2.6.15 - Tc by RAD Disk and PSresins
Introduction
99Tc (Eβmax = 293.5keV) as pure ß-emitter is a fission product of 235U with a relatively high fission yield of 6.1 %. Its presence in the environment is due to nuclear weapon tests and nuclear fuel cycle operations. Because of its long half-life (T1/2 = 2.11 x 105 a) and its high mobility in the environment, 99Tc is a significant component in the long-term storage and disposal of nuclear wastes. 99mTc (T1/2 = 6 h) as daughter of 99Mo (T1/2 = 66 h) is used in nuclear medicine for diagnostics in high-activity amounts. It can be used as well as tracer nuclide for 99Tc separation/enrichment.
Extensive procedures have been described in the literature, but are tedious and time consuming; for a comprehensive survey see [L’Annunciata 2012]. Two simple and fast methods are described below, applying tertiary amines on an inert matrix of PTFE and on microspheres.
(a) Extraction with Technetium RAD Disks
No special sample preparation like pH adjustment or disk handling steps with alcohol, as described for Strontium/Radium RAD Disks (see e.g. 2.3.1), are necessary for Technetium RAD Disks. They are stable in a pH range between 1 and 14. The water sample should not be acidified with HNO3, because of possible interferences with nitrate ions.
The particulate free water sample (100 mL or less upto several liters) is pulled through the disk (70 kPa / 0.7 bar vacuum).
For liquid scintillation measurement, the disk is placed into a vial containing scintillation cocktail and fixed on the inner glass wall (see fig. 44).
Alternatively a gas proportional counter can be used, but the sample has to be mounted in a planchet obscuring “Side Down”.
(b) Tc-99 in water with PSresins
The principle, application and general measuring procedure of PSresins are described in more detail in chapter 2.3.3.
In the case of 99Tc, the extractant used is Aliquat-336. This tertiary amine presents selectivity for pertechnetate anion at a wide pH range. Thorium interference can be avoided by cleaning the PSresin with a 0.1 M HF/HNO3 solution [Barrera 2016].
Figure 35: 99mTc plastic scintillator resin: Cartridge (left); Aliquat-336 extractant (right)
Retention of pertechnetate in the 99Tc PSresin is almost quantitative, although the extraction yield can be determined using 99mTc or ReO4-. In addition, high counting efficiencies of around 85 %, and low background values due to the low amount of scintillator used (1 g), lead to low detection limits.
This PSresin can also be used for the analysis of other anionic species, like S14CN- [Bagan 2012].
Materials and Equipment
2 mL 99Tc PSresin cartridges (TrisKem)
ReO4- carrier solution (1 mg of Rhenium per sample)
99Tc standard solution
1 M HCl
Concentrated HCl
1 M HF/HNO3
Either separation can be performed in a vacuum box or using a peristaltic pump.
Procedure
Sample 10 to 100 mL of water sample
Reduce the volume to 10 mL
Add concentrated HCl to achieve a 0.1 M concentration
Spike the solution with 1 mg of a Rhenium carrier solution
Add 2 mL of 0.1 M HCl into the 99Tc PSresin cartridge for conditioning and allow draining
Add the sample into the 99Tc PSresin cartridge
Rinse the 99Tc PSresin cartridge three times with 2 mL of deionized water or with a 0.1 M HF/HNO3 solution, if the sample may contain Thorium isotopes
Add 2 mL of deionized water into the 99Tc PSresin
Let the cartridge to drain
Put the 99Tc PSresin cartridge in a 20 mL polyethylene vial for scintillation counting
Take an aliquot (e.g. 1 mL) of the wastes collected from the 99Tc PSresin cartridge to determine the chemical yield though ICP-OES
Evaluation
The 99Tc activity A is calculated from the net counting rate RN, the counting efficiency ε, the chemical yield η and the sample volume V.
The counting efficiency ε can be obtained by running the same procedure with a sample of 10 mL with a known amount of 99Tc.
Detection Limit (MDA): 1 Bq/L (10 mL sample and 1h counting time)
40 mBq/L (100 mL sample and 3 h counting time)
L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3rd Edition 2012, Elsevier
Bagán H., Tarancón A., Stavsetra L., Rauret G. and García J.F. 2012: Determination of oil reservoir radiotracer (S14CN-) in a single step using a plastic scintillator extractive resin; Anal. Chim. Acta 736 (2012) 30–35
Barrera J., Tarancón A., Bagán H. and García J.F. 2016: A new plastic scintillation resin for single-step separation, concentration and measurement of technetium-99; Anal. Chim. Acta 936 (2016) 1–8