Three certified sugar reference materials for carbon isotope delta measurements

Reference materials for food authentication

The global food industry faces significant challenges in ensuring the safety and authenticity of food products. Economic adulteration and counterfeiting of food are estimated to cost the industry US$30-40 billion annually. Analytical testing plays a vital role in detecting food fraud. For ensuring the metrological traceability and comparability of testing results, the use of reference materials (RMs) is crucial. The article describes the role of RMs in food authenticity testing, including their applications in method validation, calibration, quality control, and the definition of conventional measurement scales. It also reviews the availability of RMs that can be used in measurement procedures to authenticate food. Furthermore, the applications of RMs in targeted adulterant detection methods, for compositional parameters used to authenticate foods and food supplements, isotopic measurements, untargeted food authenticity testing methods, and detection and quantification of genetically modified organisms (GMOs), are explored. The document concludes by recommending the development of research grade test materials or representative test materials to harmonise untargeted testing methods and improve comparability of results across laboratories and over time.

The 'LSVEC problem' for the Vienna Peedee belemnite carbon isotope-delta scale

Since 2016 there have been intense discussions within several established communities and networks regarding the consequences of the withdrawal of the LSVEC lithium carbonate reference material for carbon isotope-delta measurement calibration. These include the Isotope Ratio Working Group (IRWG) of the Consultative Committee for the Amount of Substance (CCQM, French: Comité Consultatif pour la Quantité de Matière) of the International Committee of Weights and Measures (CIPM, French: Comité International des Poids et Mesures); participants within the 2016, 2021 and 2024 Technical Meetings on International Atomic Energy Agency (IAEA) stable isotope ratio reference products; and the International Union of Pure and Applied Chemistry's Commission on Isotopic Abundances and Atomic Weights (IUPAC CIAAW) amongst others. The reasons for these intensive discussions relate to the privileged position that LSVEC has occupied for traceability of carbon isotope-delta measurements relative to Vienna Peedee belemnite (VPDB) and the potential implications of its withdrawal for all current and future measurements of carbon isotope-delta values. This perspective summarises the background information regarding the LSVEC material in the context of the VPDB carbon isotope-delta scale. The various proposals to address the LSVEC problem that have been discussed within those committees and networks are also presented. The information herein was used as a foundation for discussions at the 2024 IAEA Consultancy Meetings on the Development of IAEA Stable Isotope Reference Materials and Related Products and on the Definition and Realisation of the Isotopic Delta Scales for Light Elements (22-26 January 2024, Vienna, Austria). A summary of those 2024 meetings and associated outcomes can be found elsewhere.

Isotope ratios of total C, N, and S in particulate matter simultaneously calibrated by mixed USGS and IAEA reference materials

This study presents a method for calibrating the isotope ratios of the total carbon, nitrogen, and sulfur in particulate matter (PM) collected from the Seoul metro using an elemental analyzer-isotope ratio mass spectrometer (EA-IRMS). Mixtures of isotope reference materials (MRMs) from the U.S. Geological Survey (USGS) and International Atomic Energy Agency (IAEA) reference materials formed an input dataset for generalized least squares (GLS) regression to yield calibration lines. The analytical method proposed in this study enabled the measurement of stable isotope ratios of total carbon, nitrogen, and sulfur simultaneously. Results showed good linearity and repeatability for carbon and nitrogen isotopes, but poor results for sulfur isotopes due to peak broadening. Reference values with uncertainties for the isotope ratios of total carbon, nitrogen, and sulfur were determined for the collected PM, demonstrating twice as much uncertainty as that of the USGS and IAEA reference materials. Homogeneity was the biggest uncertainty source for the calibrated values.

Site-specific carbon isotope measurements of vanillin reference materials by nuclear magnetic resonance spectrometry

Vanillin, one of the world's most popular flavor used in food and pharmaceutical industries, is extracted from vanilla beans or obtained (bio)-synthetically. The price of natural vanillin is considerably higher than that of its synthetic alternative which leads increasingly to counterfeit vanillin. Here, we describe the workflow of combining carbon isotope ratio combustion mass spectrometry with quantitative carbon nuclear magnetic resonance spectrometry (13C-qNMR) to obtain carbon isotope measurements traceable to the Vienna Peedee Belemnite (VPDB) with 0.7‰ combined standard uncertainty (or expanded uncertainty of 1.4‰ at 95% confidence level). We perform these measurements on qualified Bruker 400 MHz instruments to certify site-specific carbon isotope delta values in two vanillin materials, VANA-1 and VANB-1, believed to be the first intramolecular isotopic certified reference material (CRMs).

Guidance for characterization of in-house reference materials for light element stable isotope analysis

RATIONALE

Preparation of in-house reference materials (RMs) is an important aspect of light element stable isotope analysis. While some relevant information is available, there is as yet no clear set of guidelines available covering all aspects of in-house production and characterization of RMs.

METHODS

To address this need, the experience of production of certified reference materials under accreditation to ISO 17034:2016 and ISO/IEC 17025:2017 has been distilled into guidance for production of in-house RMs that are fit-for-purpose.

RESULTS

The guidance provided covers five areas: (i) planning; (ii) material considerations including preparation, packaging, and storage; (iii) measurements and assessments; (iv) value and uncertainty assignment; and (v) monitoring and use.

CONCLUSIONS

In-house RMs prepared by following this guidance can be used to provide traceability to measurement results when used for normalization or for quality control and/or assurance purposes.

Discontinuity in the Realization of the Vienna Peedee Belemnite Carbon Isotope Ratio Scale

By convention, carbon isotope ratios are expressed relative to VPDB defined by the calcite standard NBS19 in the 1980s. [See T. Coplen, Pure Appl. Chem. 1994, 66, 273-276.] To improve the realization of the VPDB scale, a second fixed point (lithium carbonate, LSVEC) was introduced in 2006 [T. Coplen et al. Anal. Chem. 2006, 78, 2439-2441], which is now known to be isotopically unstable. [Assonov, S. Rapid Commun. Mass Spectrom., 2018, 32, 827-830.] With the high-quality reference materials made available in 2020, it is now possible to realize the VPDB scale with high confidence. [Assonov, S. et al. Rapid Commun. Mass Spectrom., 2020, 34, e8867; Assonov, S. Rapid Commun. Mass Spectrom. 2021, 35, e9014; Qi, H. et al. Rapid Commun. Mass Spectrosc. 2021, 35, e9006.] Here, we report the analysis of 25 reference materials using isotope ratio combustion mass spectrometry, show the discontinuity between the values measured against the new IAEA reference materials and the values currently assigned to these reference materials on the VPDB2006, and provide a link bringing these materials onto the new VPDB2020.

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

RATIONALE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Characterisation of new reference materials IAEA-610, IAEA-611 and IAEA-612 aimed at the VPDB δ13 C scale realisation with small uncertainty

RATIONALE

LSVEC, the second anchor Reference Material (RM) for the VPDB δ¹³ C scale realisation, was introduced in 2006. In 2015, its δ¹³ C value was found to be drifting and, in 2017, its use as an RM for δ¹³ C was officially discontinued by IUPAC. New RMs of low uncertainty are needed. This paper describes the preparation and characterisation of IAEA-610, IAEA-611 and IAEA-612 (calcium carbonate, of chemical origin) which shall serve as a set of RMs aimed at anchoring the VPDB scale at negative δ¹³ C values.

METHODS

The preparation and characterisation of IAEA-610, IAEA-611 and IAEA-612 were performed by addressing the contemporary technical requirements for RM production and characterisation (ISO Guide 35:2017). The three RMs were produced in large quantities, and the first batch was sealed into ampoules (0.5 g) to ensure the integrity of the RM during storage; additional batches were sealed for long-term storage. The most accurate method of CO₂ preparation and stable isotope measurements was used, namely carbonate-H₃ PO₄ reaction under well-controlled conditions combined with well-tested stable isotope ratio mass spectrometry.

RESULTS

The assigned values of δ¹³ C and associated uncertainties are based on a large number of analyses (~10 mg aliquots) performed at IAEA and address all the known uncertainty components. For aliquots down to ~100 μg, the δ¹³ C uncertainty is increased. The uncertainty components considered are as follows: (i) material homogeneity, (ii) value assignment against IAEA-603, (iii) potential storage effects, (iv) effect of the ¹⁷ O correction, and (v) mass spectrometer linearity and cross-contamination memory in the ion source.

CONCLUSIONS

The new RMs IAEA-610, IAEA-611 and IAEA-612 have been characterised on the VPDB δ¹³ C scale in a mutually consistent way. The use of three RMs will allow a consistent realisation of the VPDB δ¹³ C scale with small uncertainty to be established, and to reach metrological compatibility of measurement results over several decades.

USGS44, a new high-purity calcium carbonate reference material for δ13 C measurements

RATIONALE

The stable carbon isotopic (δ¹³ C) reference material (RM) LSVEC Li₂ CO₃ has been found to be unsuitable for δ¹³ C standardization work because its δ¹³ C value increases with exposure to atmospheric CO₂ . A new CaCO₃ RM, USGS44, has been prepared to alleviate this situation.

METHODS

USGS44 was prepared from 8 kg of Merck high-purity CaCO₃ . Two sets of δ¹³ C values of USGS44 were determined. The first set of values was determined by online combustion, continuous-flow (CF) isotope-ratio mass spectrometry (IRMS) of NBS 19 CaCO₃ (δ¹³ C_VPDB  = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li₂ CO₃ (δ¹³ C_VPDB  = -46.6 mUr exactly), and normalized to the two-anchor δ¹³ C_VPDB-LSVEC isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO₂ evolved by reaction of H₃ PO₄ with carbonates, corrected for cross contamination, and normalized to the single-anchor δ¹³ C_VPDB scale.

RESULTS

USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr. Single-anchor δ¹³ C_VPDB values of -42.08 ± 0.01 and -41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination.

CONCLUSIONS

The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in ¹³ C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ¹³ C_VPDB-LSVEC scale. It is useful in quantifying drift with time, determining mass-dependent isotopic fractionation (linearity correction), and adjusting isotope-ratio-scale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a δ¹⁸ O reference material. A δ¹³ C_VPDB-LSVEC value of -29.99 ± 0.05 mUr was determined for NBS 22 oil.

Carbon Isotopic Measurements of Nanotubes to Differentiate Carbon Sources

Stable carbon isotope (δ(¹³C)) analysis can provide information concerning the starting materials and the production process of a material. Carbon nanotubes (CNTs) are produced using a variety of starting materials, catalysts, and production methods. The use of δ(¹³C) as a tool to infer the nature of starting materials to gain insight into the mechanics of CNT growth was evaluated. The production process of NRC's SWCNT-1 was traced via the δ(¹³C) measurement of the available starting materials, intermediate products, and the final product. As isotopic fractionation is likely negligible at high temperatures, the δ(¹³C) value of the starting materials was reflected in the δ(¹³C) value of the final CNT product. For commercially available CNTs, the estimated δ(¹³C) values of identified starting materials were related to the δ(¹³C) signatures of CNTs. Using this information and the δ(¹³C) values of CNTs, the nature of unknown carbon sources was inferred for some samples. The use of δ(¹³C) analysis may be used as a tracer to differentiate between those processes that use relatively ¹³C-depleted carbon source(s) such as carbon monoxide, methane, or natural gas, and those that do not.