  Analytical measurements of the 28Si crystal used for the determination of the
Avogadro constant are essential to prevent biased results or under-estimated
uncertainties. A review of the existing data confirms the high-purity of
silicon with respect to a large number of elements. In order to obtain a direct
evidence of purity, we developed a relative analytical method based on neutron
activation. As a preliminary test, this method was applied to a sample of the
Avogadro crystal WASO 04. The investigation concerned twenty-nine elements. The
mass fraction of Au was quantified to be 1.03(18) x 10-12. For the remaining
twenty-eight elements, the mass fractions are below the detection limits, which
range between 1 x 10-12 and 1 x 10-5.
