APPLICATION OF LIQUID SCINTILLATION COUNTING TECHNIQUE TO THE DETERMINATION OF 90Sr IN MILK SAMPLES Irene Lopes1 • Maria José Madruga Instituto Tecnológico e Nuclear (ITN)/Unidade de Protecção e Segurança Radiológica (UPSR), E.N. 10, 2686-953 Sacavém, Portugal. ABSTRACT. The Nuclear and Technological Institute (ITN) has the legal responsibility to carry out the national environmental radiological survey. It was planned to provide relevant information on radioactivity levels in different components of the ecosystem (atmospheric, aquatic, and terrestrial environments). For the terrestrial environment, evaluation of the strontium-90 (90Sr) concentrations in milk samples was performed. The levels of 90Sr activities have been decreasing since it has been spread to the environment during nuclear atmospheric tests, waste discharges, and power plant accidents. Nevertheless, the measurement of 90Sr is still necessary for radiation protection purposes, because 90Sr has long physical and biological halflives and could be taken in by man via the food chain. During the environmental radioactivity survey undertaken by ITN/ UPSR, 90Sr was measured in milk samples using classical methods; however, continuous effort was put into developing new analytical methods. Determination of 90Sr on milk samples using a safer technique and a less elaborate procedure was the objective of the studies reported in this paper. A method based on the separation of 90Sr with Sr Resin (Eichrom) was used. The trials were carried out using 1 L of milk and 3 g of resin (20-mL columns). The beta measurements were performed by liquid scintillation counting (LSC) and the 90Sr activity was determined after 90Y ingrowths in the channel region 250–800 keV, under normal counting mode, using a Packard Tri-Carb 3170 TR/SL spectrometer.
INTRODUCTION
In the last few decades, strontium-90 (90Sr) has been measured in milk samples as a monitor of global fallout and possible local leakage of radioactivity in areas surrounding many nuclear facilities (Hewitt and Bronislawa 1994). However, since the 1962 nuclear weapons tests, concentration of 90Sr in food has decreased considerably (Schönhofer et al. 1992). Nowadays, 90Sr is distributed in the environment at very low levels and, consequently, the activities found in milk samples are also low. Measurement of 90Sr is nevertheless necessary for radiation protection purposes, due to its biochemical similarities with calcium. 90Sr is easily incorporated into bone tissue and has long physical and biological half-lives (28.6 and 49.3 yr, respectively), and thus is a highly radiotoxic nuclide (Torres et al. 2000). In addition to sensitive methods, efficient radiochemical techniques for extraction and purification of 90Sr from other beta emitters is required in order to measure it by gas proportional counters or liquid scintillation counting (LSC). If the purification was incomplete, the 90Sr activity could be overevaluated (Goutelard et al. 2000). Problems regarding strontium recovery could also happen because calcium is difficult to remove selectively due its similar chemical behavior with strontium. Earlier studies (JakopiË and Benedik 2005) showed that when the calcium content changes from 2000 to 2800 mg, the recovery decreases from 90 to 40%. LSC has been used for 90Sr determination for several decades (Salonen 1978) and numerous teams have employed the Sr Resin (Horwitz et al. 1992) for strontium analysis in a milk matrix due to its simplicity, rapidity, and efficiency. This resin is very specific to Sr ions and enables a rapid and simple separation of strontium from calcium, potassium, and many other elements. The rapid isolation of strontium using the supported crown-ether replaces the precipitation steps in classical methods (Hewitt and Bronislawa 1994) and combines the advantages of solvent extraction with the ease of use of column chromatography. Strontium produces very strong bonds on the chromatographic column filled with the Sr Resin. The efficiency of isolation and separation on the column is dependent on the concentration of nitric acid. 1 Corresponding
author. Email:
[email protected].
© 2009 by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p 331–337
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The strontium capacity factor increases with increasing acid concentration and decreases with increasing concentration of Na, K, and other elements, which can cause weaker strontium binding on the column. The values of distribution coefficients for different concentrations of nitric acid were reported (Grahek et al. 1999). The Sr Resin exhibits selectivity towards Sr ions and other cations (Li, Na, Mg, Al, Co, Ni, Cu, Zn, Cd) from 3M HNO3 solutions. The amounts of other elements eluted with 3M HNO3 are 5, the strontium could coprecipitate with Ca(OH)2. At pH < 4, calcium and strontium cannot be precipitated as oxalates quantitatively (Popov et al. 2006). The oxalate is filtered through a 0.45-μm ash filter and calcinated at 800 °C. Next, the residue is dissolved with concentrated nitric acid and again precipitated in an ammonium carbonate-saturated solution at pH = 10. After heating the mixture to coagulate the precipitate and to remove the excess ammonium in the solution (the distribution coefficient of strontium in nitric acid decreases by the presence of other ions such as ammonium), the residue is filtered, washed with distilled water, and dissolved with 30 mL of 3M HNO3. The sample is then loaded onto a Sr Resin column (20 mL volume, 1.5 cm inner diameter), prepared in the laboratory with 3 g of commercial Sr Resin as a suspension in 3M HNO3 (representing 5 mL of column volume). Following loading the sample solution, the column is rinsed with 30 mL of 8M HNO3 to remove Ca, K, and Ba, and with 30 mL of 3M HNO3 to remove all yttrium from the column (Saxén 2002). These 2 washing steps are important because studies performed by Vajda et al. (1992) showed that when quantities of K and/or Ca are present on the column, some losses of radiostrontium could occur due to the competition of K and Ca for the active material of chromatographic column (7% or 2.5% of
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strontium could not be retained by the column if 30 mg of K or Ca are present, respectively). With only 3 mg of K or Ca present in the sample solution, the fractions of strontium not retained became negligible (
