CINCH NucWik

Department of Chemistry
Radiochemistry
University of Helsinki

In this work, students are introduced to utilisation of ²¹⁰Pb/²¹⁰Po equilibrium in environmental samples and determination of ²¹⁰Po for obtaining ²¹⁰Pb concentration. This method is widely applied in radiolead dating of environmental samples. The radioanalytical techniques that will be learned via this exercise are sample digestion by concentrated acids and alpha sample preparation by spontaneous electrodeposition of Po.

Radium (²²⁶Ra) present in the ground produces continuously noble gas radon, part of which escapes into the atmosphere. As Rn decays, solid radioactive daughter nuclides (see the figure below) are formed in the atmosphere and attached to aerosol particles. The most important of these is the relatively long-lived ²¹⁰Pb (T1/2 = 22.3 a).

The radioactive decay chain from ²²⁶Ra to ²⁰⁶Pb.

The daughters of radon in atmosphere will return to the ground surface with rain and dust and they are taken up by plants. Transfer to animals and humans can take place through grazing and consumption of radionuclide-bearing plant or meat products. ²¹⁰Pb is a beta emitter, as well as its daughter nuclide ²¹⁰Bi (T1/2 = 5 d). Instead, the daughter nuclide of ²¹⁰Bi, ²¹⁰Po (T1/2 = 138 d) is an alpha emitter (Eα = 5.3 MeV) and it can be easily separated and measured. In fresh vegetation and peat samples there are both deposited ²¹⁰Po and ingrown ²¹⁰Po due to decay of ²¹⁰Pb. In the samples stored for at least two years, the deposited ²¹⁰Po has decayed completely and there is practically only ²¹⁰Po formed from the decay of ²¹⁰Pb. Thus, the activity concentration measured for ²¹⁰Po corresponds also the activity concentration of ²¹⁰Pb.

(If the activity of ²¹⁰Po initially in the plant is to be measured, the determination should be done well before its decay and the result should be corrected with respect to both decay of ²¹⁰Po and its in-growth from ²¹⁰Pb. This, however, is possible only if the activity concentration of ²¹⁰Pb is also measured.)

The aim of this work is to determine the concentration of ²¹⁰Po in a plant-based environmental sample. The used sample material can be vegetation, peat or lichen, depending on the availability and interest. Due to volatility of polonium, it is not possible to use hot temperature digestion for destroying organic matter and instead, a digestion with HNO₃ and HCl is carried out. The insoluble residue (e.g. silicates) is removed by filtration and excess acid is evaporated. The remaining solution is kept in contact with a silver disc for two hours. Polonium is a more noble metal than silver and is electrodeposited on the surface of a silver disc. Hydrazine or other suitable reducing reagent (e.g. ascorbic acid) is added to reduce trivalent iron to divalent. Otherwise, precipitation of trivalent iron with polonium on a silver disc causes blackening of silver, increases mass on the surface of the alpha counting sample and further worsens the quality of the measured alpha spectrum. To speed up the deposition the temperature is raised to +65–75 ^oC by using a water bath.

For determination of chemical yield, a known amount of ²⁰⁹Po tracer is added to the sample solution before the acid digestion. Alpha energy of ²⁰⁹Po is 4.88 MeV. When the activity measurement is carried out by alpha spectrometry, the peak areas of both polonium isotopes (²⁰⁹Po and ²¹⁰Po) are determined. The activity concentration of ²¹⁰Po in the sample is calculated then from the peak areas of ²⁰⁹Po and ²¹⁰Po and the activity amount of added ²⁰⁹Po.

1. 1–5 g of dry sample material (peat, lichen, moss, or vegetation) is weighed and the exact mass of the sample is recorded.

2. 0.2–0.5 Bq of ²⁰⁹Po tracer solution (T_1/2 = 103 a) is added to the sample.

3. Wet ashing can be performed either using a microwave oven or by heating on a hot plate, or in a Kjeldahl digestion apparatus. In case of using a microwave oven, smaller acid volumes can be used and the digestion time is shorter than with heating on a hot plate. However, the sample mass should not exceed ~1 gram in the microwave digestion, or then the larger sample should be divided into subsamples for digestion.

Instructions for the microwave oven option (3a) and the hot plate/Kjeldahl apparatus option (3b):

3a) The sample is transferred to a Teflon tube. 20 ml of aqua regia (3:1 conc. HCl - HNO₃) and 0.2-0.5 Bq of ²⁰⁹Po tracer solution (T_1/2 = 103 a) are added to the tube. Write down the activity concentration, reference date for the activity, and the volume of the tracer. The sample is left to stand for overnight. Next day, the sample is digested according to the instructions given by microwave oven manufacturer. After digestion, the sample is moved into a beaker and the Teflon tube is rinsed with 3 x 2 ml of conc. HCl, which is added to the beaker. For continuing sample digestion further, 15 ml of conc. HCl is added to the beaker and the sample is evaporated to 2–3 ml with gentle heating on a hot plate. The sample is left to cool down.

3b) Alternatively, the digestion can be carried out in a glass beaker of 250–500 ml, depending on the size of the original sample, which can be 1–5 grams, larger sample mass requiring larger beaker due to excessive oxidation reaction. A Kjeldahl's flask of similar volume can be also used instead of beakers, if Kjeldahl apparatus will be used instead of a hot plate. First, 50 ml of conc. HNO₃ and 0.2–0.5 Bq of ²⁰⁹Po tracer solution are added to sample in the beaker. Heating is started carefully, because vigorous foaming may appear in the beginning. The digestion is continued until the HNO₃ solution is evaporated to ~10 ml. This will take several hours. (If there still is unburned fat etc., HNO₃ can be added and the digestion continued.) The beaker or flask is let to cool well and 20 ml of conc. HCl is added. Then HNO₃ is evaporated from the sample, so that brown gases disappear. Next, the sample is evaporated to 2–3 ml and left to cool down.

4. 10 ml of 0.5 M HCl is added to the digested sample and possible remaining solid residue (silicates) is removed by filtration (for example, using glass fibre filter paper in a glass funnel). The sample vessel is washed with 2 x 5 ml of 0.5 M HCl, which is also filtered and combined with the sample solution filtrate.

5. 3 ml of saturated hydrazine monohydrochloride solution is added. The solution is heated close to the boiling point, let to cool and another 0.5 ml of hydrazine monohydrochloride solution and 1 ml of 10% ascorbic acid is added. Alternatively, instead of using hydrazine and ascorbic acid solutions with sample heating, 200–300 mg of solid ascorbic acid can be added to the sample with no heating, and then proceed to step 6. Other reductants can also be used for reducing iron, instead of toxic hydrazine monohydrochloride.

6. A silver disc is washed with alcohol or toluene to remove fats. Mark the sample name to the bottom side (the side which will not be in contact with sample solution) of the disc with a permanent marker. An example of deposition vessel made of Teflon is presented in a figure below.

Install the deposition vessel so that the silver disc is attached in the bottom, a washed side up. The deposition vessel should be checked for leakages by filling it first with water, before the actual sample is poured in the vessel. Then the sample solution is poured into the empty deposition vessel, the beaker is washed with small amount of mQ-water and the rinsing solution is poured to the deposition vessel as well. mQ-water is added to the deposition vessel until a total volume of 50 ml is reached. The sample is deposited at 65–75 ^OC (water bath) for 2 hours using a mechanical stirrer.

7. The solution is removed from the deposition vessel. Wash the vessel once with a small amount of 0.5 M HCl. Remove the silver disc from deposition vessel and rinse well with water and ethyl alcohol. When the disc is dry, the radioactivity of ²¹⁰Po is determined from the disc by alpha spectrometry.

• Describe in detail the whole separation procedure and explain why each step was made.
• Draw a separation procedure scheme.
• Draw the alpha spectrum with Excel, Origin or other suitable program, and determine the number of counts in both alpha peaks.
• Since the amount of added ²⁰⁹Po is known, the concentration of ²¹⁰Po in the sample can be calculated. Give the result as mBq/g dry weight, and calculate the uncertainty of the ²¹⁰Po activity. Calculate also the chemical yield of the separation process.
• Finally, based on the results, give a brief consideration about the effectiveness and success of the separation process: was it successful regarding the yield, and what factors may have affected to this outcome? Were there some problems or surprises during the analysis?

• During the work, lab coat, safety glasses, and gloves are used for personal protection
• Acid digestion and handling of concentrated acids are done in a ventilated fume hood
• Tweezers are used for transferring alpha samples
• MSDS (Material Safety Data Sheet) for each of the used reagent gives necessary information about working safely with that particular chemical, and its disposal after the work. The MSDSs are easily available from the websites of the chemical manufacturers and authorities
• The work includes handling of concentrated acids, so attention has to be paid for preventing accidental inhalation or splashes of the acids
• Appropriate waste containers are needed e.g. for the remaining sample solution after ²¹⁰Po deposition, and especially hydrazine waste has to be collected separately and disposed as a hazardous waste

The Lab Supervisor should study the MSDSs of the chemicals used in the work and search all supplies and reagents in advance. The volume calibration of the autopipette should be checked, if there has been a long time since the last check. It is good to get familiar with the microwave oven and alpha spectrometer before the exercise and ensure availability of the instruments.

• semiconductor alpha spectrometer
• hot plate
• if preferred, a Kjeldahl digestion instrument, or microwave oven (with Teflon tubes)
• 100–1000 µl autopipette
• scale
• thermometer for checking water bath temperature
• heating magnetic stirrers with magnets, if magnets not included in deposition vessels already
• tube-shaped deposition vessel for Po precipitation, made of e.g. glass, Teflon or other heat resistant plastic components (most importantly in any type of deposition vessel, the sample solution should be in contact with one side of a silver disc and there should be a constant stirring in the sample solution during deposition)

• concentrated acids HNO₃ and HCl
• glass fibre filter papers
• glass funnels
• 1000 ml beakers for water bath
• 250–500 ml beakers for wet digestion, in a hot plate option
• 100–1000 µl pipette tips
• silver discs with suitable diameter considering deposition vessel and alpha detector to be used
• hydrazine monohydrochloride or other suitable reductant for Fe
• ascorbic acid (can be used as such and as 10% water solution, depending on the selected reduction method)
• ethanol for washing silver discs
• tweezers for transferring alpha counting samples
• weighing plates or weighing paper and spoon/spatula for weighing samples
• tap or distilled water for water bath
• mQ water
• permanent marker
• 1–5 g of dried peat, lichen, moss, or vegetation

²⁰⁹Po tracer

Leave feedback in comments