DIDP in Oil : report of the interlaboratory comparison

The Institute for Health and Consumer Protection (IHCP) of the European Commissions Directorate-General Joint Research Centre hosts the European Union Reference Laboratory for Food Contact Materials (EURL-FCM). One of its core tasks is to organize interlaboratory comparisons (ILCs) among appointed National Reference Laboratories (NRLs). This report presents the results of the second ILC of the EURL-FCM which focused on the determination of Di-isodecyl phthalate (DIDP) in an oil matrix. The aim was to develop and perform the validation of a method for the analysis of DIDP (as model substance for a technical mixture of phthalates) from oil (as simulant for fatty foods). This exercise was used both as proficiency testing and to validate a standard operating procedure (SOP) for the determination of DIDP in oil that was written by the EURL based on the most performant methods used by NRLs in the proficiency test of 2008. Participation of local laboratories under NRLs was encouraged (by producing 60 samples). There were 28 participants to whom samples were dispatched 24 of which submitted results. From the EURL-NRL network 23 laboratories out of 24 reported results. There were 2 guests from Germany that provided results as well. Participants were invited to report four replicates measurements under repeatability conditions. The ILC was closed permanently in the middle of October for statistical interpretation. Based on the results in this precision experiment the method performance was assessed through evaluation of the repeatability and reproducibility standard deviation (SD) according to the mechanism described in ISO 5725 [11,12]. The assigned value and its uncertainty were obtained as a consensus values after applying the robust statistics to the results obtained from the participants. Laboratory results were rated with z and z scores in accordance with ISO 13528 [1]. Standard deviations for proficiency assessment (also called target standard deviations) were set based on Horwitz equation. The participation of the laboratories was regarded as satisfactory for the aim of the precision experiment with regards of the numbers of received results thanks to the proactive involvement of the NRLs-FCM. As a conclusion for participation and laboratory performance, this ILC showed: A noted increase in participation compared to the similar exercise of 2008. The number of laboratories submitting results for DIDP in oil rose from 17 to 25. This was due in part from the experience acquired in the previous year exercise as well as to the provision by the EURL of both the method description in a CEN like format as well as of the internal standard. A great increase in laboratory performance compared to 2008 with 76-92% of successful achievement of results from the participants within the tolerance limits (range 76-92% depended on concentration level considered) compared to 59% in 2008. In particular the performance at the concentration level of the SML was 80% compared to 59% for the same exercise in 2008. The harmonisation of the procedure and following a harmonised method for determination of DIDP in oil in 2009 resulted in a decrease more then 2.5 times in the reproducibility SD from 37% to 14 % for the concentration level around SML of 9 mg/kg while the repeatability SD remained almost the same at 6.5%.