Method for the detection of Tc in seaweed samples coupling the use of Re as a chemical tracer and isotope dilution inductively coupled plasma mass spectrometry

Abstract

Analysis of the artificial radionuclide ⁹⁹Tc in environmental samples requires a chemical separation due to its low concentration, and therefore the use of a chemical yield tracer is peremptory. From a practical viewpoint, Re can be used for this purpose, due to its chemical similarities with Tc. Thus, the use of a radioactive tracer for Tc recovery calculation can be avoided. However, results from a recent intercomparison exercise showed that using of Re as a chemical yield tracer appears to underestimate the Tc concentration relative to the result obtained with isotopes of Tc. In the present work, the methodology used to design a simple separation method for the measurement of ⁹⁹Tc in environmental samples is described. Tc recovery is estimated throughout the Re recovery calculation by the isotope dilution technique coupled with ICP-MS (ID-ICP-MS) technique. For chemical separation, a chromatographic resin is used. Interfering elements are removed using a resin washing step carefully designed to avoid any element fractionation between Re and Tc; the care taken in this step is of major importance to assure the equivalence of the chemical recoveries for both elements. Agreement is tested using five replicates of five seaweed samples. The average recoveries for ^95mTc and Re were 93±6 and 95±7%, respectively, those are within the uncertainty intervals for each other. The results explained here demonstrate the possibility of applying Re chemical recoveries to calculate the Tc concentrations with the advantage of not introducing systematic errors.

Introduction

Technetium-99 (2.1×10⁵ years half-life) is a low-energy β-emitter formed as a fission fragment with a relatively high fission yield (∼6%). Controlled radioactive releases from the large nuclear fuel reprocessing plants of La Hague (France) and Sellafield (UK) have been the major sources of this radionuclide in North Atlantic seawater, and they are even in competition with the global fallout source. The radioisotope is predominately present in seawater under oxic conditions in the form of TcO₄⁻ ion, which is extremely stable. It can be carried out far from these sources following the general circulation of the ocean currents [1]. Indeed, such stable characteristics allow researchers to use it as a radioactive tracer for the movement of water marine masses and marine currents. Furthermore, several biota species with a high accumulation capacity for this element have been identified [2], such as brown algae, especially Fucus vesiculosus. Their use as a bioindicator for technetium concentrations in seawater has been suggested [1].

As a fast and very sensitive alternative to radiometric detection, inductively coupled plasma (ICP) MS has been proposed for ⁹⁹Tc determination in environmental samples. Hence, quadrupole ICP-MS instruments with pneumatic [3], [4], [5], [6] or ultrasonic nebulizers [7], electrothermal vaporization units (ETV-ICP-MS, [8]), laser ablation units (LA-ICP-MS [9]) and high-resolution ICP-MS [10] have been used for ⁹⁹Tc detection in a variety of industrial and environmental samples. However, the chemical separation and purification of ⁹⁹Tc from environmental samples is needed, for which a wide variety of isotopes has been used as yield monitors. Usually, ^99mTc [11], [12], ^95mTc [13] and ^97mTc [14] have been used. These isotopes are generally determined using gamma-ray spectrometry. Beals [15] proposed the isotope dilution technique coupled with ICP-MS (ID-ICP-MS) using ⁹⁷Tc as a yield tracer. The main advantage of the ID technique is it overcomes the problems associated with instrumental drift and matrix effects. Furthermore, the ID-ICP-MS technique relies purely on isotopic ratios; therefore the problem of an incomplete extraction is negated.

It is well known that Re and Tc chemistries are similar: their analogous compounds are virtually identical from a structural standpoint. Therefore, Re has been proposed as a tracer for ⁹⁹Tc radiochemical recovery calculation from environmental samples, both for radiometric [16] and non-radiometric [17] counting methods. The first of these methods [16] follows a chemical separation based on coupling (1) the use of an anion exchange resin to concentrate Tc (and Re); (2) a precipitation step for Tc and Re as sulfides, from a NaClO₄ solution; and (3) a precipitation step for Re and Tc as tetraphenyl arsonium salts. Chemical yield is determined by gravimetric analysis of the rhenium salt. The second method [17] is applied after a chemical separation scheme based on the use of a TEVA Spec™ chromatographic resin, although several analytical details are still to be published.

Nowadays, Re is used routinely for this purpose with few problems reported. This approach avoids the use of radioactive isotopes, especially at places where these radioisotopes are strictly forbidden (e.g., on a ship for seawater sampling). Furthermore, using Re avoids the operational implications of the use of another isotope of Tc: common nuclear reactions for ⁹⁷Tc and ^95mTc generation also produce ⁹⁹Tc, which cannot be removed from the yield tracer solution [16]. An alternative reaction that produces ^95mTc free of ⁹⁹Tc has been described elsewhere [13], but this option is only available to those laboratories having a cyclotron.

However, according to the results from a recent intercomparison exercise for ⁹⁹Tc in seaweed samples, those laboratories that used Re as a chemical yield tracer systematically underestimated the Tc concentrations by more than 20% on average [18]. This fact could possibly be due to an overestimation of the recoveries for Tc.

The main difference between Re and Tc is in their respective oxidation potentials; the reduction of Re is more difficult than that of Tc [19]. Thus, slightly different co-precipitation profiles for ReO₄⁻ and TcO₄⁻ ions with TPAC, and different elution profiles from an anion exchange resin have been shown [16]. More recently, differences in the capacity factors for Tc and Re using the same chromatographic resin have been shown [20]. Those small chemical differences should be considered in detail if a new method that uses Re as a chemical yield tracer for Tc is to be designed.

In this work, a new method that uses Re ID-ICP-MS detection for Tc chemical yield calculation is proposed. In this way, data for Tc solution concentration and chemical yield can be acquired using the same analytical running with only one instrument. The advantages of this methodology are as follows: (1) radiological concerns due to radioactive tracers handling are avoided; (2) ID-ICP-MS improves the precision of the Re chemical yield calculation; (3) the method takes into account explicitly the small separation chemistry differences between Re and Tc. Hence, the previously reported problem of analyte concentration underestimation can be avoided; (4) the ID-ICP-MS technique allows the determination of both Tc concentration and chemical yield simultaneously; therefore additional techniques (such as γ-ray spectrometry for Tc isotopes or gravimetric analysis for Re isotopes) are not necessary for the recovery calculation.

This paper focuses on the methodological aspects of the separation scheme proposed, and on its justification. The equivalence for Re and Tc recoveries is tested using some seaweed samples from the North Atlantic Ocean, as there are no suitable reference materials with ⁹⁹Tc certified concentrations.