There is considerable interest in the certi®cation of
reference materials for environmental analysis [1].
However, the usefulness of a certi®ed reference material
(CRM) in validation of analytical methodology
depends critically on how well the certi®ed values are
established. When materials are prepared for certi®-
cation of total metal content it is generally relatively
straightforward to specify recommended values and
their associated uncertainties. Special dif®culties
exist, however, when the species to be determined
are isolated via an operationally-de®ned procedure.
Small operational variations can lead to non-comparability;
and it is vital that a suf®ciently detailed
description of the analytical procedure is provided,
and rigorously followed, if harmonious results are to
be obtained from any certi®cation or other interlaboratory
study.