The FIRMS Network 's PT scheme: What can be learned about inter-laboratory performance?

Demonstrating comparability of isotope-delta values between different laboratories by participation in interlaboratory exercises (ILEs) is a vital component of method validation. Since its inception, the Forensic Isotope Ratio Mass Spectrometry (FIRMS) Network has provided non-accredited ILEs to its members. Provision of an accredited proficiency testing (PT) scheme to ISO/IEC 17043:2010 was not possible until the administration of the scheme was outsourced to an accredited PT provider-in this case LGC Standards. LGC's FIRMS PT scheme has been operating since 2013 and in 2015 it was included in their scope of accreditation to ISO/IEC 17043:2010 and it remains the only such accredited PT for stable isotopic analyses. Each round of the PT scheme contains two materials that are of interest to forensic scientists. Participants report up to four isotope delta values for each material and the results of the PT scheme are interpreted using z or z'-scores calculated using a standard deviation of performance assessment (SDPA) that was initially derived from the submitted data, but now have fixed values. This paper presents and discusses results for the FIRMS PT scheme, including inter-laboratory comparability over time not just for individual laboratories but also for particular matrices that have been analysed in multiple rounds. The FIRMS PT scheme has also distributed the same physical material as earlier FIRMS ILEs providing a longer-term view. A number of interesting findings are presented highlighting the value of PT participation together with a discussion of the status of forensic bio-element stable isotope ratio analysis in the light of these outcomes.