Guidance for preparation of in-house reference materials for stable isotope analysis of HCNOS

For many years, it has been recommended that analysts performing stable isotope analysis of H, C, N, O and/or S prepare their own in-house reference materials (RMS) for daily use. These RMs can be used for calibration/normalization of instrumental data as well as for quality control and/or assurance purposes. In this way, commercially available RMs that are the source of traceability for all isotope delta analyses are preserved, ensuring that the isotope delta scales can be maintained for a longer period of time. Furthermore, in-house RMs can be prepared to supplement those that are commercially available, either by extending the available calibration range in terms of isotope delta values, or by consisting of a matrix which is not yet available from RM producers.

Some guidance is available regarding the required nature or properties of an in-house RM including stability, homogeneity, hygroscopicity and other chemical properties [e.g. Dunn & Carter 2018]. There are also a small number of publications providing some guidance on how to prepare in-house RMs for particular applications [e.g. Carter & Fry 2013, Heile & Hillarie-Marcel 2020]. There is, however, far less guidance available regarding the process of assigning an isotope delta value and associated uncertainty to an in-house RM. The guidance available to certified RM producers such as ISO/IEC 17034:2016 tend to have somewhat stricter requirements than those to be met by an in-house RM for QC purposes.

Building upon the National Measurement Laboratory’s experience as a CRM producer accredited to ISO/IEC 17034:2016, this presentation will distil the requirements for RM production into simple and clear guidelines for fit-for-purpose production and value-assignment of in-house RMs. This guidance covers five areas: (i) planning and prerequisites; (ii) material selection, preparation and storage; (iii) measurements and assessments; (iv) value assignment and uncertainty estimation; and (v) monitoring and use.

 

Dunn PJH, Carter JF. Good Practice Guide for Isotope Ratio Mass Spectrometry. 2nd ed. FIRMS; 2018. ISBN 978-0-948926-33-4. https://www.forensic-isotopes.org/gpg.html

Carter JF, Fry B. “Do it yourself” reference materials for δ¹³C determinations by isotope ratio mass spectrometry. Anal Bioanal Chem. 2013;405(14):4959-4962.

Heile J-F, Hillaire-Marcel C. Designing internal reference materials for stable H, C & O isotope measurements in CO₂ and H₂O. Rapid Commun Mass Spectrom. 2021;35(5)e9008.

ISO/IEC 17034:2016. General requirements for the competence of reference material producers. Published online 2016.