2.6.2 - Sr-90 and Pb-210 by Extraction Chromatography

Introduction

Extraction Chromatography on columns was first introduced in the 1970^th, but it has found now a broader application ever since materials as powder or in prepared columns are commercially available from EICHROM Ind. [Horwitz et al. 1992]. Among the various available products (table 7), the Strontium SPEC is probably the mostly applied column.

Due to their similar ionic radius, both Sr²⁺ and Pb²⁺ as 2-valentic cations in HNO₃ solution are retained on SPEC columns (EICHROM, TrisKem). Thus, they can easily be concentrated and separated from all other cations and from each other by the complexing affinity of Pb²⁺ with citrate.

The extraction chromatographic method described below is selective and rapid. The sample preparation procedure as preliminary step has to be selected according to the material to be analysed. The procedure below was adapted from Langer [Langer 2000] and [Horwitz 2010].

Materials and Equipment

• Sr SPEC column (EICHROM, Triskem)

• Sr, and Pb carrier solutions

• HNO₃ (8 M, 6 M, 0.05 M)

• 0.1 M ammonium citrate (pH 5)

• Gelating cocktail (long range for citrate e.g. OptiPhase HiSafe III)

Procedure

1. The Sr-SPEC-column is preconditioned with 5 mL 8 M HNO₃.

2. The preconcentrated water sample in 8 M HNO₃ is added onto the column.

3. The column is washed with 5 mL 8 M HNO₃ (separation of interfering elements e.g. ²¹⁰Bi and ⁹⁰Y). Note time, as this is the initiation of ingrowth of ⁹⁰Y!

4. The column is washed with 5 mL 6 M HNO₃ (separation of ²¹⁰Po). This step can be omitted in absence of ²¹⁰Po.

5. Elution of ⁹⁰Sr: Sr is finally eluted with 10 mL 0.05 M HNO₃ and collected in a plastic vial (mix well!).

6. For counting, 5 mL eluate are mixed with 15 mL gelating cocktail and measured immediately (only ⁹⁰Sr).

7. Another 5 mL eluate might be evaporated to dryness on a counting plate using an IR-lamp (weight before evaporation!) and measured with a proportional counter.

8. Elution of ²¹⁰Pb: ²¹⁰Pb is eluted from the column with 10 mL 0.1 M ammonium citrate (pH 5) (mix well!)

9. For counting, 4 mL eluate are mixed with 16 mL gelating cocktail.

Evaluation

The ⁹⁰Sr content is calculated from the net counting rates R_N, the measuring efficiency ε and the chemical yield η according to

The chemical yield can be controlled by comparing the amount of Sr carrier with the mass of Sr(NO₃)₂. The total emission probability of the β-decay of both ⁹⁰Sr and ²¹⁰Pb is 100 %.

The ²¹⁰Pb content is calculated according to the general scheme by taking into account the ratio between sample and total volume. For all measurements quench correction must be applied.

Lower Limit of Detection LLD without considering any preconcentration

for ⁹⁰Sr : 50 mBq per sample

for ²¹⁰Pb: 70 mBq per sample

Horwitz E.P., Chiarizia R. and Dietz M.L. 1992: A novel strontium-selective extraction chromatographic resin; Solvent Extr. Ion Exch. 10(2) (1992) 313-36

Langer M. 2000: Training Material Exercise P0005 “Extraktionschromatographie”, Research Center Karlsruhe / FTU