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1. Weigh out 10 g milk powder ash into a 400 ml beaker.

2. Add 3 ml (30 mg) Sr carrier solution (10 g/l),
200 ml distilled water,
40 ml 65 % nitric acid (HNO3).
Heat ash-bearing solution to boiling on an electric heater and
filter hot solution through a wetted filter paper into a 600 ml beaker.
The deposit on the filter is discarded.
Add 3 ml 85 % phosphoric acid (H3PO4) into the filtrate.
Add 25 % Ammonia (NH3) until the pH of the solution is about 10.
Pour the phosphate precipitate -bearing solution into centrifuge tubes, is centrifuge and discard solution. The precipitate is washed twice with a solution containing 15 ml 25 % NH3 + 1,5 ml 85 % H3PO4) in one litre of deionized water. Centrifuge after washing and discard solution.

3. Dissolve the precipitates in the centrifuge tubes with a small amount of 65% nitric acid and combine the solutions. Evaporate to near dryness and dissolve the residue in 20 ml of 8 M nitric acid.

4. Preparation of SrResin column (Eichrom/Triskem offers also prepacked 2 ml and 10 ml SrResin column, when using them packing is not necessary)
Put a small piece of fiberglass on the bottom of the column.
Fill the column to half with water.
Weigh 3 g of SrResin (100-150 µm) and stir it into a small volume of distilled water.
Transfer the resin-water mixture with a disposable pipette into the column.
Put a small piece of fiberglass on the top of the column.
Rinse the column an excess of water – keep the water table on the top of the upper fiberglass.
Rinse the column 30 mL of 3 M HNO3.

5. Pour the sample solution into the column and let it freely drop into a underlying beaker.

6. Rinse the SrResin column by
twice with 20 ml 8 M HNO3
once with 20 ml 3 M HNO3
Write down the time when the last rinsing is finished since 90Y out elutes from the column.

7. Elute strontium out of the column with 70 ml 0.05 M HNO3, collect the eluate into a 250 ml baker (high) and make the solution alkaline (pH 8-9) with 25 % NH3.
Precipitate strontium carbonate by adding 3.5 - 4 g solid ammonium carbamate (NH4CO2NH2).
Heat the solution on a water bath until a fine-grained deposit has settled down on the vial bottom.
Let the solution cool on an ice bath.

8. Filter the SrCO3 precipitate through a Millipore filter holder with suction on the Ø 25 cm filter paper.
Wash the precipitate three times with 5 ml of dist. H2O and additional three times with 5 ml of technical ethanol.
Dry the precipitate for 10 min. at 105 oC in oven, let cool in a desiccator.
Transfer the precipitate into a 20 ml LSC vial (weighed five times - take average value)
Weigh five times - take average.

9. Dissolve the precipitate in 2 ml of 1 M HCl. Mix carefully.
Add 18 ml of HiSafe 3 liquid scintillation solution. Mix carefully.
Let the sample stand for two weeks in a cool place. Within two weeks 90Y grows up in equilibrium with 90Sr.

10. Prepare a sample from a 90Sr/90Y standard solution, in which 90Sr and 90Y are in equilibrium.
Take about 2 Bq of 90Sr/90Y standard solution and add it into a mixture 2 ml 1 M HCl + 18 ml HiSafe 3 in a LSC vial. Mix carefully.

11. Prepare a background LCS sample containing 2 ml 1 M HCl + 18 ml HiSafe 3 in a LSC vial.

12. Measure the 90Sr+90Y activity of the unknown, standard and background samples with
the liquid scintillation counter.