2.6.1 - Strontium by Strontium RAD Disk

Introduction

Strontium, Lead and Radium are enriched in human bones and thus contribute with their daughter products to the long term radiation exposure to the population. Therefore their regular determination in environmental samples like surface and drinking water is mandatory. Due to the continuous β-energy of Sr-isotopes (⁹⁰Sr, ⁸⁹Sr), a selective separation step from other radionuclides as well as from matrix elements (Ca) is necessary.

Strontium and Radium RAD Disk filters are thin membranes reported to consist of Strontium and Radium selective particles held in a stable inert matrix of polytetrafluoroethylene (PTFE) fibrils. The crown-ethers 18 for Strontium and 21 for Radium have been reported to act as extractants.

The analytical procedure applying Strontium RAD Disks resembles to the procedure for Radium described in 2.2.1.5. Only methanol for preconditioning has to be introduced additionally in order not rinsing the extractant from the column.

After sample preparation by filtration, Strontium is eluted and can be detected with a Liquid Scintillation Counter or directly on the filter material being soaked with cocktail.

Materials and Equipment

• 3M Strontium RAD Disk filter (3M Empore)

• HNO₃ concentrated and 2 M

• Methanol

• 0.25 M EDTA (alkaline)

• Vacuum filtration apparatus (48 mm)

• Suction reservoir (1 L, 50 mL)

• Gelating cocktail

• ⁹⁰Sr standard solution

Procedure

Sample and Disk Preparation:

1. Acidify the sample (upto 3 L) to 2 M HNO₃ (126 mL conc. HNO₃ for 1 L water).

2. Center the Strontium RAD Disk (marked side down) on the base of the filtration apparatus and clamp the reservoir on top of the disk

3. Condition the disk by adding 10 mL of methanol

4. Apply vacuum and pull approximately 1 mL through the disk; vent the vacuum and allow the disk to soak for 1 minute

5. Apply vacuum and slowly draw the methanol through the disk, leaving 3-5 mm above the surface of the disk

6. Condition the disk with 20 mL of 2 M HNO₃ under low vacuum; leave 3-5 mm of liquid on the surface of the disk

Sample Extraction:

1. Pour the sample into the reservoir and suck with low vacuum; the flow rate should not exceed 50 mL/min.

2. Rinse the disk with 20 mL of 2 M HNO₃ and some dist. water; the end time of this rinse is recorded as the start of ⁹⁰Y in growth.

Note: The disk should not be allowed to run dry during the conditioning and the sample processing steps.

Counting: Filter Disks may be counted by either proportional or scintillation counting. Radioactive ingrowth and decay corrections must be applied. For use with a liquid scintillation counter, place disk (before disk dries) into a vial containing gelating cocktail. A Sr standard solution should be used for efficiency calibration.

Evaluation

The activity concentration is calculated according to

with f(t) = 1 – exp-(t/T_1/2 ⁹⁰Y) _* ln2

R_N = Net rate (cps)

ε = Counting efficiency of the measurement (95 % for ⁹⁰Sr)

V = Sample volume (L)

T = Time difference between rinsing and mean measuring time

T_1/2 ⁹⁰Y = 64.1 h

Detection Limit (MDA): 20 mBq/L (for 3 L sample and LL β-counter)