Forensic application of stable isotope delta values: Proposed minimum requirements for method validation

Abstract

The use of isotope ratio mass spectrometry (IRMS)-derived data in forensic contexts has increased in recent years, particularly as a tool for source inference. 1 The forensic application of analytical techniques (e.g. for resolving legal disputes or matters of law/regulation and/or for use by legal tribunals/regulators in reaching judicial or quasi-judicial decisions) requires methods that are capable of providing data of sufficient quality to be fit-for-their intended purpose thereby meeting the needs of the stakeholder(s). The quality of data can be assured via (i) the use of validated methods; (ii) suitable internal quality control (QC) and quality assurance (QA) procedures; (iii) the use of certified reference materials (CRMs); and (iv) participation in external comparative studies such as proficiency testing (PT) schemes. 2 Obtaining accreditation to the International Standard, ISO/IEC 17025:2005, 3 as interpreted for forensic science providers by, e.g., the Forensic Science Regulator in England and Wales, can only be achieved for validated methods. There are also other regulatoryrequirements where method validation can be necessary, e.g., compliance with Good Laboratory Practice. Method validation is therefore a key requirement for establishing whether or not a specific method is indeed fit-for-purpose and meets stakeholder needs in terms of the quality and utility of results produced.One definition of method validation is "confirmation by examination and the provision of objective evidence that the particular requirements for a specific intended use are fulfilled." 3 There are many references available on general aspects of "method validation" including the guidelines from the International

Addressing data comparability in the creation of combined data sets of bioapatite carbon and oxygen isotopic compositions

“…Details on data handling should be fully reported with each published isotope data set. Some guidelines on reporting isotope data are available for archaeology (Szpak et al 2017;Roberts et al 2018) and forensics (Dunn et al 2017).…”

Inter‐laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions

“…Procedures such as two‐point normalization or multiple‐point normalization are strongly recommended to achieve good long‐term accuracies and precisions in stable isotope analyses whereas single‐point calibration often failed and is no longer recommended . The quality of data can be assured via (1) the use of validated methods, (2) suitable internal quality control (QC) and quality assurance (QA) procedures, (3) the use of certified reference materials (CRMs), and (4) participation in external comparative studies such as proficiency testing (PT) schemes …”

Three certified sugar reference materials for carbon isotope delta measurements

“…In recent years, several groups have devoted much effort to providing guidelines for producing quality stable isotope Certified Reference Materials (CRMs) for use by the stable isotope community . Dunn et al have provided guidelines for IRMS method validation, while both Brand et al and Coplen et al assert the requirement for at least two RMs with distinct isotope delta values for isotopic scale anchoring and sample normalization (calibration), and the general guidelines of the principle of identical treatment – a process where the chemical composition of the reference materials chosen for calibration should be as close as possible to that of the analyzed samples (commutability of samples and reference materials), and the preparation and analytical technique are identical for both samples and reference materials. The importance of this principle was recently demonstrated when biased results were obtained for hydrogen isotope delta values when nitrogen‐containing standards were used for non‐nitrogen‐containing samples owing to the unwanted formation of HCN during the analysis .…”

“…4 In recent years, several groups have devoted much effort to providing guidelines for producing quality stable isotope Certified Reference Materials (CRMs) for use by the stable isotope community. [10][11][12] Dunn et al 10 To the best of our knowledge, there is only one sugar-based isotopic reference material currently available: BCR-657 glucose from the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium; European Commission) with δ VPDB ( 13 C) = −10.76 ± 0.04‰ (k = 2). 16 One other sugar-based isotopic reference material, sucrose, distributed by both the International Atomic Energy Agency (IAEA-CH-6; IAEA, Vienna, Austria) and the National Institute of Standards and Technology (NIST RM-8542; NIST, Gaithersburg, MD, USA), with δ VPDB ( 13 C) = −10.449 ± 0.066‰ (k = 2), was available, but it is currently out of stock at both institutions.…”