UNIVERSITY OF CALGARY
The Development of an Analytical Technique to Measure Stable and Radiogenic
Strontium Isotope Ratios Using Thermal Ionization Mass Spectrometry with the Double
Spike Method
by
Nadia Elhamel
A THESIS
SUBMITTED TO THE FACULTY OF GRADUATE STUDIES
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE
DEGREE OF MASTER OF SCIENCE
DEPARTMENT OF PHYSICS AND ASTRONOMY
CALGARY, ALBERTA
May, 2014
© Nadia Elhamel 2014
Abstract
An analytical method using a strontium “double spike” enriched in both
84
Sr and
87
Sr was developed to measure the n(87Sr)/n(86Sr) and n(88Sr)/n(86Sr) isotope ratios
using a thermal ionization mass spectrometer. The results obtained for the SRM987
reference material using this technique are in agreement with published data. However,
the results of IAPSO sea water standard revealed an enrichment in the n(88Sr)/n(86Sr)
isotope ratio relative to SRM987 of 0.04 % and n(87Sr)/n(86Sr) isotope ratio of
0.709325(27), which is significantly different from the n(87Sr)/n(86Sr) ratio of
0.709191(37), which is found when one assumes a fixed n(88Sr)/n(86Sr) isotope ratio, as
has been done in the past. The use of a double spike method for measuring strontium
isotopic composition reveals important insights into mass dependent processes that
occur as this element moves through a system as well as provide accurate radiogenic
strontium isotope amount ratios.
ii
Acknowledgments
I would like to thank my supervisor Dr. Michael Wieser for all his support and
guidance throughout my masters. I would like also to thank Kerri Miller for her
assistance in the lab work and data interpretation.
iii
Table of Contents
Abstract ............................................................................................................................ ii
Acknowledgements ......................................................................................................... iii
Table of Contents ............................................................................................................ iv
List of Tables ................................................................................................................... vi
List of Figures ................................................................................................................ viii
List of Symbols and Abbreviations .................................................................................. x
Chapter 1: Introduction .................................................................................................... 1
Chapter 2: Background ................................................................................................. 8
2.1 Isotope Fractionation ........................................................................................... 8
2.1.1 Equilibrium Isotope Fractionation ................................................................ 9
2.1.2 Kinetic Isotope Fractionation ...................................................................... 10
2.2 Sample analyses on a Thermal Ionization Mass Spectrometer ......................... 11
2.3 Correction Methods for Instrumental Isotope Mass Fractionation ..................... 12
2.3.1 Internal Normalization ................................................................................ 13
2.3.2 The Sample- Standard Bracketing Method................................................. 15
2.3.3 The Double Spike Technique .................................................................... 17
2.4 The Radiogenic Strontium Isotope System ....................................................... 17
2.5 The Stable Strontium Isotope System ............................................................... 21
Chapter 3: Measurements of Accurate Strontium Isotope Amount Ratios ................... 26
3.1 Thermal Ionization Mass spectrometry (TIMS) ................................................... 26
3.1.1 A General Overview of The TIMS ............................................................... 26
3.1.2 TIMS Measurement of Strontium Isotopes ............................................ ... 30
iv
3.2 The Strontium double Spike Technique ............................................................. 31
3.2.1 The Double Spike Algorithm ....................................................................... 32
3.2.2 Double Spike Preparation........................................................................... 39
3.2.3 Double Spike Calibration .......................................................................... 40
3.3 Samples preparation .......................................................................................... 43
3.4 Wood Samples ................................................................................................... 44
3.5 Water Samples ................................................................................................... 46
Chapter 4: Results and Discussions.............................................................................. 47
4.1 Mass Spectrometer Stability ............................................................................... 48
4.2 Output of the Algorithm Program ........................................................................ 51
4.3 The Double Spike Calibration Result.................................................................. 52
4.4 SRM987 Standard Measurements .................................................................... 55
4.5 IAPSO Seawater Standard Measurements ....................................................... 58
4.6 Sr Isotopic Composition of the Alberta Ground Water ........................................ 61
4.7 Sr Isotope Composition of Lake Vida ................................................................ 70
4.8 Sr Isotope Composition of Wood ....................................................................... 72
Chapter 5: Summary and Future work ........................................................................ 77
References .................................................................................................................... 81
Appendices A: Copies of Copyright Permissions .......................................................... 86
v
List of Tables
Table 1.1: Wood samples were analyzed for strontium isotopic composition. ................ 6
Table 2.1: Results of stable strontium measurements of selected studies.................... 24
Table 3.1: Strontium-carbonate isotopic compositions from Oak Ridge National
Laboratory .................................................................................................................... 39
Table 3.2: Measured isotopic composition of the double spike ratios by TIMS ............. 39
Table 3.3: The isotopic composition of the SRM987 carbonate reference material ...... 40
Table 3.4: Amounts of the Sr double spike (DS) and the SRM987 standard used to
calculate different 84Srspike/84SrSRM987 ratios ................................................................... 42
Table 4.1: The results of SRM987 standard measured with each sequence of samples
...................................................................................................................................... 50
Table 4.2: The Sr isotopic composition of the SREM987 standard measured in this
project ........................................................................................................................... 56
Table 4.3: The Sr isotopic composition of the IAPSO seawater standard ................... 58
Table 4.4: δ88/86Sr values of IAPSO sea water standard determined in previous studies
measured ...................................................................................................................... 59
Table 4.5: Sr concentration and the isotopic composition of Alberta ground waters ..... 63
Table 4.6: The Sr isotopic composition of some Alberta ground water samples ........... 69
Table 4.7: Sr isotopic composition of Lake Vida ........................................................... 70
Table 4.8: The Sr isotopic composition of exotic rosewood samples using the first
attempt .......................................................................................................................... 73
Table 4.9: The signal intensities and the isotope amount ratios of Sr double spike as a
result of using different ways for drying the samples ..................................................... 74
vi
Table 4.10: The Sr isotopic composition of exotic rosewood samples using the second
attempt .......................................................................................................................... 75
Table 4.11: The Sr isotopic composition of exotic rosewood samples using the third
attempt .......................................................................................................................... 76
vii
List of Figures
Figure 2.1: Schematic of MC-TIMS ............................................................................. 12
Figure 2.2: Isotope mass fractionation during TIMS measurement for n(87Sr)/n(86Sr)
ratios of the SRM987 Standard ..................................................................................... 14
Figure 2.3: Correction for instrumental mass fractionation by using the internal
normalization method .................................................................................................... 15
Figure 2.4: The seawater strontium evolution of the Phanerozoic ................................ 20
Figure 2.5: The range of δ88/86Sr values of different materials that summarized in Table
2.1 ................................................................................................................................. 25
Figure 3.1: Experimental arrangement of a single filament thermal ionization source . 27
Figure 3.2: Schematic diagram of a Faraday cup ......................................................... 30
Figure 3.3: Schematic diagram of the Sr double spike algorithm ................................. 38
Figure 3.4: Calculation of the amount of Sr double spike for the ratio 10 of
84
Sr in the
double spike to the 84Sr in the SRM987 standard ......................................................... 41
Figure 4.1a: n(87Sr/n(86Sr) ratios of the SRM987 standard measured with each
sequence of samples .................................................................................................... 49
Figure 4.1b: n(84Sr/n(86Sr) ratios of the SRM987 standard measured with each
sequence of samples .................................................................................................... 49
Figure 4.2: δ88/86Sr versus different ratios of double spike to SRM987 show no
dependence of δ88/86Sr on the different amount of double spike added ........................ 53
Figure 4.3: δ88/86Sr versus different possibilities of ratio 20 to see the best fractionation
factor can bring δ88/86Sr of ratio 20 to close to zero ...................................................... 54
viii
Figure 4.4: δ88/86Sr versus 84SrSpike/84SrSRM987 ratios using corrected isotopic
composition of Sr double spike ..................................................................................... 55
Figure 4.5 a: Variation of δ88/86Sr for the standard (SRM987) measurements .............. 57
Figure 4.5 b: Variation of n(87Sr)/n(86Sr)* for the standard (SRM987) measurements 57
Figure 4.6 a: Measurments of δ88/86Sr of the IAPSO seawater standard ...................... 60
Figure 4.6 b: Measurments of n(87Sr)/n(86Sr)* of the IAPSO seawater standard ......... 60
Figure 4.7: δ88/86Sr values versus n(87Sr)/n(86Sr)* ratios of Alberta ground water
samples ......................................................................................................................... 65
Figure 4.8: The linear relationship between δ88/86Sr values and the differences between
n(87Sr)/n(86Sr)* and n(87Sr)/n(86Sr)norm ratios for Alberta waters (𝝙) ............................ 67
Figure 4.9: Diagram of calculation of n(87Sr)/n(86Sr)*new ratios .................................... 68
Figure 4.10: δ88/86Sr values versus n(87Sr)/n(86Sr)* ratios of Lake Vida samples.......... 71
ix
List of Symbols, Abbreviations and Terminology
Abbreviations
DS = Double spiking
SSB = Sample standard bracketing
TIMS = Thermal ionization mass spectrometer
MC-ICP-MS = Multiple collector inductively coupled plasma mass spectrometer
Symbols
β or 𝝰 = Instrumental fractionation factor
sd = Standard deviation
sem= Standard error of the mean
Terminology
n(87Sr)/n(86Sr)norm: This ratio is measured by thermal ionization mass spectrometer and
normalized (corrected) to the accepted n(88Sr)/n(86Sr ) ratio of 8.375209. The subscript
“norm” is the abbreviation for normalization and n indicates an amount ratio.
n(87Sr)/n(86Sr)*: This ratio refers to the n(87Sr)/n(86Sr) ratio as measured and
normalized using the 87Sr–84Sr double spike.
n(87Sr)/n(86Sr)*new: This ratio is measured by thermal ionization mass spectrometry and
normalized to an n(88Sr)/n(86Sr)* ratio calculated from δ88/86Sr for an individual sample.
x
δ88/86Sr: This ratio refers to the n(88Sr)/n(86Sr) ratio as measured and normalized using
the 87Sr-84Sr double spike. The measured n(88Sr)/n(86Sr) ratio is presented in the δnotation in per mil with respect to the SRM987 reference material. The n(88Sr)/n(86Sr)
ratio of 8.375209 corresponds to an δ88/86Sr value of zero.
xi
Chapter 1: Introduction
The study of atoms and their isotopes is central to understanding our
environment, or more generally, of all matter in the Universe. By understanding how
isotopes form, and their relative abundances, models of how our environment is
evolving can be developed. The goal of this work was to contribute to this field of study
by developing a technique for the measurement of stable and radiogenic strontium
isotope amount ratios.
Isotopes are atoms of the same element that have the same number of protons
and different numbers of neutrons. The isotope symbol is in the form (
, where the
superscript ‘A’ denotes the mass number, which is sum of the number of protons and
neutrons, and the subscript ‘Z’ denotes the atomic number (number of protons) of an
element, E. For example,
is the isotope of strontium which has 38 protons and 50
neutrons. The atomic weight of each naturally occurring element is the mean of the
masses contributed by its isotopes.
Isotopes of an element have also the same number of electrons and therefore
they undergo the same chemical reactions. However, differences in the masses of the
isotopes can influence the reaction rates and lead to partitioning of isotopes
differentially among different phases. The separation of isotopes during chemical,
physical, or biological processes is referred to as isotope fractionation. Variations in
isotope composition of the light elements in nature such as carbon(C), nitrogen(N),
oxygen(O), hydrogen (H) and sulfur (S) have been studied widely (Hoefs,2009). These
variations are large enough to be measurable because the relative mass differences
1
between isotopes are large in low atomic number elements. The variations in isotopic
composition of an element can provide important insights into the source of an element
to a particular region or the history of a sample. This is because an element may have a
distinct isotopic composition because of nuclear, physical or biological processes
involving that element. The applications of isotope abundance data of H, C, N, O and S
to studies in the environment, Health and geology are numerous (Hoefs, 2009). Until
recently, it was not possible to determine variations in the isotopic composition of
heavier elements because these variations are typically much smaller and more difficult
to observe. Improvements in mass spectrometry opened the door to high precision
stable isotope analysis for almost all elements with more than one stable isotope on the
periodic table. Non-traditional stable isotopes have become an active field of research
over the past few years (Hoefs, 2009). There is enormous potential to use variations in
the isotopic composition of heavier elements, including Sr, to study mass dependent
processes that change isotope abundances in geological and biological systems.
Strontium has four stable isotopes at masses 84, 86, 87, and 88, with natural
abundances of approximately 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.00%) and 88Sr
(82.58%), respectively. Strontium isotopes 84Sr, 86Sr, 84Sr, and 88Sr were formed during
primordial nucleosynthesis. However, 87Sr is also formed by the radioactive beta decay
of 87Rb with a half-life of 48.8 billion years (Faure and Mensing, 2005), as follows:
87
Rb → 87Sr + β- + v+ Q
2
(1.1)
where β- is a beta particle having an electronic charge of (-1), v is an anti-neutrino, and
Q is the decay energy of 0.275 Mev. The radioactive decay of
87
Rb can be represented
by
(
(
(
)
(
where ( ) is the decay constant and (t) is the age of the system.
Variations in n(87Sr)/n(86Sr) ratios due to this nuclear process are large (i.e. 0.70
to 0.71) whereas variations in Sr isotope amount ratio due to mass dependent process
are much smaller, less than 0.1 % (Fietzke and Eisenhauer, 2006; Halicz et al., 2008;
Krabbenhoft et al, 2010; Charlier et al., 2012). Therefore, mass dependent variations in
strontium isotopic composition are expressed in the δ-notation (1.3) in units of per mil
(‰)
δ
⁄
(
)
(
)
(
)
(1.3)
where the subscript ‘SRM987’ refers to the strontium carbonate standard reference
material from the National Institute of Standards and Technology (NIST) in
Gaithersburg, MD. SRM 987 has a known isotopic composition, which is given in
Table 3.1. Mass dependent isotope fractionation processes also occur during the
measurement of Sr isotopic composition in the ion source of the mass spectrometer.
Instrumental mass fractionation is a major source of systematic errors that limit accurate
isotopic amount ratio measurements. The custom has been to ignore variations in
n(88Sr)/n(86Sr) isotope amount ratios by assuming a fixed n(88Sr)/n(86Sr) ratio of
8.375209, which was necessary to correct for instrumental mass fractionation. Recently,
3
however, it was observed that small, but significant variations in the relative amounts of
the non-radiogenic isotopes could have applications in geochemistry and archaeology.
Moreover, measuring mass dependent variations in strontium isotopic composition
(δ88/86Sr) can provide accurate data for the isotopic composition of the radiogenic
strontium isotope (n(87Sr)/n(86Sr)) .
The goal of this project is to develop an analytical method to measure the
variations in the isotopic composition of strontium caused by mass dependent
processes. This will be realized using a so-called Double Spike (DS) technique that can
correct for the instrumental mass fractionation. The result will be a measurement of
mass dependent δ88/86Sr values and accurate n(87Sr)/n(86Sr)* isotope amount ratios. In
order to achieve accurate results, it was necessary to separate Sr from interfering
elements in the sample, in particular Rb which has isobars with Sr (i.e.
87
Rb with 87Sr).
The calibration of the Double Spike was achieved using the SRM 987 standard
reference material and checked with IAPSO seawater standard. The method was then
applied to a selection of Alberta ground water samples, water from Lake Vida, and wood
samples provided by the United States Geological Survey.
Detailed information about Alberta ground water samples was not available. Lake
Vida is located in the Victoria valley in Antarctica and is one of the largest lakes in the
region. Its surface area is 6.8 km2 (Doran et al., 2003). It was originally thought to be
frozen solid. However, ground penetrating radar surveys discovered a very salty liquid
layer (brine) under 19 m of ice cover (Doran et al., 2003). Salinity of this brine is seven
times of the salinity of sea water. This high salinity leads the brine to remain liquid below
(-10 0C) (Doran et al., 2003). Lake Vida is an ice-sealed lake. The lake is completely
4
isolated from other environments due to the thick ice cover and the water does not flow
out of the lake (Doran et al., 2003). The n(13C)/n(12C) analyses on the ice cover suggest
that the brine has not been in contact with the atmosphere for more than 2800 years
(Doran et al., 2003). Microbes have been found in the lake despite the very cold, dark,
salty and isolated environment from the outside world (Murray et al., 2012). Studying
these microbes that sustain life in such harsh environments might provide insight into
other planets that have unique environment Lake Vida processes such as Jupiter's
moon Europe.
Ice cores (16.5 m in 2005, 20 and 27 m in 2010) and samples from the brine
were collected in 2005 and then again in 2010 for geochemical and microbiological
analysis (Muarry et al., 2012). Lake Vida contains high levels of carbon-based
compounds, nitrous oxide and also molecular hydrogen (H2) (Muarry et al., 2012). The
molecular hydrogen may be crucial as an energy source for life in the lake and suggest
the presence of life in an environment that has no oxygen. Lake Vida was isolated for
thousands of years from any obvious external sources of energy to help sustain
microbes (Muarry et al., 2012).
Exotic Rosewood samples were also used to test the Sr double spike method for
Sr isotopic composition measurement. Rosewood species belong to the genus of
Dalbergia. Rosewoods are dense wood and thus they are used in expensive furniture,
flooring, musical instruments, and decorative items. As a result of over exploitation,
some species from the Dalbergia genus are threatened with extinction. Therefore, they
are currently controlled by the Convention on International Trade in Endangered
Species of Wild Flora and Fauna (CITES). However, identifying timber origin is still a
5
difficult task in order to determine if the timber is a controlled species or not. Isotopic
fingerprinting is a technique that is used to distinguish among similar materials and to
trace the materials to their origin. Each region may have a characteristic isotopic
composition for strontium as well as hydrogen, nitrogen, and oxygen. Thus living
organisms possess isotopic signatures that are derived from the region in which
organisms grew. Wood samples were provided by the United States Geological Survey
(USGS) in Reston, VA. Samples were dried and ground before they were sent in sealed
containers to the Isotope Science Laboratory at the University of Calgary. These
samples are listed in Table 1.1.
Table 1.1: Wood samples were analyzed for strontium isotopic composition.
Common Name
Country of
Origin
Continent
Granadillo
Mexico
North America
3
Dalbergia cearensis
Kingwood
Brazil
South America
2
Dalbergia latifolia
Indian Rosewood
India
Asia
3
African Blackwood
Tanzania
Africa
3
Dalbergia nigra
Brazilian Rosewood
Brazil
South America
5
Dalbergia retusa
Cocobolo
Mexico
North America
1
Dalbergia spp.
Indonesian Rosewood
Malaysia
Asia
3
Dalbergia spruceana
Amazon Rosewood
Amazon Basin
South America
2
Phoebe parosa
Imbuia
Brazil
South America
2
Genus Species
Number of
Samples
Caesalpinia
enchinata
Dalbergia
Melanoxylon
6
This thesis is divided in five chapters. Chapter 2 provides an overview about the
mechanisms driving strontium isotope fractionation in nature and how to correct for
fractionation occurring during mass spectrometry measurements. Chapter 3 provides a
detailed description of the development of a strontium double spike method. Moreover,
it provides an overview about how a thermal ionization mass spectrometer (TIMS)
works and also the details of sample preparation. Chapter 4 discusses the calibration of
the Sr double spike using the SRM987 standard and also the output from the algorithm.
It also provides the strontium isotope composition of standards (SRM987 and IAPSO
seawater standard) and samples used in this study. Chapter 5 is a summary of the
results obtained from this project.
7
Chapter 2: Background
This chapter provides a general overview about isotope fractionation, which
causes changes in the isotopic composition of an element. Sample analyses on a
thermal ionization mass spectrometry will be described. Correction methods for
instrumental mass biases will be presented including internal normalization, SampleStandard Bracketing, and Double Spike technique, the latter is the focus of this
research. This section will present some background on radiogenic and stable strontium
isotope systems.
2.1
Isotope Fractionation
Isotopic fractionation is a change in the isotopic composition of an element
between two substances (i.e. oxygen exchange between CO 2 and H2O) or two phases
of the same substance (i.e. Oxygen between water vapour in gas and liquid phases).
Isotopic fractionation occurs due to physical (i.e. condensation of water vapour) or
chemical (i.e. dissolution of minerals) or biological (i.e. a process of a living organism)
processes. Isotope fractionation may be divided into two types: mass dependent
fractionation (depends on the difference in the masses of the isotopes) and mass
independent fractionation (does not depend on the difference in the masses of the
isotopes).
Two types of isotope fractionation processes, equilibrium and kinetic isotope
mass dependent fractionation, are explained in detail in the following sections (2.1.1)
and (2.1.2).
8
2.1.1 Equilibrium Isotope Fractionation
Equilibrium isotope fractionation is an isotopic exchange reaction between two
phases of a compound or between two different compounds such that equilibrium is
preserved. This process can be expressed as:
(
where A and B are elements in different phases and superscripts 1 and 2 are isotopes.
In equation (2.1), the forward and reverse reactions occur at the same rate. The
equilibrium constant can be expressed as:
(
This constant can also be expressed as isotope amount ratios (RA and RB) as
can be seen in the following equation:
(
where 𝝰
is the fractionation factor,
Equilibrium fractionation results from differences in the vibrational energies of the
molecules (Urey, 1947; Schauble, 2004). Vibrational energy is related to the zero point
energy and is dependent on temperature and the masses of the atoms. Zero point
energies of different molecules are different for different isotopes in the molecules.
Increasing temperature changes the zero point energy reducing the difference in zero
point energies between two molecules. Typically, the molecule with the lighter isotope
9
has a higher zero point energy, thus it takes less energy to break the bonds in this
molecule compared to a molecule with heavier isotopes. Therefore, isotopic
fractionation is expected to be large at low temperatures and smaller at very high
temperatures. The equilibrium constant K can also be expressed in terms of partition
functions (Q).
(
)
(
(
)
The partition function is defined as
∑
(
where
molecule,
)
with
(
at zero point energy,
is vibrational energy of a
is number of quantum states sharing the same energy level,
Boltzmann constant, and
is
is temperature. In a closed system, the vapour above the
surface of the water is enriched in lighter isotopes 1H and 16O relative to the liquid water.
2.1.2 Kinetic Isotope Fractionation
Kinetic Isotope fractionation occurs in unidirectional processes and the reaction
rate depends on the isotope amount ratios and the vibrational energies of the molecules
involved in the reaction. In general, bonds between the lighter isotopes require less
energy to break than the stronger bonds between the heavier isotopes. Hence, the
lighter isotopes become concentrated in the products of the reaction, causing the
residual reactants to become enriched in the heavy isotopes.
10
An example of a kinetic isotope fractionation is the reduction of
to
by
sulfur reducing bacteria. These living organisms break the S-O bonds between the
lighter isotopes of sulfate faster than the bonds between heavier isotopes of S and O.
Hence, the product sulfide is enriched in 32S and the remaining sulfate is enriched in
34
S.
2.2 Sample Analyses on a Thermal Ionization Mass Spectrometer
A thermal ionization mass spectrometer (TIMS) was used for this study to analyze
the strontium isotopic composition of samples. The TIMS is composed of three primary
parts: an ion source, a magnetic sector and collectors. All these parts of the mass
spectrometer are evacuated to pressures ranging from 10-7 mbar in the source to 10-9
mbar in the analyzer region (the magnetic sector and collectors) (Hoffmann et al.,
2002). Samples that are analyzed on TIMS are deposited on metal filaments, typically
made from rhenium (Re). Filaments are loaded on a sample wheel in the ion source of
TIMS which can accommodate a maximum of 21 samples. The filaments are then
heated to a high temperature (1450 0C) by passing a current through the filaments. Ions
are created by transferring electrons from the filament to the atom or from the atom to
the filament. The ions are then accelerated by a high voltage (10 kV). The focused ions
pass through a magnetic analyzer which deflects the ions into circular paths whose radii
are proportional to their mass to charge ratio. Heavy isotopes are deflected into paths
with larger radii compared to light isotopes. The separated ion beams travel to
collectors, which are positioned to intercept specific ions. The ions entering the Faraday
cups (collectors) are neutralized by electrons that flow to the collector through a high
ohmic resistor (1011 Ω). A voltage difference is generated across a resistor and then
11
amplified and measured. Figure 2.1 shows a schematic of the MC-TIMS instrument
(Thermo Fisher Triton).
Magnet
Zoom lens
Focus quad
Source lens stack
Collector array
21 sample
turret
RPQ-SEM
Fig. 2.1: Schematic of MC-TIMS, RPQ refers to the retarding quadrupole lens and SEM refers to the
secondary electron multiplier, these components are required for high sensitivity measurements
and not needed in this project (Wieser et al., 2004)
2.3
Correction Methods for Instrumental Isotope Fractionation
Kinetic isotope fractionation processes occur in the ion source when the sample
filament is heated to a high temperature causing the lighter isotopes to evaporate more
rapidly than the heavier isotopes. Fractionation processes are the major source of
systematic errors that limit accurate isotope amount ratio measurements. Therefore, it is
necessary to correct for instrumental isotope fractionation to obtain an accurate and
precise measurement for mass dependent isotope fractionation that occurs in nature,
12
δ88/86Sr, and the radiogenic Sr isotope ratio, n(87Sr)/n(86Sr). Three different techniques
can be applied in order to correct for instrumental mass fractionation: Internal
normalization, sample - standard bracketing and double spike.
2.3.1 Internal Normalization
Correction of the measured isotope amount ratios by internal normalization
adjusts the isotopic composition of all isotopes according to a fixed ratio between one
pair of isotopes. This approach is applied when one isotope is radiogenic and mass
dependent fractionation is ignored. Therefore, the radiogenic n(87Sr)/n(86Sr) isotope
ratio can be normalized to a fixed n(88Sr)/n(86Sr) ratio (8.375209) to correct mass
dependent fractionation during TIMS measurement (Nier, 1938). By rearranging the
exponential fractionation law (Equation 2.6), a fractionation factor ( ) can be calculated
from the known atomic masses, the fixed n(88Sr)/n(86Sr) amount ratio (8.375209), and
the measured n(88Sr)/n(86Sr) amount ratio. The fractionation factor ( ) is then applied for
correction of the measured n(87Sr)/n(86Sr) ratio to obtain n(87Sr)/n(86Sr)true amount ratio
(Equation 2.7), where M is the atomic mass (Russell et al.,1987).
[(
)
β
⁄(
[
⁄
)
]
(
]
β
(
)
(
)
(
)
(
To give an example for the isotope fractionation that occurs during TIMS
measurement, the data collected during one measurement of the SRM987 standard is
plotted in Figure 2.2. It is clear from the Figure 2.2 that n(87Sr)/n(86Sr) ratios of SRM987
13
standard were increasing as a function of time. This shows the evaporation of the lighter
isotope, causing the remaining sample to become enriched in the heavier isotope.
7.095E-01
87Sr/86Sr
7.090E-01
7.085E-01
7.080E-01
7.075E-01
7.070E-01
0
200
400
600
800
1000
1200
1400
Time [seconds]
87
86
Figure 2.2: Isotope mass fractionation during TIMS measurement for n( Sr)/n( Sr) ratios of the
SRM987 Standard. The true value is 0.710240.
Figure 2.3 shows the n(87Sr)/n(86Sr) isotope ratios corrected by internal
normalization to a fixed n(88Sr)/n(86Sr) isotope ratio of 8.375209 and no dependence of
n(87Sr)/n(86Sr) ratios on time was observed. There was only random scatter in the data.
Note that the precision of the measurement has improved.
14
7.1035E-01
87Sr/86Sr
norm
7.1030E-01
7.1025E-01
7.1020E-01
7.1015E-01
7.1010E-01
0
200
400
600
800
1000
1200
1400
Time [seconds]
Figure 2.3: Correction for instrumental mass fractionation by using the internal
normalization method
It is critical to understand that this normalization method completely removes any
evidence of natural isotope fractionation that may exist because a fixed n( 88Sr)/n(86Sr)
ratio is assumed. In order to investigate natural isotope fractionation and correct for
instrumental mass fractionation, either the sample-standard bracketing method or the
double spike technique must be applied, which are described below.
2.3.2 The Sample-Standard Bracketing Method
In this technique, the isotope amount ratios of a sample are found by determining
the isotope amount ratio of the sample between two measurements of a standard
material with known isotopic composition. The assumption using this method is that the
instrumental mass fractionation remains constant over all three measurements.
15
By dividing two exponential fractionation laws, one for the sample and the other
for the standard that was measured before the sample, one finds:
(
)
(
)
β
(
(
)
(
β
)
(
)
A similar equation can be written for the standard that was measured after the sample
(
)
(
)
β
(
(
)
Assuming that β
)
and β
β
(
(
)
β
(
(
(
)
)
(
(
)
(
β
)
(
)
)
β
(
(
(
)
is the mean of the β
β
β
(
(
β
)
)
β
(
)
Equation (2.13) was obtained by multiplication equations (2.11) and (2.12)
16
(
(
)
(
)
)
(
√(
)
(
)
This method depends on a constant fractionation between the sample and the
isotopic ion currents being measured. This is not the case with TIMS where the isotopic
composition of the sample changes continuously during the analysis of the sample (see
Figure 2.2). Therefore, this approach is not used.
2.3.3 The Double Spike Technique
The double spike technique is used to correct for instrumental mass fractionation
that occurs during sample preparation and during mass spectrometry measurements. It
is possible with this technique to determine both isotope ratios, the stable Sr
(n(88Sr)/n(86Sr)) and the radiogenic (n(87Sr)/n(86Sr)) isotope ratios without normalization
to a fixed n(88Sr)/n(86Sr) ratio of 8.375209. This method enables variations in
n(88Sr)/n(86Sr) to be measured and is the focus of this research project. The double
spike technique is described in detail in Chapter 3.
2.4 The Radiogenic Strontium Isotope System
Radiogenic isotopes in general are used in two different ways: providing an
absolute age of a sample and tracing the origin of a Sr source to a system. The
87
Sr
isotope is called radiogenic isotope because it is partly produced by beta decay of
87
Rb
as discussed in section 2.4. The 87Rb decay forms the basis for the rubidium- strontium
dating method (Capo et al.,1998). Rb-Sr dating method has been used to determine the
17
ages of the rocks and minerals by measurement of the rubidium and strontium
concentrations and also by measurement the ratio of n(87Sr)/n(86Sr). This technique can
only be applied if the initial isotope amount ratio of strontium in rocks is known.
Different types of rocks are formed during magmatic processes that have large
variations in the Rb/Sr ratio. For example, the Rb/Sr ratios of basalt and granite are 0.06
and 0.25, respectively (Faure and Mensing, 2005). This elemental fractionation leads to
variations in the isotopic composition of different magmatic rocks.
The amount of 87Sr found in a sample at any time is determined by many factors
including the initial 87Sr amount in the sample, the
87
Rb decay constant, the time since
the system become closed, and the 87Rb amount in the sample. Equation (1.2) presents
the absolute amount of 87Sr in the rock sample. However, because this is a small
number and it is not easy to count individual atoms, it is more practical to use isotope
amount ratios. Equation (2.14) is obtained by dividing equation (1.2) by the amount of
86
Sr.
(
)
(
)
(
)
(
Radiogenic isotopes have wide application to geochronology and isotope
geochemistry (Banner, 2004). Radiogenic strontium isotope analyses are used to
investigate and quantify the strontium budget in oceans (Palmer and Edmond, 1989;
Davis et al., 2003). Sources of radiogenic Sr to the oceans include river and ground
water discharge of Sr from continental weathering (Wickman, 1948). This could cause
an increase in the n(87Sr) /n(86Sr). Strontium with relatively lower amounts of
18
87
Sr is
input to the oceans from mid ocean ridges. The main sink for strontium is sedimentation
in marine carbonates.
Godderis and Francois (1995) focused their study on the strontium derived from
the ocean and found that seawater and marine carbonate, which was deposited from
seawater, have identical n(87Sr)/n(86Sr) isotope ratios. These ratios were calculated
using internal normalization to a fixed n(88Sr)/n(86Sr) isotope ratio (8.375209) (Nier,
1938). Therefore, any evidence of natural mass dependent isotopic fractionation was
erased by applying the internal normalization method.
Marine carbonates of different ages can be used to reconstruct the strontium
isotopic composition of seawater. Strontium isotope stratigraphy relies on recognized
variations in the ratio (n(87Sr)/n(86Sr)) of seawater through time. Burke et al. (1982)
analyzed 786 samples of marine limestones whose stratigraphic ages were known.
Thus, they reconstructed the variation of the n(87Sr)/n(86Sr) ratio of the oceans in
Phanerozoic time. The variation of n(87Sr)/n(86Sr) of seawater between 0 and 509 Ma is
shown in Figure 2.4.
19
Figure 2.4: The seawater strontium evolution of the Phanerozoic (Burke, 1982; Veizer, 1989)
The n(87Sr)/n(86Sr) ratio of seawater in Fig. 2.4 reached 0.7091 in Late Cambrian
time (500 Ma) and then fluctuated repeatedly as it decreased to 0.7068 in Late Jurassic
(158 Ma). More recently, the n(87Sr)/n(86Sr) ratio of seawater has been increasing with
only minor fluctuations toward the present value of 0.7092 (Godderis and Francois,
1995; Faure and Mensing, 2005).
20
2.5 The Stable Strontium Isotope System
The stable strontium isotope ratio (n(88Sr)/n(86Sr)) was considered a constant
value (8.375209) long time ago to correct the instrumental mass fractionation during the
radiogenic strontium isotope ratio measurement (n(87Sr)/n(86Sr)). This is largely because
mass dependent fractionation of heavier isotopes was thought to be negligible and not
possible to measure. However, in the following overview of recent research, it was
demonstrated that stable strontium isotopic composition is not constant but subtle
variations do exist.
Fietzke and Eisenhauer (2006) developed a sample-standard bracketing
technique for determining stable strontium isotope fractionation using multiple collector
inductively coupled plasma mass spectrometry (MC-ICP-MS). From their data, they
found a temperature dependence of δ88/86Sr in inorganically precipitated aragonite
(δ88/86Sr of 0.0054(5) ‰ / 0C) in the range of 10 to 50 0C and a temperature dependency
of δ88/86Sr in natural coral pavona clavus (0.033(5) ‰ / 0C) in the range of 23 to 27 0C.
Halicz et al., (2008) investigated the stable strontium isotope composition in
waters and rock materials. The porites and acropora corals investigated in their study
show δ88/86Sr values of 0.22 ± 0.07 ‰ (2sd). This result is in good agreement with
δ88/86Sr values of other coral samples reported by Fietzke and Eisenhauer in 2006. The
δ88/86Sr values of seawater samples in their study are 0.35 ± 0.06 ‰. Moreover, Halicz
et al. determined δ88/86Sr of marine and terrestrial carbonates. Thus, δ88/86Sr values
could be used as a tracer for chemical weathering and processes that lead to formation
21
of soils. The bracketing standard technique was used in their study to correct for mass
bias.
A first approach applying a sample-standard bracketing technique for
determining stable strontium isotope fractionation using MC-ICP-MS with zirconium (Zr)
as internal standard was presented by Yang et al. (2008). They used the n(90Zr)/n(91Zr)
ratio to correct for the measured Sr isotope amount ratios. They showed improvements
in precision compared to strontium isotope data obtained by only sample standard
bracketing without using Zr as internal standard. They found δ88/86Sr values of 0.207±
0.012 ‰ in fish liver samples.
Krabbenhoft et al. (2009) developed an accurate and precise method for
determination of stable strontium fractionation using TIMS with a double spike
technique. Their data for the IAPSO sea water and JCp-1 coral standards is in
agreement with previously published data and the precision improved using TIMS-DS
technique by at least a factor of 2–3 compared to MC-ICP-MS. Krabbenhoft et al. (2010)
applied this method to measure the natural variation in stable strontium isotopic
compositions in river water, hydrothermal solutions and marine carbonates to constrain
glacial or interglacial changes in the marine Sr budget.
Stable strontium isotope fractionation is also used in archaeological studies.
Knudson et al. (2010) investigated paleo- diets in archaeological human populations
and identified the trophic level (the relative position a living being occupies in food
chain) by using stable strontium data in human teeth and bones. They demonstrated
that stable strontium isotope compositions varied with increasing trophic level. Hence,
22
they showed that stable strontium isotope abundance data can be used to determine
the consumption of local and non-local strontium sources (Table 2.1).
A study by Charlier et al. (2012) showed the variations in stable strontium
isotopes in the early solar system between different planetary bodies. It is most likely
that light Sr isotopes volatilized during planetary formation at a high temperature. In
their study, stable strontium isotopes were determined by using multiple collector
inductively coupled plasma mass spectrometry (MC-ICP-MS) with zirconium (Zr) as an
internal standard. The precision they obtained for δ88/86Sr was 0.06 ‰, which is ten
times smaller than variations observed in in extra-terrestrial and terrestrial silicate
samples. This indicates that stable Sr isotopes are significantly fractionated at high
temperatures and therefore δ88/86Sr values can provide insights into planetary evolution
and magmatic processes. Some results of these studies are summarized in Table 2.1.
23
Table (2.1): Results of stable strontium measurements of selected studies, (2sd) is the error in 2
standard deviations, (2sem) is the error in 2 standard errors of the mean, (SB) is bracketing
standard method, (Zr) is Zirconium as external correction, (DS) is the double spike method.
Sample ID
Method
δ88/86Sr [‰]
Error [‰]
Reference
IAPSO seawater
SB
0.381
± 0.01 (2sem)
Fietzke and
Eisenhauer,2006
Porites
Acropora
IAPSO
Terra rossa soil
SB
SB
SB
SB
0.22
0.21
0.35
-0.18
± 0.07 (2sd)
± 0.22 (2sd)
± 0.06 (2sd)
± 0.15 (2sd)
Halicz et al., 2008
Halicz et al., 2008
Halicz et al., 2008
Halicz et al., 2008
IAPSO
JCp-1 coral
River waters
DS
DS
DS
0.386
0.197
0.24 - 0.42
± 0.005 (2sem)
± 0.008 (2sem)
± 0.01 to ± 0.02
(2sem)
Krabbenhoft et al., 2009
Krabbenhoft et al., 2009
Krabbenhoft et al., 2010
Hydrothermal fluids
DS
DS
bone and enamel
SB
-0.24
± 0.01 to ± 0.06
(2sem)
± 0.01 to ± 0.03
(2sem)
± 0.25 (2sem)
Krabbenhoft et al., 2010
Carbonate sediment
0.253 –
0.361
0.14 – 0.27
basaltic terrestrial
carbonaceous
chondrites
Martian meteorites
eucrites
Zr
Zr
0.25 - 0.32
-0.36 - 0.35
0.10 - 0.12 (2sem)
0.10 - 0.14 (2sem)
Charlier et al., 2012
Charlier et al., 2012
Zr
Zr
0.13 - 0.17
0.17 - 0.37
Charlier et al., 2012
Charlier et al., 2012
angrites
Zr
0.16 to 0.27
0.13 - 0.11 (2sem)
± 0.11 to ± 0.04
(2sem)
± 0.08 to ± 0.06
(2sem)
24
Krabbenhoft et al., 2010
Knudson et al., 2010
Charlier et al., 2012
The range of variations in δ88/86Sr values of standard materials and natural
samples determined in the previous studies are shown in Figure 2.5. Positive δ88/86Sr
values indicate a sample enriched in the heavier isotopes, whereas negative δ88/86Sr
values indicate enrichment in the lighter isotopes.
Eucrites
Martian meteorites
Carbonaceous chondrites
Basaltic terrestrial
Carbonate sediment
Hydrothermal fluids
River waters
Bone and enamel
Terra rossa soil
JCp-1
Acropora
Porites
IAPSO
Angrites
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
88/86
δ
Figure 2.5: The range of δ
88/86
0.2
0.3
0.4
0.5
0.6
Sr [‰]
Sr values of different materials that summarized in Table (2.1)
25
Chapter 3: Measurements of Accurate Strontium Isotope Amount Ratios
This chapter describes the analytical technique that was used for the strontium
isotope amount ratio measurements. A general overview of TIMS will be presented
focusing on the aspects relevant for strontium isotope measurements. Sample
preparation techniques will be discussed including sample digestion and ion exchange
chromatography. The strontium double spike technique that was used to correct for
instrumental mass bias will be discussed including preparation of the strontium double
spike enriched in both 84Sr and 87Sr, the double spike algorithm that was used to
calculate the stable and radiogenic strontium isotopic composition, and calibration of the
isotopic composition of strontium double spike.
3.1
Thermal Ionization Mass Spectrometer (TIMS)
3.1.1 A General Overview of the TIMS
A thermal ionization mass spectrometer (TIMS) is an analytical instrument that
can measure precise isotopic abundance ratios, but accurate data require correction for
mass fractionation. TIMS consists of three basic parts: an ion source, a mass analyzer,
and collectors.
Thermal Ionization Source
The thermal ionization source in TIMS is equipped with a sample turret
containing 21 positions for the filaments. An electric current passes through the metal
filament (Re) on which the sample is deposited. Ions are created by transmitting an
electron from the filament to the atom or from the atom to the filament to produce
26
negative or positive ions, respectively (Hoffmann et al., 2002). This process is known as
thermal ionization (see Figure 3.1).
Fig.3.1: Experimental arrangement of a single filament thermal ionization source (Becker, 2007)
The ions produced on the filament are accelerated through a potential difference
of 10 kV in the ion source to the magnetic sector analyzer. These ions are focused into
a beam by a series of slits and electrostatically charged plates. The ions then pass
through a magnetic sector where they are then separated according to their mass to
charge ratio.
27
Mass Analyzer (Magnet Sector)
The spread in kinetic energy of the ions is very small (~0.5 eV) in a thermal
ionization source. Therefore, it is sufficient to use a single focusing geometry that
focuses for directional (or angular) divergence only. When an ion of mass m and charge
q is accelerated by a potential difference V, it acquires Kinetic energy E as described
by:
(3.1)
Where v is the velocity of the ion. All ions of the same charge have the same kinetic
energy because they accelerated through the same potential difference. However, ions
having different masses have different velocities:
√
(
When moving ions encounter a magnetic field, they are deflected into circular
paths subject to the condition:
(
where r is the radius of arc of the ions deflected by the magnet, B is the magnetic field,
q is the charge of an ion, m is a mass of an ion and v is a velocity of an ion.
28
By elimination of the velocity of the ion from the equations (3.1) and (3.3),
equation (3.4) was obtained.
(
From Equation (3.4), the radius of arc depends only on mass, if only singly
charged ions are considered with constant magnet field and accelerating voltage.
Therefore, ions with different masses can be separated according to their mass to
charge ratio (m/q) as shown. This is the main function of the mass analyzer (magnetic
sector). Ions arriving at the high position are deflected less and are higher mass than
those which arrive at a lower positions (Herbert et al., 2003). These mass resolved
beams are then directed into collectors where the ion beam is converted into voltage.
Ion detection and measurement
A Faraday cup is utilized for the direct and accurate measurement of ion currents
of separated ion beams. A Faraday cup is a deep rectangular bucket made from
graphite and positioned behind the collector slit (see Figure 3.2). The charge on each
ion is passed to the cups on neutralization of the impacting ion. The cup is connected
with a circuit through which an electron current flows. This electron current can be
accurately measured and is directly proportional to the number of the ions that have
entered the Faraday cup. Errors in the measurement of the current are minimized with
the addition of an electron suppressor plate to the cup, as shown in Figure 3.2. The
suppressor plate suppresses the secondary electrons that may be released on ion
29
impact escaping from the cup. The voltage output of the Faraday cup amplifier is
proportional to the current input and the value of the high ohmic resistor (V=IR).
Suppress shield
High-ohmic resister
Ions beams
Faraday Cup
Amplifier
Collector slit
200V
Fig.3.2: Schematic diagram of a Faraday cup
3.1.2 TIMS Measurement of Strontium Isotopes
The strontium (Sr) extracted from samples was deposited on Re filaments along
with a solution of Ta in dilute phosphoric acid. Two microliters of the Ta 2O5 - activator
solution were added to the samples. The samples with the activator were deposited on
the filaments and heated until dryness at a current of 1 A. Finally, the filaments were
heated for one second, until the filament glowed red.
Strontium isotope measurements were performed on a Thermo-Fisher Scientific
Triton thermal ionization mass spectrometer in positive ionization mode with a 10 KV
acceleration voltage and 1011 ohm resistors in the Faraday cup amplifiers. The
instrument used five Faraday cups positioned to detect
30
84
Sr, 85Rb, 86Sr, 87Sr and 88Sr.
The TIMS cannot separate the ions 87Rb and 87Sr, therefore mass 85 was measured in
order to correct for interfering 87Rb assuming the isotope amount ratio of n(87Rb)/n(85Rb)
is 0.386000. Typically, the amount of Rb present was low (i.e. n(85Rb)/n(86Sr) of <
0.0001). Thus, this correction did not make a significant change in measurements.
Mass dependent isotope fractionation of the sample occurs in the ion source
where the strontium is heated and ionized from the filament surface. The light isotopes
evaporate leaving the remaining sample enriched in heavy isotopes. This effect is
known as instrumental mass fractionation (Habfast, 1998)
The measurements in the thermal ionization mass spectrometer started after the
filaments were heated to a current of 3.2 A corresponding to a filament temperature
about 1450 0C. Data acquisition was started when the signal intensity reached 5-10 V
for 88Sr+. For each sample 150 ratios were measured.
3.2
The Strontium Double Spike Technique
The double spike is a solution enriched in two isotopes and has a known isotopic
composition. The double spike method corrects for mass fractionation that occurs
during sample preparation and during thermal ionization mass spectrometry (TIMS)
measurements. By using this technique it is possible to determine two isotope amount
ratios at the same time, the stable isotope ratio (δ88Sr/86Sr) and the radiogenic isotope
ratio (n(87Sr)/n(86Sr)).
In the case of strontium, implementation of the double spike method requires two
measurements of isotope amount ratios for one sample. The first measurement is an
unspiked measurement for the isotopic composition of the sample and the second one
31
is a spiked measurement for a mixture of the sample combined with the spike. In this
study, an iterative routine that was adopted from Krabbenhoft et al. in (2009) is used to
find the true isotopic composition of the sample.
3.2.1 The Double Spike Algorithm
The algorithm begins by using isotope dilution equations (3.5, 3.6, and 3.7) for
(
the calculation of the n(84Srspike)/n(84Srsample) ratio (
) using values of
n(88Sr)/n(84Sr), n( 87Sr)/n(84Sr), and n( 86Sr)/n(84Sr) (3.11, 3.12 and 3.13). Note that 8xSr
represents the number of atoms of that isotope in the mixture, sample, or spike.
Substitution of
(
)
(
(
)
(
(
)
(
and
with
(
(
32
)
)
(
(
Results in
(
(
)
(
)
)
(
Rearranging equation (3.10)
(
)
(
)
(
(
(
)
(
)
Similarly, equations (3.12) and (3.13) can be obtained
(
)
(
)
(
(
(
(
)
)
(
)
(
)
(
(
(
)
(
)
In this equations the isotopic compositions of the spike is known and the three
values of the Q84 should be the same. However, mass dependent fractionation in the
measurement of the mixture result in difference results in differences among these
values. In order to determine the accurate value of the
33
84
Srspike/ 84Srsample, isotope
amount mixture ratios (
are calculated by using the mean of the 84Srspike/84Srsample
of the 86 and 88 strontium ratios as shown in equations (3.14).
(
(
(
(
Equation (3.18) is derived from equations (3.12) and (3.14) so that corrected values of
the isotopic composition of the mixture could be calculated.
(
(
((
)
(
)
(
)
(
)
)
)
(
)
(
(
(
(
(
(
(
)
(
)
(
(
(
(
)
(
)
(
(
(
)
)
(
(
)
(
)
(
(
(
(
New mixture ratios (87Sr/84Srmix(calc)) were calculated by using the exponential
fractionation law as expressed in equation (3.19), where M is the atomic mass of the Sr
isotope.
β
(
)
(
)
(
34
(
)
(
Equation (3.20) can be obtained by solving the equation (3.19) for β, the
fractionation factor to account for isotopic fractionation in the TIMS for mixture
measurements. Then, β is substituted in exponential fractionation equations to obtain
(86Sr/84Sr) mix (calc) and (88Sr/84Sr) mix (calc), equations (3.21) and (3.22).
[(
)
(
β
(
(
)
]
(
(
)
(
(
)
(
)
)
(
β
(
)
)
(3.21)
β
(
)
(3.22)
(
Now, updated values of the mixture ratios (equations 3.21 and 3.22) for the next
iteration of the routine could be obtained. In order to determine the natural fractionation
of the Sr isotopes, hence a new value for 88Sr/86Sr sample, the fractionation factor ( ) is
required. New values of the sample can be calculated from ( ) for the next iteration of
the routine. Another isotope dilution equation (3.23) can be used in the terms of
88
Sr/86Sr ratios for finding a new sample ratio of
(
)
88
Sr/86Sr (3.25).
(
)
(
(
(
(
(
(
)
)
(
(
)
)
(
(
)
(
(
(
35
)
)
)
(
(
)
(
)
(
The term (
(
)
(
(
)
can also be written in terms of (
(
(
)
(3.24).
(
(
)
(
(
(
)
A new value of 88Sr/86Sr sample is found to calculate (88Sr/86Sr)mix(calc) (3.25) from
the other mixtures (3.21) and (3.22).
(
(
)
(
)
(
(
(
)
(
The delta value (δ88/86Sr) is determined by using the 88Sr/86Sr sample (new) from
(3.26).
(
)
(
)
(
(
(
(
)
(
(
)
(
(
The fractionation factor ( ) is determined from the exponential fraction law (3.27)
to evaluate new starting values of the ratios 88Sr/84Sr, 87Sr/84Sr and 86Sr/84Sr (3.29, 3.30
and 3.31).
36
(
)
(
)
(
)
(
(
[(
)
(
)
]
(
(
(
)
The new isotope amount ratios of sample are calculated from the exponential
fractionation equations (3.29, 3.30 and 3.31).
(
)
(
)
(
)
(
(
)
(
)
(
(
)
(
)
(
(
(
)
(
(
)
(
Using the new values of the sample, the algorithm will start again. The algorithm
can end when the difference between Q86 (84) and Q88 (84) becomes less than 10-6.
(
(
The double spike algorithm was implemented using Microsoft Excel.
37
Start
Figure 3.3: Schematic diagram of the Sr double spike algorithm.
38
3.2.2 Double Spike Preparation
Two Sr-carbonates each enriched individually in
87
Sr and 84Sr were purchased
from Oak Ridge National Laboratory. The isotopic compositions of these carbonates are
listed in Table 3.1. The amount required of both
87
Sr and 84Sr to make double spike was
calculated based on the work by Rudge et al. (2009) to make a double spike ratio of
n(87Sr)/n(84Sr) around one. This ratio was chosen to optimize the precision of the
method. An 84Sr - stock solution was already available. In order to make the 87Sr-stock
solution, 8.1464 ± 0.0051 g of SrCO3 enriched in 87Sr was dissolved in 600 µL 3 % nitric
acid.
Table 3.1: Strontium-carbonate isotopic compositions from Oak Ridge National Laboratory
84
86
Sr
87
Sr
88
Sr
Sr
84-spike solution (%)
82.24 ± 0.1
3.71 ± 0.1
1.56 ± 0.1
12.49 ± 0.1
87-spike solution (%)
0.01 ± 0.00
0.82 ± 0.02
91.26 ± 0.10
7.91 ± 0.10
The concentrations of the 84Sr spike solutions and the 87Sr spike solutions were
290 ppm and 800 ppm, respectively. To obtain the n(87Sr)/n(84Sr) ratio of around one,
0.7± 0.1g of 84Sr were mixed with 0.223 ± 0.002 g of
87
Sr. The Sr isotopic composition
of the prepared double spike measured by TIMS was listed in Table 3.2.
Table 3.2: Measured isotopic composition of the double spike ratios by TIMS
86
84
0.052948 ± 0.000025
87
84
0.936621± 0.000030
88
84
0.231014 ± 0.000063
Sr/ Sr
Sr/ Sr
Sr/ Sr
39
3.2.3 Double Spike Calibration
One of the challenging aspects of this project was that the isotopic composition
of Sr double spike was not calibrated. Therefore, the first step was to determine the
accurate Sr isotopic composition of the double spike. This can be done using the
SRM987 strontium carbonate reference material, which has a known isotopic
composition, which is listed in Table 3.3. The SRM987 standard was spiked with
different amounts of the 87Sr-84Sr double spike to produce mixtures with different
strontium isotope amount ratios, n(84Srspike)/n(84SrSRM987), which ranged from 10 to 30.
This was done to calibrate the isotopic composition of the Sr double spike and to
determine the amount of Sr double spike to be mixed with samples.
Table 3.3: The Isotopic composition of the SRM987 carbonate reference material
Isotopes
Abundance (%)
84
0.5574 ± 0.0015
86
9.8566 ± 0.0034
87
7.0015 ± 0.0026
88
82.5845 ± 0.0066
Sr
Sr
Sr
Sr
In order to make different ratios of Sr double spike to SRM987, the amount of Sr
double spike that was added to the SRM987 was determined. One
n(84Srspike)/n(84SrSRM987) ratio was chosen, ratio 10, as an example to show the
calculation (Figure 3.4). One should note that in order to achieve successful analyses
on TIMS, a total of 200 ng of strontium on the Re filament are required.
40
84
Srsample =200 ng x 0.0056= 1.12 ng
(i.e. 0.56% is the abundance of 84Srsample)
84
Srspike/84Srsample = 10
84
Srspike=10 x 1.12=11.2ng
11.2 ng/0.4511=24.82197 ng
(i.e. 0.4511 is the abundance of 84Sr in Ds and
24.82197ng is the amount of Sr in DS needed)
CDs= 200 ng/g= 24.82197 ng/X
X=0.124 g
(i.e. X is the amount of Sr DS that needs to be added to samples to
make ratio 10 and CDs is Sr concentration in DS (200ng/g)
Figure 3.4: Calculation of the amount of Sr double spike for the ratio 10 of
84
double spike to the Sr in the SRM987 standard
84
Sr in the
Amounts of double spike and SRM987 standard measured in the laboratory are listed in
Table 3.4.
41
Table 3.4: Amounts of the Sr double spike (DS) and the SRM987 standard used to calculate
84
84
different n( Srspike)/n( SrSRM987) ratios
84
84
Srspike/ SrSRM987
DS amount (223 ppb) ±
SRM987 (203 ppb) ±
0.0001 g
0.0001 g
10
0.1270
1.0223
12.5
0.1682
1.0124
15
0.1844
1.0100
17.5
0.2206
1.0152
20
0.2502
1.0144
22.5
0.2788
1.0126
25
0.3153
1.0139
27.5
0.3397
1.0150
30
0.3731
1.0155
The ratios of n(84Sr spike/n( 84Sr SRM987) and δ88/86Sr value were analysed to
determine if there was a dependence of the δ88/86Sr value on the relative amount of
spike used. The δ88/86Sr value is only dependent on n(88Sr)/n(86Sr) ratios of samples
(eq.1.1), therefore the δ88/86Sr values should not depend on the different amounts of Sr
double spike that were added to the samples. If the relationship between the δ 88/86Sr
value and the ratios of n(84Srspike)/n(84SrSRM987 ) is a linear trend, than the measured
double spike isotopic composition is different from the “true” double spike isotopic
composition. If there is no relationship between δ88/86Sr values and
n(84Srspike)/n(84SrSRM987 ) ratios, then there is no dependence of δ88/86Sr on
n(84Srspike)/n(84SrSRM987 ) ratios and therefore the isotopic composition of the Sr double
spike that was used to calculate delta values is close to the “true” isotopic composition
42
of Sr double spike. The δ88/86Sr value is calculated by the double spike algorithm that
needs three TIMS measurements: (1) Unspiked measurement to calculate the Sr
isotopic composition for the SRM987 standard, (2) spiked measurements to calculate
the Sr isotopic composition for the SRM987 standard and Sr double spike mixtures, and
(3) the Sr isotopic composition for the Sr double spike.
3.3
Sample Preparation
The concentration of Sr in samples was needed in order to determine the amount
of samples required for the analyses. The Sr concentration in wood and Lake Vida
samples were known (K. Miller, personal communication, 2013). However, the Sr
concentrations in some Alberta water samples were unknown. In order to determine
accurate Sr concentrations in Alberta water samples, isotope dilution mass
spectrometry was used. In this method, a known amount of a sample is mixed with a
solution enriched in 84Sr. The isotopic composition of this spike is presented in
Table 3.1. Carefully weighed amounts of an enriched isotope solution and a sample
solution are mixed and the measured isotopic composition of the mixture can be used to
calculate the concentration of the element in the sample solution (Heumann, 1986).
The following equation (3.32) is used to calculate the concentration of strontium in the
samples:
(
(
(
))
(
)
(
where the subscripts (sam) refer to the sample and the subscripts (sp) refer to the
spike, (c) is the concentration of the element in spike or sample, (A) is the atomic mass
43
of the spike or sample, (w) is the weight of spike or sample in the mixture, (R) is the
measured, unnormalized isotope ratio of n(84Sr)/n(86Sr), and 84 and 86 refer to the
relative abundances of the natural and enriched strontium. In this equation the
concentration of the element in the sample is determined just by measuring the ratio in
mixture by mass spectrometry. All other parameters in the equation should be
measurable or known. It is assumed that the n(84Sr)/n(86Sr) of the sample is not much
different than that of the SRM987 standard and the SRM987 ratio can be used. This
assumption only introduces an error of ~1 % and is not significant in the context of this
study. Nevertheless, once an accurate isotope composition of the sample is determined,
the preparation of the mixture for IDMS can be repeated.
The enriched 84Sr spike was already available in the lab and its concentration
was calibrated against a strontium standard (SRM987), which has a Sr concentration of
203 ppb. The concentration of the spike was determined using isotope dilution equation
(3.29), where W sp=500 mg, W SRM987=1000 mg, Asp= 84.5328, ASRM987= 87.62, Rm=
4.457991 and 84Sr, 86Sr abundances of the spike and SRM987 standard are presented
in Table 3.1 and 3.3, respectively. The Sr concentration of the spike was 256.83 ng/g
with lower and upper bounds of 233.26 ng/g and 280.40 ng/g. The strontium
concentrations of the Alberta water samples was then calculated using isotope dilution
equation (3. 29), where Csp=256.83 ng/g, Wsp= 500 mg, W sam= 100 mg.
3.4
Wood Samples
In order to prepare samples for the double spike method, two measurements
(unspiked and spiked measurements) were performed. The unspiked measurements for
the wood were performed in a previous investigation (K. Miller, personal
44
communication, 2013). For the spiked measurements, 150 mg of the wood samples
(solid) was used and then 250 µl of the strontium double spike solution was added to
each of the samples. The mixture of samples and double spike were combusted in a
muffle furnace at 750 °C overnight. Five hundred microlitres of 3M HNO3 was added to
the ash. The mixture was dried under an infra-red lamp in the fume hood. Five hundred
microlitres of 3M HNO3 was then added to the residue.
Ion exchange was used to separate strontium from the sample mixture, in
particular Rb, prior to analysis. For the ion exchange process, the columns (one per
sample) were initially rinsed with 18.2 M polished (MQ) water. Five hundred
microlitres of the Eichrom Sr Spec resin was loaded in glass columns. MQ water was
added to clean the columns, followed by the addition 500 µL 3M HNO3 to rinse the
resin. Samples were then placed on the columns. The columns were rinsed with 1000
µL 3M HNO3 twice, followed by 500 µL 5 of MQ water to elute the strontium, which was
collected in 15 mL centrifuge tubes. The samples were completely dried under an infrared lamp in the fume hood.
The sample was then loaded on the zone refined rhenium (Re) filament with
about 2 µL Ta- activator solution. Samples were loaded into TIMS and heated to
1450 0C automatically. One hundred fifty ratios of n(86Sr)/n(84Sr), n(86Sr)/n(84Sr) and
n(88Sr)/n(84Sr) were measured to achieve acceptable levels of precision.
45
3.5
Water samples
In order to prepare the water samples for the Sr double spike method, unspiked
and spiked measurements were needed. For the spiked measurements, a 250 µL of
strontium double spike solution was added to the samples. Unspiked and spiked
samples in centrifuge tubes then were dried under a red lamp. Five hundred microlitres
of 3M HNO3 was added to both tubes to dissolve the samples. The Sr was isolated by
ion exchange as described in the previous section. Concentration data are provided in
Chapter 4.
46
Chapter 4: Results and Discussion
This Chapter discusses the stability of the thermal ionization mass spectrometer
to measure the Sr isotope amount ratios and output of the fractionation factor from the
algorithm to check that the algorithm had fully accounted for the isotopic fractionation in
the samples used in this project. Calibration of the isotopic composition of the Sr double
spike using SRM987 standard and results of standard measurements (SRM987 and
IAPSO seawater standard) are discussed. This section discusses and interprets the
data for different types of samples.
In order to determine the uncertainty in the measured strontium delta values and
the measured strontium radiogenic ratios, the Guide to Expression of Uncertainty in
Measurement (GUM) was used (JCGM, 2008). A method (type A) for uncertainty
analysis in the guide was used in this thesis. The experimental standard deviation (sd)
is used as the measurement uncertainty, 2sd at the 95 % confidence level. The
experimental standard deviation can be determined using the following equation (4.1).
√
∑(
̅
(
Where qi is an individual data point, ̅ is the mean value of all data points and n is the
number of data points.
47
4.1
Mass Spectrometer Stability
A strontium carbonate standard (SRM987) purchased from NIST was analyzed
with each sequence of samples to monitor the stability of the mass spectrometer. The
results for SRM987 standard collected over a period from October 2012 to September
2013 are listed in Table 4.1 .The accepted values of SRM987 standard for both ratios
n(87Sr)/n(86Sr) and n(84Sr)/n(86Sr) when normalized to the fixed n(88Sr)/n(86Sr) ratio of
8.375209 are 0.710240 and 0.056490, respectively. The values in brackets in Table 4.1
are the uncertainty associated with the last digit. The data are plotted in Figure 4.1.
48
Table 4.1: The results of SRM987 standard measured with each sequence of samples. The
88
86
uncertainty is 2sd - standard deviation. The ratios are normalized to a fixed n( Sr)/n( Sr) of
8.375209
Date
87
86
10/10/2012
0.710227(2)
0.056491(1)
25/10/2012
0.710237(3)
0.056491(1)
15/11/2012
0.710237(3)
0.056493(1)
27/02/2013
0.710230(3)
0.056485(5)
26/03/2013
0.710231(3)
0.056484(2)
12/05/2013
0.710238(2)
0.056491(1)
11/06/2013
0.710234(5)
0.056494(7)
14/06/2013
0.710238(3)
0.056484(2)
23/07/2013
0.710240(4)
0.056499(3)
09/08/2013
0.710238(6)
0.056494(3)
15/08/2013
0.710243(3)
0.056498(1)
19/08/2013
0.710251(2)
0.056488(1)
26/09/2013
0.710250(3)
0.056494(1)
Mean value
0.710238(14)
0.0564909(13)
Accepted value
0.710240
0.056490
Sr/ Sr
49
84
86
Sr/ Sr
a)
0.710255
0.710250
87Sr/86Sr
0.710245
0.710240
Historical Average
0.710235
0.710230
0.710225
0.710220
b)
0.056510
0.056505
84Sr/86Sr
0.056500
0.056495
Historical Averge
0.056490
0.056485
0.056480
0.056475
0
87
2
4
6
8
10
12
86
Fig. 4.1: a) n( Sr/n( Sr) ratios of the SRM987 standard measured with each sequence of samples
84
86
and b) n( Sr/n( Sr) ratios of the SRM987 standard measured with each sequence of samples.
The black line marks the historical mean for data collected over the period from Oct. 2012 to Sep.
2013. The broken lines are the errors (2sd).
50
14
The measurements of the standard SRM987 show change over the course of the
project (see Table 4.1 and Fig. 4.1). The reasons for this phenomenon are not fully
understood. Potential sources include the degradation of Faraday cups, changing signal
intensities or changing conditions of the vacuum pressures in the mass analyzer. The
uncertainties in the n(87Sr)/n(86Sr) and n(84Sr)/n(86Sr) ratios of SRM987 standard
(Table 4.1) were ± 14 ppm and ± 13 ppm, respectively. These uncertainties are low
enough that significant isotope abundance ratios can be measured.
4.2
Output of the Double Spike Algorithm
The main goal of this project was to develop a reliable Sr double spike method
that corrects for mass dependent isotopic fractionation that occurred during chemical
sample preparation and during mass spectrometry measurements. A Sr double spike
algorithm was used to calculate the stable and radiogenic isotopic compositions that
were reported as an δ88/86Sr value and an n(87Sr)/n(86Sr)* ratio. One of the stop criteria
of the algorithm that worked for all the samples is when the fractionation factor (𝝰) was
equal to zero. This meant the algorithm had fully accounted for the isotopic fractionation
due to chemical preparation and analysis on TIMS. The algorithm, however, never met
its initial stop criteria, which was the differences between Q86(84) and Q88(84) equal or
less than 10-6 (section 3.1.4). The differences between Q86(84) and Q88(84) in the
measurements determined in the thesis were equal or less than 10-3.
51
4.3
The Double Spike Calibration Result
Calibration of isotopic composition of Sr double spike was done to explore if the
calculated isotopic composition of a sample depended on how much Sr from double
spike was added to the sample (Double spike calibration is discussed in detail in section
3.1.3). Several different amounts of the Sr double spike were added to the SRM987
standard. The double spike algorithm was used to calculate δ88/86Sr values and
n(87Sr)/n(86Sr)* ratios simultaneously for the range of n(84Srspike)/n(84Srsample) ratios
measured in this project. Note that in order to correct for isotope fractionation that
occurred during TIMS measurements all n(88Sr)/n(84Sr), n(87Sr)/n(84Sr), and
n(86Sr)/n(84Sr) ratios were normalized to the mean of the first block of the isotope
n(87Sr)/n(86Sr) ratio. This does not produce an accurate result (i.e. true), but accounts
for the in-run isotope fractionation (e.g. see Figures 2.2 and 2.3). Three TIMS
measurements were needed to use the double spike algorithm: (1) unspiked
measurement to calculate the Sr isotopic composition for the SRM987 standard, (2)
spiked measurements to calculate the Sr isotopic composition for the SRM987 standard
and Sr double spike mixtures, and (3) the Sr isotopic composition for the Sr double
spike. Using the isotopic composition of double spike isotope amount ratios measured
by TIMS produced results showing that there was no dependence of the δ88/86Sr values
on the n(84Srspike)/(84Srsample) ratios (Fig.4.2).
52
δ88/86Sr = (0.00 ± 0.04) Ratios - 0.49
-0.6
δ88/86Sr [‰]
-0.5
-0.4
-0.3
-0.2
0
5
10
15
20
25
30
35
Different ratios of Sr double spike to SRM987
88/86
Figure 4.2: δ
88/86
δ
Sr versus different ratios of double spike to SRM987 show no dependence of
Sr on the different amount of double spike added (i.e. the ratios are in nanograms of
to nanograms of
84
84
Srspike
SrSRM987).
The δ88/86Sr values of different ratios to SRM987 obtained in this project were
very close to the expected value (δ88/86Sr= 0 ‰) and the variability of δ88/86Sr values of
different n(84SrSpike)/n(84Srsample) ratios to SRM987 was very low (about 0.09 ‰). The
exponential law in equation (4.2) was used to find the “true” isotopic composition of Sr
double spike.
β
(
)
(
)
(
)
(
where β is the fractionation factor, A and B are two different isotopes. M is the mass
number of the Sr isotopes. The fractionation factor (β) was chosen to be 0.087 in order
to shift the δ88/86Sr value of the ratio 20 of the Sr double spike to the SRM987 standard
(84Srspike/84Srsample= 20) close to zero. The reason for choosing n(84Srspike)/n(84Srsample)
53
ratio of 20 was that it provides optimal measurements with low uncertainty. Figure 4.3
shows the relation between all potential possibilities for fractionation factor (β) and
δ88/86Sr values of ratio 20.
0.08
0.06
δ88/86 Sr [‰]
0.04
0.02
0
0.083
0.084
0.085
0.086
0.087
0.088
0.089
0.09
0.091
-0.02
-0.04
-0.06
-0.08
Fractionation factors (β) of ratio 20
88/86
Figure 4.3: δ
Sr plotted versus different possibilities of ratio 20 to see the best fractionation
factor can bring δ
88/86
Sr of ratio 20 to close to zero.
The corrected δ88/86Sr value of the ratio 20 of the Sr double spike to the SRM987
standard was the closest value to zero and its value was equal to 2.4 × 10 -3 ‰. The
values of δ88/86Sr for different ratios of the Sr double spike to SRM987 were recalculated
using the “true” isotope composition of Sr double spike (Figure 4.4).
54
0.14
0.12
0.1
δ88/86Sr [‰]
0.08
0.06
0.04
0.02
0
-0.02
0
5
10
15
20
84Sr
84Sr
25
30
35
-0.04
-0.06
88/86
Figure 4.4: δ
Sr plotted versus
84
spike/
sample
84
SrSpike/ SrSRM987 ratios using corrected isotopic composition of
Sr double spike.
4.4
SRM987 Standard Measurements
Five mixtures of SRM987 and Sr double spike (with n(84Srspike)/n(84Srsample)= 20)
were prepared and the average of δ88/86Sr value, n(87Sr)/n(86Sr)* and n(87Sr)/n(86Sr)norm
ratios determined in this project were - 0.04 (4) ‰ (n=5), 0.710243 (15) (n=5), and
0.710258 (19) (n=6) respectively. The values in brackets are the uncertainty at 2sd
associated with the last digit (s).
55
Table 4.2: The Sr isotopic composition of the SRM987 standard measured in this project, the
uncertainties in brackets are 2sd
Sample
δ88/86Sr [‰]
87
Sr/86Sr*
SRM987
-0.065
0.710235
SRM987
-0.033
0.710247
SRM987
-0.048
0.710241
SRM987
-0.059
0.710237
SRM987
-0.015
0.710253
Average
-0.04 (4)
0.710243 (15)
The external precision (repeatability) for δ88/86Sr of the SRM987 standard
measured in this project is ± 0.04 (2sd), which is sufficient to measure natural Sr isotope
abundance variations. The external precision for n(87Sr)/n(86Sr)norm 19 ppm (i.e.
determined when assuming n(88Sr)n(/86Sr) ratio of 8.375209), which is slightly worse
than that for n(87Sr)/n(86Sr)*, 15 ppm (determined when Sr double spike was used). The
average n(87Sr)/n(86Sr)* amount ratio is 0.710243(15), which is in agreement with the
accepted n(87Sr)/n(86Sr) ratio of 0.710240. Measurements using the Sr double spike
technique can provide better precision for both stable and radiogenic Sr isotope amount
ratios on the instrument (TIMS) than those done by internal normalization to a fixed
n(88Sr)/n(86Sr) ratio of 8.375209.
56
0.04
0.02
0
δ88/86Sr [‰]
-0.02
-0.04
-0.06
-0.08
-0.1
-0.12
0
1
2
3
4
5
6
Sample number
0.710270
0.710260
87Sr/86Sr*
0.710250
0.710240
0.710230
0.710220
0.710210
0
1
2
3
4
5
6
Sample number
Figure 4.5: Variation of δ
88/86
87
86
Sr (a) and n( Sr)/n( Sr)* (b) for the standard (SRM987)
measurements. The black lines mark the average value of δ
87
86
88/86
Sraverge = - 0.04 ± 0.04 ‰ and
n( Sr)/n( Sr)*average = 0.710243 (15) (2sd). The broken lines are the errors (2sd).
57
4.5
IAPSO Seawater Standard Measurements
The International Association for the Physical Sciences of the Oceans (IAPSO)
distributes a seawater standard that was used in this study to represent the stable and
radiogenic strontium isotope compositions of seawater. The IAPSO standard has a
δ88/86Sr value that has been measured recently and the data are summarized in Table
4.4. However, these data are not calibrated and therefore an assessment of the
accuracy of the method cannot be done. Six replicates of the IAPSO seawater standard
were individually passed through the ion exchange columns and measured. The
isotopic composition of the IAPSO seawater standard (see Table 4.3 and Figure 4.6)
determined in this study were: δ88/86Sr =0.377(77) ‰, n(87Sr)/n(86Sr)* = 0.709325 (27)
(2sd, n=6) and n(87Sr)/n(86Sr)norm = 0.709191 (37) (2sd, n =4).
Table 4.3: The Sr isotopic composition of the IAPSO seawater standard
Sample
δ88/86Sr [‰]
87
IAPSO
0.354
0.709317
IAPSO
0.319
0.709304
IAPSO
0.421
0.709341
IAPSO
0.371
0.709323
IAPSO
0.399
0.709333
IAPSO
0.402
0.709334
Average
0.377(77)
0.709325(27)
58
Sr/86Sr*
The δ88/86Sr value of the IAPSO determined in this study is in general agreement
within uncertainty with previously published data (Table 4.4)
88/86
Table 4.4: δ
Sr values of IAPSO sea water standard determined in previous studies
δ88/86Sr [‰]
Errors [‰]
Reference
0.381
± 0.010 (2sem)
Fietzke and Eisenhauer,2006
0.35
± 0.10 (2sem)
Halicz et al., 2008
0.386
± 0.005 (2sem)
Krabbenhoft et al., 2009
0.401
± 0.010 (1sd)
JinLong et al., 2013
0.377
± 0.077 (2sd)
This project
59
a)
0.50
δ88/86Sr [‰]
0.45
0.40
(Fietzke and
Eisenhauer,2006)
0.35
δ88/86Sr= 0.381 (10)‰
0.30
0.25
0.20
0
2
4
6
8
Sample Number
b)
0.709380
0.709360
(Krabbenhoft et al., 2009)
87Sr/86Sr= 0.709312(9)
87Sr/86sr
0.709340
0.709320
0.709300
0.709280
0.709260
0
2
4
6
8
Sample Number
Fig. 4.6. a: Measurments of δ
87
88/86
Sr of the IAPSO seawater standard. b: Measurments of
86
n( Sr)/n( Sr)* of the IAPSO seawater standard. The soild lines in (a) and (b) are the averge values
of the IAPSO seawater standard measured in this project, δ
87
86
87
86
88/86
Srmean=0.377 (77) ‰ and
88/86
n( Sr)/n( Sr)* = 0.709325 (27) (2Sd), respectively. The long dash lines represent δ
Sr and
n( Sr)/n( Sr)* ratios from previously published data. The broken lines are the uncertainties (2sd)
in this project.
60
The external precision of δ88/86Sr and n(87Sr)/n(86Sr)* for IAPSO measured in this
project were ± 0.077 ‰ and ± 27 ppm, respectively. These uncertainties were
considered to be the analytical uncertainty for all other sample measurements because
of the processing that was necessary to prepare the sample.
The n(87Sr)/n(86Sr)norm value of 0.709191 (37) measured in this project is in
agreement within uncertainty with previous study (n(87Sr)/n(86Sr)norm = 0.709168(7)) by
Halicz et al. (2008). The n(87Sr)/n(86Sr)* value of 0.709325 (27) measured in this project
is in agreement within uncertainty with the n(87Sr)/n(86Sr)* value of 0.709312(9) reported
by Krabbenhoft et al. (2009). However, the n(87Sr)/n(86Sr)* value of 0.709325(27) is
different from the accepted seawater ratio (n(87Sr)/n(86Sr)norm = 0.709168 (7)) (Halicz et
al, 2008), which was normalized to a fixed n(88Sr)/n(86Sr) ratio of 8.375209. The
difference of 157 ppm might be due to the conventional normalization procedure where
the measured n(87Sr)/n(86Sr) ratio is normalized to a fixed n(88Sr)/n(86Sr) ratio of
8.375209 (δ88/86Sr = 0) neglecting natural strontium isotope mass dependent
fractionation. This was the “traditional” approach because there was no knowledge of
the natural n(88Sr)/n(86Sr) ratio.
4.6
Sr Isotopic Composition of Alberta Ground Water
Table 4.5 show the δ88/86Sr values and n(87Sr)/n(86Sr)* ratios of replicate
measurements of some Alberta ground water sample. The replicate analyses of L53
sample have a 0.004 ‰ difference between δ88/86Sr values and 1 ppm difference
between n(87Sr)/n(86Sr)* ratios. The differences between the δ88/86Sr values and in the
n(87Sr)/n(86Sr)* ratios observed in the MM3 sample were 0.02 ‰, 8 ppm, respectively.
61
The agreement between the duplicate data obtained using the double spike technique
suggests that this mass bias correction technique provides accurate results and can be
used for strontium isotopic composition analysis of ground waters. However, one
notable exception is the replicate analyses of PP7 sample where there is a 0.421 ‰
difference in δ88/86Sr values and 151 ppm difference in n(87Sr)/n(86Sr)* ratios. The
reason for this difference could be because different amounts of the double spike were
added to the sample PP7 and the duplicate. The Q(84) values (84Srspike/84Srsample)
calculated from the double spike algorithm recorded the difference in the amount of the
Sr double spike added to the samples. This effect can be seen in Figure 4.4, where the
δ88/86Sr value changes significantly as the Q(84) value (84Srspike/84Srsample) changes from
10 to 30. The “true” isotopic composition of the double spike was calculated from the
Q(84)=20, deviation from a ratio of 20 can cause significant shift in δ 88/86Sr values. In
the case of the sample PP7 and the duplicate, Q(84) values were 30 and 50,
respectively. It is not known how such a large shift in the measured δ 88/86Sr value of
ratios 30 and 50 (Q(84) = 30 or 50) could result.
62
Table 4.5: Sr concentration and the isotopic composition of Alberta ground waters, the
87
86
87
86
n( Sr)/n( Sr)* ratios calculated from double spike algorithm (Section 3.2.1) and n( Sr)/n( Sr)norm
ratios normalized to 8.375209. The symbol (𝝙) represents the differences between these ratios.
The uncertainties are in 2sd. Only one unspiked measurement was performed and therefore only
87
86
one n( Sr)/n( Sr)norm was reported for each MM3, PP7 ,and L53 samples and their duplicate.
88/86
Sr Concentration
δ
[ppm]
MM3
Samples ID
Sr
87
86
87
86
𝝙 [ppm]
n( Sr)/n( Sr)*
n( Sr)/n( Sr)norm
±0.077‰
± 27ppm
± 37ppm
0.222 ± 0.004
0.764
0.710058
0.709551
507
MM3-dup
0.222 ± 0.004
0.784
0.710066
0.709551
515
PP7
6.66 ± 0.14
1.711
0.713821
0.710708
3113
PP7-dup
6.66 ± 0.14
1.290
0.713670
0.710708
2962
L53
0.80 ± 0.02
0.586
0.710104
0.709816
288
L53-dup
0.80 ± 0.02
0.582
0.710103
0.709816
287
Miette
15.2 ± 0.3
0.818
0.708443
0.708151
292
Hot Springs Cave
0.179 ± 0.004
0.341
0.704588
0.704467
121
Soda Spring
1.221 ± 0.024
0.396
0.747244
0.747094
150
Boron Travertine Hot
1.60 ± 0.03
0.612
0.706630
0.706412
218
Banff Upper Springs
1.62 ± 0.03
2.726
0.709570
0.708608
962
Canyon Hot Springs
0.243 ± 0.004
-1.007
0.717493
0.717857
-364
Jemez Springs
639 ± 12
-0.218
0.721847
0.721926
-79
Boron Arsenic Springs
1.197 ± 0.024
-0.979
0.745973
0.746341
-368
Faywood
0.760 ± 0.016
0.137
0.716473
0.716424
49
Boron McCradie Hot
6.911 ± 0.14
0.726
0.704140
0.703884
256
Banff Middle
1.99 ± 0.04
0.327
0.708576
0.708460
116
Dewar
4.087 ± 0.082
0.512
0.709209
0.709026
183
Liard Alpha (75R)
9.49 ± 0.19
0.308
0.709318
0.709208
110
Many Springs
0.566 ± 0.012
1.498
0.709350
0.708815
535
Boron Springs (75R)
1.338 ± 0.026
0.925
0.745969
0.745621
348
Light Feather Springs
0.254 ± 0.006
0.657
0.714038
0.713803
235
Spring
Springs
63
The n(87Sr)/n(86Sr)norm and n(87Sr)/n(86Sr)* ratios of the Alberta ground water
presented in Table 4.5 and Figure 4.7 shows variations from 0.703884 to 0.747094 and
from 0.704140 to 0.747244, respectively. The highest ratio of n(87Sr)/n(86Sr)norm ratio
and n(87Sr)/n(86Sr)* ratio is recorded in the Soda Spring sample and the lowest amount
ratio is recorded at Boron McCradie Hot Springs sample. Alberta water samples also
show a wide range of δ88/86Sr values as can be seen in Table 4.5 and Figure 4.7. The
highest δ88/86Sr value of 2.726 ‰ was measured in a Banff Upper Springs sample. A
Canyon Hot Springs Boron sample has a much lower δ88/86Sr value of -1.007 ‰. The
origin of this large range of δ88/86Sr values and n(87Sr)/n(86Sr)* ratios in Alberta
groundwater samples is unknown and further information about these samples is
unavailable. However, the wide range of δ88/86Sr for Alberta spring water measured in
this project is consistent with the range of δ88/86Sr of water samples measured by others
(Neymark et al. 2013).
64
0.750
Soda Spring
Boron Springs
0.745
Boron Arsenic Springs
0.740
0.735
87Sr/86Sr*
0.730
0.725
Jemez Springs
0.720
Canyon Hot Springs
Faywood
0.715
Light Feather Spring
Liard Alpha
0.710
Banff Upper
Dewak
Many Springs
Miette
Banff Middle
Boron Travertine
0.705
Boron McCradie
Hot Springs Cave
0.700
-1.2
-0.7
-0.2
0.3
0.8
1.3
1.8
2.3
2.8
δ88/86Sr [‰]
88/86
Fig. 4.7: δ
Sr values plotted versus
bars for
87
87
86
Sr/ Sr* ratios of Alberta ground water samples. The error
86
Sr/ Sr* ratios are too small to be seen.
The differences between n(87Sr)/n(86Sr)* ratios calculated from the double spike
algorithm and n(87Sr)/n(86Sr)norm ratios normalized to an assumed and fixed
n(88Sr)/n(86Sr) ratio of 8.375209 are listed in Table 4.5. The differences in these ratios
have a wide range from -368 to 962 ppm. Figure 4.8 shows the relationship between the
differences between the n(87Sr)/n(86Sr)* and (87Sr)/(86Sr)norm ratios and δ88/86Sr values of
Alberta ground waters. It is clear from Figure 4.8 that the differences between the
65
n(87Sr)/n(86Sr)* and n(87Sr)/n(86Sr)norm ratios increase with increasing δ88/86Sr values.
This difference in the n(87Sr)/n(86Sr) ratios for each individual sample is the result of
using two different ways to calculate the n(87Sr)/n(86Sr) amount ratio. The
n(87Sr)/n(86Sr)* ratios were calculated from unspiked and spiked measurements and
n(87Sr)/n(86Sr)norm ratios were calculated from using only unspiked measurements that
normalized to the assumed and fixed n(88Sr)/n(86Sr) ratio of 8.375209. Therefore, the
mass dependent fractionation of Sr isotopes was not taken into account when the fixed
n(88Sr)/n(86Sr) ratio of 8.375209 was used to calculate n(87Sr)/n(86Sr)norm ratios. The
differences between the ratios are small when δ88/86Sr values are close to zero. Banff
Upper Springs sample has the highest δ88/86Sr value of 2.726 ‰ leading to the highest
difference between the n(87Sr)/n(86Sr)* ratio and the n(87Sr)/n(86Sr)norm ratio (962 ppm).
The lowest difference between ratios, n(87Sr)/n(86Sr)* and n(87Sr)/n(86Sr)norm, is recorded
for the Faywood sample, 49 ppm, whose has the δ88/86Sr value of 0.137 ‰.
66
1200
1000
𝝙 = 358.41δ88/86Sr - 1.5055
800
Boron Springs
𝝙 [ppm]
600
400
200
δ88/86
0
-2
-1
0
1
2
[‰]
3
-200
-400
Boron Arsenic
-600
Figure 4.8: The linear relationship between δ
87
86
87
88/86
Sr values and the differences between
86
n( Sr)/n( Sr)* and n( Sr)/n( Sr)norm ratios for Alberta waters (𝝙).
Ignoring mass dependent effects (by considering δ88/86Sr value = zero) leads to
an increase in the deviation of reported n(87Sr)/n(86Sr) ratios even more than the typical
instrumental uncertainty. The variability of δ88/86Sr values and the difference between
the n(87Sr)/n(86Sr)* ratio and the n(87Sr)/n(86Sr)norm ratio are given in Table 4.5.
The difference between the n(87Sr)/n(86Sr)* ratio and the n(87Sr)/(86Sr)norm ratio
arose because the n(87Sr)/n(86Sr)norm ratios were determined using a fixed
n(88Sr)/n(86Sr) ratio of 8.375209. Recall that the n(87Sr)/n(86Sr)* ratios were determined
using one unspiked measurement of the sample. The actual value of the n( 88Sr)/n(86Sr),
which is called (n(88Sr)/n(86Sr)*), for each individual sample can be determined from
67
their δ88/86Sr values. This is then used to find the instrumental fractionation that occurred
during the measurement of the unspiked sample. The fractionation factor found from
n(88Sr)/n(86Sr)* ratio is then applied to measured n(87Sr)/n(86Sr) ratio to produce an
accurate n(87Sr)/n(86Sr) (here called n(87Sr)/n(86Sr)*new see Table 4.6) (Figure 4.9).
to calculate β
to calculate 88Sr/86Sr*
87
86
to calculate Sr/ Sr*new
87
86
Figure 4.9: Diagram of calculation of n( Sr)/n( Sr)*new ratios, the second and third equations are
in the TIMS’s software.
The differences between the n(87Sr)/n(86Sr)* and (87Sr)/n(86Sr)*new ratios are in
the range of 0 to10 ppm (Table 4.6), which are smaller compared to the differences
between the ratios of n(87Sr)/n(86Sr)* and n(87Sr)/n(/86Sr)norm, which ranged from -368 to
962 ppm (Figure 4.8).
68
87
86
Table 4.6: The Sr isotopic composition of some Alberta ground water samples. n( Sr)/n( Sr)
87
86
ratios are calculated in two different ways. The n( Sr)/n( Sr)*new ratios calculated from
87
86
88
86
normalization of n( Sr)/n( Sr) by TIMS to n( Sr)/n( Sr)* calculated from δ
87
88/86
Sr for each
86
individual sample. n( Sr)/n( Sr)* calculated from the algorithm. The last column shows the
87
86
88
86
87
86
differences in ppm between the n( Sr)/n( Sr)* and n( Sr)/n( Sr)*new ratios for each individual
sample.
Samples ID
87
86
87
86
n( Sr)/n( Sr)*
n( Sr)/n( Sr)*
n( Sr)/n( Sr)*new
Differences
± 27ppm
± 27ppm
± 27ppm
[ppm]
Miette
8.382060
0.708443
0.708443
0
Hot Springs Cave
8.378069
0.704588
0.704588
0
Soda Spring
8.378538
0.747244
0.747244
0
Boron Travertine Hot Spring
8.380331
0.706630
0.706630
0
Banff Upper Springs
8.398045
0.709570
0.709580
-10
Canyon Hot Springs
8.366775
0.717493
0.717493
0
Jemez Springs
8.373386
0.721847
0.721847
0
Boron Arsenic Springs
8.367012
0.745973
0.745973
0
Faywood
8.376353
0.716473
0.716473
0
Boron McCradie Hot Springs
8.381288
0.704140
0.704141
-1
Banff Middle
8.377946
0.708576
0.708576
0
Dewar
8.379499
0.709209
0.709209
0
Liard Alpha (75R)
8.377786
0.709318
0.709318
0
Many Springs
8.387757
0.709350
0.709350
0
Boron Springs (75R)
8.382960
0.745969
0.745966
3
Light Feather Springs
8.380715
0.714038
0.714039
-1
69
4.7
Sr Isotope Composition of Lake Vida
Lake Vida, a unique lake located in Antarctica, is very cold, dark, salty and
remained isolated from the outside world for thousands of years by thick ice cover
(Murray et al., 2012). Three brine samples from a 2010 expedition were analyzed in this
project for strontium isotopic composition. The samples were filtered to eliminate any
precipitates when they were received in Calgary. The analytical results, including
δ88/86Sr, n(87Sr)/n(86Sr)* and n(87Sr)/n(86Sr)norm of Lake Vida samples are shown in Table
4.7 and Figure 4.10. The δ88/86Sr values measured for Lake Vida were found to be
within the range of δ88/86Sr values in the terrestrial environment.
Table 4.7: Sr isotopic composition of Lake Vida, the uncertainties are in 2sd.
Lab-ID
Pieces name
Sr concentration [ppm]
88/86
δ
Sr
± 0.077‰
87
86
87
86
n( Sr)/n( Sr)*
n( Sr)/n( Sr)norm
± 27 ppm
± 37ppm
DM-FB1
Filtered Brine 1
40
0.521
0.710880
0.710689
DM-FB2
Filtered Brine 2
40
0.515
0.711040
0.710856
DM-WB1
Whole Brine 1
40
0.526
0.710896
0.710708
70
0.71110
87Sr/86Sr
0.71105
DM-FB2
0.71100
0.71095
DM-WB1
0.71090
DM-FB1
0.71085
0.71080
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
δ88/86Sr [‰]
Figure 4.10: δ
88/86
87
86
Sr values versus n( Sr)/n( Sr)* ratios of Lake Vida samples
The variability in δ88/86Sr values of Like Vida, 0.011 ‰ (2sd) is within typical
analytical uncertainty of 0.077 ‰, which is estimated from the IAPSO uncertainty
measured in this project. However, the uncertainty in n(87Sr)/n(86Sr)* ratios, 176 ppm
(2sd) is larger than typical analytical uncertainty of 27 ppm (2sd), the IAPSO uncertainty
measured in this project. In order to determine the strontium isotopic composition for
Like Vida samples using the Sr double spike method, two measurements are required,
unspiked and spiked measurements. There is a possibility of cross-contamination when
drying samples containing spike in close proximity to unspiked samples in the fume
hood. The double spike contamination contributed to a decrease in precision in the
unspiked measurement of the sample FB2. The n(84Sr)/n(86Sr) ratio normalized to the
assumed and fixed n(88Sr)/n(86Sr) ratio of 8.375209 of the unspiked measurement for
the sample FB2 was found to be 0.0568, which is different compared to the
84
Sr/86Sr
ratio in nature that ranges from 0.0565 - 0.0567. This difference in n(84Sr)/n(86Sr) is
71
small, but it indicates Sr double spike contamination that could affect n(87Sr)/n(86Sr)
ratios more than analytical uncertainty. A repeat of unspiked measurement of the
sample FB2 is required to make sure no contamination of Sr double spike is present, but
this step was not possible because additional amounts of this sample was unavailable.
4.8
Sr Isotope Composition of Wood
In order to find the δ88/86Sr values and n(87Sr)/n(86Sr) ratios of exotic rosewood
samples using the double spike technique, many attempts were made. In all attempts,
the wood samples were ashed, mixed with 3M HNO3, and the Sr was isolated from the
samples using ion exchange. In the first attempt, the Sr double spike was added to
150 mg wood samples before combustion of the sample material. The results are
recorded in Table 4.8 and indicate that Sr double spike was lost from the mixtures
because the Q(84) (n(84SrSpike)/n(84SrSample)) values were very low. Recall that the target
Q(84)value was 20.
72
Table 4.8: The Sr isotopic composition of exotic rosewood samples using the first attempt
88/86
Lab ID
Species name
δ
G-17223
Dalbergia spruceana
G-17223-dup
Sr
87
86
87
86
n( Sr)/n( Sr)*
n( Sr)/n( Sr)norm
Q(84)
7.401
0.721710
0.718974(4)
4.2
Dalbergia spruceana
2.151
0.719816
0.718974(4)
5.5
G-17226
Dalbergia spruceana
0.976
0.719502
0.719052(3)
2.0
G-17226-dup
Dalbergia spruceana
0.730
0.719413
0.719052(3)
2.2
G-17241
Dalbergia cearensis
-34.334
0.725411
0.738163(3)
1.6
G-17242
Dalbergia cearensis
0.372
0.742416
0.742271(35)
1.6
G-17301
Dalbergia Melanoxylon
5.075
0.708137
0.706331(4)
0.7
G-17302
Dalbergia Melanoxylon
-1.054
0.709531
0.709295(44)
2.8
G-17302-dup
Dalbergia Melanoxylon
-0.089
0.709876
0.709295(44)
3.1
G-17303
Dalbergia Melanoxylon
-112
0.667897
0.708957(5)
0.3
G-17328
Dalbergia latifolia
-0.920
0.704964
0.705286(10)
2.6
G-17328-dup
Dalbergia latifolia
-2.57
0.704379
0.705286(10)
2.5
G-17329
Dalbergia latifolia
132
0.753155
0.704309(21)
1.7
G-17339
Caesalpinia enchinata
0.021
0.708857
0.708459(4)
7.9
G-17341
Caesalpinia enchinata
0.238
0.708008
0.707862
1.2
G-17342
Caesalpinia enchinata
197
0.775044
0.707995
1.4
G-17342-dup
Caesalpinia enchinata
6.040
0.710159
0.707995
1.1
G-17353
Dalbergia nigra
0.548
0.744035
0.743699
2.8
G-17353
Dalbergia nigra
0.197
0.743904
0.743699
2.7
The Q(84) values of these samples calculated from the algorithm were found to
be in the range of 0.3 – 7.9, which is less than the desired Q(84) value of 20. The small
Q(84) value indicates either more than 200 ng of strontium in the wood samples was
present or loss of the Sr double spike had occurred.
73
It was likely that the Sr double spike became volatile in the oven while burning
the mixture of samples with the Sr double spike. In order to check for this effect, two
blanks were measured. This was done by adding a known amount of the Sr double
spike to two empty beakers and drying them down in two different ways, one blank of Sr
double spike was dried under an infra-red heat lamp and the other one was dried in the
oven at 700 0C. The Sr isotope amount ratios and the signal intensities of these blanks
are listed in Table 4.9.
Table 4.9: The signal intensities and the isotope amount ratios of Sr double spike as a result of
using different ways for drying the samples, drying in the oven and drying under the heat lamp,
DS refers to Sr double spike.
DS
DS
Oven dried
Heat lamp dried
88
2.40228
0.285884
87
1.12354
0.944889
86
0.311141
0.059631
88
0.076
1.36
87
0.035
4.48
86
0.0098
0.282
84
0.032
4.75
Sr/84Sr
Sr/84Sr
Sr/84Sr
Sr [V]
Sr [V]
Sr [V]
Sr [V]
It is evident from the low signal intensities that most of the Sr double spike is lost
when evaporated in the oven at 750 0C. This loss is not seen when dried under the heat
lamp as the temperature does not exceed ~ 60-70 0C.
For the second attempt, the wood samples were ashed first and then the Sr
double spike was added. This step was done to prevent evaporation of Sr double spike
74
in the oven. The Sr isotopic compositions using this attempt are reported in Table 4.10.
The Q(84) values are still lower than the desired value (20), which means the spike did
not properly equilibrate with the sample. Adding Sr double spike to the ash then directly
drying the mixture might result in evaporation of Sr double spike before it can equilibrate
with the samples.
Table 4.10: The Sr isotopic composition of exotic rosewood samples using the second attempt
88/86
Lab ID
Species name
δ
G-17293
Dalbergia retusa
G-17293-dup
Sr
87
86
87
86
n( Sr)/n( Sr)*
n( Sr)/n( Sr)norm
Q(84)
22.277
0.712561
0.70469849
0.5
Dalbergia retusa
17.774
0.710978
0.70469849
0.5
G-17360
Dalbergia nigra
-127
0.682283
0.730257
0.9
G-17360
Dalbergia nigra
-221
0.643934
0.730257
0.8
G-17363
Dalbergia nigra
62.331
0.760261
0.737441
3.4
G-17363-dup
Dalbergia nigra
68.075
0.762328
0.737441
3.6
G-17369
Pterocarpus indicus
92.154
0.744511
0.708402
1.6
G-17369-dup
Pterocarpus indicus
-1.088
0.711793
0.708402
1.4
G-17434
Pterocarpus indicus
55.723
0.754917
0.734870
0.9
G-17434-dup
Pterocarpus indicus
150.8
0.788426
0.734870
1.1
To explore this issue a third attempt was made on the G17409 sample (Table
4.11). A larger amount of the sample (300 mg) was combusted in the oven at 650 0C in
order to perform unspiked and spiked measurements simultaneously. Five hundred
microliters of 3M HNO3 were added to the ash and left for several hours before Sr
double spike was added. This step was done to allow sufficient time to extract strontium
from the ash before adding the Sr double spike. Two hundred and fifty microliters of the
mixture of the ash with 3M HNO3 was transferred to two Teflon beakers and then 250 µl
75
of Sr double spike was added to one Teflon beaker. Both solutions were dried under the
heat Lamp. Ion exchange was used to separate Sr from the sample matrix (Section
3.3.1). This attempt worked for the G17409 sample and its duplicate and Q(84) values
of 15 and 19 were obtained, respectively. The differences in the δ 88/86Sr and 87Sr/86Sr of
the G17409 sample were 0.02 ‰ and 5 ppm, respectively, which is within typical
analytical uncertainty. Further trials are needed to confirm the reliability of this method.
Table 4.11: The Sr isotopic composition of exotic rosewood samples using the third attempt. The
uncertainties are in 2sd
Lab ID
Species name
88/86
δ
Sr
87
86
87
86
n( Sr)/n( Sr)*
n( Sr)/n( Sr)norm
±0.077‰
± 27ppm
± 37ppm
Q(84)
G-17409
Phoebe parosa
-0.129
0.713484
0.713530
15
G-17409-dup
Phoebe parosa
-0.114
0.713489
0.713530
19
76
Chapter 5: Summary and Future Work
The goal of this project was to develop an analytical technique to measure the
stable and radiogenic strontium isotope amount ratios in samples. Measuring stable
strontium isotopic composition reveals important insights into mass dependent
processes as well as provides accurate data for the isotopic composition of the
radiogenic strontium isotope. Measuring radiogenic strontium isotopic composition
provides an absolute age of a sample and traces the origin of a Sr source to a system.
Thermal ionization mass spectrometry (TIMS) was used to analyze strontium in two
standards, SRM987 and IAPSO seawater standard, and variety of samples, including
Alberta ground water, water from Lake Vida, and exotic rosewood. All samples were
mixed with strontium double spike enriched in 87Sr and 84Sr to correct for isotope mass
dependent fractionation that occurs during sample preparation and during TIMS
measurement. Strontium was separated from the sample mixture and other unwanted
elements such as 87Rb through ion exchange procedures. Samples with were then
loaded onto Re filaments along with a Ta activator solution. The samples were
deposited on the filaments and then loaded into TIMS’s carousel to measure the isotope
amount ratios of samples.
Two standards were analyzed to test the robustness of the analytical technique.
The results obtained in this project for the SRM987 and the IAPSO sea water standards
are in accordance with previously published data. The strontium isotope composition of
the SRM987 standard was determined as δ88/86Sr= - 0.04 (4) ‰, n(87Sr)/n(86Sr)*=
0.710243 (15) and a corresponding conventionally normalized (i.e. normalized to a fixed
n(88Sr)/n(86Sr) ratio of 8.375209) n(87Sr)/n(86Sr)norm= 0.710258 (19) (all uncertainties
77
2sd). The IAPSO sea water standard results were δ88/86Sr= 0.377 (77) ‰,
n(87Sr)/n(86Sr)* = 0.709325(27) and n(87Sr)/n(86Sr) = 0.709191(37) (all uncertainties
2sd). The results of the SRM987 and IAPSO sea water standards indicate that the
precision using the Sr double spike technique is better than that when a fixed
n(88Sr)/n(86Sr) isotope ratio is used. The IAPSO uncertainty was taken to be the
analytical uncertainty for all other samples measured in this project.
Two different types of samples were analyzed for the Sr isotopic composition
using the analytical technique developed. The first type of sample included Alberta
ground water and water from Lake Vida. Alberta ground water highlighted that the
deviation in measuring n(87Sr)/n(86Sr) ratios can increase more than the typical
instrumental uncertainty when mass dependent effects were ignored (δ88/86Sr = zero).
Moreover, the similarity between the n(87Sr)/n(/86Sr)* and n(87Sr)/n(86Sr)*new ratios for
each individual Alberta ground water demonstrated the robustness of the algorithm that
was used to calculate the n(87Sr)/n(86Sr)* ratio accurately for a given n(88Sr)/n(86Sr)
ratio. Some Alberta ground water samples also showed success of the Sr double spike
method to be used for Sr isotopic composition analyses. The mass bias correction
technique provided results that were identical within typical analytical uncertainties
using Alberta ground water samples, L53 and MM3. However, the large difference
observed in the analysis of the PP7 sample and the duplicate suggests that adding an
accurate amount of Sr double spike to samples is required in order to obtain an
accurate precision for Sr isotopic composition measurement. Lake Vida samples also
showed success of the Sr double spike method to be used for Sr isotopic composition
measurement. The variability in δ88/86Sr values of Like Vida was 0.011 ‰ (2sd).
78
However, the uncertainty in measurements of the n(87Sr)/n(86Sr)* ratios was larger than
typical analytical uncertainty of 27 ppm (2sd). The large uncertainty observed in Lake
Vida samples in n(87Sr)/n(86Sr)* ratios could be a result of presence of crosscontamination that had occurred while drying samples containing spike in close
proximity to unspiked samples under the heat lamp. Therefore, the double spike
contamination contributed to decrease in the precision in the unspiked measurement of
the sample FB2.
The second type of samples investigated was wood samples and many attempts
were made in order to measure Sr isotopic compositions in woods. The first attempt
showed loss of Sr double spike when evaporated in the furnace, most likely from the
high temperature used to ash the wood. Loss of Sr double spike was also observed in
the second attempt when Sr double spike was added to the ash after burning the
woods. The reason for this could be that Sr double spike evaporated before it
equilibrated with the samples. A third attempt was applied on one wood sample and its
duplicate and showed success of the Sr double spike method. In the third attempt,
spiked and unspiked measurements were done at the same time and the 3M HNO 3 was
added to the ash several hours before the Sr double spike was added. However, the
success of the Sr double spike method on woods cannot be confirmed unless this
method is applied on additional samples.
The results of this study demonstrate that changes in n(88Sr)/n(86Sr) isotope
amount ratios can result in offsets of n(87Sr)/n(86Sr) ratios when a constant
n(88Sr)/n(86Sr) value of 8.375209 is assumed for the mass bias internal correction during
TIMS measurement. For example, IAPSO sea water showed the offset of up to
79
157 ppm. This could be a serious issue when high accuracy in n(87Sr)/n(86Sr) amount
ratios is required. Therefore, accurate measurement of δ88/86Sr is important to obtain the
accurate n(87Sr)/n(86Sr), (n(87Sr)/n(86Sr)*), which incorporates mass dependent isotopic
fractionation of n(88Sr)/n(86Sr).
Future work will focus on using a thermal ionization mass spectrometer with
double spike technique to measure the Sr isotopic composition in woods from different
locations. These analyses can be used to identify the geographical origin of the wood.
Also, it will be interesting to see if samples have the same n(87Sr)/n(86Sr) ratios and
different δ88/86Sr values, which indicates to different sources of Sr. However, if mass
depended fractionation is ignored, these samples will appear to have the same source
of Sr.
80
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85
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