Radiochemical separation and anti-compton analysis of Ni, Sn, Te and Zn in lead standard reference materials at ultra-trace levels

In this work results are reported on the determination of some elements, Ni, Sn, Te and Zn, present at ultratrace levels (ng g-1) in three different samples of metallurgical lead, i.e. electrolytically refined lead, thermally refined lead and lead with added impurities, assigned to be employed as standard reference materials by BCR. The Instrumental Neutron Activation Analysis was used for its characteristics to be a primary analytical method and for reaching high sensitivity and accuracy. The γ-spectrometry measurements have been performed by means of a counter system operating in anticoincidence; for the Sn and Te measures a radiochemical separation of the main interfering radionuclides was necessary. The irradiation was performed in the central channel of the Triga Mark II reactor of the R.C.-Casaccia, ENEA, at a neutron flux of 2.68 1013 n cm-2 s-1; the irradiation time was protracted to reach a total integrated flux of 2×1019 n×cm-2. In this way, 42.2 ng g-1 of Ni, 1.7 ng g-1 of Sn, 7.6 ng g-1 of Te and 35.5 ng g-1 of Zn in the Electrolytically Refined Lead and 24.2 ng g-1 of Ni, 5.7 ng g-1 of Sn, 8.2 ng g-1 of Te and 18.5 ng g-1 of Zn in the Thermally Refined Lead, respectively, were measured. The third sample was used as reference: it was obtained by adding impurities to a Thermally Refined Lead. In this case each element was present at μg g-1 level. It should be underlined that for some elements at ultra-trace levels (i.e. Ni, Sn and Te) only few measurements were useful in the relative certification: in particular, only using a radiochemical separation and an anti-Compton suppression a very low limit of detection was reached.