Milk is an important component of our daily nutrition. Therefore, the measurement of 90Sr in milk samples has been used as a monitor of global fallout from nuclear weapon tests and Chernobyl accident and possible local leakage of radioactivity in areas surrounding nuclear facilities. While 131I and 137Cs can easily be quantified by γ-spectrometry after preconcentration, the pure β-emitting Sr-isotopes need to be purified from both other radionuclides and Ca as co-present in larger mass amounts. The defined reporting level in the Commission recommendations 2000/473/Euratom for 90Sr in milk is 0.2 Bq/L.
A commonly used method for determining Sr in aqueous solutions, wet depositions and soil samples comprises as central step its separation and concentration on EICHROM SPEC columns as described in 2.3.2. A crutial part in milk samples is the breakthrough of Sr due to the high Ca amount present. Therefore only smaller milk volumes of 30 mL can be applied unless further separation steps or larger extraction chromatographic materials are introduced.
The procedure below, adapted from [Landstetter 2009] and [Lopes 2009], summarizes the sample preparation procedure for 300 mL raw milk; for further column elution details see 2.3.2.
Materials and Equipment
- Sr carrier
- HNO3 conc., 8 M HNO3, 3 M HNO3, 0.05 M HNO3
- 0.05 M oxalic acid
- Gelating LS cocktail (e.g. OptiPhase HiSafe III)
- Sr-specific resin material (5.4 g) (EICHROM)
- Raw milk (300 mL) is dried at 220 °C and then ashed at 550 °C.
- A Sr carrier is added (chemical yield recovery) and the ash is leached with conc. HNO3.
- Ammonium oxalate is precipitated at pH 4 to 5 (separation of alkaline metals).
- The precipitate is washed with water and then the oxalate is destroyed by fuming with conc. HNO3.
- The residue is dissolved in 12 mL of 8 M HNO3 and loaded ont the column (5.4 g of the 100-150 mm Sr-specific resin).
- Ca, K, Ba are removed with 25 mL of 8 M HNO3.
- The column is washed with 10 mL of 3 M HNO3 + 0.05 M oxalic acid, with 10 mL of 3 M HNO3 and then with 10 mL of 0.05 M HNO3.
- 90Sr is eluted with 45 mL of 0.05 M HNO3.
- The solution is evaporated to dryness. The residue is dissolved in 8.25 mL of 0.05 M HNO3.
- 0.25 mL is taken for the determination of the chemical yield (e.g. ICP-MS).
- 90Sr is measured by LS in equilibrium with 90Y (2 weeks storage) after adding 12 mL of Optiphase HiSafe III.
The 90Sr activity concentration is calculated according to the general scheme, taking into account the chemical yield, the sample volume, and the measuring efficiency for both 90Sr and 90Y (about 180 %).
Detection Limit (MDA): 150 mB/L (LL β-counter)
Landstetter C., Cernohlawek N. and Katzlberger C. 2009: Determination of Sr-90 in milk in Austria; in: J. Eikenberg et al. “LSC 2008 Advances in Liquid Scintillation Spectrometry”, pp 313-319, Radiocarbon 2009, Tucson
Lopes I. and Madruga M. J. 2009: Application of liquid scintillation counting technique to the determination of Sr-90 in milk samples; in: J. Eikenberg et al. “LSC2008 Advances in Liquid Scintillation Spectrometry”, pp 331-337, Radiocarbon 2009, Tucson