R02012: Assessment of UK laboratory performance: tritium in fish and milk

Study Duration: September 1999 to June 2002

Contractor: Environmental Radioactivity and Bioassay, Harwell Scientifics Ltd

This is the fourth in a series of laboratory proficiency exercises funded by the Agency (previously MAFF) for the measurement of radionuclides in food. Recent observations of high levels of organically bound tritium (OBT) in marine biota in the Severn estuary made fish (plaice) an obvious sample choice for this exercise. Milk was also selected since it is one of the most commonly monitored materials in the foodchain and has been used as a sample material in previous exercises.

Tritiated thymidine was selected as the spike material since its complex combustion characteristics would give an added degree of difficulty for those laboratories that use furnace-based analytical procedures (because the high temperature might drive off some of the substances to be analysed).

Dried fish and milk samples were sourced from reputable suppliers and checked for ambient levels of tritium. The materials were spiked with tritiated thymidine, freeze-dried and homogenised. The materials were sub-sampled and an independent sub-contractor made homogeneity checks. The materials were then distributed to UK laboratories for analysis. The results were returned to the Agency, which made the results anonymous before returning them to the contractor for completion of the report.

This work again highlighted the problem of preparing homogenous spiked materials for use in intercomparisons.

Thymidine, chosen because of its complex combustion characteristics and tendency to 'foul' catalysts used in furnace combustion methods, is potentially unstable and can be degraded by either radioloysis or metabolism from a range of micro-organisms that may be present in the spiking solution.

This, together with uncertainty as to tritiated water losses on freeze-drying the spiked material, produced an intercomparison material that was less than ideal from a homogeneity viewpoint. Given these problems the majority of laboratories returned satisfactory results to the Agency.

The results will be used by FSA as a guide to assessing contractors', both individually and collectively, capability to measure organically bound tritium.

Final report is available from the FSA Library and Information Centre. To obtain a copy, please contact the Enquiry Desk, Dr Elsie Widdowson Library and Information Services, Food Standards Agency (Tel: 020 7276 8181/8182 or by email to: infocentre@foodstandards.gsi.gov.uk).

Contact: Nick Wood
Tel: 020 7276 8767
Email: nick.wood@foodstandards.gsi.gov.uk