DOI: 10.1007/s10967-008-1702-7 Journal of Radioanalytical and Nuclear Chemistry, Vol. 278, No.3 (2008) 789–794 0236–5731/USD 20.00 Akadémiai Kiadó, Budapest © 2008 Akadémiai Kiadó, Budapest Springer, Dordrecht Micro-PIXE characterization of reference samples intended for QA/QC of k 0 NAA T. Bučar, B. Smodiš,* P. Pelicon, J. Simčič, R. Jaćimović Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia (Received July 10, 2008) Cellulose cylinders and circular filter papers spiked with known amounts of standard element solutions were prepared for studying some aspects of assessing measurement uncertainty of NAA and the elemental distribution measured by micro-PIXE analysis. Results for the cylinders showed strongly non-homogeneous distribution of the elements, both in radial and vertical directions, dominantly caused by osmosis driven transport of added liquid solution from the centre to the edges. Results for the thin cellulose filter paper disks exhibited weaker peaking of the standard element concentrations at the edges in comparison with the thick cylinders. Introduction Neutron activation analysis (NAA) is an analytical method, which allows the determination of elements present, by irradiation of the sample with neutrons and measurement of the induced radioactivity. The energy intensity of a particular gamma-line involved is a direct measure of the amount of the concerned radionuclide present in the sample. The k 0 -based NAA (k 0 -NAA) is a variant of the method that is particularly flexible with respect to its multi-element capability and applicability as an efficient analytical tool. 1,2 The samples analyzed are usually in a solid form, and for easier handling and measurement pressed in a form of cylinder. Reporting a k 0 -NAA measurements result requires, like with any other analytical result, accurate knowledge of its uncertainty. Therefore, continuous efforts have been made in our laboratory to assess the measurement uncertainty of k 0 -NAA and to pinpoint/evaluate its specific sources. 3–5 One of the approaches used to address these issues is application of certified reference materials and calibration standards. 6 When applying calibration standards, a uniform distribution of known amount of chosen element within the reference sample is required. Possible method to achieve this, is transferring of known quantity of the solution of the reference element onto the appropriate solid material. One of available materials is cellulose. Therefore, cellulose cylinders and filter papers spiked with known amounts of standard element solutions were prepared and measured by micro-PIXE (particle induced X-ray emission) for their homogeneity. * E-mail: Borut.smodis@ijs.si Experimental Preparation of cellulose cylinders The cylinders were prepared so as to reproduce the geometrical shape of real samples used in routine analyses. About 0.2 g of cellulose microcrystalline powder (supplied by Merck) was pressed by the force of 50 kN. The resulting shape of the pellet was a cylinder having a diameter of 10 mm and a thickness of about 2 mm. Single-element samples were prepared from the Merck CertiPUR ® ICP standard solutions: Cr 1000 mg/L, Sb 1000 mg/L and Au 1000 mg/L. The expanded uncertainties for the element content ranged from 0.3 to 0.5% relative, which is hardly available as solid natural matrix certified reference materials. Antimony and gold standards were in hydrochloric acid solutions, and chromium in nitric acid solution. Known amounts of standard solution were pipetted on cylinder surfaces, at the centre of its cross section area. The masses of the sample solutions, applied onto the cylinders were determined by weighing the pipettes before and after the application. Filter papers A known amount of the single element standard solutions described earlier in the text was applied on a cellulose qualitative filter paper (supplied by Whatman). The diameter of the filter paper disks was 10 mm and the thickness about 0.2 mm. The standard solutions were (1) pipetted onto the paper or (2) transferred onto the paper by means of capillary.