Separation and Analysis of Sr-90 and Zr

 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
© Ana Paula Zattoni, 2015
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.
iii iv 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 Sr90, but also from Y-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.
v vi Table of Contents
RÉSUMÉ ................................................................................................................ III ABSTRACT ............................................................................................................ V TABLES LIST ........................................................................................................ IX PICTURES LIST .................................................................................................... XI ABBREVIATIONS LIST ....................................................................................... XIII ACKNOWLEDGMENTS ...................................................................................... XIX INTRODUCTION..................................................................................................... 1
1. RADIOSTRONTIUM .............................................................................................................................................. 5 1.1. Occurrence and radiological properties of strontium-­‐90 ........................................................... 5 1.2. Applications of strontium-­‐90 .................................................................................................................... 8 1.3. Instability of strontium-­‐90 and the origin of its radioactivity .................................................. 9 1.4. Hazardous effects of strontium-­‐90 ..................................................................................................... 10 2. NUCLEAR THREATS OF SR-­‐90 AND RADIOCHRONOMETRY FOR AGE-­‐DATING APPLICATIONS .......... 13 2.1. Nuclear threats and risks involving orphaned sources ............................................................. 13 2.2. Radiochronometry for nuclear forensic applications ................................................................ 15 3. ANALYTICAL TECHNIQUES TO QUANTIFY SR-­‐90 AND ZR-­‐90 .................................................................. 21 3.1. Principles of mass spectrometry .......................................................................................................... 21 3.1.1. Advantages and disadvantages of MS for the analysis of Zr-­‐90 ........................................ 23 3.1.2. Triple quadrupole mass spectrometers to minimize isobaric interferences ............... 24 3.1.3. Separation of Sr-­‐90 from Zr-­‐90 using reaction cells ............................................................... 26 3.2. Analysis of Sr-­‐90 by liquid scintillation ............................................................................................. 27 4. CHROMATOGRAPHIC TECHNIQUES TO SEPARATE SR-­‐90 AND ZR-­‐90 .................................................... 31 4.1. Principles of chromatography............................................................................................................... 31 4.2. Distribution ratio (D) ................................................................................................................................ 33 4.3. Column performance and efficiency of separation ...................................................................... 34 4.4. Measurement of peak asymmetry ....................................................................................................... 36 4.5. Ion exchange chromatography (IEC) ................................................................................................ 37 4.5.1. Ion exchange resins ................................................................................................................................ 39 4.6. Extraction chromatography (EXC) ..................................................................................................... 40 4.6.1. Extraction process in EXC .................................................................................................................... 41 4.7. IEC and EXC for radiochemical separations and potential applications for Sr-­‐90 and Zr-­‐90 43 5. EXPERIMENTAL ................................................................................................................................................ 47 5.1. Chemicals ........................................................................................................................................................ 47 5.2. Digestion of SrTiO3 ..................................................................................................................................... 47 5.3. Separation tests ........................................................................................................................................... 48 5.4. Omnifit® glass column preparation .................................................................................................. 49 5.5. Methodology .................................................................................................................................................. 49 vii 5.6. Mass spectrometry analysis .................................................................................................................... 50 5.6.1. Performance of reaction cells to separate strontium from zirconium ........................... 52 5.7. Analysis of Sr-­‐90 by liquid scintillation ............................................................................................. 54 6. RESULTS AND DISCUSSION .............................................................................................................................. 55 6.1. Digestion of SrTiO3 ..................................................................................................................................... 55 6.2. Separation of Sr and Zr using a cation-­‐exchange resin ............................................................. 57 6.3. Resin shrinkage and issues for Zr recovery ..................................................................................... 61 6.3.1. Effect of method downscaling on separation efficiency ........................................................ 62 6.4. Separation of Sr and Zr using an anion-­‐exchange resin ........................................................... 64 6.5. IEC versus EXC for the separation of Sr and Zr ............................................................................. 66 6.6. Addition of HF in samples ........................................................................................................................ 69 6.7. Summary of the efficiency of all resins tested ................................................................................ 72 6.8. Performance of DGA method for the recovery of trace levels of Zr ...................................... 73 6.9. Determining the age of a radiostrontium source ......................................................................... 74 6.10. Potential of reaction cell to separate strontium from zirconium ...................................... 76 CONCLUSIONS .................................................................................................... 81 REFERENCES ...................................................................................................... 83 ANNEXE 1............................................................................................................. 87 viii Tables List
Table 1.1 – Radiological properties of threatening radionuclides ..................
Table 2.1 – Accidents involving RTGs reported by the IAEA ........................
Table 2.2 – Radiological information from nuclear or radioactive materials ..
Table 3.1 – Minimum resolution required to discriminate isobaric
interferences at m/z 90 for the analysis of Zr-90 in MS .................................
Table 3.2 – Typical chemical reactions in reaction cells................................
Table 3.3 – Theoretical binding properties of Zr and Sr with oxygen atoms..
Table 4.1 – Common commercial IEC resins ................................................
Table 4.2 – Common commercial EXC resins...............................................
Table 4.3 – Distribution ratios (D) for strontium and zirconium in the
AG50W-X8 resin ...........................................................................................
Table 5.1 – Instrumental setting for SrTiO3 digestion (Mars 5, Easy
PrepTM vials) ..................................................................................................
Table 5.2 – Acquisition parameters for analysis of Sr and Zr by ICPQQQMS .................................................................................................................
Table 5.3 – Comparison of ionization energies between measured
elements and internal standard .....................................................................
Table 5.4 – Acquisition parameters for the analysis of Sr and Zr using
reaction cell and O2 as reaction gas ..............................................................
Table 5.5 – Acquisition parameters for the analysis of Sr-90 by liquid
scintillation.....................................................................................................
Table 6.1 – Acid mixtures used for SrTiO3 digestion tests ............................
Table 6.2 – Digestion efficiency of SrTiO3 under different acidic conditions..
Table 6.3 – Performance of alternative eluents for Zr ...................................
Table 6.4 – Sample loading volumes according to the mass of dry resin
used for separations......................................................................................
Table 6.5 – Recovery of Zr in DGA Resin according to HNO3/HF ratio in
samples .........................................................................................................
Table 6.6 – Summary of resins performance to isolate Zr prior MS
analyses ........................................................................................................
P.7
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P.16
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P.72 ix x Pictures List
Figure 1.1 – Brief description of the origin of radioactivity in the
environment ..................................................................................................
Figure 1.2 – Decay chain of strontium-90 .....................................................
Figure 1.3 – Means of uptake and bioaccumulation for strontium-90 ............
Figure 2.1 – Number of nuclear and radioactive incidents reported by the
IAEA for the last years ..................................................................................
Figure 2.2 – Forensic Science.......................................................................
Figure 2.3 – Decay process of Sr-90 as function of elapsed time .................
Figure 3.1 – Basic components of ordinary mass spectrometers ..................
Figure 3.2 – Quadrupole mass spectrometer ................................................
Figure 3.3 – Triple quadrupole mass spectrometer mechanism....................
Figure 3.4 – Mechanism of energy transfer and detection of beta particles
by liquid scintillation ......................................................................................
Figure 3.5 – Growth rate of Y-90 and secular equilibrium with Sr-90 ............
Figure 4.1 – Equilibrium in chromatographic separations .............................
Figure 4.2 – In column chromatography technique .......................................
Figure 4.3 – Experimental variables to determine resolution in
chromatography ............................................................................................
Figure 4.4 – Parameters for the determination of peak asymmetry...............
Figure 4.5 – Separation of cations and anions by IEC ..................................
Figure 4.6 – Schema of extraction chromatography......................................
Figure 4.7 – D values for strontium and zirconium in the Dowex 1-X10
resin ..............................................................................................................
Figure 4.8 – Capacity factor for strontium and zirconium in the DGA resin ...
Figure 5.1 – AF Omnifit® Column Design .....................................................
Figure 5.2 – Method applied for separation tests ..........................................
Figure 6.1 – Reproducibility of SrTiO3 digestion using HNO3/HF mixture .....
Figure 6.2 – Elution profile of Sr and Zr in 4M HCl (10 g AG50W-X8, 100200 mesh) .....................................................................................................
Figure 6.3 – Elution profile of Sr and Zr in 3M HCl (10 g AG50W-X8, 100200 mesh) .....................................................................................................
Figure 6.4 – Separation of Sr and Zr using a 2M to 6M HCl gradient (10 g
AG50W-X8, 100-200 mesh) ..........................................................................
Figure 6.5 – Elution curves of Sr as function of HCl molarity (10 g
AG50W-X8, 100-200 mesh) ..........................................................................
Figure 6.6 – Elution curves of Sr at 2M HNO3 and 2M HCl (10 g AG50WX8, 100-200 mesh)........................................................................................
Figure 6.7 – Separation of Sr and Zr using a 2M HNO3 to 6M HCl
gradient in 2 g AG50W-X8 (100-200 mesh) ..................................................
Figure 6.8 – Volume of eluent for Sr elution as function of mass of
AG50W-X8 ....................................................................................................
Figure 6.9 – Separation efficiency for Sr and Zr using Dowex1-X8 resin ......
P.6
P.8
P.11
P.13
P.16
P.19
P.21
P.23
P.25
P.28
P.29
P.32
P.33
P.35
P.37
P.38
P.40
P.44
P.46
P.49
P.50
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P.58
P.59
P.59
P.60
P.61
P.63
P.63
P.65
xi Figure 6.10 – Zirconium retention in Dowex1-X8 as function of HCl
concentration ................................................................................................
Figure 6.11 – Maximum recovery of Zr according to HCl concentration in
Dowex1-X8 ...................................................................................................
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) ..................................................................................................
Figure 6.13 – Proposed extraction mechanism for Sr for its separation
from Zr by EXC .............................................................................................
Figure 6.14 – Tailing effect as a function of Sr concentration (AG50W-X8) .
Figure 6.15 – Separation of Sr and Zr using Dowex1-X8 for samples
containing HF ...............................................................................................
Figure 6.16 – Separation of Sr and Zr using DGA for samples containing
HF (a. 0.01%, b. 0.2%) .................................................................................
Figure 6.17 – Complete methodology to separate Sr and Zr using DGA
resin ..............................................................................................................
Figure 6.18 – Comparative between experimental and expected results
for the recovery of trace levels of Zr using DGA resin ..................................
Figure 6.19 – Procedure for determining the age of a radiostrontium
source ...........................................................................................................
Figure 6.20 – Comparative between theoretical and experimental
concentrations for the analysis of Sr-90 by liquid scintillation .......................
Figure 6.21 – Zr and Sr oxides formation in mass spectrometry as
function of O2 concentration in the reaction cell ............................................
Figure 6.22 – Predominant species of Zr (a) and Sr (b) at 6% O2 in the
reaction cell ..................................................................................................
Figure 6.23 – Correlation between results for the analysis of Zr at m/z 90
and m/z 106 ..................................................................................................
Figure 6.24 – Predominant species of Y at 6% O2 in the reaction cell ..........
xii P.65
P.66
P.67
P.68
P.69
P.70
P.71
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P.74
P.75
P.76
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P.79 Abbreviations List
D – Alpha particles
D – Separation factor
E – Beta particles
ߣ – Decay constant
a – asymmetry portion of a peak
‫ – ܣ‬Final activity
‫ܣ‬଴ – Initial activity
‫ܣ‬ௌ – Peak asymmetry
A+ – Charged analyte
AG50W-X8 – Cationic resin
AMS – Accelerator Mass Spectrometry
b – Back portion of a peak
‫ – ܥ‬Final 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
‫ – ܧ‬Extractant
EPA – Environmental Energy Agency
EXC – Extraction chromatography
F – Force
F- – Fluoride
G – Gas
G+ – Charged gas
‫ – ܪ‬Height 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
i.d – Internal diameter
xiii 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
‫ – ܮ‬Length
‫ – ܮ‬Ligand
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
‫ – ܯܯ‬Molar 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
‫ – ݍ‬Charge
Q – Quadrupole
ܴௌ – Resolution
‫ – ݎ‬Distance 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
SrO+ – Charged strontium oxide
xiv SrTiO3 – Strontium Titanate
‫ – ݐ‬Age 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
ZrO+ – Charged zirconium oxide
xv xvi “Science can only be created by those who are thoroughly imbued with the
aspiration toward truth and understanding
”
x Albert Einstein
xvii xviii 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.
xix xx 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 highenriched 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.
1 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.
In practice, the age of a radioactive source could be determined using principles of
2 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 [Zr90]/[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
3 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.
4 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].
5 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].
As for any other isotopes of strontium, Sr-90 can form other chemical compounds
6 (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, Co60, Po-210, and Ir-192.
Table 1.1 – Radiological properties of threatening radionuclides [3]
Specific
Half-life
Radionuclide
Activity
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 halflife, 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
7 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 neutronrich 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).
8 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:
࡭
ࢆࢄ
ଽ଴
ଷ଼ܵ‫ݎ‬
ื
ื
ଽ଴
ଷଽܻ
࡭
ࢆା૚ࢄ
+ ࢼି
+ ߚି ื
ଽ଴
ସ଴ܼ‫ݎ‬
+ ߚି
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.
9 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 Sr90, it also undergoes E- decay to form Zr-90, which this time is a stable nonradioactive 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 betaemitters, 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).
10 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].
11 12 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).
300
243
250
200
215
222
171
172
163
155
2011
2012
2013
150
100
50
0
2007
2008
2009
2010
Figure 2.1 – Number of nuclear and radioactive incidents
reported by the IAEA for the last 7 years
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
14 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
15 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
Appearance
Dimensions
U, Pu content
Isotopic composition
Impurities
18
O/16O ratio
Surface roughness
Microstructure
Age
16 Information
Material type (powder, pellet)
Reactor type
Chemical composition
Enrichment (reactor type)
Production process, geolocation
Geolocation
Production plant
Production process
Date of production or last purification
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 (‫ )ܣ‬is 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 (‫ )ݐ‬in nuclear forensic applications
(Equation 2.1).
࡭ = ࡭૙ ࢋିࣅ࢚
(2.1)
Here, the age (‫ )ݐ‬can be isolated in the equation and be expressed in terms of both
final and initial activities (Equation 2.2):
૚
࡭
࢚ = െ ࣅ ‫ ܖܔ‬ቀ࡭ 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 (‫ )ܥ‬as follows (equation 2.3):
17 ࡭=
࡯×ࡺ࡭ ×࢒࢔૛
ࡹࡹ×࢚૚/૛
(2.3)
Where, ܰ஺ is the Avogadro’s number, ‫ ܯܯ‬is the molar mass, and ‫ݐ‬ଵ/ଶ is half-life.
So, Equation 2.2 can be expressed in terms of final ( ‫ ) ܥ‬and initial ( ‫ܥ‬଴ )
concentrations of the radioactive species (Equation 2.4).
૚
࡯
࢚ = െ ࣅ ‫ ܖܔ‬ቀ࡯ 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.
18 Figure 2.3 – Decay process of Sr-90 as function of elapsed time
For that reason, equation 2.4 could also be expressed as:
૚
[࢙࢚ࢇ࢈࢒ࢋ ࢏࢙࢕࢚࢕࢖ࢋ]
࢚ = െ ࣅ ‫ ܖܔ‬ቀ[࢘ࢇࢊ࢏࢕ࢇࢉ࢚࢏࢜ࢋ ࢏࢙࢕࢚࢕࢖ࢋ]ቁ
(2.5)
Or, in the case of Sr-90, as:
૚
[ࢆ࢘ିૢ૙]
࢚ = െ ࣅ ‫ ܖܔ‬ቀ [ࡿ࢘ିૢ૙]ቁ
(2.6)
19 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.
20 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)
21 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).
22 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.
23 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
Resolution required
Atomic mass
࢓
Ions
(u)
ቀࡾࡿ =
ቁ
ο࢓
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).
24 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
‫ ܫ‬+‫ ܩ‬՜‫ ܩ‬+‫ܫ‬
x Formation of uncharged interferences
exchange
that are not detected.
Proton
transfer
(a) ‫ ܪܫ‬ା + ‫ ܩ‬՜ ‫ ܪܩ‬ା + ‫ܫ‬
(b) ‫ ܫ‬ା + ‫ ܪܩ‬՜ ‫ ܪܫ‬ା + ‫ܩ‬
Adduct
Formation
‫ܣ‬ା + ‫ ܩ‬՜ ‫ ܩܣ‬ା
x Formation of uncharged interferences
that are not detected (a).
x Formation of charged interferences
that are heavier than the analyte (b).
x Formation of analytes heavier than
the interferences.
‫ = ܫ‬Interference, ‫ = ܩ‬Reaction gas, ‫ = ܣ‬Analyte
25 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.).
26 Table 3.3 – Theoretical binding properties of Zr and Sr with oxygen
atoms [32]
ǻHR
+
(M + O2 o MO+)
Bond length
Electronic
Ion
(Å)
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).
27 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.
28 Figure 3.5 – Growth rate of Y-90 and secular equilibrium with Sr-90
29 30 Chapter 4
4. Chromatographic Techniques to Separate Sr-90 and Zr90
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 (‫ ) ܯ‬and in the stationary phase (ܵ ) is given by the distribution
coefficient (‫( )ܭ‬Equation 4.1).
஼
‫ = ܭ‬஼ ೔,ೄ
೔,ಾ
(4.1)
31 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).
32 Figure 4.2 – In column chromatography technique
4.2. Distribution ratio (D)
The distribution ratio (‫ )ܦ‬is 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
33 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(௧ᇱమ ି௧ᇱభ )
௪మ ା௪భ
(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
through the chromatogram obtained for a given chromatographic separation.
34 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:
௧
ܰ = 16 ቀ௪೔ 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
35 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 (‫ )ܮ‬of 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, ‫ ܪ‬is 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].
As defined by Bayne, peak asymmetry can be expressed as:
36 ௕
‫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).
37 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 (‫ݍ‬ଶ )
competing for the charge on the surface (‫ݍ‬ଵ ) (Equation 4.8).
|‫݇ = |ܨ‬௘
|௤భ ௤మ |
௥మ
(4.8)
In equation 4.6 ݇௘ is the Coulomb’s constant and ‫ ݎ‬the distance between ‫ݍ‬ଵ and ‫ݍ‬ଶ .
Briefly, for a negligible distance (‫ )ݎ‬in 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
38 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.
39 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.
40 Table 4.2 – Common commercial EXC resins
Resin
Sr
DGA
18-crown-6 ether
N,N,N’,N’-tetra-n-octyldiglycolamide
Extractant
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
41 [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௔௤
+
‫ିܮݖ‬௔௤
‫ܮܯ‬௓,௔௤
֎ ‫ܮܯ‬௓,௔௤
֎ ‫ܮܯ‬௓,௢௥௚ (4.9)
(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௔௤
+ ‫ିܮݖ‬௔௤ ֎ ‫ܯ‬௢௥௚
+ ‫ିܮݖ‬௢௥௚
ା௓
ି
‫ܯ‬௢௥௚ + ‫ܮݖ‬௢௥௚ ֎ ‫ܮܯ‬௓,௢௥௚ (4.11)
(4.12)
In both cases, however, the general equation for the extraction process can be
expressed as follows:
42 ‫ܮܯ‬௓,௢௥௚ + ‫ ܧݕ‬ ௢௥௚ ֎ ‫ܮܯ‬௓ ‫ܧ‬௬,௢௥௚
(4.13)
‫ܮܯ‬௓ ‫ܧ‬௬,௢௥௚ ֎ ‫ܮܯ‬௓ ‫ܧ‬௬,௔௤
(4.14)
where, ‫ܯ‬ା represents a metal, ‫ ିܮ‬a ligand, ‫ ܮܯ‬the 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 multielement 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].
43 Table 4.3 – Distribution ratios (D) for strontium and zirconium
in the AG50W-X8 resin
0.5 M
1.0 M
2.0 M
3.0 M
4.0 M
Eluent
Zr
Sr
Zr
Sr
Zr
Sr
Zr
Sr
Zr
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].
Figure 4.7 – D values for strontium and zirconium in the Dowex 1-X10 resin
44 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.
45 Figure 4.8 – Capacity factor for strontium and zirconium
in the DGA resin [66]
46 Chapter 5
5. Experimental
This chapter describes the methodology and list the materials used throughout the
present work in order to determine the best condition to separate and quantify
strontium and zirconium for age-dating applications.
5.1. Chemicals
Certified standard solutions (PlasmaCal ICP/ICPMS, 4% HNO3) of strontium,
zirconium, and yttrium have been purchased from SPC Science. Stock solutions
for separation tests and calibration curves have been prepared using high-purity
deionized water (18.2 M:*cm) from Millipore Bedford, MA, USA, and
environmental grade acids (Anachemia Science). All solutions have been
conserved at 4°C in centrifuge polypropylene tubes until their utilization.
5.2. Digestion of SrTiO3
SrTiO3 (Aldrich, 99%) has been used for digestion tests to simulate true solid
samples containing radiostrontium in the titanate form. The digestion protocol has
been adapted from Parker et al [67]. The tests have been performed under highpressure (Easy PrepTM vials) using a microwave oven (Mars 5) from CEM
Corporation with temperature and pressure controls. The digestion program is
presented in Table 5.1.
47 Table 5.1 – Instrumental setting for SrTiO3 digestion
(Mars 5, Easy PrepTM vials)
Stage
1
2
Power
Max
1600
1600
Ramp
%
100
100
Min
30
30
Pressure Temperature
Psi
250
250
°C
160
200
Hold
Min
5
40
Precise masses of SrTiO3 were first weighed in 5 mL Teflon vials and then diluted
in concentrated nitric acid before being transferred to the microwave vessels. Poor
recoveries have been observed when SrTiO3 was weighed directly in the
microwave vessels. Actually, the strong electrostatic interactions between SrTiO3
and the surface of microwave vessels resulted in significant losses of the product
during sample preparation.
5.3. Separation tests
Four different commercial resins (AG50W-X8, DOWEX1-X8, Sr Resin, and DGA)
have been tested to separate strontium and zirconium. The performance of these
resins has been assessed using a glass column from Omnifit®. In some cases,
pre-packed columns (Eichrom) have also been applied to compare or validate
results. Different HCl, HNO3, H2SO4, and H2C2O4 solutions at different molarities
have been tested as eluents. Before elution, mixtures containing strontium,
zirconium, and eventually yttrium that would be expected to be present in real
samples were evaporated to dryness and then diluted in the appropriate solvent
that could contain traces of HF depending on the nature of the test.
48 5.4. Omnifit® glass column preparation
The Omnifit® glass column was dismantled following manufacturer’s instructions
and washed using a laboratory detergent. The column was then well rinsed and
packed with the resin suspended in water. A vacuum was applied to slowly drain
the excess of water. Dryness was avoided to prevent air bubbles in the column. A
plunger and an adjusting nut (Figure 5.1) at the upper side of the column helped to
compact the resin. Strong compaction has shown to affect flow rate. Finally, the
column was connected to a peristaltic pump and flow rate was set to 2 mL/min.
Figure 5.1 – AF Omnifit® Column Design
5.5. Methodology
After packing, resins have been washed with water and then conditioned with an
acid solution, usually the eluent used to elute the first component from the column.
Before injecting the mixture to be separated, a blank was recovered. It consisted of
49 the same eluent used for the conditioning step. After sample injection, strontium
and zirconium were recovered in polypropylene centrifuge tubes using the
appropriate eluent (Figure 5.2).
Figure 5.2 – Method applied for separation tests
5.6. Mass spectrometry analysis
Analyses of fractions recovered during separation tests were performed in a triple
quadrupole mass spectrometer (Agilent 8800). After the separation, fractions were
evaporated to dryness to eliminate any trace of HF possibly present that could
damage glass pieces in the instrument. Fractions were then diluted with 4% HNO3
as much as necessary to fit the concentration within the quantification range. Table
50 5.2 summarizes optimal acquisition settings for the quantification purpose.
Table 5.2 – Acquisition parameters for analysis of Sr and Zr by
ICPQQQ-MS
Scan Type
Single Quad
Plasma Mode
Hot
Lenses
Extract 1
Extract 2
Omega Bias
Omega Lens
Q1 Entrance
Q1 Exit
Cell Focus
Cell Entrance
Cell Exit
Deflect
Plate Bias
Q1
Q1 Bias
Q1 Prefilter Bias
Q1 Postfilter Bias
Q1 Ion Guide
SLS Factor
SLG
Cell
Gas mode
OctP Bias
OctP RF
Energy Discrimination
Wait Time Offset
Wait Time Offset
Spectrum mode options
Replicates
Sweeps
5.3 V
-225.0 V
-200 V
28.0 V
3.0 V
-2.0 V
1.0 V
-50 V
-50 V
15.2 V
-50 V
-6.0 V
-20.0 V
-30.0 V
0.40
0.90 V
No gas
-8.0 V
180 V
5.0 V
0 msec
3
100
Strontium and zirconium recoveries have been monitored respectively at m/z 88
and m/z 90. Blanks and standard solutions were used to ensure quality control of
results. Signal fluctuations from the instrument were corrected through addition of
51 Indium as internal standard. According to Kozuka et al., ideal internal standards
should have ionization energies close to that for measured elements [68]. Table
5.3 presents the ionization energy for strontium, zirconium, and indium.
Table 5.3 – Comparison of ionization energies between
measured elements and internal standard
Energy
(eV)
Element
Strontium
5.69
Zirconium
6.63
Indium (internal standard)
5.79
Both the detection limit (‫ )ܦܱܮ‬and the quantification limit (‫ )ܱܳܮ‬were estimated
through the determination of the standard deviation (‫ )ݏ‬obtained for the analysis of
10 blanks in 4% nitric acid (Equation 5.1 and 5.2) .
‫ = ܦܱܮ‬3‫ݏ‬
(5.1)
‫ = ܱܳܮ‬10‫ݏ‬
(5.2)
5.6.1. Performance of reaction cells to separate strontium from zirconium
The potential to use oxygen in reaction cells to separate strontium and zirconium
has been assessed through the analysis of standard solutions at different gas
compositions. In this case, strontium has been monitored at m/z 88 (Sr+) and m/z
104 (SrO+) as well as zirconium at m/z 90 (Zr+) and m/z 106 (ZrO+). The interaction
between yttrium and oxygen has also been evaluated and both m/z 89 and m/z
52 105 have also been monitored. The acquisition parameters used during reactioncell tests are presented in Table 5.4.
Table 5.4 – Acquisition parameters for the analysis of Sr and Zr
using reaction cell and O2 as reaction gas
Scan Type
MS/MS
Plasma Mode
Hot
Lenses
Extract 1
Extract 2
Omega Bias
Omega Lens
Q1 Entrance
Q1 Exit
Cell Focus
Cell Entrance
Cell Exit
Deflect
Plate Bias
Q1
Q1 Bias
Q1 Prefilter Bias
Q1 Postfilter Bias
Cell
Gas mode
4th Gas Flow (O2)
OctP Bias
OctP RF
Energy Discrimination
Wait Time Offset
Wait Time Offset
Spectrum mode options
Replicates
Sweeps
5.3 V
-225.0 V
-200 V
28.0 V
3.0 V
-2.0 V
1.0 V
-50 V
-50 V
15.2 V
-50 V
-4.0 V
-22.0 V
-20.0 V
Use gas
0 – 100 %
-8.0 V
180 V
5.0 V
60 msec
3
100
53 5.7. Analysis of Sr-90 by liquid scintillation
For separation tests implicating a solution of radiostrontium (NIST Standard, 30
Bq/mL, 14/04/2000), recovered fractions have been diluted in water and then
analysis of Sr-90 has been conducted by liquid scintillation using a Perkin Elmer
Tri-Carb 2900TR instrument. Acquisition parameters are listed in Table. 5.5.
Table 5.5 – Acquisition parameters for the
analysis of Sr-90 by liquid scintillation
Quench Indicator
tSIE/AEC
External Std Terminator (sec)
0.5 2s%
Pre-Count Delay (min)
0.00
Quench Set
n/a
Count Time (min)
240.00
Count Mode
Normal
Assay Count Cycles
1
Repeat Sample Count
1
#Vials/Sample
1
Calculate % Reference
Off
Background Subtract
Off
Low CPM Threshold
Off
2 Sigma % Terminator
Off
LLa 0.0
Region A (keV)
ULb 50.0
LL
0.0
Region B(keV)
UL
100.0
LL
0.0
Region C(keV)
UL
2000.0
Counting corrections
Static Controller
Luminescence Correction
Colored Samples
Heterogeneity Monitor
Coincidence Time (nsec)
Delay Before Burst (nsec)
Half Life Correction
a. lower limit
b. Upper limit
54 On
Off
n/a
n/a
18
75
Off
Chapter 6
6. Results and Discussion
This chapter highlights the performance of the ion exchange and extraction resins
for the separation of strontium and zirconium. It also presents the potential of using
a reaction cell in mass spectrometry to eliminate the need for a prior separation by
chromatography. A method for SrTiO3 digestion has also been suggested as a
sample preparation step for solid sources of strontium-90.
6.1. Digestion of SrTiO3
Due to its refractory character, strontium titanate is a compound usually very
difficult to decompose. For this reason, four different mixtures with different H2O2,
HNO3, and HF acid ratios were tested to assess the best condition to achieve
successful SrTiO3 digestion (Table 6.1). As reported by Packer et al., addition of
HF would help the solubilisation of Ti while H2O2 would contribute to reduce the
formation of N2O4 as well as to provide a cloudless solution [67].
Table 6.1 – Acid mixtures used for SrTiO3 digestion tests
HNO3
H2O2
HF
Mixture ID
(% v/v)
(% v/v)
(% v/v)
1
100
Absent
Absent
2
89
11
Absent
3
84
11
5
4
95
Absent
5
As presented in Table 6.2, efficient digestion has been obtained using mixtures
containing HF. Even if concentrated nitric acid is considered as a strong oxidizing
agent, addition of HF is usually necessary for complete dissolution of oxide
compounds [69].
55 Table 6.2 – Digestion efficiency of SrTiO3 under different acidic conditions
HNO3
Mixture 1
(%)
HNO3/H2O2
Mixture 2
(%)
HNO3/HF/H2O2
Mixture 3
(%)
HNO3/HF
Mixture 4
(%)
92
101
100
101
98
96
98
94
97
92
97
109
Average
95 ± 3
97 ± 4
98 ± 1
101 ± 8
Ti
0
31 ± 8
96 ± 1
96 ± 6
Element
Sr
The presence of H2O2 does not improve the performance of SrTiO3 digestion.
Likewise, the appearance of the SrTiO3 solution after digestion in HNO3/HF
demonstrates that samples containing approximately 10,000 mg/L of SrTiO3 were
very transparent, which has made the usage of H2O2 unnecessary to obtain
satisfactory results.
In general, digestion using only HNO3/HF has not only been proven to be the most
convenient and efficient method and also a very reproducible approach. For nine
replicates, obtained recovery for Sr was 102 r 2 %, while for Ti, 97 r 1 %, which
means that up to 3% of titanium could not be dissolved under those conditions
(Figure 6.1).
56 % in solution
110
100
90
80
70
60
50
40
30
20
10
0
Sr
Ti
1
2
3
4
5
6
Replicate ID
7
8
9
Figure 6.1 – Reproducibility of SrTiO3 digestion using HNO3/HF mixture
Due to the difficulty to generate SrTiO3 pellets in the laboratory, digestion tests
have been performed using only SrTiO3 under the powder form.
6.2. Separation of Sr and Zr using a cation-exchange resin
The method to separate zirconium from strontium using the cationic AG50W-X8
resin has been adapted from the method proposed by Strelow [70]. A similar
method has been already proven to provide good reproducibility making possible
its application in geochronology work for age determination [71].
Preliminary tests have been performed using about 10 g of AG50W-X8 resin, 100200 mesh, H+ form (Eichrom) poured into a 15 i.d. X 100 mm glass column.
Experimental elution curves have been obtained using weighed solutions
!#
containing equivalent amounts of Sr and Zr (50 µg/mL) eluted in 2, 3 and 4M HCl.
Molarities lower than 2M have been disregarded due to the high distribution ratio
values reported in the literature [9]. In practice, extremely long retention times
would be expected for these conditions.
As presented in Figure 6.2, incomplete separation has been achieved using 4M
HCl and separation factor (D) of about 2 seemed not to be sufficient to obtain
Recovery (%)
complete resolution of peaks.
50
45
40
35
30
25
20
15
10
5
0
Sr
Zr
0
10
20
30
40
50
60
70
80
90
100
4M HCl (mL)
Figure 6.2 – Elution profile of Sr and Zr in 4M HCl
(10 g AG50W-X8, 100-200 mesh)
Although significant improvement has been obtained for a separation factor of 6
using 3M HCl, incomplete separation has also been observed (Figure 6.3).
58 Recovery (%)
50
45
40
35
30
25
20
15
10
5
0
Sr
Zr
0
50
60
70
80
90
100 110 120 130 140 150
3M HCl (mL)
Figure 6.3 – Elution profile of Sr and Zr in 3M HCl
(10 g AG50W-X8, 100-200 mesh)
Briefly, 2M HCl has shown to be the most efficient condition to separate Sr and Zr
among the three scenarios tested (Figures 6.4). A separation factor of 27 has been
50
45
40
35
30
25
20
15
10
5
0
2M HCl
6M HCl
Sr
Zr
0
50
100
110
120
130
140
150
160
170
180
190
200
250
300
350
400
450
500
550
Recovery (%)
calculated in this case.
HCl (mL)
Figure 6.4 – Separation of Sr and Zr using a 2M to 6M HCl gradient
(10 g AG50W-X8, 100-200 mesh)
59 Apparently, in 2M HCl Zr is completed retained in the column during Sr elution and
Zr removal was only possible after the addition of 6M HCl. Actually, the increase in
solvent concentration increased the number of H+ ions in the system and to
displace zirconium that was then easily stripped off the column.
As presented in Figure 6.5, the usage of 2M HCl, however, resulted in longer
retention times and broader peaks for Sr. Likewise, the volume of eluent required
to completely elution of Sr increased from about 80 mL to about 200 mL.
45
40
35
4M HCl
Sr (%)
30
3M HCl
25
2M HCl
20
15
10
5
0
0
20
40
60
80
100 120 140 160 180 200
HCl (mL)
Figure 6.5 – Elution curves of Sr as function of HCl molarity
(10 g AG50W-X8, 100-200 mesh)
Since the distribution ratio of Sr in nitric acid eluent was theoretically two times
smaller than for HCl (Table 4.3), better performance has been obtained by
replacing HCl by HNO3. As demonstrated in Figure 6.6, 2M HNO3 eluent produced
narrower peaks and it shifted the maximum peak from about 150 mL to about 80
mL.
60 25
Sr (%)
20
15
2M HCl
10
2M HNO3
5
0
0
50
100
150
200
Eluent (mL)
Figure 6.6 – Elution curves of Sr at 2M HNO3 and 2M HCl
(10 g AG50W-X8, 100-200 mesh)
250
6.3. Resin shrinkage and issues for Zr recovery
In general, poor recoveries for Zr have been obtained using the AG50W-X8 resin
probably resulting from problems of resin shrinkage after increasing the eluent
concentration from 2M to 6M. To avoid sudden changes that could disturb
equilibrium in the column, the usage of stronger eluents at low molarities was also
assessed. Higher cross-linking resins, however, could also help reducing shrinkage
issues.
Tests have demonstrated that sulphuric and oxalic acids could be used as
alternative eluents of Zr. Based on results presented in Table 6.3, 2M H2SO4, 3M
H2SO4, and 0.2M H2C2O4 have provided both peak asymmetry and reasonable
recovery.
61 Table 6.3 – Performance of alternative eluents for Zr
Peak Asymmetry
Zr Recovery
(%)
2M H2SO4
1.2
104 ± 3
3M H2SO4
1.0
101 ± 2
0.2M H2C2O4
1.2
123 ± 3
Eluent
Utilization of oxalic and sulphuric acids, however, has been limited to elution tests
involving high concentrations of Zr where the evaporation steps were not required
to concentrate the analyte. Due to the high boiling point of H2SO4, the utilization of
Teflon vials became impracticable and glass beakers have been avoided since
they could contain significant amounts of Zr able to contaminate the samples.
6.3.1. Effect of method downscaling on separation efficiency
As presented in Figure 6.7, efficient separation of strontium and zirconium was
successfully achieved using a smaller column bed containing approximately 2 g of
AG50W-X8 resin.
62 70
2M HNO3
Recovery (%)
60
6M HCl
50
40
Sr
30
Zr
20
10
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Eluent (mL)
Figure 6.7 – Separation of Sr and Zr using a 2M HNO3 to 6M HCl gradient in 2
g AG50W-X8 (100-200 mesh)
Among the advantages, downscaling allowed to reduce the volume of eluent
required and consequently elution time for complete separation. As presented in
Figure 6.8, the volume of 2M HNO3 required for the elution of Sr was decreased
from about 150 mL to about 20 mL.
150
2M HNO3 (mL)
130
110
90
70
50
30
10
10g
2g
1g
0.5g
AG50W-X8 (g)
Figure 6.8 – Volume of eluent for Sr elution as function of mass of AG50W-X8
"
Reducing column beds, however, has demanded the adjustment of the volume of
loaded sample in order to prevent extra-column effects [72]. In general, compatible
loading volumes have been proved to ensure peak shape and avoid peak overlap.
Suggested loading volumes according to the mass of resin used are presented in
Table 6.4.
Table 6.4 – Sample loading volumes according to the mass
of dry resin used for separations
Mass of resin
(g)
Sample
(µL)
2
1
0.5
1000
500
250
6.4. Separation of Sr and Zr using an anion-exchange resin
As demonstrated in Figure 6.9 satisfactory separation has also been achieved
using the Dowex1-X8 resin (Acros Organics, 100-200 mesh).
64 Recovery (%)
120
110
100
90
80
70
60
50
40
30
20
10
0
11M HCl
4M HCl
Sr
Zr
0
5
10
15
20 25 30
HCl (mL)
35
40
45
50
Figure 6.9 – Separation efficiency for Sr and Zr using Dowex1-X8 resin
In agreement with the literature, separation was possible as Zr uptake increased
with HCl concentration while strontium that does not have any affinity with the resin
was immediately eluted from the column [55]. As presented in Figure 6.10,
maximum Zr uptake could be obtained at 11M HCl.
Zr uptake (%)
100
90
80
70
60
50
8
9
10
11
HCl (M)
Figure 6.10 – Zirconium retention in Dowex1-X8 as function of HCl
concentration
"!
At the end, 4M HCl has been proved to be the best condition for Zr elution. Under
this condition, however, recoveries higher than 100% have been observed, which
was interpreted as possible column contaminations or interferences in mass
Zr recovery (%)
spectrometry caused by traces of HCl in samples (Figure 6.11).
130
120
110
100
90
80
70
60
50
40
30
20
0
1
2
3
4
HCl (M)
Figure 6.11 – Maximum recovery of Zr according to HCl concentration in
Dowex1-X8
6.5. IEC versus EXC for the separation of Sr and Zr
The performance of ion exchange and extraction resins has been carried out using
approximately 0.5 g of resin, 250 µL of sample loading, and 40 mL of eluent (i.e. 20
mL for Sr eluent and 20 mL for Zr eluent). In general, eluents have been chosen
according to their capability to provide the best resolution and/or maximum
recovery. Optimal results for each one of the four resins tested (AG50W-X8 – 100200 mesh, Dowex1-X8 – 100-200 mesh, Sr – 50-100 µm, and DGA – 50-100 µm)
are presented in Figure 6.12.
66 Recovery (%)
100
3M H2SO4
2M HNO3
80
60
Sr
40
Y
20
Zr
a
0
0
5
Recovery (%)
100
10
15
20
25
30
11M HCl
35
40
45
50
45
50
45
50
4M HCl
80
60
40
20
b
0
0
Recovery (%)
100
10
15
20
25
30
0.05M HNO3
80
35
40
3M HNO3
60
40
20
0
c
0
100
Recovery (%)
5
5
10
15
25
30
35
40
1M HNO3
0.05M HNO3
80
20
H2O
60
40
20
d
0
0
5
10
15
20
25
30
Eluent (mL)
35
40
45
50
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)
"#
Contrary to results obtained for IEC resins, Zr and Sr have exhibited opposite
behaviours in both Sr and DGA resins. Actually, the ability of zirconium to form
charged complexes has certainly increased its affinity with the aqueous mobile
phase, which probably contributed to reduce its retention time in EXC. Such an
effect could not be possible for Sr since it cannot form charged complexes and, for
that reason, it has shown stronger affinity with the organic stationary phase and a
longer retention time in Sr and DGA resins (Figure 6.13).
Figure 6.13 – Proposed extraction mechanism for Sr for its separation from
Zr by EXC
In practice, the possibility to recover zirconium before strontium reduced the risks
of peak overlap caused by tailing problems at high concentrations of Sr potentially
found in real samples. Results presented in Figure 6.14 show that tailing problems
started to become important once the Sr concentration increased from 10 µg/mL to
1000 µg/mL.
68 Sr (%)
100
90
80
70
60
50
40
30
20
10
0
10 µg/mL
50 µg/mL
100 µg/mL
1000 µg/mL
40
60
80
100
120
140
160
180
200
2M HNO3 (mL)
Figure 6.14 – Tailing effect as function of Sr concentration (AG50W-X8)
In terms of efficiency to eliminate possible isobaric interferences from yttrium,
limitations have been encountered specially for the AG50W-X8 and Sr resins,
where peak overlapping has been observed. The main issue for Dowex1-X8 and
DGA resins, on the other hand, has been associated to insufficient Zr recovery. For
DGA, Zr concentration has been found to be below the quantification limit.
6.6. Addition of HF in samples
Since the separation of Sr and Zr in real samples could be performed in presence
of trace levels of HF from digestion step, hydrofluoric acid has been added to
samples in order to assess its impact on separation efficiency.
For the anionic resin, separation was compromised after addition of 0.01% HF to
the samples. As demonstrated in Figure 6.15, peak overlapping has been
69 observed. Repulsion forces between neutral or positive complexes of zirconium
Recovery (%)
and the active site on resin surface probably contributed to accelerate Zr elution.
100
90
80
70
60
50
40
30
20
10
0
Sr
Zr
0
5
10
15
20
Eluent (mL)
Figure 6.15 – Separation of Sr and Zr using Dowex1-X8 for samples
containing HF
For DGA resin, on the other hand, the utilization of HF in samples has not only
helped to increase Zr recovery but also ensured separation efficiency. As
demonstrated in Figure 6.16, recovery of zirconium was increased from 11 1% to
102 10% and no overlapping has been detected after HF concentration was
increased from 0.01% to 0.2%.
#
100
1M HNO3
0.05M HNO3
Recovery (%)
80
60
40
20
0
a
0
5
10
15
20
25
30
35
40
45
50
100
Recovery (%)
80
60
40
Sr
20
Zr
b
0
0
5
10
15
20 25 30
Eluent (mL)
35
40
45
50
Figure 6.16 – Separation of Sr and Zr using DGA for samples containing HF
(a. 0.01%, b. 0.2%)
In general, the HNO3/HF ratio of 5 has proved to be efficient to obtain a satisfactory
recovery of zirconium (Table 6.5). According to results, about 50% of zirconium
recovery was lost after increasing HNO3/HF ratio from 5 to 10. It was assumed that
the increase in NO3- concentration in the system has provoked competition against
F- ions to form zirconium complexes. Actually, even if fluoride complexes formation
was theoretically more favoured, in this case nitrate complexes were more likely to
form as a consequence of an excess of nitrate ions in the sample.
#
Table 6.5 – Recovery of Zr in DGA Resin according to
HNO3/HF ratio in samples
HNO3/HF
Ratio
[HNO3]
(M)
[HF]
(M)
Recovery
(%)
5
1
0.2
100
5
0.5
0.1
112
10
1
0.1
52
10
2
0.2
59
Due to the toxicity and risks of damaging glass components in mass
spectrometers, minimal HF concentrations have been considered and the
possibility of using HF as pure eluent has been totally discarded.
6.7. Summary of the efficiency of all resins tested
Table 6.6 summarizes the performance of the fours resins tested to separate
strontium from zirconium. The respective recoveries for each element under all the
experimental conditions tested over this work can be find in the Annexe 1.
Table 6.6 – Summary of resins performance to isolate Zr prior MS analyses
Separation
efficiency in
Resin
Sr elimination
Y elimination
presence of HF in
samples
AG50W-X8
Dowex1-X8
Sr-Resin
DGA-Resin
72 In general, DGA resin has proven to be the most efficient alternative to obtain
satisfactory separation and acceptable zirconium recovery. As presented in Table
6.6, DGA was the only resin able to eliminate both potential interferences from Sr
and Y at m/z 90 even under conditions where HF was present. Due to its superior
performance over other resins, a complete methodology using DGA resin for
isolating zirconium prior to MS analysis has been proposed (Figure 6.17).
Figure 6.17 – Complete methodology to separate Sr and Zr using DGA resin
Before starting the separation, the resin was usually cleaned with 50 mL of water
and conditioned with 5 mL 1M nitric acid. A blank was then recovered before
loading the sample. The sample load solution, was prepared in a mixture of 1M
HNO3 / 0.2M HF and finally, zirconium was recovered using a 10 mL 1M HNO3 as
the eluent.
6.8. Performance of DGA method for the recovery of trace levels of Zr
The method presented in Figure 6.17 has been tested in standard solutions
73 containing zirconium concentrations at ppt levels and concentrations for strontium
100 times higher than those for Zr (Figure 6.18).
Zr (ng/mL)
20
Expected values
E
Experimental Results
15
10
5
0
1
2
3
4
5
6
Sample ID
Figure 6.18 – Comparative between experimental and expected results for the
recovery of trace levels of Zr using DGA resin
Except for samples 1 and 2, other samples exhibited a good correlation with
expected values. Average recovery obtained for zirconium was 94 ± 6%. The
detection limit for zirconium has been determined as being 92 pg/mL. No strontium
has been detected in the zirconium fractions. The detection limit for strontium has
been determined as being 48 pg/mL.
6.9. Determining the age of a radiostrontium source
In order to determine the age of radioactive source, the proposed separation
method using DGA-Resin was also applied to isolate zirconium-90 from its parent
Sr-90. The complete procedure used for the separation and analyses of Sr-90 and
Zr-90 is presented in Figure 6.19.
# Figure 6.19 – Procedure for determining the age of a radiostrontium source
As indicated, a solution containing 30 Bq/mL of radiostrontium (Sr-90 NIST
Standard, 14/04/2000) has been first separated in two different portions. The first
fraction was used to isolate Zr-90 with the DGA resin for further analyses by ICPMS, while the second was simply diluted in 15 mL of water for the analysis of Sr-90
by liquid scintillation.
As the exact concentration of Zr-90 was not available, blanks and standard
solutions have been used to ensure the quality of results for Zr analysis. For Sr-90,
it was observed that the experimental concentration was about 30 % lower than the
theoretical concentration (Figure 6.20).
75 y = 220.5756x
R² = 0.9993
40
cpm
30
20
10
0
0.00
Theoretical [Sr-90]
Experimental [Sr-90]
0.05
0.10
0.15
0.20
Bq/mL
Figure 6.20 – Comparative between theoretical and experimental
concentrations for the analysis of Sr-90 by liquid scintillation
Although the true age of the radiostrontium source was unknown, an age of 68 ±
11 years was proposed. Calculations have been based on the principles of the
decay law treated in section 2. The error of 11 years, however, has been
considered too high for nuclear forensic applications. Problems associated to the
quality of Sr-90 solution have been considered as the main cause to the lack of
accuracy encountered.
6.10. Potential of reaction cell to separate strontium from zirconium
As zirconium could be analyzed at m/z 106 to avoid isobaric interferences at m/z
90, some tests to assess the ability of Zr and Sr to form oxides in mass
spectrometry under different O2 concentrations have been carried out. Results for
standard solutions prepared in 4% HNO3 containing 5 ng/mL of Sr and 5 ng/mL of
Zr is presented in Figure 6.21.
#"
6000
Signal (cps)
5000
4000
3000
SrO+
2000
ZrO+
1000
0
0
10
20
30
40
50
60
70
80
90
100
O2 (%)
Figure 6.21 – Zr and Sr oxides formation in mass spectrometry as function of
O2 concentration in the reaction cell
The formation of ZrO+ (m/z 106) showed to be favoured at 10% oxygen, where a
maximum peak was detected. At this point, a decontamination factor of about 80%
against strontium was obtained.
Maximum decontamination factor has been achieved at 6 % O2. Under this
condition, there was about 17% of Zr that was transformed in zirconium oxide
(90Zr16O+) and about 2% of strontium oxide (88Sr16O+) that could be formed (Figure
6.22a and 6.22b). Such condition has allowed increasing decontamination factor
from 80% to 98 % to meet the minimum of 97% expected for date-aging
applications.
##
Zr species (%)
100
90
80
70
60
50
40
30
20
10
0
90Zr+
90Zr16O+
90Zr16O1H+
90Zr16O1H +
2
a
Sr species (%)
85
100
90
80
70
60
50
40
30
20
10
0
90Zr16O +
2
90
95
100
105
110
115
120
125
88
Sr+
Sr16O2+
88
Sr16O+
88
88
Sr16O1H+
b
85
90
95
100
105
110
115
120
125
Figure 6.22 – Predominant species of Zr (a) and Sr (b) at 6% O2 in the
reaction cell
Although only 17% of Zr was expected to be formed at 6% O2, Figure 6.23 shows
that comparable results to m/z 90 could always be obtained. After analyzing a
series of solutions containing different concentrations of Zr, a good correlation (R2
= 0.997) between results obtained at m/z 90 and m/z 106 has been achieved.
#$
Zr at m/z = 106
(ng/mL)
100
90
80
70
60
50
40
30
20
10
0
R² = 0.997
0
10
20
30
40
50
60
70
80
90
100
Zr at m/z = 90
(ng/mL)
Figure 6.23 – Correlation between results for the analysis of Zr at m/z 90
and m/z 106
The only limitation found at 6% O2 was the maximum decontamination factor of
90% achieved to eliminate potential isobaric interferences from yttrium. In this
case, approximately 10 % of yttrium has demonstrated to react with oxygen to form
yttrium oxide (89Y16O+) that could interfere in the analysis of Zr at m/z 106 (Figure
Y species (%)
6.24).
100
90
80
70
60
50
40
30
20
10
0
89 +
Y
89 16
Y O+
Y O1H+
89 1
Y H+
85
90
Y O2+
89 16
89 16
Y O1H2+
89 16
95
100
105
m/z
110
115
120
125
Figure 6.24 – Predominant species of Y at 6% O2 in the reaction cell
#%
80 Conclusions
The present work, demonstrates that the utilization of collision cells in MS without
prior chromatographic separation does not provide sufficient resolution to
completely isolate Zr from all its isobaric interferences. However, the results
suggest that triple quadrupole instruments (ICPQQQ-MS) have a potential to
significantly minimize the level of isobaric interferences while reducing both the
duration and the complexity of sample preparation procedures.
The technique was demonstrated to eliminate 98% of interferences from Strontium
and 90% of interferences from Yttrium that are eventually present at high levels in
seized sources of radiostrontium. Although the present work has concentrated all
the efforts on the development of a radiochronometric method to isolate Sr-90 from
Zr-90, it is believed that such a technique could also been applied to other longerlived radionuclides also of interest for nuclear security experts (e.g. Cs-137 (t1/2 =
30 y), Pu-238 (t1/2 = 88 y)).
Among four commercial resins tested, DGA has been proved to provide the best
performance for the radiological separation. Recoveries higher than 99% for Zr
have been obtained. The DGA approach has also been demonstrated to be the
faster approach and the more efficient to eliminate both the isobaric interferences
from Strontium and Yttrium. None of these two elements have been detected by
mass spectrometry after the chromatographic separation.
Finally, the proposed method using DGA combined to MS and liquid scintillation for
the respective analysis of Zr-90 and Sr-90 has been applied to determine the age
of an aqueous solution of radiostrontium. Although the true age of the radioactive
81 source was unknown, an age of 68 ± 11 years was calculated. The uncertainty of
11 years observed, however, has been considered too high for nuclear forensic
applications. Problems associated to the quality of Sr-90 solution have been
considered as the main cause to the lack of accuracy encountered.
More tests using sources of radiostrontium having well known ages would help to
identify the major sources of error. They would also validate the proposed method
while helping to determine the precision of the age determined experimentally.
Likewise, the utilization of Teflon material and ultra-pure acidic solutions would
help to minimize systematic errors by preventing any trace of unwanted
contamination.
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85 86 Annexe 1
87