Polonium testing

Polonium unwittingly became element of the year in 2006. Now, researchers in Italy have devised a new method for the determination of the highly toxic polonium-210 isotope as well as lead-210 in rock and soil samples. The procedure validates against two certified International Atomic Energy Authority reference materials and could allow enforcement agencies a quicker and simpler method of analysis for environmental and security purposes.

Guogang Jia and Giancarlo Torri of the Agency for Environmental Protection and Technical Services (APAT) in Rome explain that radionuclides of the uranium series and thorium series, for instance, occur naturally throughout the environment. Their radioactivity plays a significant role in our general exposure to radioactivity. Of particular interest are polonium-210 and lead-210, which both accumulate in the food chain. Some marine organisms and grazing animals, such as caribou and reindeer are most affected.

Scientists working in environmental science, the field of health physics, geochronology, and other areas, require robust analytical methods for the determination of these two radionuclides in various sample types. Moreover, refractory samples such as rock and soil present a particular problems because of their chemical complexity and the low concentrations of these radionuclides.

The most commonly used test, originally developed by Jia and colleagues is the spontaneous deposition of polonium on a silver disc from a weakly acidic solution and determination by alpha spectrometry. Lead-210 detection is more complicated, involving long-winded separation and time delays to allow radioactive daughter progeny to accumulate for indirect detection of the parent isotope.

The team has now developed a more efficient protocol for sampling otherwise inaccessible materials such as rock and soil samples. First, they fuse the sample with sodium carbonate and sodium oxide at 600 Celsius and then leach the polonium-210 and lead-210 from the fused sample at 200-250 Celsius with a highly corrosive brew of HNO₃+HF, HClO₄ and HCl.

Next, about ten percent of the leaching solution is used for polonium-210 determination, with the standard deposition of polonium on a silver disc from a weakly acidic solution containing hydroxylamine hydrochloride, sodium citrate and a polonium-209 tracer. Determination is carried out using alpha spectrometry. The remainder of the solution is used for the determination of lead-210 content.

The lead determination is carried out by precipitation as the sulfate, purification with Na₂S as PbS in six molar ammonium acetate. The researchers then separate the resulting product from any alpha-emitting particles by anion-exchange. Finally, lead-210 concentration is measured by beta-counter.

The approach allows for a detection limit of 0.75 Bq per kilogram for polonium-210 and 2.2 Bq per kilogram for lead-210 for a 1 gram sample of soil or rock.