Fifty-three participants from twenty-nine countries registered to the exercise. Seven participants did not report results. The
test item used was a seawater sample containing the trace elements As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Se and Zn.
Laboratories with demonstrated experience in the field provided results to establish the assigned values (Xref). The
standard uncertainties associated to the assigned values (uref) were calculated according to ISO Guide 35. Laboratory
results were rated with z- and zeta (ζ-) scores in accordance with ISO 13528. The standard deviation for the proficiency
assessment, , for all elements was based on the experience in the Water Framework Directive and was set at 25 %. The
trace elements were present in very low concentration levels (low μg L-1 range equal to natural contamination levels) and
therefore many laboratories reported "lower than" values. The percentage of satisfactory z-scores ranged from 41 % (Cr,
Fe) to 86 % (Mo). The low concentration levels of the trace elements in a difficult matrix (high saline content) need to be
taken into consideration to understand the general low rate of satisfactory scores. Laboratories that score systematically
too high should examine the cause of this positive bias.JRC.D.5-Standards for Food Bioscienc
