Demonstrating comparability of isotope-delta values between different laboratories by participation in inter-laboratory 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.

通过参与实验室间练习（ILE）来证明不同实验室之间同位素-δ值的可比性是方法验证的重要组成部分。自成立以来，法医同位素比质谱（FIRMS）网络已向其成员提供未经认可的ILE。在将计划的管理工作外包给认可的PT提供商之前，无法向ISO / IEC 17043：2010提供认可的能力验证（PT）计划。LGC的FIRMS PT计划自2013年开始运作，2015年被纳入ISO / IEC 17043：2010的认证范围，并且它仍然是唯一获得此类稳定同位素分析认证的PT。

PT方案的每一轮都包含法医科学家感兴趣的两种材料。参与者报告每种材料最多具有四个同位素δ值，并且使用最初根据提交的数据得出的性能评估标准偏差（SDPA）计算的z或z得分来解释PT方案的结果。具有固定值。

本文介绍并讨论了FIRMS PT计划的结果，包括实验室间随着时间的可比性，不仅是各个实验室的情况，还包括经过多轮分析的特定基质的时间。FIRMS PT方案还分发了与早期FIRMS ILE相同的物理材料，从而提供了更长远的视野。提出了许多有趣的发现，突出了PT参与的价值，并根据这些结果讨论了法医生物元素稳定同位素比分析的现状。