established for traditional beekeeping practices, without feeding, in the range -22.48 to -27.90‰ for δ(13)C. For these samples, the δ(15)N values range from -1.58 to 7.98‰, depending on the plant sources of pollen and nectar. The δ(13)C values of the commercial samples vary from -18.54 to -26.58‰. High δ(13)C values are typical of sugar cane or corn syrups which have distinctive isotopic (13)C signatures because both plants use the C4 photosynthetic cycle, in contrast to most RJs which are derived from C3 plants. These differences in the (13)C-isotopic composition allow the detection of the addition of such sugars. RJs from traditional sources and from industrial production by sugar feeding are thus successfully distinguished.

Enriquecimento e alocação de 13C em plantas de eucalipto

Nas últimas décadas, a utilização de isótopos estáveis em várias áreas de pesquisa vem se destacando, como na análise de fluxos e rotas metabólicas, análise de efeitos de estresses em plantas e, em grande escala, no estudo da matéria orgânica do solo (MOS). Estudos de alterações e dinâmica da MOS usando a variação da abundância natural do 13C requerem mudanças na razão isotópica do C. Quando não existe essa possibilidade, uma das alternativas é enriquecer o material vegetal (planta) com 13C, via fixação de 13CO2, de modo que a razão isotópica seja distinta daquela da MOS original. O objetivo deste trabalho foi investigar a magnitude e a homogeneidade do enriquecimento em 13C em diferentes componentes da planta de eucalipto. No processo de marcação, três plantas de eucalipto, com 4 meses de idade, cultivadas em solução nutritiva foram expostas a uma atmosfera enriquecida com 13CO2, em uma câmara de vidro (448 dm³), com temperatura em torno de 24 ºC. A concentração de CO2 e a razão 13C/12C foram monitoradas por um espectrômetro de massa de razão isotópica (IRMS) em amostras de ar retiradas ao longo do processo (126 dias com três pulsos de 13CO2 semanais). Após o período de marcação, as plantas foram separadas em folha (folha-fonte e folha-dreno), galho, casca, lenho e raiz e analisadas em IRMS. O resultado foi expresso em partes por mil (‰) em relação ao padrão internacional de C denominado Pee-Dee Belemnite (PDB), obtendo-se a δ13C PDB delas: folha-fonte (828,07 ‰), folha-dreno (645,72 ‰), galho (672,49 ‰), casca (691,86 ‰), lenho (632,02 ‰) e raiz (536,55 ‰). O padrão de alocação e enriquecimento de 13C entre os componentes das plantas foi homogêneo, embora com diferenças numéricas da ordem de 291 ‰ na δ13C PDB. As plantas de eucalipto mantiveram alta taxa de absorção de CO2 e, consequentemente, alta taxa fotossintética em concentrações de CO2 muito acima (180,4 mmol L-1 - 7.934 ppmv) da encontrada na atmosfera (8,64 mmol L-1 - 380 ppmv). O 13C fixado durante o dia foi liberado em menor escala na respiração noturna, em comparação com o 12C. O grau de enriquecimento com 13C obtido indica que a técnica empregada permite o enriquecimento suficiente do material para traçar o C em estudos de decomposição e estabilização de litter de eucalipto em frações da MOS.

Stable isotope profiles of nitrogen gas indicate denitrification in oxygen‐stratified humic lakes

Mid-summer N(2) profiles were analyzed from nine oxygen-stratified, humic-acid-rich lakes using a continuous flow isotope ratio mass spectrometer and a Gasbench II device. Sample preparation steps were performed under water to avoid air contamination. The instrument precision for the δ(15)N measurement was high (0.03‰), but for the whole sampling and analysis procedure the mean deviation between replicate samples was 0.13‰ for the δ(15)N measurements and 5.5% for the N(2) gas concentration analysis. The results show that the Gasbench peripheral was suitable for measurement of the (15)N natural abundance of dissolved nitrogen gas, with denitrification indicated by the oversaturation and slightly (<1‰) depleted δ(15)N values of the dissolved N(2) gas in the suboxic zones of some of the study lakes. Calculated values for the denitrified (excess) N(2) varied between -5.3 and 0.7‰. The denitrification potential was determined using the (15)N tracer method, with results showing nitrate-inducible denitrification and no signs of anaerobic ammonium oxidation (anammox).

Laser ablation isotope ratio mass spectrometry for enhanced sensitivity and spatial resolution in stable isotope analysis

Stable isotope analysis permits the tracking of physical, chemical, and biological reactions and source materials at a wide variety of spatial scales. We present a laser ablation isotope ratio mass spectrometry (LA-IRMS) method that enables δ(13)C measurement of solid samples at 50 µm spatial resolution. The method does not require sample pre-treatment to physically separate spatial zones. We use laser ablation of solid samples followed by quantitative combustion of the ablated particulates to convert sample carbon into CO(2). Cryofocusing of the resulting CO(2) coupled with modulation in the carrier flow rate permits coherent peak introduction into an isotope ratio mass spectrometer, with only 65 ng carbon required per measurement. We conclusively demonstrate that the measured CO(2) is produced by combustion of laser-ablated aerosols from the sample surface. We measured δ(13)C for a series of solid compounds using laser ablation and traditional solid sample analysis techniques. Both techniques produced consistent isotopic results but the laser ablation method required over two orders of magnitude less sample. We demonstrated that LA-IRMS sensitivity coupled with its 50 µm spatial resolution could be used to measure δ(13) C values along a length of hair, making multiple sample measurements over distances corresponding to a single day's growth. This method will be highly valuable in cases where the δ(13)C analysis of small samples over prescribed spatial distances is required. Suitable applications include forensic analysis of hair samples, investigations of tightly woven microbial systems, and cases of surface analysis where there is a sharp delineation between different components of a sample.

Marine sediment pore‐water profiles of phosphate d18O using a refined micro‐extraction

Phosphorus cycling in the ocean is influenced by biological and geochemical processes that are reflected in the oxygen isotope signature of dissolved inorganic phosphate (Pi). Extending the Pi oxygen isotope record from the water column into the seabed is difficult due to low Pi concentrations and small amounts of marine porewaters available for analysis. We obtained porewater profiles of Pi oxygen isotopes using a refined protocol based on the original micro‐extraction designed by Colman (2002). This refined and customized method allows the conversion of ultra‐low quantities (0.5 – 1 µmol) of porewater Pi to silver phosphate (Ag3PO4) for routine analysis by mass spectrometry. A combination of magnesium hydroxide co‐precipitation with ion exchange resin treatment steps is used to remove dissolved organic matter, anions, and cations from the sample before precipitating Ag3PO4. Samples as low as 200 µg were analyzed in a continuous flow isotope ratio mass spectrometer setup. Tests with external and laboratory internal standards validated the preservation of the original phosphate oxygen isotope signature (δ18OP) during micro extraction. Porewater data on δ18OP has been obtained from two sediment cores of the Moroccan margin. The δ18OP values are in a range of +19.49 to +27.30‰. We apply a simple isotope mass balance model to disentangle processes contributing to benthic P cycling and find evidence for Pi regeneration outbalancing microbial demand in the upper sediment layers. This highlights the great potential of using δ18OP to study microbial processes in the subseafloor and at the sediment water interface.

Stable carbon isotope ratio analyses on trace methane from ice samples

Measurement of the concentration and stable isotope ratios of methane from air enclosures in ancient ice can aid in understanding the temporal dynamics of methane sources and sinks. These measurements can reconstruct past atmospheric compositions on scales of tens-to-hundreds of years with adequate precision and accuracy. We present an improved micro-extraction gas-chromatography continuous-flow isotope ratio mass spectrometer (CF-IRMS) method to extract and measure the methane 13C/ 12C ratio (δ 13CH 4) of air occluded in glacial ice. The technique described uses a unique post-combustion trapping of the methane signal to produce a high amplitude sample peak increasing the signal to noise ratio of the sample. The technique requires only ca. 165 pmol of CH 4 (ca. 30–150 g of ice depending on methane concentration). The small sample requirement permits very fine temporal sampling resolution and the possibility of routine duplicate measurements from multi-parameter ice core samples. The technique was applied to atmospheric air, artificial ice samples, and samples from the GISP2 ice core. The precision of our technique was evaluated using both atmospheric air and artificial ice samples and is less than ± 0.3‰ (1σ). Our test measurements of shallow, poorer quality GISP2 ice from 225 ± 20 yr BP give a δ 13CH 4 value of − 47.37 ± 0.6‰ (corrected for gravitational fractionation and referenced to VPDB), which is in excellent agreement with other published data.

Authentication of Bergamot Essential Oil by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometer (GC-C-IRMS)

The quality assessment of bergamot essential oils was established employing the gas chromatography-combustion- isotope ratio mass spectrometer (GC-C-IRMS) technique. An authenticity range was obtained investigating the carbon stable isotope ratio of genuine Italian bergamot essential oils (harvest period 2008–2009), in order to compare the GC-C-IRMS data of several industrial, commercial and foreign bergamot essential oil samples. Moreover, with the aim to test the efficiency and the sensibility of IRMS device, self-adulterated in laboratory bergamot oil samples were analyzed. The data were compared with those achieved by conventional enantioselective gas chromatography (Es-GC) and high resolution gas chromatography (GC-FID). Results of this work indicated that GC-C-IRMS was able not only to detect the presence of adulterants in the samples, but also to discriminate the bergamot oil samples according to their geographic provenance and the nature of the adulterants added.

Hydrogen isotope correction for laser instrument measurement bias at low water vapor concentration using conventional isotope analyses: application to measurements from Mauna Loa Observatory, Hawaii

The hydrogen and oxygen isotope ratios of water vapor can be measured with commercially available laser spectroscopy analyzers in real time. Operation of the laser systems in relatively dry air is difficult because measurements are non-linear as a function of humidity at low water concentrations. Here we use field-based sampling coupled with traditional mass spectrometry techniques for assessing linearity and calibrating laser spectroscopy systems at low water vapor concentrations. Air samples are collected in an evacuated 2 L glass flask and the water is separated from the non-condensable gases cryogenically. Approximately 2 µL of water are reduced to H(2) gas and measured on an isotope ratio mass spectrometer. In a field experiment at the Mauna Loa Observatory (MLO), we ran Picarro and Los Gatos Research (LGR) laser analyzers for a period of 25 days in addition to periodic sample collection in evacuated flasks. When the two laser systems are corrected to the flask data, they are strongly coincident over the entire 25 days. The δ(2)H values were found to change by over 200‰ over 2.5 min as the boundary layer elevation changed relative to MLO. The δ(2)H values ranged from -106 to -332‰, and the δ(18)O values (uncorrected) ranged from -12 to -50‰. Raw data from laser analyzers in environments with low water vapor concentrations can be normalized to the international V-SMOW scale by calibration to the flask data measured conventionally. Bias correction is especially critical for the accurate determination of deuterium excess in dry air.

Inter‐laboratory calibration of new silver orthophosphate comparison materials for the stable oxygen isotope analysis of phosphates

Stable oxygen isotope compositions (δ(18)O values) of two commercial and one synthesized silver orthophosphate reagents have been determined on the VSMOW scale. The analyses were carried out in three different laboratories: lab (1) applying off-line oxygen extraction in the form of CO(2) which was analyzed on a dual inlet and triple collector isotope ratio mass spectrometer, while labs (2) and (3) employed an isotope ratio mass spectrometer coupled to a high-temperature conversion/elemental analyzer (TC/EA) where Ag(3)PO(4) samples were analyzed as CO in continuous flow mode. The δ(18)O values for the proposed new comparison materials were linked to the generally accepted δ(18)O values for Vennemann's TU-1 and TU-2 standards as well as for Ag(3)PO(4) extracted from NBS120c. The weighted average δ(18)O(VSMOW) values for the new comparison materials UMCS-1, UMCS-2 and AGPO-SCRI were determined to be + 32.60 (± 0.12), + 19.40 (± 0.12) and + 14.58 (± 0.13)‰, respectively.

Measurement of 13C and 15N isotope labeling by gas chromatography/combustion/isotope ratio mass spectrometry to study amino acid fluxes in a plant–microbe symbiotic association

We have developed a method based on a double labeling with stable isotopes and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses to study amino acid exchange in a symbiotic plant-microbe association. Isotopic precision was studied for 21 standards including 15 amino acid derivatives, three N-protected amino acid methyl esters, three amines and one international standard. High correlations were observed between the δ(13)C and δ(15)N values obtained by GC/C/IRMS and those obtained by an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (R(2) = 0.9868 and 0.9992, respectively). The mean precision measured was 0.04‰ for δ(13)C and 0.28‰ for δ(15)N (n = 15). This method was applied in vivo to the symbiotic relationship between alfalfa (Medicago sativa L.) and N(2)-fixing bacteria. Plants were simultaneously labeled over 10 days with (13)C-depleted CO(2) ((12)CO(2)), which was assimilated through photosynthesis by leaves, and (15)N(2) fixed via nodules. Subsequently, the C and N isotope compositions (i.e. δ(13)C and δ(15)N) of free amino acids were analyzed in leaves and nodules by GC/C/IRMS. The method revealed the pattern of C and N exchange between leaves and nodules, highlighting that γ-aminobutanoic acid and glycine may represent an important form of C transport from leaves to the nodules. The results confirmed the validity, reliability and accuracy of the method for assessing C and N fluxes between plants and symbiotic bacteria and support the use of this technique in a broad range of metabolic and fluxomic studies.

A method for analyzing the δ18O of resin‐extractable soil inorganic phosphate

Improved tools for tracing phosphate transformations in soils are much needed, and can lead to a better understanding of the terrestrial phosphorus cycle. The oxygen stable isotopes in soil phosphate are still not exploited in this regard. Here we present a method for measuring the oxygen stable isotopes in a fraction of the soil phosphate which is rapidly available to plants, the resin-extractable P. This method is based on extracting available phosphate from the soil with anion-exchange membranes, soil organic matter removal by a resin, purification by precipitation as cerium phosphate, and finally precipitation as silver phosphate. The purified silver phosphate samples are then measured by a high-temperature elemental analyzer (HT-EA) coupled in continuous flow mode to an isotope ratio mass spectrometer. Testing the method with Mediterranean and semi-arid soils showed no artifacts, as well as good reproducibility in the same order as that of the HT-EA analytical uncertainty (0.3‰).

An Inexpensive Auto-injection Preprocessing System for N2O by Isotope Ratio Mass Spectrometer to Measure δ15N and δ18O of Nitrate

硝酸イオンの窒素・酸素安定同位体比(δ15N，δ18O)はその高い起源推定能から海洋，湖沼，河川，地下水での物質循環の研究に用いられてきた。1990年代半ばに開発された脱窒菌法により，硝酸イオンのδ15N，δ18Oを微量で同時に測定できるようになった。脱窒菌法で多量のサンプルを短時間に精度よく測定するには前処理ライン(N2Oガスを質量分析計に導入するための前処理)が必要不可欠である。しかしながら，こうした前処理ラインの分析機器は高額(数千万円程度)である。著者らは多くの研究室に設置されている通常の固体粉末試料の同位体測定ライン(質量分析計と元素分析計がConflo IIIで連結された測定システム)を活用し，測定精度や処理速度を落とすことなく，安価に自動前処理できるラインを開発することに成功したのでここに報告する。