Development of a Method for 129I Determination Using Radiochemical Neutron Activation Analysis

Andrej Osterc, Radojko Jaćimović, Vekoslava Stibilj*

Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
Tel.: +386 1 5885 352,
Fax: +386 1 5885 346,
E-mail: vekoslava.stibilj@ijs.si

Abstract
In this work an RNAA method for ¹²⁹I determination was developed. The decomposition of the samples (up to 150 g of soil or plant material) was made by alkaline fusion, where the ratio of sample: KOH and the heating conditions were crucial. The sample was then leached with water and then neutralised with H₂SO₄ (96%). The iodine present in the leachate was oxidised by the addition of NaNO₂ (10%) and H₂SO₄ (2.5 mol L^–1) and extracted into chloroform, followed by the reduction and stripping of iodine with Na₂SO₃ (10%) into an aqueous phase. Iodine was again oxidised, purged from the system with nitrogen and adsorbed onto activated charcoal. We optimised the flow rate of nitrogen, the duration of the process, the amount of charcoal needed to trap iodine and improved the purity of the charcoal, because its chemical composition (traces of K, Na, Br) influences the activity levels (dose rate) of the irradiated charcoal and prolongs the cooling time required before the radiochemical separation of the radioisotope ¹³⁰I, induced by the nuclear reaction ¹²⁹I(n,γ)130I (t½ = 12.36 h, Eγ = 536 keV), can be made. The chemical yield for the whole procedure was determined spectrophotometrically and via the nuclear reaction ¹²⁷I(n,2n)¹²⁶I. The method was developed, optimised and validated using the reference material IAEA-375 Soil and a good agreement between the obtained and recommended values was found. The measurement uncertainty of the method was also assessed. The method developed was used to investigate the content of ¹²⁹I in brown algae (Fucus virsoides) collected
along the Slovene coast and the results found were in the order of 4 × 10^–7 µg g^–1.

Keywords: ¹²⁹I, RNAA, IAEA-375 Soil, measurement uncertainty, brown marine algae