Separation and analysis of Sr-90 and Zr-90 for nuclear forensic applications

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Separation and Analysis of Sr-90 and Zr-90 for Nuclear

Forensic Applications

Mémoire

Ana Paula Zattoni

Maitrise en chimie

Maître ès sciences (M.Sc.)

Québec, Canada

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Résumé

Le présent travail porte sur le développement technologique pour déterminer l'âge des sources de radiostrontium à travers du rapport [Zr-90]/[Sr-90], en utilisant les techniques de spectrométrie de masse et scintillation liquide pour quantifier les deux isotopes. Parce que Sr-90 et Zr-90 sont des interférences isobariques en spectrométrie de masse, une séparation radiochimique est nécessaire pour isoler du Zr-90 avant son analyse. Parmi quatre résines commerciales, la résine DGA a fourni la meilleure performance pour isoler le Zr-90 du Sr-90. Des récupérations supérieures à 99% pour le Zr-90 ont été obtenues. La résine DGA était aussi l'approche la plus rapide et la plus efficace pour éliminer les interférences isobariques du Sr-90 et aussi de l’Y-90 potentiellement présents dans des échantillons contenant des niveaux élevés de radioactivité. Des expériences impliquant l’utilisation d’une cellule de collision pour éliminer des interférences isobariques ont fourni des facteurs de décontamination insuffisants pour des applications en criminalistique nucléaire.

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Abstract

In this work, a technological development to determine the age of radioactive strontium sources through the [Zr-90]/[Sr-90] ratio using mass spectrometry and liquid scintillation to quantify both isotopes is presented. Because Sr-90 and Zr-90 are isobaric interferences in mass spectrometry, a radiochemical separation to isolate Zr-90 has been shown to be mandatory prior to analysis. Four commercial resins (AG50W-X9, Dowex1-X8, Sr and DGA resins) were tested to isolate Zr-90 from Sr-90. Best performance was observed for the DGA resin, including recoveries higher than 99% for Zr-90. DGA has also demonstrated to be the faster approach and the most efficient not only to eliminate isobaric interferences from Sr-90, but also from Y-Sr-90, potentially present in samples containing high levels of radioactivity. Experiments using a collision cell to eliminate isobaric interferences in a triple quadrupole mass spectrometer (ICP-QQQMS) have also been carried out, but results have demonstrated insufficient decontamination factors for nuclear forensic applications.

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Tables List

Table 1.1 – Radiological properties of threatening radionuclides ... P.7 Table 2.1 – Accidents involving RTGs reported by the IAEA ... P.15 Table 2.2 – Radiological information from nuclear or radioactive materials .. P.16 Table 3.1 – Minimum resolution required to discriminate isobaric

interferences at m/z 90 for the analysis of Zr-90 in MS ... P.24 Table 3.2 – Typical chemical reactions in reaction cells ... P.25 Table 3.3 – Theoretical binding properties of Zr and Sr with oxygen atoms .. P.27 Table 4.1 – Common commercial IEC resins ... P.39 Table 4.2 – Common commercial EXC resins... P.41 Table 4.3 – Distribution ratios (D) for strontium and zirconium in the

AG50W-X8 resin ... P.44 Table 5.1 – Instrumental setting for SrTiO3 digestion (Mars 5, Easy

PrepTM vials) ... P.48 Table 5.2 – Acquisition parameters for analysis of Sr and Zr by

ICPQQQ-MS ... P.51 Table 5.3 – Comparison of ionization energies between measured

elements and internal standard ... P.52 Table 5.4 – Acquisition parameters for the analysis of Sr and Zr using

reaction cell and O2 as reaction gas ... P.53

Table 5.5 – Acquisition parameters for the analysis of Sr-90 by liquid

scintillation... P.54 Table 6.1 – Acid mixtures used for SrTiO3 digestion tests ... P.55

Table 6.2 – Digestion efficiency of SrTiO3 under different acidic conditions.. P.56

Table 6.3 – Performance of alternative eluents for Zr ... P.62 Table 6.4 – Sample loading volumes according to the mass of dry resin

used for separations ... P.64 Table 6.5 – Recovery of Zr in DGA Resin according to HNO3/HF ratio in

samples ... P.72 Table 6.6 – Summary of resins performance to isolate Zr prior MS

analyses ... P.72

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Pictures List

Figure 1.1 – Brief description of the origin of radioactivity in the

environment ...P.6 Figure 1.2 – Decay chain of strontium-90 ...P.8 Figure 1.3 – Means of uptake and bioaccumulation for strontium-90 ...P.11 Figure 2.1 – Number of nuclear and radioactive incidents reported by the

IAEA for the last years ...P.13 Figure 2.2 – Forensic Science...P.16 Figure 2.3 – Decay process of Sr-90 as function of elapsed time ...P.19 Figure 3.1 – Basic components of ordinary mass spectrometers ...P.21 Figure 3.2 – Quadrupole mass spectrometer ...P.23 Figure 3.3 – Triple quadrupole mass spectrometer mechanism...P.25 Figure 3.4 – Mechanism of energy transfer and detection of beta particles

by liquid scintillation ...P.28 Figure 3.5 – Growth rate of Y-90 and secular equilibrium with Sr-90 ...P.29 Figure 4.1 – Equilibrium in chromatographic separations ...P.32 Figure 4.2 – In column chromatography technique ...P.33 Figure 4.3 – Experimental variables to determine resolution in

chromatography ...P.35 Figure 4.4 – Parameters for the determination of peak asymmetry ...P.37 Figure 4.5 – Separation of cations and anions by IEC ...P.38 Figure 4.6 – Schema of extraction chromatography ...P.40 Figure 4.7 – D values for strontium and zirconium in the Dowex 1-X10

resin ...P.44 Figure 4.8 – Capacity factor for strontium and zirconium in the DGA resin ...P.46 Figure 5.1 – AF Omnifit® Column Design ...P.49 Figure 5.2 – Method applied for separation tests ...P.50 Figure 6.1 – Reproducibility of SrTiO3 digestion using HNO3/HF mixture ...P.57

Figure 6.2 – Elution profile of Sr and Zr in 4M HCl (10 g AG50W-X8,

100-200 mesh) ...P.58 Figure 6.3 – Elution profile of Sr and Zr in 3M HCl (10 g AG50W-X8,

100-200 mesh) ...P.59 Figure 6.4 – Separation of Sr and Zr using a 2M to 6M HCl gradient (10 g

AG50W-X8, 100-200 mesh) ...P.59 Figure 6.5 – Elution curves of Sr as function of HCl molarity (10 g

AG50W-X8, 100-200 mesh) ...P.60 Figure 6.6 – Elution curves of Sr at 2M HNO3 and 2M HCl (10 g

AG50W-X8, 100-200 mesh) ...P.61 Figure 6.7 – Separation of Sr and Zr using a 2M HNO3 to 6M HCl

gradient in 2 g AG50W-X8 (100-200 mesh) ...P.63 Figure 6.8 – Volume of eluent for Sr elution as function of mass of

AG50W-X8 ...P.63 Figure 6.9 – Separation efficiency for Sr and Zr using Dowex1-X8 resin ...P.65

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Figure 6.10 – Zirconium retention in Dowex1-X8 as function of HCl

concentration ... P.65 Figure 6.11 – Maximum recovery of Zr according to HCl concentration in

Dowex1-X8 ... P.66 Figure 6.12 – Comparative of separation of Sr and Zr using ion exchange

and extraction resins (a. AG50W-X8, b. DOWEX1-X8, c. Sr-Resin, d.

DGA-Resin) ... P.67 Figure 6.13 – Proposed extraction mechanism for Sr for its separation

from Zr by EXC ... P.68 Figure 6.14 – Tailing effect as a function of Sr concentration (AG50W-X8) . P.69 Figure 6.15 – Separation of Sr and Zr using Dowex1-X8 for samples

containing HF ... P.70 Figure 6.16 – Separation of Sr and Zr using DGA for samples containing

HF (a. 0.01%, b. 0.2%) ... P.71 Figure 6.17 – Complete methodology to separate Sr and Zr using DGA

resin ... P.73 Figure 6.18 – Comparative between experimental and expected results

for the recovery of trace levels of Zr using DGA resin ... P.74 Figure 6.19 – Procedure for determining the age of a radiostrontium

source ... P.75 Figure 6.20 – Comparative between theoretical and experimental

concentrations for the analysis of Sr-90 by liquid scintillation ... P.76 Figure 6.21 – Zr and Sr oxides formation in mass spectrometry as

function of O2 concentration in the reaction cell ... P.77

Figure 6.22 – Predominant species of Zr (a) and Sr (b) at 6% O2 in the

reaction cell ... P.78 Figure 6.23 – Correlation between results for the analysis of Zr at m/z 90

and m/z 106 ... P.79 Figure 6.24 – Predominant species of Y at 6% O2 in the reaction cell ... P.79

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Abbreviations List

D – Alpha particles

D – Separation factor E – Beta particles ߣ – Decay constant

a – asymmetry portion of a peak ܣ – Final activity

ܣ_଴ – Initial activity ܣ_ௌ – Peak asymmetry A+ – Charged analyte AG50W-X8 – Cationic resin

AMS – Accelerator Mass Spectrometry b – Back portion of a peak

ܥ – Final concentration ܥ_଴ – Initial concentration

ܥ_௘ – Concentration of a solute in the extractant phase ܥ_௜,ெ – Concentration in the mobile phase

ܥ௜,ௌ – Concentration in the stationary phase

cpm – Count per minute cps – Count per second D – Distribution ratio

DGA – Diglycolamide resin Dowex1-X8 – Anionic resin ܧ – Extractant

EPA – Environmental Energy Agency EXC – Extraction chromatography F – Force

F-_{– Fluoride}

G – Gas

G+ – Charged gas

ܪ – Height of the theoretical plate H+ – Proton H2C2O4 – Oxalic acid H2O – Water H2O2 – Hydrogen peroxide H2SO4 – Sulphuric acid HCl – Hydrochloric acid

HEU – High-enriched uranium HF – Hydrofluoric acid

HNO3 – Nitric acid

ǻHR – Enthalpy ݅ – Given compound I – Interference

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I+ – Charged interference

IAEA – Internation Atomic Energy Agency

ICP-MS – Inductively Coupled Plasma Mass Spectrometry

ID – Identification

IEC – Ion exchange chromatography

IUPAC – International Union of Pure and Applied Chemistry

K – Distribution coefficient ݇Ԣ – Capacity factor ݇_௘ – Coulomb’s constant ܮ – Length ܮ – Ligand LL – Lower limit LOD – Detection limit LOQ – Quantification limit m – Mass ο݉ – Mass difference M – Molar m/z – Mass-to-charged ratio M+ – Charged metal ܯܯ – Molar mass MS – Mass spectrometry M: – Megaohm

ܰ – Number of theoretical plates N/Z – Neutron-to-proton ratio N2O4 - Nitrogen tetroxide NO3- – Nitrate ܰ஺ – Avogadro’s number O – Atomic oxygen O2 – Molecular oxygen Pb – Lead

ppt – part per trillion Psi – lbf/square inch Pu – Plutonium ݍ – Charge Q – Quadrupole ܴௌ – Resolution

ݎ – Distance between two charges R2 – Correlation factor

RDDs – Radiological dispersion devices RF – Radio-frequency RTGs – Radiothermal generators s – Standard deviation S – Stationary phase SI – International system Sr – Strontium

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SrTiO3 – Strontium Titanate

ݐ – Age of a radioactive source t½ – Half-life

Ti – Titanium

ݐ௠ – Dead time

ݐԢ௜ – Adjusted retention time

TIMS – Thermal Ionization Mass Spectrometry ݐ௜ – Retention time

u – Mass unit U – Uranium

U.S – United States UP – Upper limit

ܸ_ெ – Volume of the mobile phases ܸ_ௌ – Volume of the stationary phases

v/v – Volume-to-volume ratio

y – Years Y – Yttrium

ݓ௜, – Width at the base of a peak

Zr – Zirconium

Zr+ – Charged zirconium

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“Science can only be created by those who are thoroughly imbued with the aspiration toward truth and understanding ”

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Acknowledgments

My completion of this project would not have been possible without the kind support of my director Dominic Larvière. So, I would like to thank him to be always open to discuss and share ideas while guiding me to successfully achieve the goals of this project.

I also would like to thank Serge Groleau for all the support in the laboratory, my office partners Annie Michaud, Pablo Lebed, and Marie-Ève Lecavalier for their pleasant company during all the time we spent together. Charles Labrecque, Kenny Nadeau, Jean-Michel Benoit, Solange Schneider, Laurence Whitty-Léveillé, Sabrina Potvin, Julien Légaré Lavergne, Justyna Florek, and Maela Choimet who I had the opportunity to work with and, in some cases, the opportunity to struck up a close friendship.

Likewise, I would like to thank Health Canada, the Research and Technology Initiative, and Agilent to make this project possible. And finally, I would like to thank Sherrod Maxwell for the interest in this project as well as for the suggestions and the encouragement, which have been all important to the accomplishment of this work.

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Introduction

Incidents involving illicit trafficking and smuggling of nuclear and radioactive material have been object of concern since the early 90s, when the first cases involving unauthorized activities started being reported in Switzerland and Italy, then years later in Germany, Czech Republic, and Hungary [1]. Today, more than 2,400 cases have been already confirmed since 1995, and 155 cases have been reported for the period between July 2012 and June 2013 [2].

Before the 90s, the main concern for nuclear security was to protect only high-enriched uranium (HEU) and plutonium in nuclear facilities. However, the increasing number of cases implicating illegal possession, theft, or loss involving other radioactive sources since 1995, forced authorities to establish a new concept of nuclear security, while triggering efforts towards eliminating nuclear and radioactive threats.

Such new concept became synonym of both protection and control over not only nuclear but also any kind of radioactive material that could give rise to malicious actions, including unpredictable terrorist activities and utilization of radiological weapons known as dirty bombs.

Contrary to nuclear bombs, dirty bombs are relatively easier to fabricate and are mainly characterized by their dispersive effect. The purpose of dirty bombs is not to destroy but contaminate, while spreading a radioactive material through the utilization of conventional explosives.

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Usually, radionuclides that have long half-lives or a high specific activity are potentially more interesting for the production of radiological dispersion devices (RDDs). Ranked in a short list of these radionuclides, strontium-90 has a half-life of about 29 years and a specific activity of about 518 X 1010 Bq/g. This corresponds, for example, to a specific activity of about 1.5 times higher than for cesium-137, which has an equivalent half-life (i.e. 30 years) [3].

Sources of strontium-90 can be found in laboratories of research or hospitals for the production of yttrium-90 and cancer treatment as well as in wastes of nuclear facilities. The main concern, however, is associated to sources of strontium-90 found in orphaned radiothermal generators (RTGs) widely used in the 50s to provide energy in areas of difficult accessibility. It is estimated that hundreds of orphaned RTGs containing high levels of activity are still lost around the world. Actually, the lower degree of security surrounding these sources is assumed to be appealing for nuclear terrorists.

Following an alleged terrorist attack, where the presence of a nuclear or a radioactive source is detected, a nuclear forensic investigation takes place. Working with other forensic sciences, nuclear forensics aims not only to answers questions about the radiological hazard but also provide complementary radioisotopic information to determine the origin of a seized source. Isotopic composition, for example, could provide information about the fabrication date or last purification (i.e. age of the radioactive source) and, in conjunction with other chemical and physical data, provide clues about the facility responsible for its production.

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the decay law. Actually, Sr-90 is an unstable radioisotope that undergoes beta decay to form Y-90 which in turn decays into Zr-90, a stable nucleus. Thus, the age of a source containing strontium-90, for example, would be a function of the [Zr-90]/[Sr-90] ratio, where concentrations of both isotopes could be determined, respectively, by liquid scintillation and mass spectrometry, as demonstrated over the present work. To be successfully used to date nuclear materials, however, this approach requires an efficient method for radionuclide separation to isolate Sr-90 from Zr-90 from the radioactive source as well as a sensitive method of analysis to provide accurate results while reducing age uncertainty.

Mass spectrometry has been widely used for analytical purposes because of its sensitivity, accuracy, and possibility to discriminate isotopic species. The major inconvenient of this technique is the isobaric interferences caused by ionic atoms or molecules having the same m/z, as for strontium-90 and zirconium-90. Such interferences cause peak overlap and an overestimation of compounds of interest. Sometimes, even high-resolution devices are not sufficient to overcome this problem and pre-treatments (e.g. separation) are often mandatory prior to analysis.

Besides liquid-liquid extraction and precipitation techniques, ion exchange (IEC) and extraction chromatography (EXC) have gained extensive attention in the past years, especially because of their potential to be used in radiochemical separations. Previous works have demonstrated, for example, their efficiency for removing and determining trace amounts of Sr-90 in environmental, food, and seawater samples [4-7].

In terms of age-dating applications, Charbonneau et al. have recently reported results for the separation of Co-60 from Ni-60 using both anionic and extraction

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resins [3]. Likewise, Steeb et al. have presented a method to separate Sr-90 from Zr-90 using the Sr Resin [8]. For this last procedure, however, no information has been found regarding the possibility of using cationic, anionic, or DGA resins. Actually, different distribution coefficients available in the literature for both elements suggest that high levels of selectivity could also be achieved using those resins [9-14].

In this context, this work aims to compare the performance of the AG50W-X8, Dowex1-X8, DGA, and Sr Resin and, eventually, propose one or more alternatives to separate Sr-90 and Zr-90 for nuclear forensic applications. To make a good comparison, experimental conditions like mass of resin and volume of eluents have been kept constant to assess recovery and resolution of peaks after chromatographic separations. Assuming that real samples could contain high levels of radioactivity, significant amounts of yttrium-90 could cause isobaric interferences that should not be neglected. For that reason, the possibility to completely isolate Y-90 has also been considered to evaluate the efficiency of resins.

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Chapter 1

1. Radiostrontium

As previously mentioned, potential interest in radiostrontium for nuclear threats is a consequence of peculiar radiological characteristics of Sr-90. Thus, this chapter aims to detail these characteristics, while explaining the radiological risks associated to Sr-90 and its hazardous effects for humans and for the environment.

1.1. Occurrence and radiological properties of strontium-90

Radioactive sources can exist in the environment naturally (i.e. primordial and cosmogenic radionuclides) or via accidental or deliberate anthropogenic activities (Figure 1.1). According to astrophysics theories, primordial radionuclides have been produced in the course of nucleosynthesis and have been presented on Earth from the beginning.

Cosmogenic radionuclides, on the other hand, are continuously produced by the interaction of cosmic irradiation with gases in the atmosphere (e.g. N2, O2, Ar, etc.),

and brought to the earth by rainwater. In general, both primordial and cosmogenic radionuclides contribute to the harmless levels of radioactivity in the environment.

The occurrence of worrisome levels of radioactivity, however, is a consequence of the release of significant amounts of radioisotopes through nuclear tests or nuclear accidents. It has been reported, for example, that about 8,000 TBq of Sr-90 have been released around the Chernobyl area in 1986 causing damage that, even almost 30 years later, still holds the attention of numerous scientists [15,16].

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Figure 1.1 – Brief description of the origin of radioactivity in the environment

As presented in Figure 1.1, the origin of radionuclides in the environment is multifaceted. In the case of Sr-90, it has an anthropogenic origin. Actually, Sr-90 is a by-product of the fission of uranium and plutonium, continuously produced in nuclear power plants. According to the U.S Environmental Energy Agency (EPA), strontium-90 is considered one of the more hazardous constituents of nuclear wastes [17].

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(e.g. halides, oxides, sulphides) and its dispersion through the environment would be strongly influenced by the chemical form and solubility.

In terms of radiological properties (Table 1.1), Sr-90 shows a specific activity of about 140 Ci/g. Comparatively to other threatening radioisotopes, it accumulates more reactivity per unit of mass than Ra-226, Am-241, Pu-238, and Cs-137. Also, Sr-90 has a half-life (i.e. time that takes for the radioactivity to decay to one-half of its original value) of about 29 years, which is longer than the half-life of Cf-252, Co-60, Po-210, and Ir-192.

Table 1.1 – Radiological properties of threatening radionuclides [3]

Radionuclide

Specific

Activity Half-life Decay mode

(Ci/g) (y) Ra-226 1 1600 D Am-241 3.5 430 D Pu-238 17 88 D Cs-137 88 30 E Sr-90 140 29 E Cf-252 540 2,6 D Co-60 1100 5.271 E Po-210 4500 0.4 (140d) D Ir-192 9200 0.2 (74d) E

As presented in Table 1.1, the specific activity is inversely proportional to its half-life, which means that the higher is the specific activity, the shorter is the half-life. In practice, short-lived isotopes are less harmful to the environment than long-lived isotopes as they decay away faster and completely. However, short-lived isotopes can be fatal, once humans have been directly exposed to the high-energy emitted. For Sr-90, which is considered a long-lived isotope, long-term damage is expected due to its slower decay rate that will take years. For those radionuclides, human

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exposure to ionizing radiation occurs over an extended period of time due to the fact that lower but more persistent quantities of radioactivity will remain in the environment [18,19].

1.2. Applications of strontium-90

Currently, controlled amounts of strontium-90 have been extensively used in medicine as radioactive tracers. As illustrated in the Figure 1.2, Sr-90 is a neutron-rich nucleus that, through a decay process, forms yttrium-90, an intermediate decay product that is often used for cancer treatment.

Figure 1.2 – Decay chain of strontium-90

Due to its capacity to produce heat, Sr-90 in the form of strontium titanate (SrTiO3),

has also been widely used in the past for the production of portable power supplies. Known as radioisotope thermoelectric generators (RTGs), these devices have been manufactured to provide energy in remote sites where electricity was quite limited (i.e. navigational beacons, weather stations, and space vehicles).

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1.3. Instability of strontium-90 and the origin of its radioactivity

In general, two factors including nucleus mass and neutron-to-proton ratio (N/Z) contribute to nucleus instability and, in practice, to influence the mode of radiation emitted.

It is normally observed, for example, that heavier nuclei (i.e. usually heavier than Pb) are more likely to emit alpha particles (D), while lighter nuclei tend to achieve stability through the emission of positive beta particles or positrons (E+_{) to}

compensate repulsive forces caused by an excess of protons. Also, it is noticed that when the number of neutrons becomes more important than the number of protons (i.e. increase in the N/Z ratio), it is the emission of negative beta particles (E-_{) or electrons that are rather detected.}

As already mentioned, strontium-90 is an unstable neutron-rich nucleus and for that reason it undergoes E-_{decay, which is generally represented as follows:}

ࢄ ࢆ ࡭ _ื _ࢄ ࢆା૚࡭ + ࢼି ܵݎ ଷ଼ ଽ଴ _{ื ܻ} ଷଽ ଽ଴ _{+ ߚ}ି _{ื ܼݎ} ସ଴ ଽ଴ _{+ ߚ}ି

Thus, to achieve stability, Sr-90 liberates the excess of neutrons in the form of protons and very energetic E-_{particles. Each rearrangement per second}

corresponds to the activity of the radioactive source in Becquerel (Bq) according to SI.

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As indicated above, the proton gives rise to a decay product, in this case, Y-90, an intermediate decay product. Yttrium-90 is also an unstable nucleus and, as for Sr-90, it also undergoes E-_{decay to form Zr-90, which this time is a stable}

non-radioactive isotope.

1.4. Hazardous effects of strontium-90

Major radiological risks and hazardous effects of strontium-90 sources are associated to the energetic contributions of Sr-90 and Y-90 beta particles. As beta-emitters, Sr-90 and Y-90 penetrates the skin, while interacting with cells and discharging their energy that are, respectively, 546 keV and 2280 keV [20].

In practice, strontium-90 absorption in humans can result from direct exposure to radiation, inhalation of fine particles in air or, as in most situations, from the consumption of both contaminated food and water (Figure 1.3).

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Figure 1.3 – Means of uptake and bioaccumulation for strontium-90

Chemically, strontium-90 demonstrates analogue properties with calcium and, once in the organism, it tends to be incorporated in bones and teeth increasing risks of cancer. Actually, a major portion of absorbed strontium-90 is excreted during the first year after exposure with a biological half-life (i.e. the time an organism takes to eliminate one half the amount of a compound or chemical on a strictly biological basis) of 40 days. However, there is about 10% of Sr-90 that is tightly bound to the bones and with a biological half-life of 50 years it is slowly excreted from human’s body [21].

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Chapter 2

2. Nuclear Threats of Sr-90 and Radiochronometry for

Age-Dating Applications

The significant number of incidents involving nuclear and radioactive material has forced authorities not only to increase the control over those materials but also motivated nuclear forensic experts to develop techniques able to provide important radiochemical information for criminal investigations. In this context, this chapter aims to present the terrorist potential involving orphaned sources, including those of Sr-90, as well as to explain the role of nuclear forensics and how radiochronometry could help to determine the origin of a seized source eventually used in nuclear attacks.

2.1. Nuclear threats and risks involving orphaned sources

Despite international’s effort to monitor and regulate the utilization of nuclear and radioactive materials, the number of incidents and illicit trafficking involving them is still significant (Figure 2.1).

Figure 2.1 – Number of nuclear and radioactive incidents reported by the IAEA for the last 7 years

171 243 215 222 172 ₁₆₃ 155 0 50 100 150 200 250 300 2007 2008 2009 2010 2011 2012 2013

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In total, the International Atomic Energy Agency (IAEA) has already reported 2407 incidents from 1995 to 2013, including cases of illegal possession or attempts to sell nuclear or radioactive material, theft or loss, and unauthorized activities apparently without criminal nature [2]. Actually, millions of radioactive sources are available worldwide and inadequate control over usage, storage, and production in different countries seems to contribute to the number of incidents.

One of the biggest issues is probably associated to orphaned sources, that means sources that were abandoned, lost, or misplaced in the past without authorization and, today, are outside of regulatory control. Thousands of radiothermal generators (RTGs) like those using Sr-90 (Chapter 1), for example, have been discovered in the Russia coast containing extremely high levels of radioactivity. Unfortunately, there are about nearly a hundred pieces that have not been yet recovered and remain unprotected against unauthorized interference [22].

In practice, only a few numbers of accidents involving RTGs have been reported (Table 2.1) [23], but authorities do not rule out the risks of nuclear threats resulting from the lower degree of security surrounding these sources. Main concerns started arising after the United States discovered documents in Afghanistan with real intentions of Al Qaeda in developing radiological dispersion devices (RDDs), vulgarly known as dirty bombs [24].

As previously described, dirty bombs consist of conventional explosives combined with a radioactive material. Once detonated, the radioactive material is dispersed, while contaminating the environment, killing, injuring, and exposing people directly to radiation. The degree of damages would depend on many factors like physical and chemical form of the radioactive material, size of explosives, and proximity of

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people to the explosion.

Table 2.1 – Accidents involving RTGs reported by the IAEA

Year Case

1999 A stolen radioactive heat source was found emitting radioactivity at a bus stop in Kingisepp, in Russia. The source was then recovered.

2001 Three radioisotope heat sources were stolen from lighthouses located in the Kandalaksha Bay area, in Russia. After being found, the sources were sent to Moscow.

2001 Three woodsmen have been diagnosed with radiation sickness after finding two unshielded radioactive heat sources near the Inguri River valley, in Georgia. Two victims have experienced nausea, vomiting, and dizziness after hours of exposure to sources of Sr-90 containing about 30,000 Ci. They were treated for many months before recovering from severe radiation burns. The sources were recovered in 2002.

2002 Three shepherds were exposed to high radiation doses after they stumbled upon a number of RTGs in the Tsalenjikha region. Eight generators were recovered.

2003 An RTG was found 200 meters in the shoals of the Baltic Sea, which was recovered later by a team of experts.

2003 The theft of metals from an RTG has been discovered in the White Sea region, in Russia. The six radioactive sources have not been taken.

2.2. Radiochronometry for nuclear forensic applications

Nuclear forensics is the science responsible for providing radiological properties of radioactive sources that could be complementary to other biological, digital, and chemical properties used in criminal investigations.

Assuming, for example, that a terrorist attack takes place and the presence of a nuclear or a radioactive material is confirmed, nuclear forensic experts are put in

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charge to work in conjunction with other forensic sciences to identify the alleged responsible (Figure 2.2).

Figure 2.2 – Forensic Science

In general, radiological information of nuclear or radioactive material includes the appearance, structure, and isotopic composition. As indicated in Table 2.2, an important parameter is the age, which can provide valuable information about the date of fabrication or the last purification [25].

Table 2.2 – Radiological information from nuclear or radioactive materials

Parameter Information

Appearance Material type (powder, pellet)

Dimensions Reactor type

U, Pu content Chemical composition Isotopic composition Enrichment (reactor type)

Impurities Production process, geolocation

18_O/16_{O ratio} _Geolocation

Surface roughness Production plant Microstructure Production process

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To determine the age, radiochronometry is a technique often used in fields such archaeology, anthropology, and geology to date samples like human bones, corals, and other artefacts preserved even over a billions of years. This technique has also been widely used in environmental research for tracing climate changes [26] and recently started receiving increasingly attention in nuclear forensics.

The principle of the radiochronometry technique is based on the fact that activity of a radionuclide decays exponentially with time. According to the decay law, the activity of a radioactive source (ܣ) is a function of three variables: the initial activity of the radioisotope (ܣ_଴), its decay constant (ߣ) also represented by ln2/t1/2 ratio,

and the elapsed time or also called the age (ݐ) in nuclear forensic applications (Equation 2.1).

࡭ = ࡭૙ࢋିࣅ࢚ (2.1)

Here, the age (ݐ) can be isolated in the equation and be expressed in terms of both final and initial activities (Equation 2.2):

࢚ = െ૚_ࣅܔܖ ቀ_࡭࡭

૙ቁ (2.2)

When the radioactive source is unknown, however, it is impossible to know its original activity (Charbonneau, 2012) and a change of variables in the equation becomes necessary. In this case, respectively activities can be converted in units of concentration (ܥ) as follows (equation 2.3):

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࡭ =࡯×ࡺ࡭×࢒࢔૛

ࡹࡹ×࢚૚/૛ (2.3)

Where, ܰ_஺ is the Avogadro’s number, ܯܯ is the molar mass, and ݐ_ଵ/ଶ is half-life.

So, Equation 2.2 can be expressed in terms of final (ܥ ) and initial (ܥ଴) concentrations of the radioactive species (Equation 2.4).

࢚ = െ૚_ࣅܔܖ ቀ_࡯࡯

૙ቁ (2.4)

As in Figure 2.3, as the time passes, the radioactive species (in this case Sr-90) tends to decay at the same time a more stable decay product (Zr-90) is build up.

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Figure 2.3 – Decay process of Sr-90 as function of elapsed time

For that reason, equation 2.4 could also be expressed as:

࢚ = െ૚_ࣅܔܖ ቀ_{[࢘ࢇࢊ࢏࢕ࢇࢉ࢚࢏࢜ࢋ
࢏࢙࢕࢚࢕࢖ࢋ]}[࢙࢚ࢇ࢈࢒ࢋ ࢏࢙࢕࢚࢕࢖ࢋ] ቁ (2.5)

Or, in the case of Sr-90, as:

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Briefly, the decay process as that illustrated for Sr-90 could serve as a chronometer, where the age of an unknown source could be estimated by the determination of respective concentrations of both the radioactive and the stable isotopes at a given time ݐ.

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Chapter 3

3. Analytical Techniques to Quantify Sr-90 and Zr-90

In order to achieve maximum accuracy and precision for age-dating purposes, the analysis of high levels of radioactivity from Sr-90 and trace levels of Zr-90 could be performed using, respectively, liquid scintillation and mass spectrometry techniques. In this chapter, principles, advantages, and/or limitations of these two techniques have been discussed.

3.1. Principles of mass spectrometry

Mass spectrometry is a multi-element technique widely used for obtaining quantitative or qualitative information about a sample containing inorganic or organic material. This technique covers nearly all the elements that are discriminated by their difference in the mass-to-charge ratio (m/z).

Basically, all mass spectrometers are composed of an inlet and an ionization system, a mass analyzer, and a detector (Figure 3.1).

Figure 3.1 – Basic components of ordinary mass spectrometers

Depending on its nature, inlet systems can accommodate samples under solid, liquid, or gas state. In typical setups, liquid samples (usually more homogeneous)

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pass through a nebulizer to transform the sample into an aerosol that is driven towards the ion source.

Established mass spectrometry techniques such as Accelerator Mass Spectrometry (AMS), Thermal Ionization Mass Spectrometry (TIMS), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) have been proved to reduce the amount of sample necessary for the analysis of inorganic compounds. It has been reported that ICP-MS has become a dominating technique especially for the determination of long-lived radionuclides (t1/2 > 10 years) present at trace levels

in different samples (e.g. water, soils, biological, and medical samples) [27,28].

In ICP-MS, for example, inorganic materials are positively ionized the ion source. Usually, the ionization takes place in an inert atmosphere using argon, under a set radio frequency, and a plasma temperature of up to 8,000 K.

Argon is commonly used because its first ionization potential (15.8 eV) is higher than the first ionization potential of almost all other elements (except fluorine, neon, and helium), which ensure the maximum ionization of the elements of interest during analyses. Since the energy required for the second ionization is usually too high for most part of elements, second ionization is less likely to happen.

After elements have been converted into ions, they are sent to the mass analyser that acts as a mass filter to separate different masses. Common mass analysers are called quadrupoles, which are consisted of four rods operating in an oscillating electrical field capable of guiding ions towards the detector (Figure 3.2).

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Figure 3.2 – Quadrupole mass spectrometer [29]

Once the ions reach the detector, they are measured as a current and, then, converted into a series of peaks that form the mass spectrum.

3.1.1. Advantages and disadvantages of MS for the analysis of Zr-90

Mass spectrometry offers some advantages such as short analysis time, low sample consumption, high sensitivity, reduced background interference due to the possibility of using efficient mass analyzers as filters [30], and the ability of discriminating different isotopes.

The major disadvantage, however, consists of isobaric interferences caused by ions or molecules having the same mass-to-charge (m/z) ratio. Actually, maximum resolution provided by typical quadrupoles (less than 5,000) can be not sufficient to avoid peak overlapping and overestimation of the compound of interest.

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For example, isobaric interferences for the analysis of Zr-90 could be caused by the presence of its parent isotopes like Sr-90 and Y-90 [31,32]. In this case, even high-resolution instruments that provide resolutions of about 15,000 would not be sufficient to discriminate between the peaks of those three isotopes (Table 3.1), suggesting the use of a pre-treatment strategy, usually a chromatographic separation prior to analysis to chemically separate them.

Table 3.1 – Minimum resolution required to discriminate isobaric interferences at m/z 90 for the analysis of Zr-90 in MS

Ions Atomic mass _(u) Resolution required _ቀࡾ

ࡿ = ࢓ ο࢓ቁ Sr-90 89.908 29,668 Y-90 89.907 36,772 Zr-90 _89.905 _-

݉ = Mass to be analyzed (In this case, 90 for Zr-90) ο݉ = Difference between two atomic masses

3.1.2. Triple quadrupole mass spectrometers to minimize isobaric interferences

Recently designed, a triple quadruple is a tandem mass spectrometer consisted of two quadrupoles (Q1 and Q3) placed at the two extremities of a reaction cell (Q2). The first quadrupole is normally used as a filter to reduce the number of species entering in Q2 containing a reaction gas such as He, H2, O2 etc. At the end,

products from the reaction cell are driven to Q3 that serves to eliminate remaining interferences and guide only the isotope or compound(s) of interest towards the detector (Figure 3.3).

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Figure 3.3 – Triple quadrupole mass spectrometer mechanism

As for any quadrupole, a reaction cell operates in a radio-frequency (RF) mode. However, RF is usually adjusted to focus ions and favour either a collision or a chemical reaction with the reaction gas.

Using nonreactive gases, collisions are favoured and the species are discriminated by the difference in their kinetic energy [33]. Using highly reactive gases on the other hand, different reactions can take place to produce different polyatomic species that are discriminated according to their different masses (Table 3.2) [34].

Table 3.2 – Typical chemical reactions in reaction cells

Mechanism General Form(s) Advantage

Charge

exchange ܫ

ା_{+ ܩ ՜ ܩ}ା_{+ ܫ} _{x Formation of uncharged interferences}

that are not detected. Proton

transfer

(a) ܫܪା _{+ ܩ ՜ ܩܪ}ା_{+ ܫ}

(b) ܫା_{+ ܩܪ ՜ ܫܪ}ା_{+ ܩ}

x Formation of uncharged interferences that are not detected (a).

x Formation of charged interferences that are heavier than the analyte (b). Adduct

Formation ܣ

ା_{+ ܩ ՜ ܣܩ}ା _{x Formation of analytes heavier than}

the interferences.

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In general, interferences can be converted into non-detectable species or in species of different masses that are shifted from the region of interest. When the interaction with the reaction gas is stronger with the analyte, this last can be converted in a heavier compound to be shifted to a less overladen region of the spectrum.

Briefly, reaction cells technology had been developed not only to improve the performance of mass spectrometers, but also to reduce background or eliminate isobaric interferences impossible to be removed using ordinary instruments or high-resolution devices.

3.1.3. Separation of Sr-90 from Zr-90 using reaction cells

Successful applications using O2 as a reaction gas for solving problems of isobaric

interferences between Zr-90 and Sr-90 have already been reported [31,35-40]. Actually, the formation of zirconium oxide is more likely to happen than the formation of strontium oxide, which makes possible to perform the analysis of Zr at m/z = 106 (ZrO+) rather than m/z = 90 (Zr+).

According to Eiden et al., Zr seems to react at least 200 times faster with oxygen than Sr through addition of O2 into the reaction cell. Theoretical positive enthalpy

YDOXHV ǻHR) for strontium oxides formation suggest that this reaction is less

favourable than that for zirconium oxides6. Also, the covalent bond for ZrO+ seems to be stronger than that for SrO+ due to the differences in both electronic densities and metal–oxygen bond lengths (Table 3.3.).

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Table 3.3 – Theoretical binding properties of Zr and Sr with oxygen atoms [32] Ion ǻHR (M+ + O2 o MO+₎ Bond length (Å) Electronic Density* Experimental Calculated ZrO+ -249 -186 1.74 SrO+ +199 +215 2.35

*Zr and Sr atoms on top; O atom at the bottom

3.2. Analysis of Sr-90 by liquid scintillation

Even if it is possible to analyse radioactive isotopes by mass spectrometry, conventional counting techniques, such as liquid scintillation, are still recommended for radioisotopes like Sr-90 that have high specific activities.

In typical liquid scintillation methods, the radioactive sample is mixed with a cocktail containing a solvent and a soluble scintillator. In the cocktail, the solvent occupies from 60-99% of the total solution while the scintillator, only 0.3-1% [41]. For this reason, beta particles usually transfer their energy first to solvent molecules. The energy passes between solvent molecules until it reaches the scintillator. Once excited, the scintillator releases the absorbed energy in the form of photons that can be easily detected (Figure 3.4).

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Figure 3.4 – Mechanism of energy transfer and detection of beta particles by liquid scintillation [41]

In some cases, however, beta particles are enough energetic (i.e. energy higher than 0.6 MeV) to cause disturbance of adjacent molecules in matter followed by a photon emission that can be detected without the need to introduce a scintillator in the sample. This phenomenon known as Cerenkov effect occurs when a charged particle travels at constant velocity in a medium characterized by its index of refraction markedly larger than 1 at a speed exceeding that of the light in that medium. Actually, in gaseous, liquid, or solid media, the velocity of light will be less than its velocity in a vacuum, and the beta particle will be able to travel in such media at speeds exceeding that of light [42].

In practice, Sr-90 is not enough energetic to produce Cerenkov radiation. In this case, Sr-90 is detected indirectly through the signal emitted by Y-90 eventually present at secular equilibrium in the sample, which means, with the same activity of Sr-90.

As indicated in Figure 3.5 [42], secular equilibrium between Sr-90 and Y-90 is achieved after approximately 20 days. Once activities have been determined, they can be converted in units of mass or concentration using equation 2.3 presented in chapter 2.

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Chapter 4

4. Chromatographic Techniques to Separate Sr-90 and

Zr-90

Due to the importance to separate Sr-90 from Zr-90 to avoid isobaric interferences in mass spectrometry, this chapter presents the potential of both ion exchange and extraction chromatography techniques to be used in the separation of those isotopes. Principles of chromatography, theoretical definitions of distribution coefficients, retention factors, distribution ratios, resolution, and the number of theoretical plates to assess the performance of separation have also been addressed.

4.1. Principles of chromatography

Chromatography is the term used to designate a set of techniques implicating a mobile phase (i.e. gas or liquid) and a stationary phase (i.e. solid and/or liquid) for the separation of mixtures. In practice, the mobile phase carries the sample through the stationary phase that interacts with species in the sample.

Thermodynamically, chromatographic separations consist in an equilibrium process (Figure 4.1) where the number of ions of a given species (݅) in either the mobile phase (ܯ) and in the stationary phase (ܵ) is given by the distribution coefficient (ܭ) (Equation 4.1).

ܭ = ஼೔,ೄ

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Figure 4.1 – Equilibrium in chromatographic separations

In general, species having less affinity with the stationary phase (i.e. lower K values) experience shorter retention times and tend to move faster than those having stronger affinities (i.e. higher K values).

Sometimes, the time by which a component is retarded by the stationary phase is expressed in terms of capacity or retention factor (݇Ԣ) as follows:

݇ᇱ ₌ ஼೔,ೄ௏ೄ

஼೔,ಾ௏ಾ (4.2)

Where, ܥ_௜,ௌ and ܥ_௜,ெ are the component concentration, ܸ_ௌ and ܸ_ெ are the respective volumes of the stationary and mobile phases.

In typical column chromatographic techniques, the sample is usually introduced at the top of a column packed with the stationary phase. Once the mobile phase is poured into the column, compounds in the sample move at different speeds as a consequence of the magnitude of interaction with the stationary phase. Finally, each compound can be recovered in a different fraction. A chromatogram is usually the visual output of a chromatographic separation, where each different peak generated corresponds ideally to a specific compound in the mixture (Figure 4.2).

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Figure 4.2 – In column chromatography technique

4.2. Distribution ratio (D)

The distribution ratio (ܦ) is commonly used to express the distribution of a solute between two phases for a specific mobile phase. According to IUPAC, ܦ corresponds to the ratio of the total concentration of a solute in the extractant phase (ܥ௘) to the total initial concentration (ܥ଴) (Equation 4.3) [43].

ܦ = ஼೐

஼బ (4.3)

In practice, distribution ratios are very useful to compare the degree of selectivity, for example, of a resin for two different species dissolved in the same solvent. The selectivity is usually expressed in terms of separation factors (ߙ), which correspond

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to the ratio of D values of two different compounds that should be separated (Equation 4.4).

ߙ_஺,஻ = ஽ಲ

஽ಳ (4.4)

By convention, ܦ_஺ > ܦ_஻.

4.3. Column performance and efficiency of separation

One way to assess the performance of a chromatographic column is determining, for example, the resolving power or the ability of a column to separate two or more peaks (Equation 4.5).

ܴ_ௌ =ଶ(௧ᇱమି௧ᇱభ)

௪మା௪భ (4.5)

In the above equation, ݐԢ_௜ corresponds to the adjusted retention time (ݐԢ_௜ = ݐ_௜ - ݐ_௠) of each compound and ݓ௜, the respective widths at the base of each peak. ݐ௠ is

usually subtracted and corresponds to the time required for the mobile phase to travel the length of the column without any interaction with the stationary phase.

As demonstrated in Figure 4.3, both ݐԢ௜ and ݓ௜ could be determined experimentally

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Figure 4.3 – Experimental variables to determine resolution in chromatography

Theoretically, a separation is considered complete when ܴௌ > 1.5.

The efficiency of separation, on the other hand, could be assessed through the determination of the number of theoretical plates (ܰ) using equation 4.6:

ܰ = 16 ቀ௧೔

௪೔ቁ

ଶ

(4.6)

Again, ݐ_௜ and ݓ_௜ are respectively, the retention time and the width at the base of the peak for a given compound ݅.

The notion of theoretical plates was introduced in 1941 by Martin and Synge through the Plate Theory that supposes that a chromatographic column contains a large number of imaginary and thin sections called plates within each analyte is found to be at equilibrium between the stationary and mobile phase. As the notion

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of the theoretical plate is now well established and it is applicable to all types of chromatographic columns, it is convenient to express the performance of chromatographic columns in terms of number of theoretical plates [44-46].

More efficient methods are obtained for greater ܰ values. In general, ܰ is associated to the length (ܮ) of the column (Equation 4.7), but can be affected by experimental factors such as: technique of column and sample preparation, solute property, temperature, and flow rate.

ܰ =_ு௅ (4.7)

Here, ܪ is the height equivalent to one theoretical plate.

4.4. Measurement of peak asymmetry

Normally, perfect Gaussian peaks are rarely obtained. In general, peak asymmetry is frequently observed and the main causes include at least one of the following conditions: nature of the packing material, nature of the analytes to be separated, and chromatographic system [47].

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ܣௌ =௕_௔ (Equation 4.8)

where ܾ and ܽ represent, respectively, the back and the front portion at 10% of the peak height (Figure 4.4).

Figure 4.4 – Parameters for the determination of peak asymmetry

In practice, symmetrical peaks have asymmetry factors between 0.9 and 1.2.

4.5. Ion exchange chromatography (IEC)

Among numerous chromatographic techniques, ion exchange chromatography is used to separate ions based on electrostatic interactions between a charged surface and the ionic species in the sample. Repulsive electrostatic forces are expected for charges of the same sign and attractive forces, for opposite signs.

In practice, this technique allows the separation, for example, of anions from cations in a mixture. Counter-ions tend to be attracted to the surface while co-ions tend to be repelled (Figure 4.5).

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Figure 4.5 – Separation of cations and anions by IEC [48]

In a system formed only by counter-ions, the magnitude of interaction with the stationary phase will be proportional to the magnitude of the free charges (ݍ_ଶ) competing for the charge on the surface (ݍଵ) (Equation 4.8).

|ܨ| = ݇௘|௤భ_௥௤మమ| (4.8)

In equation 4.6 ݇_௘ is the Coulomb’s constant and ݎ the distance between ݍ_ଵ and ݍ_ଶ.

Briefly, for a negligible distance (ݎ) in a chromatographic column and a constant value for the surface charge (ݍଵ), ions carrying larger charges tend to be stronger

retained by the stationary phase than smaller charges. In this scenario, separations become possible as the stoichiometric process implicated allows counter-ions to be replaced by equivalent amounts of other counter-ions to preserve electrical

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neutrality of the system [49]. Efficient separations could be achieved through the reversible exchanges of counter-ions at the surface of the stationary phase.

4.5.1. Ion exchange resins

Ion exchange resins are solid materials containing active and charged sites covalently bounded to the stationary phase. Depending on the group attached, those resins can be classified as cationic or anionic resins. Anionic resins carry positive charges and are designed to uptake negative counter-ions, while cationic resins, carrying negative charges, are designed to up take positive counter-ions [30] (Table 4.1).

Table 4.1 – Common commercial IEC resins Resin AG50W-X8 _(Cationic) DOWEX1-X8 _(Anionic) Active site - SO3- - N(CH3)3+

Structure

Normally, active sites are arranged to form cross-linked chains. Resins with high crosslink percentages show a more rigid structure and provide a greater number of active groups. Common resins are usually available from 2% up to 12% or even 16% of crosslink percentage. In practice, performance of resins is mainly affected by crosslink percentage since it has an impact on the degree of selectivity.

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4.6. Extraction chromatography (EXC)

Another chromatographic technique that has been receiving increasingly attention in recent years is the extraction chromatography. This technique combines the selectivity of liquid-liquid extractions with the speed, resolving power, and simplicity of chromatographic procedures.

Figure 4.6 – Schema of extraction chromatography [50]

As presented in figure 4.6, the liquid stationary phase or organic extractant is usually adsorbed on the surface of an inert solid support, usually porous silica or an organic polymer. The nature of the extractant usually determines the selectivity of the resin, but diluents are often employed to change the selective properties of the resin. Two examples of commercial extractants are presented in Table 4.2.

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Table 4.2 – Common commercial EXC resins

Resin Sr DGA

Extractant

18-crown-6 ether N,N,N’,N’-tetra-n-octyldiglycolamide

Extraction chromatography differs from partitioning chromatography because equilibrium takes place between an aqueous solution that corresponds to the mobile phase and an organic solution, in this case, the stationary phase.

Extraction chromatography is also different from ordinary liquid-liquid extractions due to the presence of the solid support that influences both the distribution coefficient (K) and the efficiency of extraction.

4.6.1. Extraction process in EXC

The basis of successful separations in extraction chromatography depends to a great extent on the ability of some species to undergo chemical transformations while other species do not. For example, metals are usually found in aqueous solutions under their ionic form. However, some metals in the presence of ligands can form neutral complexes that can be further solvated in the organic phase.

Different models have already been proposed to describe the overall mechanism and equilibrium processes implicated in the extraction chromatography technique

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[51]. A first model, for example, assumes that the neutral complex is first formed in the aqueous phase (Equation 4.9) and then transferred to the organic phase (Equation 4.10), where the extraction process takes place.

Model 1: Complex formation in aqueous phase

ܯ௔௤ା௓+ ݖܮି௔௤֎ ܯܮ௓,௔௤ (4.9)

ܯܮ_௓,௔௤ ֎ ܯܮ_{௓,௢௥௚} (4.10)

A second model suggests that the species are first transferred to the organic phase (Equations 4.11) under their ionic form and then they form the neutral complex in the organic phase (Equation 4.12) to be extracted.

Model 2: Complex formation in organic phase ܯ_௔௤ା௓_{+ ݖܮ} ௔௤ ି _{֎ ܯ} ௢௥௚ା௓ + ݖܮି௢௥௚ (4.11) ܯ௢௥௚ା௓ + ݖܮ௢௥௚ି ֎ ܯܮ௓,௢௥௚ (4.12)

In both cases, however, the general equation for the extraction process can be expressed as follows:

ܯܮ_{௓,௢௥௚}+ ݕܧ_௢௥௚֎ ܯܮ_௓ܧ_{௬,௢௥௚} (4.13) ܯܮ_௓ܧ_{௬,௢௥௚}֎ ܯܮ_௓ܧ_௬,௔௤ (4.14)

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where, ܯା_{represents a metal, ܮ}ି_{a ligand, ܯܮ the neutral complex formed, and ܧ}

the extractant that usually has an electron donor property.

4.7. IEC and EXC for radiochemical separations and potential applications for Sr-90 and Zr-90

Besides precipitation and solvent extraction, ion exchange chromatography is one of the most traditional methods used for radiochemical separations especially for the separation of actinides. In general, ion exchange chromatography has a multi-element character and usually shows better performance and higher recovery rates than other separation techniques [52, 53].

Earlier studies have demonstrated the possibility, for example, of using IEC resins to isolate fission products to evaluate their toxicity even when they are presented at trace levels in samples [54]. Some studies have also showed the efficiency of using ion exchange resins to separate radiostrontium from a variety of matrix [31, 36, 37].

Specifically for a given Sr-Zr system, Strelow had showed that regardless the acidic conditions, zirconium usually experiences stronger affinity with a cationic resin than numerous other elements, including Sr (Table 4.3) [9, 10].

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Table 4.3 – Distribution ratios (D) for strontium and zirconium in the AG50W-X8 resin

Eluent _Zr0.5 M _Sr _Zr1.0 M _Sr _Zr2.0 M _Sr _Zr3.0 M _Sr _Zr4.0 M _Sr HCl 105 217 7250 60.2 489 17.8 61 10 14.5 7.5 HNO3 104 146 6500 39.2 652 8.8 112 6.1 30.7 4.7

Likewise, zirconium seems to have a stronger affinity with anionic resins while strontium, does not have any affinity in hydrochloric or nitric acid conditions (Figure 4.7) [55].

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Recent studies, however, have demonstrated that extraction chromatography is now starting to compete with ion exchange in many separation problems, including radiochemical applications where trace levels of analytes are eventually implicated.

Most part of those applications has been focused on the separation and analysis of radionuclides in environmental samples [56,62]. However, Maxwell and Culligan have reported the separation performance of extraction resins for urine samples containing actinides and Sr-90 [63]. Likewise, Kim et al. have presented a separation method to isolate Sr-89 and Sr-90 from calcium, barium, and yttrium in milk samples [64].

Currently, there are extraction resins designed to extract specific radionuclides. This is the case of the Sr resin that has been developed to extract strontium while other elements could be easily eluted from the chromatographic column. It has been demonstrated that in a solution of 3M HNO3 - 0.01M oxalic acid, Sr is

completely retained, while Zr, for example, is rapidly eluted from the column [65].

As for ion exchange resins, distribution ratios or also capacity factors for most part of elements in extraction resins have already been reported [14]. As an example, figure 4.8 presents the difference between the capacity factors of strontium and zirconium in the DGA resin. As demonstrated, the potential to separate those two elements using nitric acid solutions becomes possible, as the capacity factor for Sr at 1M HNO3 is at least three times higher than that for Zr. In other words, Sr is

more likely to experience a longer retention time than Zr, while this last can be faster eluted from the column.

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Figure 4.8 – Capacity factor for strontium and zirconium