Accurate Isotope Ratio Measurements Research Articles

Rapid, precise and accurate Os isotope ratio measurements of nanogram to sub-nanogram amounts using multiple Faraday collectors and amplifiers equipped with 1012 Ω resistors by N-TIMS

We present a new approach to obtain rapid and precise (0.1 % or better) Os isotopic compositions for small (nanogram to sub-nanogram) amounts of Os extracted from geological samples using static collection with Faraday cups and amplifiers equipped with 1012 Ω resistors, by negative thermal ionization mass spectrometry (N-TIMS/Triton Plus). The results show that the measurement repeatability of Os isotopic ratios (190Os/188Os, 187Os/188Os and 186Os/188Os) changes as a function of signal intensity that varied from 0.005 to 0.05V for 192OsO3− (here and throughout the text, given intensities are equivalent signals that would be obtained using a 1011 Ω standard amplifier; 0.001V (1mV) is equivalent to ~62500 cps). At 192OsO3− ion beams greater than 0.02V with 50 ratios measured over a 10min acquisition time, the repeatability on 187Os/188Os, as well as 186Os/188Os and 190Os/188Os, is better than that obtained by the conventional peak-hopping electron multiplier (SEM) at 192OsO3−=~200,000 cps with 500–100 ratios measured over a 30–60min acquisition time. At 192OsO3− ion beams of ~0.04V or above, the 187Os/188Os and 186Os/188Os data for loads of 1 and 0.1ng Os reference materials can be measured with a repeatability of <0.1% (2σ) and deviate by <0.1% from the accepted values. Similar results can be achieved for Os load sizes in geological samples as low as~0.025ng. In addition, duplicate Faraday measurements of six spiked peridotitic samples (total Os loaded=0.3–1.2ng) yield results within uncertainty of data obtained by peak-hopping SEM, including both 187Os/188Os and Os concentration. At much higher 192OsO3− ion beams (~0.5–1V) with 300 ratios taken (total acquisition time circa one hour), the Faraday measurements of 1 to 3ng Os loads of reference material solutions can generate high-precision 186Os/188Os data with a repeatability of 30–50ppm and a mean intermediate precision of 10–30ppm. Collectively, our tests demonstrate that the use of Faraday cups equipped with 1012 Ω amplifiers in a static collection mode by N-TIMS can rapidly produce precise and accurate (within 0.1 %) Os isotopic data (both 187Os/188Os and 186Os/188Os) for small amounts of Os (as low as 0.025ng) extracted from geological samples. The optimum 192OsO3− signal is 0.04V or above to achieve the best results. While the smaller signal intensities used in concert with 1012 Ω amplifiers necessarily yield lower precision measurements than the best achievable with larger signals on 1011 Ω amplifiers, the repeatability we have achieved on reference materials can expand the application of the 190Pt–186Os decay system to geological samples containing significantly less Os than those normally measured on 1011 Ω amplifiers, if isotopic variations in excess of 50ppm are present.

Isotope dilution mass spectrometry for quantitative elemental analysis of powdered samples by radiofrequency pulsed glow discharge time of flight mass spectrometry

In recent years particular effort is being devoted to the development of pulsed glow discharges (PGDs) for mass spectrometry because this powering operation mode could offer important ionization analytical advantages. However, the capabilities of radiofrequency (RF) PGD coupled to a time of flight mass spectrometry (ToFMS) for accurate isotope ratio measurements have not been demonstrated yet. This work is focused on investigating different time positions along the pulse profile for the accurate measurement of isotope ratios. As a result, a method has been developed for the direct and simultaneous multielement determination of trace elements in powdered geological samples by RF-PGD-ToFMS in combination with isotope dilution mass spectrometry (IDMS) as an absolute measurement method directly traceable to the International System of Units. Optimized operating conditions were 70W of applied radiofrequency power, 250Pa of pressure, 2ms of pulse width and 4ms of pulse period, being argon the plasma gas used. To homogeneously distribute the added isotopically-enriched standards, lithium borate fusion of powdered solid samples was used as sample preparation approach. In this way, Cu, Zn, Ba and Pb were successfully determined by RF-PGD-ToF(IDMS) in two NIST Standard Reference Materials (SRM 2586 and SRM 2780) representing two different matrices of geological interest (soil and rock samples). Cu, Zn, Ba and Pb concentrations determined by RF-PGD-ToF(IDMS) were well in agreement with the certified values at 95% confidence interval and precisions below 12% relative standard deviation were observed for three independent analyses. Elemental concentrations investigated were in the range of 81–5770mg/kg, demonstrating the potential of RF-PGD-ToF(IDMS) for a sensitive, accurate and robust analysis of powdered samples.

Isotope ratio measurements: new developments and applications

After 2004 and 2008 (Vols. 378/2 and 390/2) this is the third issue of Analytical and Bioanalytical Chemistry to contain papers on new developments and applications in isotope ratio measurements. Whereas Klaus Heumann was guest editor for the two previous special issues, in this issue Torsten Schmidt joins him as guest editor for the first time. His involvement with this collection of papers strongly reflects the increasing importance of compound-specific stable isotope analysis, one of his core research areas, on which, with Maik Jochmann, he has recently published a monograph “Compound-specific Stable Isotope Analysis” (see corresponding book review in this issue). A total of 24 manuscripts describing new instrumentation, methodological questions, for example normalization and the use of certified isotope reference materials, isotope-labeling experiments, determination of natural isotope signatures, applications in archeology and in clinical and nuclear chemistry, and isotope dilution analysis demonstrate the broad range of modern precise and accurate isotope ratio measurements. This great variety of applications also clearly shows that increasing attention is being focused on isotope ratio determinations in modern analytical chemistry. A recently published textbook “Isotopic Analysis” by Frank Vanhaecke and Patrick Degryse, which is also reviewed in this issue, confirms this trend. Another interesting trend compared with the earlier collections of papers on this topic is that thermal ionization mass spectrometry (TIMS) is no longer predominant in current applications. Although TIMS still has some important niches of application in nuclear and geochemistry, it has frequently been substituted by ICP–MS, because of its advantages of simpler sample preparation and multi-element capability and also because some elements, for example refractory and precious metals, are not easily accessible to TIMS. On the other hand, gas chromatography coupled with an isotope ratio mass spectrometer (GC–IRMS) and a combustion interface for converting organic molecules into CO2 and N2 for C/C and N/N ratio measurements, respectively, by electron impact ionization is increasingly used for new applications in compound-specific isotope analysis. Furthermore, the more recently introduced hyphenation with liquid chromatography for carbon-isotope analysis of compounds not or hardly amenable to GC separation has been demonstrated. One “Technical Note” reports the first isotope ratio measurements with a commercially available Mattauch– Herzog ICP–MS, which was constructed for simultaneous mass detection. However, future improvements of this instrument will show whether this type of mass spectrometer can become an alternative to high-resolution multi-collector systems for precise isotope ratio determinations. A Feature Article by Willi Brand on “Atomic weights: not so constant after all” may be of general interest to all natural scientists. This article explains why IUPAC’s Commission on Isotopic Abundances and Atomic Weights recently decided to substitute fixed atomic weights for ten elements by atomic weight intervals. More precise isotope ratio measurements of the isotope variation of the corresponding natural and commercial substances during the last two decades led to this decision. Whereas for the light elements, for example hydrogen, carbon, and oxygen, isotope variations have been known for a long time, those for heavy metals, for example thallium, which is one of the ten elements for which the change has been made, were discovered by highly precise isotope ratio measurements especially by use of multi-collector (MC)-ICP–MS systems. This substantial improvement in isotope ratio determinations is Published in the topical collection Isotope Ratio Measurements: New Developments and Applications with guest editors Klaus G. Heumann and Torsten C. Schmidt.

A Hydrogen Gas-Water Equilibration Method Produces Accurate and Precise Stable Hydrogen Isotope Ratio Measurements in Nutrition Studies ,

Stable hydrogen isotope methodology is used in nutrition studies to measure growth, breast milk intake, and energy requirement. Isotope ratio MS is the best instrumentation to measure the stable hydrogen isotope ratios in physiological fluids. Conventional methods to convert physiological fluids to hydrogen gas (H 2 ) for mass spectrometric analysis are labor intensive, require special reagent, and involve memory effect and potential isotope fractionation. The objective of this study was to determine the accuracy and precision of a platinum catalyzed H 2 -water equilibration method for stable hydrogen isotope ratio measurements. Time to reach isotopic equilibrium, day-to-day and week-to-week reproducibility, accuracy, and precision of stable hydrogen isotope ratio measurements by the H 2 -water equilibration method were assessed using a Thermo DELTA V Advantage continuous-flow isotope ratio mass spectrometer. It took 3 h to reach isotopic equilibrium. The day-to-day and week-to-week measurements on water and urine samples with natural abundance and enriched levels of deuterium were highly reproducible. The method was accurate to within 2.8 o /oo and reproducible to within 4.0 o /oo based on analysis of international references. All the outcome variables, whether in urine samples collected in 10 doubly labeled water studies or plasma samples collected in 26 body water studies, did not differ from those obtained using the reference zinc reduction method. The method produced highly accurate estimation on ad libitum energy intakes, body composition, and water turnover rates. The method greatly reduces the analytical cost and could easily be adopted by laboratories equipped with a continuous-flow isotope ratio mass spectrometer.

Broadband non‐selective excitation of plutonium isotopes for isotope ratio measurements in resonance ionization mass spectrometry: a theoretical study

Making isotope ratio measurements with minimum isotope bias has always been a challenging task to mass spectrometrists, especially for the specific case of plutonium, owing to the strategic importance of the element. In order to use resonance ionization mass spectrometry (RIMS) as a tool for isotope ratio measurements, optimization of the various laser parameters and other atomic and system parameters is critical to minimize isotopic biases. Broadband simultaneous non-selective excitation of the isotopes of plutonium in the triple resonance excitation scheme with λ(1) = 420.77 nm, λ(2) = 847.28 nm, and λ(3) = 767.53 nm based on density matrix formalism has been theoretically computed for the determination of isotope ratios. The effects of the various laser parameters and other factors such as the atomization temperature and the dimensions of the atomic beam on the estimation of isotope ratios were studied. The effects of Doppler broadening, and time-dependent excitation parameters such as Rabi frequencies, ionization rate and the effect of non-Lorenztian lineshape have all been incorporated. The average laser powers and bandwidths for the three-excitation steps were evaluated for non-selective excitation. The laser intensity required to saturate the three-excitation steps were studied. The two-dimensional lineshape contour and its features were investigated, while the reversal of peak asymmetry of two-step and two-photon excitation peaks under these conditions is discussed. Optimized powers for the non-selective ionization of the three transitions were calculated as 545 mW, 150 mW and 545 mW and the laser bandwidth for all the three steps was ~20 GHz. The isotopic bias between the resonant and off-resonant isotope under the optimized conditions was no more than 9%, which is better than an earlier reported value. These optimized laser power and bandwidth conditions are better than in the earlier experimental work since these comprehensive calculations yield simultaneous and much more accurate isotope ratios than those in the sequential and less accurate determination reported earlier. Application of these theoretical calculations to minimize the isotopic biases under these conditions for the rapid, efficient and accurate isotope ratio measurements using RIMS has been outlined.

The measurement of Pb isotope ratios in sub-ng quantities by fast scanning single collector sector field-ICP-MS

This work describes an analytical protocol for the measurement of isotope ratios in samples containing small amounts of lead (i.e. ≤0.2ng total Pb) using the Nu Attom single collector-ICP-MS. The method was evaluated using the NIST SRM 981 isotopic standard and standard-sample bracketing for correction of the instrumental mass fractionation. The long-term precision (n=32, ±2SD) was 0.11% for the 208Pb/206Pb and 207Pb/206Pb ratios and 0.21–0.23% for the 20xPb/204Pb ratios (where x=208, 207, 206). The precision of the measured Pb isotope ratios reported here (for equivalent total Pb quantities) is comparable to multi-collector (MC) methodologies, using either an array of ion counters (Cocherie and Robert, 2007, Chem. Geol. 243, 90–104) or conventional Faraday collectors coupled with a high efficiency sample cone (Makishima and Nakamura, 2010, J. Anal. At. Spectrom. 25, 1712–1716). The accuracy of the measured Pb isotope ratios and the effect of the sample matrix were evaluated for a series of USGS rock standard digests and doped solutions of the NIST SRM 981 standard. For total cation concentrations <10μgmL−1 and concomitant low concentrations (<1.5μgmL−1) of easily ionisable elements (i.e. Na and K), matrix effects were not resolvable at the level of precision reported. However, significant matrix effects were associated with sample compositions typical of natural waters, and removal of (at least) the major cations is required for accurate isotope ratio measurements.

A Correction Method for the Peak Tailing Backgrounds for Accurate Isotope Ratio Measurements of Uranium in Ultra Trace Levels using Thermal Ionization Mass Spectrometry

A new method in thermal ionization mass spectrometry (TIMS) was developed to correct peak tailing backgrounds in the isotope ratio measurements of uranium in ultra trace levels for higher accuracy. Two different uranium standard reference materials (U005 and U030) were used to construct databases of signal intensities at mass 234 u and mass 236 u, which correspond to the two uranium minor isotopes, and signal intensity of . Correlations between peak tailing backgrounds and were obtained by least-squares regression on calculated backgrounds at mass 234 u and mass 236 u with respect to the signal intensity of followed by separation of the peak tails of the two major isotopes of uranium ( and ), which enables us to obtain a master equation for peak tailing background correction on all kinds of samples. Verification of the correction method was carried out using U010 and IRMM-040a.

Species‐specific stable isotope analysis by the hyphenation of chromatographic techniques with MC‐ICPMS

This work reviews the basis and all the existing publications on the hyphenation of chromatography-based techniques to MC-ICPMS for isotopic studies that were published until the end of 2010. A brief historical retrospective of the measurement of isotope ratios from transient signals by ICPMS with different sample introduction techniques is also included. The most important experimental parameters and data reduction strategies affecting the accurate and precise measurement of compound-specific isotope ratios by either HPLC or GC coupled to MC-ICPMS are discussed. All the applications are reported and critically reviewed in terms of analytical characteristics, performances, optimization, advantages and disadvantages and future applicability to the environmental, geochemical, or bioinorganic studies.

The use of total evaporation method using Channeltron electron multipliers by thermal ionization mass spectrometry for europium isotope ratio measurements on picogram sample amounts

Thermal ionization mass spectrometry (TIMS) is a well established instrumental technique for providing accurate and precise isotope ratio measurements for elements with low first ionization potential. In the nuclear domain, particularly for the study of spent nuclear fuel sample solutions, the reduction of quantities is an important issue in order to decrease the analyst exposition to the radioactive samples. This paper presents experimental results obtained for picogram quantities of samples using electron multipliers (Channeltrons) implanted on the new generation of TIMS associated with the total evaporation (TE) acquisition method. TE method presents some advantages for the measurement of low quantities of samples in relation to the whole sample consumption and the integration of the whole signal. For this study, a Thermal Ionization Mass Spectrometer from IsotopX (Isoprobe-T) equipped with a multi-collector system including Faraday cups and four movable Channeltrons is used. This work is focused on lanthanide elements, and particularly europium, in relation to the importance of their characterization in the spent fuel to validate neutronic calculation codes. In a first approach, europium isotopic ratios by total evaporation method using Faraday cups are obtained for quantities down to 250 pg. The results obtained by TE (Faraday cups only) on such low quantities confirm the potentialities of the method. A reproducibility of 2.5‰ ( k = 2), was obtained on the 153Eu/ 151Eu ratio ( n = 11), for natural samples with an accuracy around 1‰. In a second approach the same procedure is tested on Channeltron detectors by reducing the amounts of samples (from 50 pg to 1 pg). In order to obtain accurate isotope results, intrinsic parameters such as applied voltages, dead time and noise were determined. A Channeltron specific intercalibration gain method was also developed, as this parameter was found to be the major source of uncertainties using this type of detectors. The reproducibility on the 153Eu/ 151Eu ratio was respectively found around 3% ( k = 2). These results show the potentialities of isotopic measurements of sub-nanograms quantities and the different sources of uncertainties using total evaporation and Channeltrons on the Isoprobe-T.

Determination of ultra-trace levels of priority PBDEs in water samples by isotope dilution GC(ECNI)MS using 81Br-labelled standards

A gas chromatography electron capture negative ionization mass spectrometry (GC(ECNI)MS) procedure for the determination of priority polybrominated diphenyl ethers (PBDEs; congeners 28, 47, 99, 100, 153 and 154) in water samples at regulatory EU levels has been developed. The method is based on the use of (81)Br-labelled PBDEs for isotope dilution analysis and the measurement of (79)Br/(81)Br isotope ratios in gas chromatography peaks with the electron capture negative ionization technique. The suitability of this ion source for the precise and accurate measurement of bromine isotope ratios has been demonstrated. The general ECNI-IDMS procedure was evaluated by the analysis of NIST SRM 1947 (Lake Michigan fish tissue) with satisfactory results. For the analysis of water samples, 500 mL of the samples were spiked with the labelled PBDEs and extracted with 10 mL isooctane for 30 min. The extract was evaporated down to ca. 100 μL and injected in the GC(ECNI)MS. Detection limits ranged from 0.014 (-1) to 0.089 pg mL(-1) depending on the congener. Recoveries from real water samples, spiked at a level of 0.5 pg mL(-1), ranged from 77% to 102%.

PH-Dependent Equilibrium Isotope Fractionation Associated with the Compound Specific Nitrogen and Carbon Isotope Analysis of Substituted Anilines by SPME-GC/IRMS

Solid-phase microextraction (SPME) coupled to gas chromatography/isotope ratio mass spectrometry (GC/IRMS) was used to elucidate the effects of N-atom protonation on the analysis of N and C isotope signatures of selected aromatic amines. Precise and accurate isotope ratios were measured using polydimethylsiloxane/divinylbenzene (PDMS/DVB) as the SPME fiber material at solution pH-values that exceeded the pK(a) of the substituted aniline's conjugate acid by two pH-units. Deviations of δ(15)N and δ(13)C-values from reference measurements by elemental analyzer IRMS were small (<0.9‰) and within the typical uncertainties of isotope ratio measurements by SPME-GC/IRMS. Under these conditions, the detection limits for accurate isotope ratio measurements were between 0.64 and 2.1 mg L(-1) for δ(15)N and between 0.13 and 0.54 mg L(-1) for δ(13)C, respectively. Substantial inverse N isotope fractionation was observed by SPME-GC/IRMS as the fraction of protonated species increased with decreasing pH leading to deviations of -20‰ while the corresponding δ(13)C-values were largely invariant. From isotope ratio analysis at different solution pHs and theoretical calculations by density functional theory, we derived equilibrium isotope effects, EIEs, pertinent to aromatic amine protonation of 0.980 and 1.001 for N and C, respectively, which were very similar for all compounds investigated. Our work shows that N-atom protonation can compromise accurate compound-specific N isotope analysis of aromatic amines.

Isotope Ratio Analysis of Lead in Blood and Environmental Samples by Multi-collector Inductively Coupled Plasma Mass Spectrometry

It is widely recognized that lead (Pb) affects children's cognitive function, even at relatively low blood lead levels (<10 µg dL(-1)). The determination of the source of Pb in children is essential for effective risk management. The use of multi-collector ICPMS (MC-ICPMS) for isotope ratio measurements of Pb in environmental and biological samples was examined for this purpose. MC-ICPMS with an instrumental mass fractionation correction by Tl allowed accurate isotope ratio measurements of the Pb isotopic reference material NIST SRM 981. However, the presence of matrix elements (Al, Ca, Fe and Na) at more than 10 mg kg(-1) in the sample solution significantly deteriorated the accuracy. The separation of Pb from the matrix is necessary for accurate measurements of the isotope ratio of Pb in environmental and biological samples. Bromide-complexation, followed by anion exchange was found to be satisfactory in terms of the recovery of Pb (90 to 104%) and the efficiency of matrix separation. The procedure was applied to a preliminary source analysis of Pb in the blood of Japanese children, and a significant contribution of indoor dust was demonstrated.

Accurate and High-Precision Determination of Boron Isotopic Ratios at Low Concentration by MC-ICP-MS (Neptune)

We report an approach for the accurate and reproducible measurement of boron isotope ratios in natural waters using an MC-ICP-MS (Neptune) after wet chemistry sample purification. The sample matrix can induce a drastic shift in the isotopic ratio by changing the mass bias. It is shown that, if no purification is carried out, the direct measurement of a seawater diluted one hundred times will induce an offset of −7‰ in the isotopic ratio, and that, for the same concentration, the greater the atomic mass of the matrix element, the greater the bias induced. Whatever the sample, it is thus necessary to remove the matrix. We propose a method adapted to water samples allowing purification of 100 ng of boron with a direct recovery of boron in 2 ml of 3% v/v HNO3, which was our working solution. Boron from the International Atomic Energy Agency IAEA-B1 seawater reference material and from the two groundwater reference materials IAEA-B2 and IAEA-B3, was chemically purified, as well as boron from the certified reference material NIST SRM 951 as a test. The reproducibility of the whole procedure (wet chemistry and MC-ICP-MS measurement) was ± 0.4‰ (2s). Accuracy was verified by comparison with positive-TIMS values and with recommended values. Seawater, being homogeneous for boron isotope ratios, is presently the only natural water material that is commonly analysed for testing accuracy worldwide. We propose that the three IAEA natural waters could be used as reference samples for boron isotopes, allowing a better knowledge of their isotopic ratios, thus contributing to the certification of methods and improving the quality of the boron isotopic ratio measurements for all laboratories. Nous présentons ici une approche permettant la mesure juste et reproductible des rapports isotopiques du bore dans les eaux naturelles par l’utilisation d’un MC-ICP-MS (Neptune) après purification chimique de l’échantillon. Comme nous le démontrons, la matrice de l’échantillon peut induire un fractionnement important des rapports isotopiques. La mesure directe d’une eau de mer non purifiée, simplement diluée cent fois, entraine une différence de −7‰ sur le rapport isotopique. Il est également montré, qu’à teneur identique, plus l’élément est de masse atomique élevée, plus le biais sera important. Quel que soit l’échantillon, il s’avère nécessaire de le purifier. Nous proposons une méthode adaptée aux eaux naturelles permettant la purification de 100 ng de bore avec une récupération directe du bore dans 2 ml de 3% v/v HNO3 qui est la solution utilisée en routine pour la mesure sur notre MC-ICP-MS. La purification chimique du bore a été réalisée sur une eau de mer (IAEA-B1) et deux eaux souterraines (IAEA-B2 et IAEA-B3) ainsi que sur le matériau de référence NIST SRM 951. La reproductibilité de la procédure (purification chimique et analyse au NEPTUNE) est de ± 0.4‰ (2s). La justesse est vérifiée par comparaison avec des valeurs obtenues en P-TIMS ainsi qu’avec les valeurs recommandées. L’eau de mer étant homogène pour les rapports isotopiques du bore, elle est actuellement la seule eau naturelle analysée couramment pour tester la justesse des mesures au sein de différents laboratoires. Nous proposons que les trois eaux naturelles de l’IAEA soient utilisées en tant que solutions de référence pour les isotopes du bore, permettant ainsi une meilleure connaissance de leur rapport isotopique, et par conséquent aidant à la certification des méthodes et améliorant la qualité de la mesure des rapports isotopiques du bore dans les différents laboratoires.

Hydropyrolysis over a platinum catalyst as a preparative technique for the compound‐specific carbon isotope ratio measurement of C27 steroids

Compound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is an important method for the determination of the (13)C/(12)C ratios of biomolecules such as steroids, for a wide range of applications. However, steroids in their natural form exhibit poor chromatographic resolution, while derivatisation adds carbon thereby corrupting the stable isotopic composition. Hydropyrolysis with a sulphided molybdenum catalyst has been shown to defunctionalise the steroids, while leaving their carbon skeleton intact, allowing for the accurate measurement of carbon isotope ratios. The presence of double bonds in unsaturated steroids such as cholesterol resulted in significant rearrangement of the products, but replacing the original catalyst system with one of platinum results in higher conversions and far greater selectivity. The improved chromatographic performance of the products should allow GC/C/IRMS to be applied to more structurally complex steroid hormones and their metabolites.

Experimental investigation of the ionization mechanisms of uranium in thermal ionization mass spectrometry in the presence of carbon

Thermal ionization mass spectrometry (TIMS) is a well established instrumental technique for providing accurate and precise isotope ratio measurements of elements with reasonably low first ionization potential. In nuclear safeguards and in environmental research, it is often required to measure the isotope ratios in small samples of uranium. Empirical studies had shown that the ionization yield of uranium and plutonium in a TIMS ion source can be significantly increased in the presence of a carbon source. But, even though carbon appeared crucial in providing high ionization yields, processes taking place on the ionization surface were still not well understood. This paper describes the experimental results obtained from an extended study on the evaporation and ionization mechanisms of uranium occurring on a rhenium mass spectrometry filament in the presence of carbon. Solid state reactions were investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Additionally, vaporization measurements were performed with a modified-Knudsen cell mass spectrometer for providing information on the neutral uranium species in the vapor phase. Upon heating, under vacuum, the uranyl nitrate sample was found to turn into a uranium carbide compound, independent of the type of carbon used as ionization enhancer. With further heating, uranium carbide leads to formation of single charged uranium metal ions and a small amount of uranium carbide ions. The results are relevant for a thorough understanding of the ion source chemistry of a uranyl nitrate sample under reducing conditions. The significant increase in ionization yield described by many authors on the basis of empirical results can be now fully explained and understood.

Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results

Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH 4 to C 4H 10) and CO 2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N 2, O 2, Ar,…). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm 3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ 13C measurements ranges from ± 0.2‰ to ± 1.5‰, depending on the hydrocarbon molecule.

Accurate and Precise Zinc Isotope Ratio Measurements in Urban Aerosols

We developed an analytical method and constrained procedural boundary conditions that enable accurate and precise Zn isotope ratio measurements in urban aerosols. We also demonstrate the potential of this new isotope system for air pollutant source tracing. The procedural blank is around 5 ng and significantly lower than published methods due to a tailored ion chromatographic separation. Accurate mass bias correction using external correction with Cu is limited to Zn sample content of approximately 50 ng due to the combined effect of blank contribution of Cu and Zn from the ion exchange procedure and the need to maintain a Cu/Zn ratio of approximately 1. Mass bias is corrected for by applying the common analyte internal standardization method approach. Comparison with other mass bias correction methods demonstrates the accuracy of the method. The average precision of delta(66)Zn determinations in aerosols is around 0.05 per thousand per atomic mass unit. The method was tested on aerosols collected in Sao Paulo City, Brazil. The measurements reveal significant variations in delta(66)Zn(Imperial) ranging between -0.96 and -0.37 per thousand in coarse and between -1.04 and 0.02 per thousand in fine particular matter. This variability suggests that Zn isotopic compositions distinguish atmospheric sources. The isotopic light signature suggests traffic as the main source. We present further delta(66)Zn(Imperial) data for the standard reference material NIST SRM 2783 (delta(66)Zn(Imperial) = 0.26 +/- 0.10 per thousand).

Measurement of strontium isotope ratios by MC-ICP-MS after on-line Rb–Sr ion chromatography separation

Large volume injection ion chromatography has been evaluated for the precise and accurate measurement of strontium isotope ratios by multicollector ICP-MS. On-line coupling of the ion chromatograph to the MC-ICP-MS instrument allowed the complete separation of isobaric rubidium and other matrix elements (Ca, Mg, Na and K) and the measurement of strontium isotope ratios on flat-topped chromatographic peaks of ca. 3 minutes duration. No drift in the strontium isotope ratios was observed during strontium elution and mass bias could be corrected using either the 86Sr/88Sr isotope ratio (internal correction) or the observed 87Rb/85Rb isotope ratio (external correction) in the real samples. Typical internal precisions obtained with the selected measurement and correction conditions were about 2 times lower than the precisions obtained with continuous sample introduction for a solution 50 ng g–1 of Sr NIST SRM 987 in HNO3 2% (typical standard deviations of 0.00022–0.00036 for the corrected 87Sr/86Sr ratio in comparison to 0.00014–0.00020 for continuous sample introduction). The method was applied to the measurement of strontium isotope ratios in cider samples and the results agreed with previous data obtained after an off-line Rb–Sr separation procedure using the Sr-Spec chelating material. The precision obtained for the real samples was poorer than previous data due to the low concentration of strontium in the measured samples. The method was also applied to other sample types (soil leachates, apple leaves and apples) to study the transference of strontium isotope ratios in the soil–tree–cider system.

A Simplified, Accurate and Fast Method for Lithium Isotope Analysis of Rocks and Fluids, and δ7Li Values of Seawater and Rock Reference Materials

We report a new approach to conduct fast and accurate lithium isotope ratio measurements by MC‐ICP mass spectrometry after wet chemical sample preparation. In contrast to most previously published methods our MC‐ICP‐MS set‐up did not use a desolvating system to achieve appropriate ion beam intensities and, therefore, was less affected by matrix‐induced shifts of the instrumental mass bias. As the total lithium background and build‐up in the sample introduction system was low, previous sample residues could be washed out by an extended uptake of the new sample. Elimination of a nitric acid rinse step increased the sample throughput by a factor of two and allowed the instrumental mass bias drift to be tracked more precisely. δ7Li values of powdered silicate rock reference materials and seawater obtained in this study revealed good accuracy and an overall analytical uncertainty of typically 0.5‰ (2s). On the basis of a comparison between our lithium isotope data and compiled literature data, we recommend preliminary average δ7Li values for seawater (+30.8‰) and several silicate rock reference materials (BHVO‐1: +5.0‰; JA‐1: +5.6‰; JB‐2: +4.8‰). The compilation of published δ7Li values for seawater suggests that the observed large lithium isotope differences are due to inter‐method and/or interlaboratory bias. Most recently published δ7Li values for seawater show little variation and confirm a constant lithium isotope composition (at the sub ‰ level) of seawater in well mixed ocean basins.

Chromatographic Purification for the Determination of Dissolved Silicon Isotopic Compositions in Natural Waters by High-Resolution Multicollector Inductively Coupled Plasma Mass Spectrometry

A procedure is described for accurate Si isotope ratio measurements by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Dissolved silicon was preconcentrated and separated from other elements present in natural surface waters using anion-exchange chromatography. The optimized procedure provides virtually complete elimination of major inorganic constituents while maintaining Si recovery in excess of 97%. High-resolution capabilities of MC-ICPMS used in this study allow interference-free measurements of 28Si and 29Si isotopes using conventional solution nebulization sample introduction without aerosol desolvation. Owing to the magnitude of polyatomic ion contributions in the region of mass 30, mostly from 14N16O+, measurements of the 30Si isotope can be affected by tailing of the interference signals, making exact matching of analyte and nitric acid concentrations in all measurement solutions mandatory. Isotope abundance ratio measurements were performed using the bracketing standards approach and on-line correction for mass-bias variations using an internal standard (Mg). Uncertainties, expressed as 95% confidence intervals, for replication of the entire procedure are better than +/-0.18/1000 for delta29Si and +/-0.5/1000 for delta30Si. For the first time with MC-ICPMS, the quality of Si isotope abundance ratio measurements could be verified using a three-isotope plot. All samples studied were isotopically heavier than the IRMM-018 Si isotopic reference material.