Absolute Isotope Ratios Research Articles

Absolute silicon molar mass measurements, the Avogadro constant and the redefinition of the kilogram

The results of an absolute silicon molar mass determination of two independent sets of samples from the highly 28Si-enriched crystal (AVO28) produced by the International Avogadro Coordination are presented and compared with results published by the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Research Council (NRC, Canada) and the National Metrology Institute of Japan (NMIJ, Japan). This study developed and describes significant changes to the published protocols for producing absolute silicon isotope ratios. The measurements were made at very high resolution on a multi-collector inductively coupled plasma mass spectrometer using tetramethylammonium hydroxide (TMAH) to dissolve and dilute all samples. The various changes in the measurement protocol and the use of TMAH resulted in significant improvements to the silicon isotope ratio precision over previously reported measurements and in particular, the robustness of the 29Si/30Si ratio of the AVO28 material. These new results suggest that a limited isotopic variability is present in the AVO28 material. The presence of this variability is at present singular and therefore its significance is not well understood. Fortunately, its magnitude is small enough so as to have an insignificant effect on the overall uncertainty of an Avogadro constant derived from the average molar mass of all four AVO28 silicon samples measured in this study. The NIST results confirm the AVO28 molar mass values reported by PTB and NMIJ and confirm that the virtual element–isotope dilution mass spectrometry approach to calibrated absolute isotope ratio measurements developed by PTB is capable of very high precision as well as accuracy. The Avogadro constant NA and derived Planck constant h based on these measurements, together with their associated standard uncertainties, are 6.02214076(19) × 1023 mol−1 and 6.62607017(21) × 10−34 Js, respectively.

Calibration strategies for the determination of stable carbon absolute isotope ratios in a glycine candidate reference material by elemental analyser-isotope ratio mass spectrometry.

We report a methodology for the determination of the stable carbon absolute isotope ratio of a glycine candidate reference material with natural carbon isotopic composition using EA-IRMS. For the first time, stable carbon absolute isotope ratios have been reported using continuous flow rather than dual inlet isotope ratio mass spectrometry. Also for the first time, a calibration strategy based on the use of synthetic mixtures gravimetrically prepared from well characterised, highly (13)C-enriched and (13)C-depleted glycines was developed for EA-IRMS calibration and generation of absolute carbon isotope ratio values traceable to the SI through calibration standards of known purity. A second calibration strategy based on converting the more typically determined delta values on the Vienna PeeDee Belemnite (VPDB) scale using literature values for the absolute carbon isotope ratio of VPDB itself was used for comparison. Both calibration approaches provided results consistent with those previously reported for the same natural glycine using MC-ICP-MS; absolute carbon ratios of 10,649 × 10(-6) with an expanded uncertainty (k = 2) of 24 × 10(-6) and 10,646 × 10(-6) with an expanded uncertainty (k = 2) of 88 × 10(-6) were obtained, respectively. The absolute carbon isotope ratio of the VPDB standard was found to be 11,115 × 10(-6) with an expanded uncertainty (k = 2) of 27 × 10(-6), which is in excellent agreement with previously published values.

Is the coefficient of variation a valid measure for variability of stable isotope abundances in biological materials?

The isotopic compositions of low-mass elements such as carbon are commonly expressed on the δ scale, relative to the isotopic ratio of an international standard (RST). Carbon stable isotope measurements of plant material are interpreted with an apparently biological meaning as an estimator of water use efficiency, and this has been used widely in recent years to screen plant material for variation. To compare the observed variability with other traits, the coefficient of variation (CV) is often used as an intrinsic variability measure. Theoretical considerations as well as an example dataset were used to test the independence of the CV from the RST on which the values of isotopic composition on the δ scale are based. It can be shown mathematically that the CV of a dataset of δ(13)C values is directly dependent on RST. The exploration of the example dataset confirmed this but also showed that the conversion of absolute isotopic ratios into atom fraction does not change the resulting CV. Similarly, a discrimination calculated between two δ(13)C values is independent of RST, but depends on accurate knowledge of both δ(13)C values. It was also shown that results of statistics (e.g. ANOVA) are robust among different units of isotopic composition. As CVs estimated from isotopic compositions expressed on the δ scale depend on the underlying standard, they should not be compared with those of other traits, and even comparisons of CVs among isotopic datasets should be interpreted with care.

An Improved Measurement of Isotopic Ratios by High Resolution Mass Spectrometry

The study of protein kinetics requires an accurate measurement of isotopic ratios of peptides. Although Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometers yield accurate mass measurements of analytes, the isotopologue ratios are consistently lower than predicted. Recently, we demonstrated that the magnitude of the spectral error in the FT-ICR mass spectrometer is proportional to the scan duration of ions. Here, we present a novel isotopic ratio extrapolation (IRE) method for obtaining accurate isotopic ratio measurements. Accuracy is achieved by performing scans with different duration and extrapolation of the data to the initial moment of the ion rotation; IRE minimizes the absolute isotopic ratio error to ≤1%. We demonstrate the application of IRE in protein turnover studies using (2)H(2)O-metabolic labeling. Overall, this technique allows accurate measurements of the isotopic ratios of proteolytic peptides, a critical step for enabling routine studies of proteome dynamics.

Precise Determination of the Absolute Isotopic Abundance Ratio and the Atomic Weight of Chlorine in Three International Reference Materials by the Positive Thermal Ionization Mass Spectrometer-Cs2Cl+-Graphite Method

Because the variation in chlorine isotopic abundances of naturally occurring chlorine bearing substances is significant, the IUPAC Inorganic Chemistry Division, Commission on Isotopic Abundances and Atomic Weights (CIAAW-IUPAC) decided that the uncertainty of atomic weight of chlorine (A(r)(Cl)) should be increased so that the implied range was related to terrestrial variability in 1999 (Coplen, T. B. Atomic weights of the elements 1999 (IUPAC Technical Report), Pure Appl. Chem.2001, 73(4), 667-683; and then, it emphasized that the standard atomic weights of ten elements including chlorine were not constants of nature but depend upon the physical, chemical, and nuclear history of the materials in 2009 (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem.2011, 83(2), 359-396). According to the agreement by CIAAW that an atomic weight could be defined for one specified sample of terrestrial origin (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem.2011, 83(2), 359-396), the absolute isotope ratios and atomic weight of chlorine in standard reference materials (NIST 975, NIST 975a, ISL 354) were accurately determined using the high-precision positive thermal ionization mass spectrometer (PTIMS)-Cs(2)Cl(+)-graphite method. After eliminating the weighing error caused from evaporation by designing a special weighing container and accurately determining the chlorine contents in two highly enriched Na(37)Cl and Na(35)Cl salts by the current constant coulometric titration, one series of gravimetric synthetic mixtures prepared from two highly enriched Na(37)Cl and Na(35)Cl salts was used to calibrate two thermal ionization mass spectrometers in two individual laboratories. The correction factors (i.e., K(37/35) = R(37/35meas)/R(37/35calc)) were obtained from five cycles of iterative calculations on the basis of calculated and determined R((37)Cl/(35)Cl) values in gravimetric synthetic mixtures. The absolute R((37)Cl/(35)Cl) ratios for NIST SRM 975, NIST 975a, and ISL 354 by the precise calibrated isotopic composition measurements are 0.319876 ± 0.000067, 0.319768 ± 0.000187, and 0.319549 ± 0.000044, respectively. As a result, the atomic weights of chlorine in NIST 975, NIST 975a, and ISL 354 are derived as 35.45284(8), 35.45272(21), and 35.45252(2) individually, which are consistent with the issued values of 35.453(2) by IUPAC in 1999.

Quantifying absolute carbon isotope ratios by AMS

Our AMS produced a ratio of instrument transmissions for 14C and 13C equal to 13/14 to 0.22±0.06% accuracy at a specific charge-changing energy of 235keV and giving absolute isotope ratios with that mass correction. The ratio for 13C and 12C transmissions was 12/13 at 215keV. A differential equation model of energetic ion and atom transport through the gas collision cell was constructed with functions representing experimental cross-sections for all collision effects. This model showed cation yield dependencies that were linear with energy for the monoenergetic isotopic ions at the energies that we measured, explaining the inverse mass dependence. The model predicted multiple lower energies that have this dependence and, more importantly, two low energies at which cation yields were equal for pairs of isotopes, presaging potential absolute isotope ratio measurements at low MS energies.

A Feasibility Study to Determine Cooling Time and Burnup of Advanced Test Reactor Fuel Using a Nondestructive Technique and Three Types of Gamma-ray Detectors

Abstract The goal of this work was to perform a feasibility study and establish measurement techniques to determine the burnup of the Advanced Test Reactor (ATR) fuels at the Idaho National Laboratory (INL). Three different detectors of high purity germanium (HPGe), lanthanum bromide (LaBr3), and high pressure xenon (HPXe) in two detection system configurations of below and above the water pool were used in this study. The last two detectors were used for the first time in fuel burnup measurements. The results showed that a better quality spectra can be achieved with the above the water pool configuration. Both short and long cooling time fuels were investigated in order to determine which measurement technique, absolute or fission product ratio, is better suited in each scenario and also to establish what type of detector should be used in each case for the best burnup measurement. The burnup and cooling time calibrations were established using experimental absolute activities or isotopic ratios and ORIGEN burnup calculations. A method was developed to do burnup and cooling time calibrations using fission isotopes activities without the need to know the exact geometry.

Determining the 17O/ 16O ratio of water using a water–CO 2 equilibration method: Application to glacial–interglacial changes in 17O-excess from the Dome Fuji ice core, Antarctica

Recent studies show that oxygen three isotope measurement ( 16O, 17O, and 18O) of water provides additional information for investigating the hydrological cycle and paleoclimate. For determining the 18O/ 16O value of water, a conventional CO 2–water equilibration method involves measurement of the ratios of CO 2 isotopologues which were equilibrated with water. However, this long-established technique was not intended to measure the 17O/ 16O ratio, primarily because the historic ion correction scheme does not allow for possible deviations from a fixed (and mass-dependent) relationship between 17O/ 16O and 18O/ 16O isotope ratios. Here, we propose an improved method for obtaining the 17O/ 16O isotope ratio of fresh water by the equilibration method and measurement of the 45/44 CO 2 ion abundance ratio. Equations which we formulated for 17O/ 16O measurement have two features: first, instead of absolute isotope ratio (R), all equations are formulated in δ values, measured by isotope ratio mass spectrometry. Second, we include two “assigned” δ values of water standards in the equations, because the δ 18O are commonly measured against two working standards to normalize the span of the δ scale. This approach clarifies that the contribution from 17O ( 12C 16O 17O +) to the molecular ion current at mass-to-charge ratio m/z 45 signal depends not on the absolute 13C/ 12C ratio, but on the relative δ 13C differences between the working standards and the sample. The pH value of water affects δ 17O estimation because δ 13C of CO 2 was changed in the water–CO 2 system. We reevaluated this effect using a set of equations, which explicitly includes CO 2 partial pressure effect on pH value. Our new estimation of pH effect is significantly smaller than previously reported value, but it does not alter the main conclusions in the previous study. The method was verified by δ 17O measurements of an international standard reference water (GISP) provided by the IAEA. We applied the method to investigate 17O-excess of the ice core drilled at the Dome Fuji station, Antarctica. A total of 1320 samples from a 130 m section around Marine Isotope Stage 9.3 (∼330,000 years before present) were measured. The error of a measurement for δ 17O is 0.175‰ and that of 17O-excess is 184 per meg. Although these analytical uncertainties hampered accurate estimation of the changes in 17O-excess, the averaged data indicate that 17O-excess around MIS 9.3 was higher than during the subsequent glacial period. This approach can be applied only to fresh water samples, and additional improvements will be needed to measure samples which contains significant amount of carbonate minerals.

Correction of mass spectrometric isotope ratio measurements for isobaric isotopologues of O2, CO, CO2, N2O and SO2

Gas isotope ratio mass spectrometers usually measure ion current ratios of molecules, not atoms. Often several isotopologues contribute to an ion current at a particular mass-to-charge ratio (m/z). Therefore, corrections have to be applied to derive the desired isotope ratios. These corrections are usually formulated in terms of isotope ratios (R), but this does not reflect the practice of measuring the ion current ratios of the sample relative to those of a reference material. Correspondingly, the relative ion current ratio differences (expressed as delta values) are first converted into isotopologue ratios, then into isotope ratios and finally back into elemental delta values. Here, we present a reformulation of this data reduction procedure entirely in terms of delta values and the 'absolute' isotope ratios of the reference material. This also shows that not the absolute isotope ratios of the reference material themselves, but only product and ratio combinations of them, are required for the data reduction. These combinations can be and, for carbon and oxygen have been, measured by conventional isotope ratio mass spectrometers. The frequently implied use of absolute isotope ratios measured by specially calibrated instruments is actually unnecessary. Following related work on CO2, we here derive data reduction equations for the species O2, CO, N2O and SO2. We also suggest experiments to measure the required absolute ratio combinations for N2O, SO2 and O2. As a prelude, we summarise historic and recent measurements of absolute isotope ratios in international isotope reference materials.

Use of Zr for mass bias correction in strontium isotope ratio determinations using MC-ICP-MS

Isotope abundance ratios and isotopic composition of strontium in a biological sample were determined using MC-ICP-MS whereby zirconium was admixed with solutions of digested NIST SRM 987 and samples and used for mass bias correction with implementation of a combination of standard-sample-standard bracketing and internal normalization. In this manner, the certified value of 8.37861 for 88Sr/86Sr in SRM 987 was used for mass bias correction of 90Zr/91Zr in two adjacent spiked solutions of SRM 987. Their average was then used to calculate mass bias corrected Sr isotope ratios in the sample. An approximate 2.5-fold improvement in precision of determination of 87Sr/86Sr and 88Sr/86Sr was obtained compared to that based on only the standard-sample-standard bracketing technique, although close matching of Sr and Zr concentrations is required in the standard and sample. Absolute isotope ratios of 0.0564240 ± 0.0000042, 0.709362 ± 0.000013 and 8.38034 ± 0.00010 (1SD) and δx/86Sr-values of −2.228 ± 0.075‰, −1.377 ± 0.018‰ and 0.207 ± 0.012‰ (1SD) for 84Sr/86Sr, 87Sr/86Sr, 88Sr/86Sr relative to SRM 987, respectively, were obtained characterizing a fish liver sample. In agreement with previous studies, evidence is presented for variation of 88Sr/86Sr in samples. Estimation of the measurement uncertainty confirmed that the major source of imprecision arises from the uncertainty in the certified value of 88Sr/86Sr in SRM 987 used for mass bias correction.

Measurements of the 3ν 3 band of 14N15N16O and 15N14N16O using continuous-wave cavity ring-down spectroscopy

The absorption spectra of the 3ν3 band of nitrous oxide isotopologues, 14N15N16O and 15N14N16O, have been measured using diode laser cavity ring-down spectroscopy in 6400–6463 and 6465–6532 cm-1, respectively. Spectroscopic parameters and the rotational line intensities of the bands have been determined. We have applied this spectroscopic technique to the measurements of the absolute isotope ratio of those isotopologues using the absolute line intensities.

Stable isotope profiles in sperm whale teeth: variations between areas and sexes

Sperm whale (Physeter macrocephalus) teeth were used to investigate whether variation in the chronological profiles of carbon and nitrogen stable isotope ratios along dentine growth layers could reflect differences in ontogenetic movements and/or dietary shifts in animals from different regions and sexes, as well as to show the differences in the isotopic environments experienced by these animals. Absolute isotopic ratios ranged from -14.1 to -11.0‰ for carbon and 10.8 to 18.1‰ for nitrogen, with the whale from the Indian Ocean, the two from the Mediterranean Sea and the female from the Azores presenting the most different median isotopic ratios. The Icelandic and the Indian Ocean males showed the expected decrease in δ13C around the age of ten, denoting male segregation from natal groups. For the latter, this was larger by almost twofold compared to other teeth, probably due to the much stronger latitudinal gradient in planktonic δ13C in the southern hemisphere. The Mediterranean Sea whales exhibited the lowest median δ15N values, probably reflecting the oligotrophy of this sea, while the male showed a marked change in isotopes around the age of 20 that could indicate a move to the eastern basin or a temporal change in basal isotopic signatures. The Atlantic females did not show a marked change in δ13C as expected since they stay in low latitudes throughout their lives. Stable isotope profiles in whale teeth can be used to investigate differences in the timings of ontogenetic movements and dietary history between individuals and sexes, and the biogeochemistry of different regions inhabited, and have the potential to allow inferences to be made about population substructure.

Seven Golden Rules for heuristic filtering of molecular formulas obtained by accurate mass spectrometry.

BackgroundStructure elucidation of unknown small molecules by mass spectrometry is a challenge despite advances in instrumentation. The first crucial step is to obtain correct elemental compositions. In order to automatically constrain the thousands of possible candidate structures, rules need to be developed to select the most likely and chemically correct molecular formulas.ResultsAn algorithm for filtering molecular formulas is derived from seven heuristic rules: (1) restrictions for the number of elements, (2) LEWIS and SENIOR chemical rules, (3) isotopic patterns, (4) hydrogen/carbon ratios, (5) element ratio of nitrogen, oxygen, phosphor, and sulphur versus carbon, (6) element ratio probabilities and (7) presence of trimethylsilylated compounds. Formulas are ranked according to their isotopic patterns and subsequently constrained by presence in public chemical databases. The seven rules were developed on 68,237 existing molecular formulas and were validated in four experiments. First, 432,968 formulas covering five million PubChem database entries were checked for consistency. Only 0.6% of these compounds did not pass all rules. Next, the rules were shown to effectively reducing the complement all eight billion theoretically possible C, H, N, S, O, P-formulas up to 2000 Da to only 623 million most probable elemental compositions. Thirdly 6,000 pharmaceutical, toxic and natural compounds were selected from DrugBank, TSCA and DNP databases. The correct formulas were retrieved as top hit at 80–99% probability when assuming data acquisition with complete resolution of unique compounds and 5% absolute isotope ratio deviation and 3 ppm mass accuracy. Last, some exemplary compounds were analyzed by Fourier transform ion cyclotron resonance mass spectrometry and by gas chromatography-time of flight mass spectrometry. In each case, the correct formula was ranked as top hit when combining the seven rules with database queries.ConclusionThe seven rules enable an automatic exclusion of molecular formulas which are either wrong or which contain unlikely high or low number of elements. The correct molecular formula is assigned with a probability of 98% if the formula exists in a compound database. For truly novel compounds that are not present in databases, the correct formula is found in the first three hits with a probability of 65–81%. Corresponding software and supplemental data are available for downloads from the authors' website.

Mass Discrimination During MC-ICPMS Isotopic Ratio Measurements: Investigation by Means of Synthetic Isotopic Mixtures (IRMM-007 Series) and Application to the Calibration of Natural-Like Zinc Materials (Including IRMM-3702 and IRMM-651)

A long known way of anchoring isotope ratio values to the SI system is by means of gravimetrically prepared isotopic mixtures. Thermal ionization mass spectrometry (TIMS) is the traditionally associated measurement technique, but multi-collector double focusing inductively coupled plasma (MC-ICP)-MS now appears to be an attractive alternative. This absolute calibration strategy necessitates that mass discrimination effects remain invariant in time and across the range of isotope ratios measured. It is not the case with MC-ICPMS and the present work illustrates, in the case of Zn isotopic measurements carried out using locally produced synthetic Zn isotope mixtures (IRMM-007 series), how this calibration strategy must be adjusted. First, variation in mass discrimination effects across the measurement sequence is propagated as an uncertainty component. Second, linear proportionality during each individual measurement between normalized mass discrimination and the average mass of the isotope ratios is used to evaluate mass discrimination for the ratios involving low abundance isotopes. Third, linear proportionality between mass discrimination and the logarithm of the isotope ratio values for n(67Zn)/n(64Zn) and n(68Zn)/n(64Zn) in the mixtures is used iteratively to evaluate mass discrimination for the same ratios in the isotopically enriched materials. Fourth, ratios in natural-like materials (including IRMM-3702 and IRMM-651) are calibrated by external bracketing using the isotopic mixtures. The relative expanded uncertainty (k = 2) estimated for n(68Zn)/n(64Zn) and n(67Zn)/n(64Zn) ratio values in the synthetic isotopic mixtures and the natural-like zinc samples was in the range of 0.034 to 0.048%. The uncertainty on the weighing (0.01%, k = 1) was the largest contributor to these budgets. The agreement between these results and those obtained with a single detector TIMS and with another MC-ICPMS further validated this work. The absolute isotope ratio values found for IRMM-3702-material also proposed as "delta 0" for delta-scale isotopic measurements-are n(66Zn)/n(64Zn) = 0.56397 (30), n(67Zn)/n(64Zn) = 0.082166 (35), n(68Zn)/n(64Zn) = 0.37519 (16), and n(70Zn)/n(64Zn) = 0.012418 (23). The derived Zn atomic weight value Ar(Zn) = 65.37777 (22) differs significantly from the current IUPAC value by Chang et al. [1]. Remeasurement, with isotopic mixtures from the IRMM-007 series, of the Zn isotope ratios in the same Chang et al. [1] material have revealed large systematic differences (1.35 (27)% per atomic mass unit) that suggest unrecognized measurement biases in their results.

Determination of Intramolecular δ13C from Incomplete Pyrolysis Fragments. Evaluation of Pyrolysis-Induced Isotopic Fractionation in Fragments from the Lactic Acid Analogue Propylene Glycol

Intramolecular carbon isotope ratios reflect the source of a compound and the reaction conditions prevailing during synthesis and degradation. We report here a method for determination of relative (Deltadelta13C) and absolute (delta13C) intramolecular isotope ratios using the volatile lactic acid analogue propylene glycol as a model compound, measured by on-line gas chromatography-pyrolysis coupled to GC-combustion-isotope ratio mass spectrometry. Pyrolytic fragmentation of about one-third of the analyte mass produces optimal fragments for isotopic analysis, from which relative isotope ratios (Deltadelta13C) are calculated according to guidelines presented previously. Calibration to obtain absolute isotope ratios is achieved by quantifying isotope fractionation during pyrolysis with an average fractionation factor, alpha, and evaluated by considering extremes in isotopic fractionation behavior. The method is demonstrated by calculating ranges of absolute intramolecular isotope ratios in four samples of propylene glycol. Relative and absolute isotope ratios were calculated with average precisions of SD(Deltadelta13C) <0.84 per thousand and SD(delta13C) <3.0 per thousand, respectively. The various fractionation scenarios produce an average delta(13)C range of 2 per thousand for each position in each sample. Relative isotope ratios revealed all four samples originated from unique sources, with samples A, B, and D only distinguishable at the position-specific level. Regardless of pyrolysis fractionation distribution, absolute isotope ratios showed a consistent pattern for all samples, with delta13C(3) > delta13C(2) > delta13C(1). The validity of the method was determined by examining the difference in relative isotope ratios calculated through two independent methods: Deltadelta13C calculated directly using previous methods and Deltadelta13C extracted from absolute isotope ratios. Deviation between the two Deltadelta13C values for all positions averaged 0.1-0.2 per thousand, with the smallest deviation obtained assuming equal fractionation across all fragment positions. This approach applies generally to all compounds analyzed by pyrolytic PSIA.

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide.

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N2O) has been developed. It is based on the addition of different amounts of doubly labeled 15N2O to an N2O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines' inherent high precision of the order of 10(-5) can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to 14N15N16O and 15N14N16O in the 15N2O sample and due to 15N2+ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 per thousand (1 sigma). Using tropospheric N2O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of 15N at the central nitrogen atom over 15N at the terminal nitrogen atom than measured previously for tropospheric N2O based on a chemical conversion method: 46.3 +/- 1.4 per thousand as opposed to 18.7 +/- 2.2 per thousand. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air-N2 and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.

Selected ion flow tube mass spectrometry analyses of stable isotopes in water: isotopic composition of H 3O + and H 3O +(H 2O) 3 ions in exchange reactions with water vapor

A new method has been developed for the determination of the isotope abundance ratios of deuterium, D, and oxygen-18, 18O, in water vapor (and water) using selected ion flow tube mass spectrometry (SIFT-MS). H 3O + ions are injected into the helium carrier gas where they associate with the H 2O and HDO molecules in a sample of water introduced into the carrier gas. The D and 18O contents of the product cluster ions H 8DO 4 + and H 9 18OO 3 + at m/e = 74 and 75, respectively, are determined by reference to the majority cluster ion H 9O 4 + at m/e = 73. Allowance is made for the contribution of the H 8 17OO 3 + ions to the m/z = 74 ions. Absolute isotopic ratios are measured within seconds without the need for precalibration of the SIFT-MS instrument, currently to an accuracy of better than 2%.

Atmospheric helium isotopic ratio

A recent study of the isotopic ratio of helium (3He/4He) in the terrestrial atmosphere led the individuals who made the investigation to suggest that this ratio is decreasing with time, probably due to anthropogenic release of low isotopic ratio crustal helium from oil and gas wells. The measurements made over an 11 year period from 1977 to 1988 led to the prediction that the rate of decrease was of the order of 1 × 10−9/yr. Several absolute isotopic ratio measurements made between 1969 and the present time also have been reported. These show a slight decrease in the helium isotopic ratio (much less than the 11 year data set shows). In the present paper, we report an unpublished, absolute helium isotopic ratio measurement the authors made in 1956. This value, when combined with the previously reported results, extends the time over which such measurements were made to a 32 year period and does not confirm that there is any decrease in the helium isotopic ratio in the atmosphere over this time.

Absolute Isotopic Abundance Ratios and Atomic Weight of a Reference Sample of Nickel.

Absolute values have been obtained for the isotopic abundance ratios of a reference sample of nickel (Standard Reference Material 986), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated nickel isotopes, were used to calibrate the mass spectrometers. The resulting absolute isotopic ratios are: 58Ni/60Ni=2.596061±0.000728, 61Ni/60Ni=0.043469±0.000015,62Ni/60Ni=0.138600±0.000045, and 64Ni/60Ni=0.035295±0.000024, which yield atom percents of 58Ni=68.076886 ±0.005919, 60Ni = 26.223146±0.005144,61Ni=1.139894±0.000433, 62Ni =3.634528±0.001142, and 64Ni =0.925546±0.000599. The atomic weight calculated from this isotopic composition is 58.693353 ±0.000147. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error.

Detection of 32Si and 36Cl with the ETH/SIN EN-Tandem

Improvements made at the Zürich AMS-facility in the detecting technique of heavier cosmogenic radioisotopes are described. It is possible to perform now 36Cl measurements of natural samples with isotope ratios of greater than 10 −14. The reproducibility of 36Cl measurements with a beam energy of 48 MeV is discussed. In the case of 32Si successful efforts were made to determine absolute isotope ratios of artificially enriched silicon samples.