The requirement for accreditation is to ensure the validity of results, by monitoring the performance of your laboratory’s results through comparison to that of other laboratories. Participation in Proficiency testing (PT) is one of the ways to demonstrate technical competence. As in your case, this is a challenge that a number of laboratories are faced with, where PT is not suitable, not practical or does not exist. It is then important to find the best means possible, to assess and monitor the performance of a method. Where no suitable PT scheme is available, your laboratory should compare results between yourselves and one or more laboratories (interlaboratory comparison). If such an option does not exist, you could evaluate a method by analysing matrix matched reference material, if available. If such materials are not available, packaging material can be spiked with certified Extractables and Leachable compounds of interest and recovery evaluated against certified values. Either way, all PT and related activities must be planned, documented and evaluated for suitability. The performance of an accredited method must be demonstrated on an ongoing basis, as part of you documented PT Plan.
The following ISO 17025 document templates, available for purchase, may be of interest: