Stable Isotope Reference Research Articles

A community-led calibration of the Zr isotope reference materials: NIST candidate RM 8299 and SRM 3169

We introduce a zirconium stable isotope reference material (RM) adhering to community-defined best practices: NIST RM 8299. Its calibration provides insights into mass-fractionation laws and accurate determination of absolute ratios using MC-ICP-MS.

Reduction of multiple reaction monitoring protein target list using correlation analysisa.

High mass resolution mass spectrometry provides hundreds to thousands of protein identifications per sample, and quantification is typically performed using label-free quantification. However, the gold standard of quantitative proteomics is multiple reaction monitoring (MRM) using triple quadrupole mass spectrometers and stable isotope reference peptides. This raises the question how to reduce a large data set to a small one without losing essential information. Here we present the reduction of such a data set using correlation analysis of bovine dairy ingredients and derived products. We were able to explain the variance in the proteomics data set using only 9 proteins across all major dairy protein classes: caseins, whey, and milk fat globule membrane proteins. We term this method Trinity-MRM. The reproducibility of the protein extraction and Trinity-MRM methods was shown to be below 5% in independent experiments (multi-day single-user and single-day multi-user) using double cream. Further application of this reductionist approach might include screening of large sample cohorts for biologically interesting samples before analysis by high-resolution mass spectrometry or other omics methodologies.

Predicting isoscapes based on an environmental similarity model for the geographical origin of Chinese rice

The authentication of geographical origin of food is important using stable isotope analysis. However, the isotopic databank is still short of comprehensive. The isoscapes model based on environmental similarity is used for the first time to predict the geospatial distribution of δ13C, δ2H and δ18O in Chinese rice in 2017 and 2018. 794 rice samples in 2017 were used to build isoscapes model. Independent verification shows that the predicted isotope distribution from this new approach is of high accuracy, with a root mean square error (RMSE) of 0.51 ‰, 7.09 ‰ and 2.06 ‰ for δ13C, δ2H and δ18O values for 2017, respectively. Our results indicate that it is possible to predict the spatial distribution of stable isotopes in rice using an isoscapes model based on environmental similarity. This novel strategy can enrich and complement a stable isotope reference database for rice origin identification at regional scale.

Isotopically characterised N2 O reference materials for use as community standards.

RationaleInformation on the isotopic composition of nitrous oxide (N2O) at natural abundance supports the identification of its source and sink processes. In recent years, a number of mass spectrometric and laser spectroscopic techniques have been developed and are increasingly used by the research community. Advances in this active research area, however, critically depend on the availability of suitable N2O isotope Reference Materials (RMs).MethodsWithin the project Metrology for Stable Isotope Reference Standards (SIRS), seven pure N2O isotope RMs have been developed and their 15N/14N, 18O/16O, 17O/16O ratios and 15N site preference (SP) have been analysed by specialised laboratories against isotope reference materials. A particular focus was on the 15N site‐specific isotopic composition, as this measurand is both highly diagnostic for source appointment and challenging to analyse and link to existing scales.ResultsThe established N2O isotope RMs offer a wide spread in delta (δ) values: δ 15N: 0 to +104‰, δ 18O: +39 to +155‰, and δ 15NSP: −4 to +20‰. Conversion and uncertainty propagation of δ 15N and δ 18O to the Air‐N2 and VSMOW scales, respectively, provides robust estimates for δ 15N(N2O) and δ 18O(N2O), with overall uncertainties of about 0.05‰ and 0.15‰, respectively. For δ 15NSP, an offset of >1.5‰ compared with earlier calibration approaches was detected, which should be revisited in the future.ConclusionsA set of seven N2O isotope RMs anchored to the international isotope‐ratio scales was developed that will promote the implementation of the recommended two‐point calibration approach. Particularly, the availability of δ 17O data for N2O RMs is expected to improve data quality/correction algorithms with respect to δ 15NSP and δ 15N analysis by mass spectrometry. We anticipate that the N2O isotope RMs will enhance compatibility between laboratories and accelerate research progress in this emerging field.

Reference Materials: gas mixtures to support measurements for climate change studies

This work aims at presenting some activities carried out at INRiM for the development of new reference materials (RMs) of greenhouse gases to support measurements for climate change studies. Since the preindustrial era, greenhouse gases emissions due to human activities have dramatically increased, mostly due to economic and demographic growth and their concentrations in the atmosphere have grown continuously since 2011, reaching highest levels in 2019. The increase of greenhouse gases concentration in atmosphere due to anthropogenic emissions is the main cause of global warming and carbon dioxide (CO2) is one of the main culprits for this phenomenon. In this context, the capability of discriminating between anthropogenic and natural emissions is of utmost importance. The determination of the isotopic composition of CO2 can support the assessment of the uptake of CO2 in the environmental compartments, i.e. atmosphere and hydrosphere, and can help to distinguish natural from anthropogenic carbon in the atmosphere. The possibility to establish metrological traceability of the measurements of CO2 isotopic composition is a key aspect, and greatly relies on the availability of gaseous reference materials having compositions and uncertainties that are fit for purpose. In the past years, INRiM participated in the EMPIR project 16ENV06 SIRS “Metrology for stable isotope reference standards”, and is now taking part in the EMPIR project 19ENV05 STELLAR “Stable isotope metrology to enable climate action and regulation”. INRiM has experience in the preparation of gas standards by primary methods, namely gravimetry and dynamic dilution, which can be applied to the realisation of reference materials of CO2 at known isotopic composition. The present work describes the preliminary results obtained by INRiM in the preparation of pure CO2 RMs and at atmospheric amount fraction. Examples of uncertainty budgets and identification of the uncertainty sources are also given.

On the metrological traceability and hierarchy of stable isotope reference materials aimed at realisation of the VPDB scale: Revision of the VPDB δ13 C scale based on multipoint scale-anchoring RMs.

In recent years, the primary reference material (RM) for the VPDB scale, NBS19, has become unavailable, and the RM used for low-end scale-anchoring, LSVEC, was found unsuitable due a drift in the δ13 C value. Given these problems, new RMs aimed at realising the VPDB δ13 C scale with low uncertainty were produced. Establishing the consistency of the new RMs with the "old" RMs prompted our revision of the underlying principles of RM value assignments, and the VPDB δ13 C scale realisation and its long-term sustainability. Analysis of major developments of the VPDB scale, a review of the contemporary requirements for RMs, and comparison with well-established measurement scales have been performed, with the aim of revising the VPDB δ13 C scale, principles of RM value assignments, and calibrator hierarchy. Requirements for scale-anchoring RMs with low uncertainty and measures to establish the scale sustainability have been formulated. The revised scale realisation is based on multiple reference points, well-defined calibration hierarchy and the use of well-understood methods for value assignment. The realisation scheme includes the new primary RM IAEA-603 and scale-anchoring RMs IAEA-610, IAEA-611 and IAEA-612, covering δ13 C from +2.46 to -36.7 ‰ VPDB, with uncertainties, including inhomogeneity and stability assessment, of less than 0.015 ‰. The values of these four RMs were assigned in a mutually consistent way; agreement between measurements made using this realisation with those made using the VPDB scale of 2006 has been demonstrated on NIST CO2 RMs 8562-8564. Multipoint-anchoring of the VPDB δ13 C scale provides several distinct "points" on the scale as means for cross-measurements to check the stability and viability of RMs and detect drift of values, if any. This ensures that the δ13 C scale is suitable for the most demanding applications, and provides options for developing further RMs with high accuracy inside a robust scale realisation scheme.

Enzymatic Cascades for Tailored 13C6 and 15N Enriched Human Milk Oligosaccharides.

Several health benefits, associated with human milk oligosaccharides (HMOS), have been revealed in the last decades. Further progress, however, requires not only the establishment of a simple “routine” method for absolute quantification of complex HMOS mixtures but also the development of novel synthesis strategies to improve access to tailored HMOS. Here, we introduce a combination of salvage-like nucleotide sugar-producing enzyme cascades with Leloir-glycosyltransferases in a sequential pattern for the convenient tailoring of stable isotope-labeled HMOS. We demonstrate the assembly of [13C6]galactose into lacto-N- and lacto-N-neo-type HMOS structures up to octaoses. Further, we present the enzymatic production of UDP-[15N]GlcNAc and its application for the enzymatic synthesis of [13C6/15N]lacto-N-neo-tetraose for the first time. An exemplary application was selected—analysis of tetraose in complex biological mixtures—to show the potential of tailored stable isotope reference standards for the mass spectrometry-based quantification, using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) as a fast and straightforward method for absolute quantification of HMOS. Together with the newly available well-defined tailored isotopic HMOS, this can make a crucial contribution to prospective research aiming for a more profound understanding of HMOS structure-function relations.

Two new defatted beef reference materials, CAAS-1801 and CAAS-1802, for carbon and nitrogen stable isotope ratio measurements.

Isotope reference materials are essential to enable reliable and comparable isotope data across multiple laboratories. Although many reference materials already exist, the best reference materials should mimic the unknown samples, so new reference materials continue to evolve with the development of isotope research in new product areas. Two defatted beef reference materials, CAAS-1801 and CAAS-1802, with substantially different δ13 C values (due to difference in dietary intake), have been prepared as reference materials for stable C and N isotope analysis of meat tissue. Homogeneity, and short- and long-term stability tests of these reference materials have been performed. The δ13 C and δ15 N values of both materials were measured for two-point isotopic normalization against international reference materials by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). A total of nine international laboratories were selected for the joint evaluation. Cochran statistical analysis yielded the values reported here. The defatted beef reference material CAAS-1801 from Heilongjiang province has a δ13 C value of -13.58 ± 0.56‰ relative to VPDB and a δ15 N value of 4.23 ± 0.56‰ relative to N2 in air. The defatted beef reference material CAAS-1802 from Sichuan province has a δ13 C value of -25.03 ± 0.45‰ and a δ15 N value of 4.36 ± 0.69‰. The two defatted beef reference materials were found to be isotopically stable across a range of ambient temperatures, and to have low volatility and toxicity, which enables them to be useful as stable isotope reference materials in the field of authentication and traceability of meat.

Correction to: Metrology for stable isotope reference materials: 13C/12C and 18O/16O isotope ratio value assignment of pure carbon dioxide gas samples on the Vienna PeeDee Belmnite-CO2 scale using dual-inlet mass spectrometry.

The authors would like to call the reader's attention to the fact that unfortunately the formula for the O17 correction parameter "a" is reported incorrectly as.

A guide for proper utilisation of stable isotope reference materials

ABSTRACTMany scientific publications about stable isotope ratios suffer from flawed practices regarding calibration and normalisation of raw δ values in conjunction with prescribed δ values of reference materials. Violations of the identical treatment principle with regards to samples and standards (i.e. reference materials) and lack of adherence to SI-mandated and IUPAC-recommended nomenclature exacerbate the widespread problem of lackadaisical analytical practice and reporting. Science is supposed to strive for exactness, whereas ambiguity and jargon confound interdisciplinary communication. This contribution aims to expose typical misconceptions and avoidable errors and offers guidance toward the reproducible generation of isotope data, isotopic scale normalisation, and proper data reporting. We offer a comprehensive overview of sources of light stable isotope reference materials to best match sample matrices encountered by stable isotope practitioners with chemically similar reference materials.

Metrology for stable isotope reference materials: 13C/12C and 18O/16O isotope ratio value assignment of pure carbon dioxide gas samples on the Vienna PeeDee Belemnite-CO2 scale using dual-inlet mass spectrometry.

Isotope ratio measurements have been conducted on a series of isotopically distinct pure CO2 gas samples using the technique of dual-inlet isotope ratio mass spectrometry (DI-IRMS). The influence of instrumental parameters, data normalization schemes on the metrological traceability and uncertainty of the sample isotope composition have been characterized. Traceability to the Vienna PeeDee Belemnite(VPDB)-CO2 scale was realized using the pure CO2 isotope reference materials(IRMs) 8562, 8563, and 8564. The uncertainty analyses include contributions associated with the values of iRMs and the repeatability and reproducibility of our measurements. Our DI-IRMS measurement system is demonstrated to have high long-term stability, approaching a precision of 0.001 parts-per-thousand for the 45/44 and 46/44 ion signal ratios. The single- and two-point normalization bias for the iRMs were found to be within their published standard uncertainty values. The values of 13C/12C and 18O/16O isotope ratios are expressed relative to VPDB-CO2 using the [Formula: see text] and [Formula: see text] notation, respectively, in parts-per-thousand (‰ or per mil). For the samples, value assignments between (-25 to +2) ‰ and (-33 to -1) ‰ with nominal combined standard uncertainties of (0.05, 0.3) ‰ for [Formula: see text] and [Formula: see text], respectively were obtained. These samples are used as laboratory reference to provide anchor points for value assignment of isotope ratios (with VPDB traceability) to pure CO2 samples. Additionally, they serve as potential parent isotopic source material required for the development of gravimetric based iRMs of CO2 in CO2-free dry air in high pressure gas cylinder packages at desired abundance levels and isotopic composition values. Graphical abstract CO2 gas isotope ratio metrology.

Summary and recommendations from the International Atomic Energy Agency Technical Meeting on the Development of Stable Isotope Reference Products (21-25 November 2016).

Summary and recommendations from the International Atomic Energy Agency Technical Meeting on the Development of Stable Isotope Reference Products (21-25 November 2016).

Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective

Accurate age control of the late Tortonian to early Messinian (8.3–6.0 Ma) is essential to ascertain the origin of benthic foraminiferal δ18O trends and the late Miocene carbon isotope shift (LMCIS), and to examine temporal relationships between the deep-sea, terrasphere and cryosphere. The current Tortonian–Messinian Geological Time Scale (GTS2012) is based on astronomically calibrated Mediterranean sections; however, no comparable non-Mediterranean stratigraphies exist for 8–6 Ma suitable for testing the GTS2012. Here, we present the first high-resolution, astronomically tuned benthic stable isotope stratigraphy (1.5 kyr resolution) and magnetostratigraphy from a single deep-sea location (IODP Site U1337, equatorial Pacific Ocean), which provides unprecedented insight into climate evolution from 8.3–6.0 Ma. The astronomically calibrated magnetostratigraphy provides robust ages, which differ by 2–50 kyr relative to the GTS2012 for polarity Chrons C3An.1n to C4r.1r, and eliminates the exceptionally high South Atlantic spreading rates based on the GTS2012 during Chron C3Bn. We show that the LMCIS was globally synchronous within 2 kyr, and provide astronomically calibrated ages anchored to the GPTS for its onset (7.537 Ma; 50% from base Chron C4n.1n) and termination (6.727 Ma; 11% from base Chron C3An.2n), confirming that the terrestrial C3:C4 shift could not have driven the LMCIS. The benthic records show that the transition into the 41-kyr world, when obliquity strongly influenced climate variability, already occurred at 7.7 Ma and further strengthened at 6.4 Ma. Previously unseen, distinctive, asymmetric saw-tooth patterns in benthic δ18O imply that high-latitude forcing played an important role in late Miocene climate dynamics from 7.7–6.9 Ma. This new integrated deep-sea stratigraphy from Site U1337 can act as a new stable isotope and magnetic polarity reference section for the 8.3–6.0 Ma interval.

STABLE ISOTOPE (δ13C, δ18O) ANALYSIS OF SULFIDE-BEARING CARBONATE SAMPLES USING LASER ABSORPTION SPECTROMETRY

Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered δ 18 O and δ 13 C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H 2 S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO 2 . Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition ( δ 13 C, δ 18 O) of mixed CO 2 -H 2 S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO 3 stable isotope reference materials. Analyses of mixed CO 2 -H 2 S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.

Influence of the marine feeding area on the muscle and egg fatty‐acid composition of Atlantic salmon Salmo salar spawners estimated from the scale stable isotopes

Fatty acids in muscle tissue and eggs of female Atlantic salmon Salmo salar spawners were analysed to evaluate the dietary quality of their final feeding areas in the Baltic Sea. The final likely feeding area was identified by comparing stable carbon and nitrogen isotope composition of the outermost growth region (final annulus) of scales of returned S. salar with that of reference S. salar caught from different feeding areas. Some overlap of stable-isotope reference values among the three areas, in addition to prespawning fasting, decreased the ability of muscle tri-acylglycerols to discriminate the final likely feeding area and the area's dietary quality. Among three long-chained polyunsaturated fatty acids, docosahexaenoic acid (DHA; 22:6n-3), eicosapentaenoic acid (EPA; 20:5n-3) and arachidonic acid (ARA; 20:4n-6), the proportions of ARA in total lipids of spawning S. salar muscle and eggs showed a significant negative correlation with increasing probability of S. salar having returned from the Baltic Sea main basin (i.e. the Baltic Sea proper). The results suggest that ARA in muscle and eggs is the best dietary indicator for dietary characteristics of final marine feeding area dietary characteristics among S. salar in the Baltic Sea.

Quantifying uncertainty in stable isotope mixing models

Mixing models are powerful tools for identifying biogeochemical sources and determining mixing fractions in a sample. However, identification of actual source contributors is often not simple, and source compositions typically vary or even overlap, significantly increasing model uncertainty in calculated mixing fractions. This study compares three probabilistic methods, Stable Isotope Analysis in R (SIAR), a pure Monte Carlo technique (PMC), and Stable Isotope Reference Source (SIRS) mixing model, a new technique that estimates mixing in systems with more than three sources and/or uncertain source compositions. In this paper, we use nitrate stable isotope examples (δ15N and δ18O) but all methods tested are applicable to other tracers. In Phase I of a three-phase blind test, we compared methods for a set of six-source nitrate problems. PMC was unable to find solutions for two of the target water samples. The Bayesian method, SIAR, experienced anchoring problems, and SIRS calculated mixing fractions that most closely approximated the known mixing fractions. For that reason, SIRS was the only approach used in the next phase of testing. In Phase II, the problem was broadened where any subset of the six sources could be a possible solution to the mixing problem. Results showed a high rate of Type I errors where solutions included sources that were not contributing to the sample. In Phase III some sources were eliminated based on assumed site knowledge and assumed nitrate concentrations, substantially reduced mixing fraction uncertainties and lowered the Type I error rate. These results demonstrate that valuable insights into stable isotope mixing problems result from probabilistic mixing model approaches like SIRS. The results also emphasize the importance of identifying a minimal set of potential sources and quantifying uncertainties in source isotopic composition as well as demonstrating the value of additional information in reducing the uncertainty in calculated mixing fractions.

Outcome of a public consultation on the Draft Scientific Opinion of the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) on Dietary Reference Values for vitamin A

Outcome of a public consultation on the Draft Scientific Opinion of the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) on Dietary Reference Values for vitamin A

Thirteenth international symposium on biological and environmental reference materials (BERM-13)

The 13th international symposium on biological and environmental reference materials (BERM-13) was held at the International Atomic Energy Agency (IAEA) Headquarters in Vienna, Austria, from 25 to 29 June 2012. The BERM-13 carried on a series of BERM Symposia, which was launched in 1983 and has been held on a regular basis since then [1–3]. As one of the main producers of reference materials (RMs) characterized for radionuclides and stable isotope amount ratios, the IAEA has a specific interest in biological and environmental reference materials and has been actively involved in BERM symposia since their inception. However, this was the first time that the symposium was organized by the IAEA. The main symposium topics were: recent developments, current trends and future needs for RMs; selection and proper use of RMs; the role of RMs in assuring harmonization and comparability of measurement results; availability and distribution of RMs; and education and training related to RMs production and their use. The symposium was organized in several sessions, namely food and reference materials; standardization, metrology and international cooperation; health, medicine and forensic; reference materials production, trends and techniques; environmental reference materials; including plenary sessions each day and two poster sessions. Taking the advantage of the fact that BERM-13 was organized as an IAEA conference, three special sessions were also organized on topics of high importance to the IAEA and in general, namely special sessions on environmental nuclear fingerprints, stable isotope reference materials and reference materials for emergency situations. Altogether there were 92 oral presentations (4 opening remarks, 23 invited, and 65 oral lectures), 77 posters, and 3 round table discussions. The symposium was attended by 197 participants from 49 countries and five international organizations (the International Bureau of Weights and Measures—BIPM, the European Commission/Joint Research Centre—EC/JRC, the European Directorate for the Quality of Medicines & Health Care—EDQM, the IAEA and the International Organization for Standardization—ISO). The highlights of the BERM-13 symposium were invited presentations, covering RMs related to food and human nutritional status and biomarkers (S. Wise, C. M. Pfeifer, USA), metabolomics (K. W. Phinney, USA), RMs required to support harmonized human biomonitoring programs (M. Horvat, Slovenia), RMs needed for key enabling technologies (H. Emons, EC/JRC/IRMM), RMs for pharmaceuticals (A. Lodi, CE/EDQM), elemental speciation (O. Donard, France) and nanoscale RMs (D. L. Keiser, USA), new organic stable isotope RMs (A. Schimmelmann, USA), as well as lectures related to large RM programs (T. Saito, Japan; Z. Mester, Canada; M. Betti, IAEA), legislation (A. Held, EC/JRC/IRMM), top metrology (M. Kuehne, BIPM), and certain technical requirements, for example, stability testing and monitoring strategies (S. L. R. Ellison, UK). The special sessions have, among others, revealed a strong need for new environmental RMs suitable for low-level radioactivity measurements. This is an area where new very sensitive measurement techniques have been developed for characterization of ‘hot particles’ but face a lack of appropriate quality assurance tools. A similar situation was presented in the case of RMs for emergency preparedness, where the Gulf oil spill (M. M. Schantz, USA; A. R. Wagener, Brazil) A. Fajgelj (&) International Atomic Energy Agency, Department of Nuclear Sciences and Applications, Vienna International Centre, PO Box 100, 1400 Vienna, Austria e-mail: A.Fajgelj@iaea.org

Techniques for measuring vitamin A activity from β-carotene

Dietary β-carotene is the most important precursor of vitamin A. However, the determination of the efficiency of in vivo conversion of β-carotene to vitamin A requires sensitive and safe techniques. It presents the following challenges: 1) circulating β-carotene concentration cannot be altered by eating a meal containing ≤6 mg β-carotene; 2) because retinol concentrations are homeostatically controlled, the conversion of β-carotene into vitamin A cannot be estimated accurately in well-nourished humans by assessing changes in serum retinol after supplementation with β-carotene. In the past half-century, techniques using radioisotopes of β-carotene and vitamin A, depletion-repletion with vitamin A and β-carotene supplements, measurement of postprandial chylomicron fractions after consumption of a β-carotene dose, and finally, stable isotopes as tracers to follow the absorption and conversion of β-carotene in humans have been developed. The reported values for β-carotene to vitamin A conversion showed a wide variation from 2 μg β-carotene to 1 μg retinol (for synthetic pure β-carotene in oil) and 28 μg β-carotene to 1 μg retinol (for β-carotene from vegetables). In recent years, a stable isotope reference method (IRM) was developed that used labeled synthetic β-carotene. The IRM method provided evidence that the conversion of β-carotene to vitamin A is likely dose dependent. With the development of intrinsically labeled plant foods harvested from a hydroponic system with heavy water, vitamin A activity of stable isotope-labeled biosynthetic β-carotene from various foods consumed by humans was studied. The efficacy of plant foods rich in β-carotene, such as natural (spinach, carrots, spirulina), hybrid (high-β-carotene yellow maize), and bioengineered (Golden Rice) foods, to provide vitamin A has shown promising results. The results from these studies will be of practical importance in recommendations for the use of pure β-carotene and foods rich in β-carotene in providing vitamin A and ultimately in preventing either overconsumption or poor intake of vitamin A by humans.

A strategy for selection of reference materials in stable oxygen isotope analyses of solid materials

The propagation of uncertainties associated with the stable oxygen isotope reference materials through a multi-point normalisation procedure was evaluated in this study using Monte Carlo (MC) simulation. We quantified the normalisation error for a particular selection of reference materials and their number of replicates, when the choice of standards is restricted to either nitrates, sulphates or organic reference materials alone, and in comparison with when this restriction was relaxed. A lower uncertainty in stable oxygen isotope analyses of solid materials performed using High-Temperature Pyrolysis (HTP) can be readily achieved through an optimal selection of reference materials. Among the currently available certified reference materials the best performing pairs minimising the normalisation errors are USGS35 and USGS34 for nitrates; IAEA-SO-6 and IAEA-SO-5 for sulphates; and IAEA-601 and IAEA-602 for organic materials. The normalisation error can be reduced further--by approximately half--if each of these two analysed reference materials is replicated four times. The overall optimal selection among all nine considered reference materials is the IAEA-602 and IAEA-SO-6 pair. If each of these two reference materials is replicated four times the maximum predicted normalisation error will equal 0.22‰, the minimum normalisation error 0.12‰, and the mean normalisation error 0.15‰ over the natural range of δ(18)O variability. We argue that the proposed approach provides useful insights into reference material selection and in assessing the propagation of analytical error through normalisation procedures in stable oxygen isotope studies.