The European Reference Laboratory on Heavy Metals in Feed and Food (EURL-HM) organised a proficiency test to determine total arsenic (As), cadmium (Cd), lead (Pb), methyl mercury (MeHg), mercury (Hg) and inorganic arsenic (iAs) mass fractions in a dry fish powder.
It was to support Commission Regulation (EC) 1881/2006 setting maximum levels for contaminants in food.
Laboratory results were rated using z- (z'- for MeHg) and zeta (ζ-) scores in accordance with ISO 13528:2015.
More than 90% had satisfactory results (according to the z-score) for total As, Cd, Hg and MeHg, but only nine labs analysed MeHg.
Large discrepancies in limits of detection (LOD) were seen even among labs using the same technique, according to the report.
The reference material, bought from the International Atomic Energy Agency (IAEA), Tuna fish flesh homogenate was used as a test item.
Maximum levels (MLs) in European regulation for muscle meat of tuna are expressed as mg kg-1 wet weight: 0.3 for Pb; 0.10-0.15 for Cd; and 1 for Hg. A lower ML (0.5 mg kg-1) is set for Hg in other fishery products and fish muscle meats.
Mercury mistake
Participants were requested to assess compliance of the fish powder, without knowing the type of it.
“Thirty eight labs reported mass fraction above 3 mg kg-1 dry mass for total Hg. Most of them concluded that the test item was non-compliant, stating that their results (referred to dry mass) exceeded the MLs set in the legislation.
“Only four labs stated correctly that compliance could not be properly assessed, due to the lack of information about the fish species analysed and the water content in the natural (nonlyophilised) fish.”
Forty two National Reference Laboratories (NRLs) from 30 countries registered but one could not report results due to technical instrumental problems.
They were asked to perform two or three independent measurements, report calculated mean (xi) referring to dry mass, the corresponding expanded measurement uncertainty (U(xi)) together with the coverage factor (k) and the analytical technique used.
Most participants performed satisfactorily: above 92% for the z-score and 85% for the ζ-scores.
Twenty nine labs performed satisfactorily for the determination of three measurements (total As, Cd and Hg) and most provided realistic estimates of measurement uncertainties.
Since there were no assigned values for total Pb and iAs, the lab performances could not be evaluated.
Help to measure methylmercury
Meanwhile, Joint Research Centre (JRC) scientists have developed a certified reference material (CRM) to improve accuracy of methylmercury measurements in food.
The CRM (ERM-AE671) consists of isotopically labelled methylmercury with certified values for the isotopic composition.
JRC said use of an isotopically labelled internal standard can correct for problems in determination of element species.
Regulation 466/2001/EC sets limits for the total mercury content in food.