Precise Isotope Ratio Determination Research Articles

Direct Uranium Isotopic Analysis of Swipe Surfaces by Microextraction-ICP-MS.

The ability to directly measure uranium isotope ratios on environmental swipes has been achieved through a solution-based microextraction process and represents a significant advancement toward the development of a rapid method to analyze international nuclear safeguard samples. Here, a microextraction probe is lowered and sealed onto the swipe surface, and analytes within the sampling site (∼8 mm2) are dissolved and extracted into a flowing solvent of 2% nitric acid (HNO3). The mobilized species are subsequently directed into an inductively coupled plasma-mass spectrometer (ICP-MS) for accurate and precise isotope ratio determination. This work highlights the novelty of the sampling mechanism, particularly with the direct coupling of the microextraction probe to the ICP-MS and measurement of uranium isotope ratios. The preliminary method detection limit for the microextraction-ICP-MS method, utilizing a quadrupole-based MS, was determined to be ∼50 pg of 238U. Additionally, precise and accurate isotope ratio measurements were achieved on uranium reference materials for both the major (235U/238U) and minor (234U/238U and 236U/238U) ratios. While the present work is focused on directly measuring uranium isotopic systems on swipe surfaces for nuclear safeguards and verification applications, the benefits would extend across many applications in which direct solid sampling is sought for elemental and isotopic analysis.

Precise determination of low abundance isotopes (174Hf, 180W and 190Pt) in terrestrial materials and meteorites using multiple collector ICP-MS equipped with 1012 Ω Faraday amplifiers

Analytical protocols for measurements of isotope ratios involving low-abundance isotopes 174Hf (~0.16%), 180W (~0.12%), and 190Pt (~0.014%) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at ca. tenfold improved precision compared to previous studies are presented. The improvements in precision result in part from the use of Faraday amplifiers equipped with 1012Ω resistors for collection of the isotope beams of interest, as well as for the isotope beams used to monitor isobaric interferences. For ion currents >5×10−13 A (>50mV on an amplifier with 1011Ω resistor), uncertainties (2s.d.) of better than 100ppm are typically achieved for isotope ratios involving 174Hf, 180W and 190Pt. Terrestrial isotope abundances (atomic percent ±2s.d.) of 174Hf (0.16106±0.00006; n=15); 180W (0.11910±0.00009; n=10); and 190Pt (0.01286±0.00005; n=9) were determined on reference solutions throughout multiple analytical sessions over ~3years. The results, however, rely on the accuracy of the reference value chosen for the isotope ratios employed for mass bias correction and, to a lesser extent, on the accuracy of the exponential mass fractionation law used for mass bias correction. For the precise determination of isotope ratios involving 174Hf, 180W and 190Pt in natural materials, efficient chromatographic separation schemes for Hf and W from silicate-rich matrices, and for Pt from iron meteorite matrices, are presented that provide effective separation of the element of interest from interfering elements. First results for Pt suggest that IAB, IIAB and IIIAB iron meteorites exhibit terrestrial 190Pt abundances within uncertainty. Three chondrite samples exhibit indistinguishable 174Hf from terrestrial Hf. The isotope abundance of 180W in meteorites is more variable due to radiogenic 180W that was produced by 184Os-decay.

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.

Isotope ratio determination of objects in the field by portable laser ablation sampling and subsequent multicollector ICPMS

Laser ablation coupled to multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) allows accurate and precise isotope ratio determination of solid samples in a quasi-nondestructive manner, making the technique interesting for isotopic provenance studies of valuable historical artifacts. In order to enable LA-MC-ICPMS analysis of objects that cannot be transported to the laboratory for analysis, a portable laser ablation sampling device was applied. Ablated material was collected on filters, which were subsequently analyzed in the laboratory by means of MC-ICPMS. Filters were analyzed by: (I) acid digestion followed by solution nebulization MC-ICPMS and (II) fast scanning filter ablation coupled to MC-ICPMS. For both procedures, the analytical performance was investigated for lead isotope ratio measurements in ceramics, lead ore (galena) and the metallic lead standard NIST SRM 981. Filter digestion followed by solution analysis allowed determination of lead isotope ratios with external precisions of better than 0.10‰. Mass-dependent deviations towards lighter isotope ratios of 0.10‰ per u were observed for metallic lead, which indicates to a low extent isotopic fractionation during the sampling procedure. Filter ablation coupled to MC-ICPMS resulted in reduced precision compared to the solution measurements. However, it enabled straightforward simultaneous isotopic and elemental analysis by splitting the carrier gas flow to a MC-ICPMS and quadrupole ICPMS. In summary, portable LA sampling followed by MC-ICPMS analysis was shown to enable lead isotope ratio determinations of objects in the field with an analytical performance similar to a laboratory-based analysis. The approach was applied to lead isotope provenancing of ancient Chinese ceramics.

Accurate and precise determination of isotopic ratios by MC‐ICP‐MS: A review

For many decades the accurate and precise determination of isotope ratios has remained a very strong interest to many researchers due to its important applications in earth, environmental, biological, archeological, and medical sciences. Traditionally, thermal ionization mass spectrometry (TIMS) has been the technique of choice for achieving the highest accuracy and precision. However, recent developments in multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) have brought a new dimension to this field. In addition to its simple and robust sample introduction, high sample throughput, and high mass resolution, the flat-topped peaks generated by this technique provide for accurate and precise determination of isotope ratios with precision reaching 0.001%, comparable to that achieved with TIMS. These features, in combination with the ability of the ICP source to ionize nearly all elements in the periodic table, have resulted in an increased use of MC-ICP-MS for such measurements in various sample matrices. To determine accurate and precise isotope ratios with MC-ICP-MS, utmost care must be exercised during sample preparation, optimization of the instrument, and mass bias corrections. Unfortunately, there are inconsistencies and errors evident in many MC-ICP-MS publications, including errors in mass bias correction models. This review examines "state-of-the-art" methodologies presented in the literature for achievement of precise and accurate determinations of isotope ratios by MC-ICP-MS. Some general rules for such accurate and precise measurements are suggested, and calculations of combined uncertainty of the data using a few common mass bias correction models are outlined.

Mercury isotope fractionation during volatilization of Hg(0) from solution into the gas phase

The objective of this work was to determine the magnitude of mercury isotope fractionation during volatilization of Hg(0) dissolved in aqueous solution to the gas phase and to develop a method for the precise determination of isotope ratios of gaseous Hg(0) by multi-collector ICP-MS. Hg(0) was generated in situ by reducing Hg(II) using SnCl2 and subsequently purged into a trapping solution. A 316 μM KMnO4 trapping solution, acidified by 0.9 M sulfuric acid, recovered 96.1 ± 4.0% of Hg (2 SD, n = 20). The observed δ202Hg of −0.049 ± 0.065‰ (2 SD, n = 17) relative to the initial Hg standard was not greater than the daily external precision of the isotope ratio measurement, which is typically <0.100‰ (2 SD). The volatilization experiments were conducted by reducing Hg(II) completely using SnCl2, letting the developed Hg(0) evaporate from solution and purging the Hg(0) vapor in the headspace over the solution into the trapping solutions. The concentration and isotope ratios of Hg in both the trapping solutions and the Hg(0) remaining in solution were measured at different time intervals. The kinetics of the volatilization was found to be a first order process. The pattern of isotope fractionation during volatilization followed a Rayleigh fractionation with an observed maximum δ202Hg value of 1.48 ± 0.07‰ (2 SD). Fractionation factors of 1.00044 and 1.00047 were obtained in two independent experiments. These results provide the first experimental proof for, and quantified the pattern and magnitude of, the Hg isotope fractionation during volatilization of Hg(0) from solution into the gas phase.

Precise isotope-ratio determination by CGC hyphenated to ICP-MCMS for speciation of trace amounts of gaseous sulfur, with SF6 as example compound.

Capillary gas chromatography coupled to an inductively coupled plasma mass spectrometer with multiple-collector detection (GC-ICP-MCMS) has been used to assess the precision and instrumental mass bias in sulfur isotope-ratio determination for the gaseous sulfur species SF6. The isotopic composition of the compound was certified by the institute for reference materials and measurements (IRMM, Belgium) and is available as PIGS 2010. Integration of the peaks (peak half-width 1.4 s) was performed using a special peak-integration method based on definition of the integration area by assessment of a uniform isotope-ratio area within the chromatographic peak. Instrumental mass bias was determined to be approximately 12% per mass unit and proved to be stable in the concentration range measured. Replicate injections of 2, 10, 20, and 30 ng (as S) SF6 diluted in argon gave precision for the 32S/34S ratio from 0.6% RSD for 2-ng injections to 0.03% RSD for 30-ng injections. The 32S/33S and 33S/34S isotope-ratio precision was better than 0.4% RSD for injections of 10 ng (as S) and higher. Detection limits were in the absolute pg range for all measured sulfur isotopes.

Precise isotope ratio determination of common Pb using quadrupole LA-ICP-MS with optimized laser sampling conditions and a robust mixed-gas plasma

This work presents a method for the precise (0.2% RSE) determination of common (i.e. non-radiogenic) Pb isotope ratios using quadrupole Nd:YAG (266 nm) LA-ICP-MS at low (∼2 ppm) Pb concentrations. Laser sampling conditions significantly influence the precision of Pb isotope ratio measurements. This paper presents a set of optimum sampling conditions for the described system. Setting the laser focus above the sample surface significantly improves the precision of ratio measurements due to increased count rates and a reduction in the heterogeneity of particulate matter produced by fracturing at the site of ablation. In addition, using a more robust, mixed-gas (Ar–N2) plasma significantly increases sensitivity and reduces mass bias. With a mixed Ar–N2 plasma and optimized laser sampling conditions, single collector quadrupole LA-ICP-MS can be superior, for some applications (e.g., where micrometre-scale spatial resolution is important), to TIMS, and at low Pb concentrations is a cost-effective alternative to LA-MC-ICP-MS. The application of single detector, quadrupole LA-ICP-MS to the precise determination of common Pb isotope ratios in minerals has not been previously documented. This method is a powerful tool for use in isotope tracer studies of ore deposits and has potential applications to a range of environmental problems.

Magnetic Sector ICPMS with Desolvating Micronebulization: Interference-Free Subpicogram Determination of Rare Earth Elements in Natural Samples

A new method has been developed combining desolvating micronebulization with magnetic sector inductively coupled plasma mass spectrometry (ICPMS) for the analysis of all 14 stable rare earth elements (REEs) in small samples of marine particulate matter. Application is demonstrated for REEs in suspended particles from a deep ocean hydrothermal vent plume and a geological reference material. A 100-fold reduction in oxide formation, relative to standard nebulizer-spray chamber sample introduction, makes oxide interference correction negligible, even for samples that are very enriched in Ba and light REEs. Enriched isotopes for one light and one heavy REE ((145)Nd and (171)Yb) are used as both isotope dilution and internal standards, providing determination of all the REEs in one analysis. This standardization scheme eliminates the need for multimass drift correction used previously to achieve acceptable accuracy with external standardization techniques. Instead, the method exploits capabilities for accurate and precise determination of isotope ratios, a principal strength of ICPMS, and the mass-independent sensitivity of electric field scans on our double-focusing instrument. We demonstrate overall precision of ≤2% (1σ) and accuracy better than 6% for all the REEs (except Er = 8.7%), based on comparison to recommended values for USGS certified reference material BHVO-1 (basalt). This performance is similar to that obtained by full isotope dilution mass spectrometric techniques, but the new method is far simpler, requires 5 min sample(-)(1), and avoids interferences introduced by complex mixtures of enriched isotopes. Sensitivity of (1.2-1.4) × 10(6) counts s(-)(1) ppb(-)(1) and background intensities of 2-60 counts s(-)(1) provide excellent detection limits of 1-40 ppq, a 100-fold improvement on established ICPMS methods. The low sample introduction rate (100 μL min(-)(1)) allows unprecedented absolute detection limits of 1-20 fg.

Determination of Cu and Mo Isotope Fractionations in Natural Materials Using MC-ICP-MS

Our aim is to determine the deviations (8) in the isotopic compositions of Cu and Mo between samples and standards, using the VG Plasma54 multiple-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) at the University of Rochester. MC-ICP-MS systems are well-suited to this research because they feature high ionization efficiency for all transition metals, simple sample introduction, simultaneous collection of multiple ion beams for precise determination of isotope ratios, and no fractionation of the sample reservoir during analysis (Halliday et al., 1995). As a result, the largest source of uncertainty in isotopic analyses by MC-ICP-MS is the variability of instrumental mass bias with time. We can correct for this effect if we take advantage of another unique characteristic of MC-ICP-MS: The minimal variation in instrumental mass bias among different elements over a limited mass range (Hirata, 1996). As a result, if samples and standards are 'spiked' with an element of similar mass to the element of interest, the magnitude of mass bias, ~, can be determined by comparing the measured isotopic composition of the spike element to the 'true' value. The measured composition of the element of interest is then corrected using cz and the appropriate mass fractionation law. This 'elemental spike' method, analogous to use of an isotopic 'double spike', has proven useful in precise and accurate determination of Pb isotopic compositions (Longerich et al., 1987; Hirata et al., 1996). For fractionation studies, the use of elemental spikes avoids some of the complexities of a double spike. It also facilitates isotopic analysis of elements with fewer than four stable isotopes. Significantly, 8 values do not depend on cc being identical for the element of interest (cci) and the spike element (%). Although differences in ~z produce inaccuracies in isotope ratio determinations, such errors cancel in the 8 notation as long as ~i/CCs is constant. Similarly, inaccuracy in the 'true' value of the spike element will not affect 8, as long as the isotopic composition of the spike is identical in samples and standards.

Applicability of High-Resolution ICP−Mass Spectrometry for Isotope Ratio Measurements

The present paper reports on the capability of high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS) for the accurate and precise determination of isotope ratios of which at least one of the isotopes involved is spectrally interfered when measured at low resolution, using Cu as a typical example. When a commercially available Finnigan MAT Element high-resolution ICP−mass spectrometer operated at a resolution setting of 3000 is used, careful selection of the measurement conditions allows a 63Cu/65Cu or 206Pb/207Pb isotope ratio precision of ∼0.1% (RSD for n = 10) at sufficiently high count rates (≥100 000 counts/s). In this work, Cu isotope ratios were determined both in (i) an Antarctic sediment digest and in (ii) a human serum reference material. Even at the low Cu concentration level in the Antarctic sediment digests (∼10 μg/L), the 63Cu/65Cu isotope ratio could be measured with an RSD of ≤0.6% (n = 5). Although both isotopes involved were severely spectrally interfered when measured at low resolution, measurement at R = 3000 resulted in an isotope ratio that agreed within the measurement uncertainty with the expected natural isotopic abundance ratio. For the human serum reference material, the sample preparation was limited to 10-fold dilution with 0.14 M HNO3, resulting in solutions containing ∼100 μg/L Cu. The 63Cu/65Cu isotope ratio was measured with an RSD of 0.3% (n = 5) and agreed within the measurement uncertainty with the value expected on the basis of the natural isotopic abundances. Finally, on the basis of the corresponding mass spectra at R = 3000, for both samples an attempt was made to identify the polyatomic ions that spectrally interfere with the Cu ion signals at low resolution. For the Antarctic sediment digest, these interfering ions were speculatively identified as 40Ar23Na+, 28Si35Cl+, and 23Na216O1H+ for 63Cu+ and 30Si35Cl+, 28Si37Cl+, and 23Na219F+ for 65Cu+. For the human serum reference material, the interfering signals were attributed to 40Ar23Na+, 31P16O2+, and 23Na216O1H+ for 63Cu+ and to 32S16O21H+ for 65Cu+.

Recent developments in thermal ionization mass spectrometric techniques for isotope analysis. A review

Thermal ionization mass spectrometry in the positive or negative ionization mode can be used for precise isotope ratio determinations of most elements. The preferably formed ions are M+, M– and MOx–(x= 1–4). Recent interesting developments in this field, which are relevant in geology, are the U/Th and Re/Os dating methods, redeterminations of the relative atomic masses of elements by more precise and more accurate isotope ratio measurements, the determination of isotope variations in geological and cosmic samples and the analysis of trace and ultra-trace amounts of elements and elemental species in the environment. In order to measure accurately a low abundant isotope in addition to a high abundant isotope (up to ratios of 109), thermal ionization mass spectrometers with deceleration lens systems have been constructed which fulfil the required abundance sensitivities. Chromium (III)/chromium(VI) speciation below the µg l–1 level in different river water samples is reported as an example of the application of positive thermal ionization mass spectrometry in connection with the isotope dilution technique. Rhenium/osmium determinations in iron meteorites of group IIA and IAB are performed by using a corresponding separation procedure with an OsO4 distillation step and rhenium isolation by anion-exchange chromatography. The isochron for the IIA iron meteorites results in an age of (4.54 ± 0.08)× 109 years. These precise measurements also indicate small deviations of the IAB silicate-rich iron meteorite data from the IIA isochron. Precise boron isotope ratio measurements by negative thermal ionization mass spectrometry are applied to determine anthropogenic influences in groundwater samples.

Determination of Stable Isotope Ratios of Atmospheric Water Vapor and Carbon Dioxide.

A method to determine stable isotope ratios of atmospheric water vapor and carbon dioxide is presented. Simplified sampling and careful purification allow precise determination of isotope ratios. Air sample is collected in a plastic bag. The water vapor and carbon dioxide are recovered by pumping through cryogenic traps under atmospheric pressure. Hydrogen and oxygen isotope ratios are determined for small amount of atmospheric water vapor. Carbon and oxygen isotope ratios are determined for carbon dioxide purified from nitrous oxide. The analytical errors of the present method are practically similar to those of conventional methods.

An improved selected ion recording system for precise isotope ratio determination.

An improved analog four-channel selected ion recording system is described, in which major modifications permit a decrease in the dwell time to 33 ms per channel, thus minimizing the mass cycling error. Synchronization with a sinusoidal sweep voltage superimposed on the normal accelerating voltage (8 kV) enables two channels to be monitored simultaneously in real time and each mass to be recorded continuously. These improvements allow measurement of ion current ratios with a precision of 0.2% over a wide dynamic range, permitting accurate determination of isotopic enrichment in biomedical assays even when this enrichment is derived from a single label. Use of the system is illustrated by the analysis of palmitate turnover in dogs (using [1--13C] palmitic acid) with an average standard deviation corresponding to the detection of 0.04% excess of [1--13C] palmitate.

Precise measurement of isotope ratios with a single collector mass spectrometer

A simple electronic circuit is described, making possible a precise determination of isotope ratios measured with a single collector mass spectrometer. For13C/12C measurements an accuracy of better than 1‰ is obtained, corresponding to an absolute sensitivity of better than 1 ∶ 105.