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PAPER
Cite this: J. Anal. At. Spectrom., 2014,
29, 1444
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Ultra-precise titanium stable isotope
measurements by double-spike high resolution
MC-ICP-MS
Marc-Alban Millet†* and Nicolas Dauphas
In this contribution, we present a new technique for the ultra-precise determination of titanium stable
isotope composition (expressed as d49Ti or deviation of the
49
Ti/47Ti ratio to the reference standard) of
geological samples by multi-collection plasma source mass spectrometry (MC-ICPMS) using the double
spike method to correct for instrumental mass bias. Tails of polyatomic spectral interferences on 46Ti are
accounted for by using sample-standard bracketing in high-resolution mode. Choice of ideal double and
triple spike composition is investigated and results show that analytical error for a single measurement is
optimised for a
47
Ti–49Ti double spike composed of ca. 50% of each spike and mixed with ca. 52% of
sample. Measurements of pure Ti solution show that internal error on single measurements of ca.
0.010& (95% c.i.) is attainable on d49Ti, in agreement with the error model. Due to the lack of a widely
available reference isotopic standard for titanium, all results are expressed as deviations relative to newly
created reference material (OL-Ti standing for Origins Laboratory – titanium) prepared from an ultra-
Received 10th March 2014
Accepted 22nd May 2014
pure titanium metal rod. A range of analytical tests demonstrates the robustness of our method. An
external reproducibility of ca. 0.020& (2sd) is routinely achievable for Ti stable isotopes. Data for a range
of basaltic rock standards as well as a subduction zone basalt reference suite is presented and show that
the Ti stable isotope compositions of terrestrial basalt show resolvable variations but are overall very
DOI: 10.1039/c4ja00096j
close to the OL-Ti reference standard. The average Ti isotopic composition of the basalts studied here is
www.rsc.org/jaas
the present best estimate of the upper mantle composition; d49Ti ¼ +0.004 0.062& (2sd).
1. Introduction
Over the last decade, advances in plasma source multi-collection mass spectrometry have allowed isotope geochemists to
address questions relevant to both high and low temperature
geochemistry using new stable isotope systems.1–4 Although
abundant in most terrestrial and extra-terrestrial silicate rocks,
titanium isotopes have received limited attention. Titanium has
5 stable isotopes (46Ti: 8.01%, 47Ti: 7.33%, 48Ti: 73.81%, 49Ti:
5.50%, 50Ti: 5.35%). These have been studied primarily for
isotopic anomalies as tracers of nucleosynthetic processes, early
solar system processes, and cosmogenic effects,5–25 whereas few
studies have yet taken interest in the stable isotope variations of
titanium.12,26,27 Titanium is a refractory element that is also
immobile in uids. Although Ti3+ is present in meteorites,28,29
titanium is consistently present as Ti4+ in terrestrial rocks and
differentiated meteorites, and resides in both silicate and oxide
mineral phases. This seemingly simple chemical behaviour, as
Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi
Institute, The University of Chicago, 5734 S Ellis Avenue, Chicago, IL 60637, USA.
E-mail: [email protected]
† Present address: Durham University, Department of Earth Sciences, South
Road, Durham DH1 3LE, United Kingdom.
1444 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
well as its abundance, makes it an appealing element for
studies of magmatic and planetary differentiation. However,
because the potential applications of Ti stable isotopes mainly
reside in high temperature geochemistry, isotopic fractionations are expected to be small, which mandates the development of a high-precision method.
Pitfalls for generating highly precise and accurate data for Ti
stable isotopes are: (i) non-quantitative yield during Ti purication by ion exchange chromatography, which can generate
mass dependant isotope fractionation; (ii) large instrumental
mass bias inherent to plasma source mass spectrometry and;
(iii) presence of isobaric interferences.
Double-spike is a powerful method that has been shown to
efficiently deal with stable isotope fractionation occurring
during sample processing (digestion and purication) and
mass spectrometry.30–37 An inherent issue to this methodology is
the potential large error propagation during calculation of
sample composition from raw data. The amount of error
propagation is highly dependent on the choice of spike
composition and the relative proportion of double-spike and
sample in the analysed solution. Turning to polyatomic interferences, the most recent generations of MC-ICPMS have the
ability to measure in high-resolution mode, which takes
advantage of small mass differences between the ion of interest
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and polyatomic interference by measuring ion beams on the
side of at top peaks.38,39 Furthermore, polyatomic interferences
can be reduced by the use of desolvating sample introduction
systems.
Here, a method is presented to measure Ti stable isotope
ratios to a precision level of 0.020& (2sd) by double-spike MCICPMS in high resolution mode. We rst investigate the
optimum composition of the Ti spike required for minimal
error propagation, then carry out a range of tests to assess the
accuracy and precision of this method. Finally, data are presented for an Island Arc Basalt reference suite (New Britain
basalts) as a preliminary investigation of the Ti stable isotope
composition of igneous rocks.
2.
Optimisation of spike composition
Double or triple spiking has been a method of choice to
generate ultra-precise isotope ratios by either thermal ionisation or plasma source mass spectrometry [e.g. ref. 37–49]. The
method was rst introduced by Dodson30,31 and, in the case of
stable isotope ratios, consists in resolving the following nonlinear equation (eqn (1) and ref. 34):
Rm ¼ [((1 f)(RStandard(ix/in)a)) + fRSpike](ix/in)b
(1)
where Rm, RStandard and RSpike are the values of the measured,
standard and spike isotope ratio, in is the normalising isotope
for ratios whereas ix is any other isotope of the element of
interest, f is the proportion of the normalising isotope coming
from the spike in the sample–spike mixture, a is the natural
fractionation factor between the sample and the standard of
reference, and b is the instrumental fractionation factor.
Unknowns in this equation are f, a and b, which implies that a
minimum of 3 isotope ratios is required, so that at least three
versions of eqn (1) can be written and the system thus produced
can be resolved.
The assumption behind this calculation is that the sample
and standard of reference are related to each other by mass
dependent fractionation following an exponential law for all 3
input isotope ratios, which seems appropriate for instrumental
mass fractionation by TIMS and MC-ICPMS.50,51 Departures
from mass-dependent fractionation, either due to nucleosynthetic processes, cosmogenic effects, radioactive decay, nuclear
eld shi, or uncorrected analytical interference in one (or
several) isotope involved in the calculation would result in
inaccurate results. In meteorites and lunar samples, 50Ti shows
departures from mass-dependent fractionation due to the
presence of nucleosynthetic anomalies and cosmogenic
effects.11,16,23,25 Another difficulty with this isotope is the
possible presence of a gas-based isobaric interference from
36 14 +
Ar N . The accumulation of potential issues related to 50Ti
makes it a non-ideal choice for resolution of double/triple spike
equations and thus it should not be included in the resolution
of eqn (1) for Ti isotope fractionation measurements.
It should be noted that 46Ti has also been shown to be in
variable abundance in meteorites.24 However, the recorded
range of variation for d46Ti* (0.03 to +0.06) is much less
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important than for d50Ti* (0.13 to +0.37). This study is concerned with measurements of terrestrial rocks that do not
exhibit 46Ti anomalies and should always be related to each
other by mass-dependent isotope fractionation in the
46
Ti–47Ti–48Ti–49Ti space. Nevertheless, it is advisable that extraterrestrial samples displaying isotope anomalies are measured
both spiked and unspiked in order to measure their true isotope
composition.12,44
The spike composition and mixing proportions have to be
optimized in order to minimise error propagation on the
natural fractionation factor a. Recently, Rudge et al.35 have
presented optimum double spike compositions for all elements
measurable by MC-ICP-MS containing four isotopes or more. In
the case of titanium, the best potential double spikes all involve
50
Ti. However these calculations did not consider potential
complications arising from the presence of isotope anomalies
or potential interferences. In order to rene the choice of our
spike composition, we have developed a Mathematica® code to
explore all double and triple spikes using 46Ti, 47Ti, 48Ti, 49Ti by
Monte-Carlo simulation. Practically, individual spike compositions (available from Oak Ridge National Lab) are rst mixed to
create triple spikes. All triple spikes are then mixed with a
reference Ti isotope composition. Relative abundances of each
isotope are then turned into beam intensities by setting the
most abundant isotope of each mixture at 30 volts. Errors on
each beam are then calculated as the quadratic sum of the
Johnson noise and counting statistics (eqn (2) and (3)):
sbeam2 ¼ sJohnson2 + scounting2
(2)
which can be expressed in the following way:
sbeam2 ¼ (4kTR + V eR)/Dt
(3)
where k is the Boltzmann constant, T is the temperature (In
Kelvins), R is the collector resistance (in ohms), V is the average
voltage measured on the collector (in volts), e is the elementary
charge (1.60217646 1019 C) and Dt is the integration time (in
seconds). Although plasma icker has been highlighted as a
potentially signicant source of error,52 it is related to mass bias
instability. Since the double spike method accounts for instrumental mass bias, it has been neglected in these calculations.
We then simulate an MC-ICP-MS analysis by generating 500
random integrations (8 seconds integration time) with a standard deviation sbeam. All integrations are passed through the
double spike calculation to solve for f, a and b. For each
computed a a corresponding d49Ti can then be calculated.
Finally, the 2se of all d49Ti (calculated on the basis of a typical 50
integration cycle analysis) is mapped for all triple spikes-sample
mixtures investigated. All results can be represented within a
tetrahedron formed by the three pure spikes at the base, and the
reference Ti isotope composition at the top (Fig. 1a).
Fig. 1b shows an error map for all possible 46Ti–47Ti–49Ti
triple spikes, all mixed in the same proportion with the Ti
standard. This specic section of the tetrahedron has been
chosen as it contains the optimal spike composition that
minimizes the error propagation on a. Low error areas are
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Fig. 1 (a) Schematic representation of all the possible spike–sample mixtures investigated in our triple spike Monte-Carlo simulation. All
compositions are enclosed in a tetrahedron which top apex is the standard composition (i.e., natural stable isotope composition) and base
apexes (light grey area) are the individual spikes. In this tetrahedron, sections parallel to the base (represented in dark grey) contain all possible
triple spike mixtures mixed with the same amount of natural sample. (b) Ternary diagram showing the results of the error simulation on d49Ti for
all triple spikes containing 46Ti–47Ti–49Ti. The resolution space considered is 46Ti–47Ti–48Ti–49Ti. Assumed run conditions are: 50 integrations of
8.389 s duration, maximum ion beam set at 30 V on the most abundant isotope in each mixture, T ¼ 300 K, all collectors connected to 1011 U
amplifiers. The optimal spike composition is a double spike composed of 47Ti 0.50 : 0.50 49Ti mixed with 52% of sample (Fig. 1b–d). Note that
errors systematically increase away from the edges of the diagram, indicating that double spikes are more favourable than triple spikes. White
zones at the centre of the error map indicate extremely high error (out of scale).
consistently found on the edges of the ternary diagram, and
errors always increase towards the centre of the diagram. This
means that double spikes always provide better errors than
triple spikes (Fig. 1b and c), similarly to what has been
previously observed.35,53 The superiority of double spikes
relative to triple spikes stems from the topology of the error
map and reects the fact that ideal spike mixtures are
necessarily away from the apexes (otherwise it reduces to a
1446 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
single-spike) and away from the natural isotopic composition,
which lies somewhere in the ternary diagram. The optimized
spike mixtures are therefore always along edges of the ternary
diagrams, corresponding to two-isotope spike mixtures. In this
case, the optimal calculated spike is a double spike consisting
of 50% of 47Ti and 50% of 49Ti, which gives minimal errors
when mixed in 48 : 52 proportions with natural samples
(Fig. 1c and d).
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3.
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Analytical methods
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3.1 Preparation of reference standard and
spike
47
Ti–49Ti double
Measurement of Ti stable isotope ratios requires a standard of
reference, which is currently lacking. A potential candidate for
this purpose is the NIST SRM3162a pure Ti solution. However,
preliminary Ti stable isotope data for this pure Ti solution24
show that it is heavily fractionated from igneous rock standards
(BCR-2, BIR-1) and thus make it inappropriate for study of
magmatic processes. Instead, Zhang et al. showed that a Ti
metal wire obtained form Alfa Aesar had a Ti stable isotope
composition much closer to that of igneous rock standards.24 In
order to improve standard availability, we purchased a 500 g
ultra-pure Ti metal rod from Alfa Aesar (99.999% purity, 12.7
mm Ø, 25 cm length, Lot# E06X030, Stock# 36681), which we
hereaer refer to as the Origins Laboratory-titanium standard,
or OL-Ti. Five wafers of 1–2 mm thickness (ca. 500 mg each)
were taken at the ends and at quarter-length to check for
homogeneity of the starting material. Wafers were then gently
leached in 5 ml of concentrated HCl for 10 minutes at 100 C
then washed in Milli-Q water. Weight loss during this process
was ca. 0.2–0.3%. Leached wafers were then digested in 1 mL of
concentrated HF, and 19 mL of 3 M HNO3 were added aer
digestion to each solution to achieve less concentrated solutions. Other pure Ti solutions measuring during this study
include the ultrapure Ti wire prepared by Zhang et al. (2011) and
a Ti solution purchased from Claritas (Lot#CL2-129TI).
Isotopes 47Ti and 49Ti enriched to ca. 85% were purchased in
oxide form from Trace Sciences International. Aer digestion in
concentrated Optima® HF, both spikes were mixed in equal
proportion and then diluted with 3 M HNO3 so that the nal Ti
concentration of the double spike is 192 ppm and HF concentration is ca. 0.3 M.
3.2
Rock standard preparation and Ti separation
Sample preparation and Ti purication methods are almost
identical to those outlined by Zhang et al.24 Between 50 to a 100
mg of silicate rock standard is rst digested in concentrated
HNO3 and HF (1 mL of concentrated HF, 3 mL of concentrated
HNO3) at 120 C for 24–48 hours. Perchloric acid is added before
evaporation of digestion acid and samples are taken to incipient
dryness before further addition of concentrated nitric acid.
These repeated evaporation steps in presence of perchloric acid
are to minimise the presence of uorides that can sequestrate Ti
and generate stable isotope fractionation. Once all perchloric
acid is evaporated, samples are then cycled three times through
concentrated HNO3 dissolution–evaporation. They are then
taken up in 5 mL of 12 M HNO3 to which 30 mg of H3BO3 is
added to dissolve any potential remaining uoride. At this
point, an aliquot containing 20 mg of titanium is taken from the
samples and mixed in ideal proportion with the 47Ti–49Ti
double spike. The sample–DS mixture is then dried down,
cycled twice through concentrated nitric acid evaporation and
then taken up in 5 mL of 12 M HNO3, ready for chemical
purication.
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Titanium purication is achieved in two steps, with the
second one being repeated twice to achieve optimum separation. First, samples are passed through a TODGA column made
with 2 mL cartridges (Eichrom®). The resin bed was conditioned with 15 mL of 12 M HNO3. The matrix was eluted with
another 10 mL of the same acid before Ti was stripped off the
column in 10 mL of 12 M HNO3 + 1 wt% H2O2. Samples were
then dried down and taken up in 2.5 mL of 4 M HF, ready for the
second step of purication. This step is mainly used to remove
Mo and limit potential interferences of doubly charged Mo
isotopes on Ti. Long and narrow columns (10 cm length, 0.32
cm Ø) are lled with ca. 0.8 mL of AG1-X8 resin and conditioned
with 6 mL of 4 M HF before sample loading. Matrix (including
Mo) is then further washed with another 10 mL of 4 M HF and Ti
is then collected with 5 mL of 9 M HCl + 0.01 M HF. All acids
used in this procedure are made of concentrated acids that are
either purchased ultrapure acids (HF, H3BO3, H2O2) or puried
through double distilling (HNO3, HCl). Total procedural blank
is ca. 10–15 ng, which is insignicant compared to the amount
of Ti processed on columns (ca. 30–40 mg).
3.3
Mass spectrometry
Titanium stable isotope measurements were carried out on a
Neptune MC-ICPMS installed at Origins laboratory, University
of Chicago. This instrument is tted with a jet interface pump
and with Jet sample cones and X skimmer cones in order to
optimise sensitivity. Samples are introduced through an Apex
HF desolvating nebuliser used with Ar + N2. Isotopic measurements consisted of 50 integrations of 8.389 s each. The baseline
was corrected by subtracting on-peak zeros measured in clean
acids every 10 sample measurements. Samples and standards
were measured in 0.3 M HNO3 + 0.0014 M HF. The washouts
were carried out in 0.45 M HNO3 + 0.005% HF for 120–180 s,
with a typical background of ca. 5–7mV of 48Ti. All sample
measurements were interspersed with measurements of standards that were spiked and treated similarly to the samples.
Measurements were carried either in medium or high-resolution mode. The main difference being that the 36Ar14N
interference on 50Ti can only be resolved using high resolution,
to the cost of a drop in sensitivity from 200 to 100 V ppm1,
whereas interference of 30Si16O on 46Ti can be resolved in
medium resolution. Tails of interference peaks beneath the Ti
peaks can affect isotopic measurements but these are accounted for by sample-standard bracketing all double-spiked
measurements.41 We rst used high-resolution measurements
to calibrate both reference standard and double spike, and then
tested the technique at both medium and high resolutions.
Measurement solutions are diluted to 250 ppb of total titanium, with 48Ti being the largest ion beam at 30 V. The
amount of Ti required for analysis is readily achievable by
digesting only minute amount (a few milligrams) of natural
rock samples (see Section 3.2).
Minor amounts of Ca in sample solution can cause interferences on mass 46 (46Ca 0.003%) and 48 (48Ca 0.19%).
Although all three chemical purication steps separate Ca from
Ti, residual interferences may still be present. Accurate
J. Anal. At. Spectrom., 2014, 29, 1444–1458 | 1447
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Composition of the OL-Ti reference standard and of the
Ti–49Ti double spike used in this study
Table 1
47
Ti/47Ti
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OL-Ti
2se
Double spike
2se
46
48
Ti/47Ti
1.092874
0.000011
0.032663
0.000001
10.070565
0.000086
0.270783
0.000007
49
Ti/47Ti
0.749766
—
0.945969
0.000013
50
Ti/47Ti
0.729437
0.000009
0.023709
0.000001
interference correction would require measuring two Ca
isotopes in order to estimate the instrumental mass bias for Ca,
and use it to calculate the intensity of the 46Ca and 48Ca beams.
While 44Ca (2.1%) is abundant enough to be well measured,
the very low abundance of 43Ca (0.14%) and consequential
large errors would imply unreliable interference correction.
Instead, we assume that Ca and Ti have identical instrumental
mass fractionation factors. These are calculated during double
spike inversion for each Ti standard solutions (Ca free and
measured without correction).
4. Calibration of reference standard
and double spike
In the absence of previous isotopic data on the OL-Ti standard,
the only available mean of measuring its true isotopic composition would be external calibration with a previously calibrated
standard of a neighbouring element such as Fe (e.g., IRMM014). However, this is impossible to do in static mode due to the
large spacing required between all collectors. Instead we
decided to internally normalise its isotope composition to a
49
Ti/47Ti ratio of 0.749766 with an exponential law (see Table 1),
similarly to what is done for 50Ti anomaly measurements.20,23,25
The double spike was calibrated by measuring the pure
standard and double spike as well as nine standard–DS
mixtures spaced every 10% in proportion. Measurements were
carried out in 100 cycles of 8.389 s each, at high resolution.
Every measurement was preceded by three blank measurements
that were then interpolated to remove on-peak zeros. Calculated
double-spike composition can be found in Table 1. Double
spike corrected values for each standard–DS mixture show that
our 47Ti–49Ti calibration is robust for a range of standard–DS
mixing proportions from 0.3 to 0.8, while the calculated
optimum is at 52% sample, exactly in the middle of that range.
5.
5.1
Optimization of isotopic analyses
Homogeneity of the OL-Ti reference standard
All results presented in this study are reported as d49Ti, which is
the deviation in permil units of the 49Ti/47Ti ratio of the sample
relative to the OL-Ti standard. The rst step to establishing Ti
stable isotopes as a useful tracer of geological processes is to
certify the homogeneity of this reference standard. The four
wafers taken from the extremities and quarter lengths of the
ultrapure Ti metal rod were measured three times relative to the
wafer sampled from the half-length (Fig. 2). The half-length
wafer was also measured 13 times relative to itself in the same
sequence in order to check the quality of the data. The weighted
mean of each wafer (Table 2, calculated using Isoplot54) plots
within error of zero (at the 95% c.i. level) and shows that the OLTi standard is homogeneous in solid form. All ve solutions
were subsequently mixed together to create a single standard
solution. All data presented in the rest of this study are relative
to the nal OL-Ti standard solution. The OL-Ti standard is
available in both solid and solution form upon request to the
authors.
5.2
Effect of peak position
Accuracy and precision of double-spike measurements
strongly dependent on alignment of peaks and presence
interference peak tails. Due to the presence of traces of HF
the measurement acid, interference of 28Si19F on 47Ti
is
of
in
is
Fig. 2 Homogeneity of the OL-Ti reference standard. Wafers taken from the titanium ultrapure metal rod at extremities and quarter lengths were
measured relative to that wafer sampled at half-length. No wafer shows any deviation from 0 with respect to errorbars and the standard is
therefore considered to be homogeneous in solid form.
1448 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
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Table 2 Ti stable isotope results for analytical tests carried out during this study. All weighted means and 95% confidence intervals calculated
using Isoplot (Ludwig, 2003)
d49Ti
OL-Ti rod
Extremity
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1/4 length
3/4 length
Extremity
Half length
#1
#2
#3
#1
#2
#3
#1
#2
#3
#1
#2
#3
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
#11
#12
#13
Peak should plateau atness
150 ppm
130 ppm
320 ppm
250 ppm
Doping tests
K 10%
Mg 10%
Al 10%
Na 10%
Fe 10%
Ca 10%
Ca 1%
Ca 0.1%
Mo 0.1%
0.001
0.000
0.009
0.004
0.003
0.007
0.003
0.002
0.014
0.003
0.013
0.015
0.005
0.000
0.001
0.009
0.004
0.001
0.010
0.005
0.012
0.007
0.001
0.002
0.001
0.021
0.017
0.017
0.018
0.018
0.020
0.020
0.018
0.017
0.034
0.018
0.023
0.016
0.018
0.019
0.019
0.025
0.018
0.019
0.020
0.017
0.020
0.020
0.029
0.019
0.039
0.016
61.1
24626
0.017
0.021
0.4
82
0.014
0.007
0.001
0.001
0.007
0.090
0.003
0.014
0.011
0.020
0.017
0.019
0.019
0.019
0.024
0.019
0.029
0.026
possible. Also, formation of 30Si16O in the torch area can affect
measurements of 46Ti. While the measurement position is
typically set in the middle of the peak shoulder plateau, it is
important to examine the sensitivity of double spike data with
respect to the measurement position. Tests measurements
carried out in medium resolution at 320, 150, +130 and +250
ppm amu of the typical measurement point can be found in
Table 2. Although analyses carried out at 320 and +250 ppm
amu display extremely large deviation from 0, those made at
150 and +130 ppm amu show values within error bars of 0 and
with error bars typical of samples ran in normal conditions.
This indicates that a mass range of 300 ppm amu around the
usually set mass is adequate for ultra-high precision analysis,
which is much larger than typical magnet stability.
This journal is © The Royal Society of Chemistry 2014
d49Ti
95% c.i.
2sd
0.0003
0.000
0.0009
0.002
0.0170
0.011
Av.
0.003
0.009
0.013
Av.
0.000
0.008
0.017
Av.
0.001
0.022
0.028
Av.
0.0001
0.0004
0.0119
95% c.i.
Weighted mean
Av.
5.3
Effect of trace impurities
Presence of trace impurities in measurement solutions can
create adverse effects on stable isotope measurements by MCICP-MS.34,55 These can be especially important when using
external correction methods for instrumental mass bias. To test
if these affect double-spike measurements of Ti stable isotopes,
we carried out measurements of the pure OL-Ti standard doped
with major elements typically found in silicate rocks aer HF
digestion (Fig. 3 and Table 2). Results show that measurements
are still accurate within 0.02& for K, Mg, Al, Na, and Fe up to
element to Ti contributions of 10%. This is consistent with
previous analytical methods for stable isotope measurements
by MC-ICPMS using double spike41,42,48 and shows that matrix
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Results of doping tests to test the reliability of the titanium double spike technique to account for matrix effect. Double spiked OL-Ti
solutions doped with 10% element: Ti of Mg, Fe, Na, K and Al do not show any deviations, showing the robustness of the technique. Doping with
Ca and Mo was also carried out to check for the effect of direct isobaric interferences of 46Ca and 48Ca as well as doubly charged Mo ions. Results
show that Ca correction introduced in our data reduction routine can deal with as much as 1% residual Ca, whereas trace amount of Mo (0.1%)
does not impact Ti stable isotope measurements. Error bars represent the internal precision of each measurement.
Fig. 3
effect can be accurately accounted for by double-spike
methodologies.
Another issue is the incomplete removal of elements with
potential for isobaric interference, such as Ca (46Ca and 48Ca on
46
Ti and 48Ti) and Mo (doubly charged ions on all Ti isotopes).
We tested this by doping our Ti reference solution with Ca and
Mo (Fig. 3). We limited the Mo doping to 0.1% to reect the low
abundance of Mo relative to Ti in most geological samples and
results show that doubly charged Mo ions do not generate
detectable offsets. However, doping experiments with 0.01%,
1% and 10% Ca relative to Ti shows that the Ca interference
correction in the data reduction routine is only effective up to
1% Ca impurity.
6. Discussion
Fig. 4 Internal precision on d49Ti for all single measurements included
in this study. Errors typically range from ca. 0.009& to 0.015& (95% c.i.)
although some larger errors can occur when the mass spectrometer is
unstable. Once errors on bracketing standards are taken into account,
errors on sample data typically range 0.013 to 0.030& (95% c.i.).
6.1 Internal precision, reproducibility and accuracy of
double spike measurements
Internal precision of single d49Ti analysis typically ranges from
0.009 to 0.015&, although some higher errors may occur if the
MC-ICPMS is unstable (Fig. 4). This internal precision is slightly
higher than the optimal modelled error of 0.009& (95% c.i.) for
typical mass spectrometry conditions (eqn (3)). This small offset
is either due to small secular variations in the intensity of the
30 16
Si O interference tail on the 46Ti peak or to non-exponential
mass bias occurring in the Apex HF or at the interface region.
Once the errors of bracketing standards are propagated, typical
error on bracketed sample measurements vary from 0.013 to
0.030& (95% c.i.).
Repeated measurements of the Ti-wire standard and a pure
Ti solution from Claritas® over 15 weeks (Table 3 and Fig. 5)
give a reproducibility for all three solutions of ca. 0.02& (2sd)
on d49Ti. Both standards also show measurable deviations
relative to the OL-Ti standard, with the Ti-wire displaying a
lighter composition (d49Ti ¼ 0.098 0.023&, 2sd, n ¼ 43),
1450 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
while the Claritas Ti solution is shied towards a heavier
composition (d49Ti ¼ +0.145 0.021&, 2sd, n ¼ 23).
Finally, accuracy of double-spiked measurements can be
checked by simple sample-standard bracketing measurements
of the Ti-Wire standard (Table 3), as is most commonly done for
stable isotopic analysis by MC-ICPMS.39 Measurements of the
49
Ti/47Ti ratio made at low and medium resolution all give
results within error of the double-spiked measurements and
therefore demonstrates the accuracy of our Ti stable isotope
measurements as the double-spike and sample-standard
bracketing methods are sensitive to different biases.
Furthermore, double spike measurements of the Claritas Ti
solution carried out in medium and high-resolution give similar
precision (Table 3). Consequently, we recommend that doublespike measurements on terrestrial samples be carried out in
medium resolution in order to benet from higher sensitivity
and diminish sample consumption.
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Table 3 Ti stable isotope data for a pure Ti solution from Claritas and the Ti Wire standard used by Zhang et al. (2011) and Zhang et al. (2012). All
weighted means and 95% confidence intervals calculated using Isoplot (Ludwig, 2003)
d49Ti
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Claritas Ti
HR
MR
d49Ti
95% c.i.
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
#11
#12
#13
#14
#15
#16
#17
#18
#19
#20
#21
#22
#23
0.137
0.139
0.142
0.158
0.165
0.156
0.145
0.125
0.147
0.153
0.146
0.132
0.155
0.146
0.126
0.135
0.135
0.146
0.160
0.141
0.139
0.140
0.157
0.024
0.026
0.025
0.024
0.021
0.023
0.019
0.020
0.024
0.022
0.023
0.018
0.030
0.030
0.016
0.017
0.017
0.020
0.021
0.019
0.018
0.027
0.034
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
#11
#12
#13
#14
#15
#16
#17
#18
#19
#20
#21
#22
#23
#24
#25
#26
#27
#28
#29
#30
#31
#32
#33
#34
#35
#36
#37
0.100
0.110
0.096
0.093
0.095
0.100
0.090
0.095
0.098
0.088
0.100
0.101
0.095
0.093
0.090
0.084
0.103
0.097
0.116
0.097
0.069
0.096
0.080
0.083
0.099
0.100
0.087
0.081
0.100
0.108
0.110
0.089
0.095
0.101
0.089
0.106
0.123
0.017
0.021
0.018
0.020
0.025
0.056
0.032
0.035
0.024
0.026
0.024
0.014
0.015
0.026
0.025
0.017
0.024
0.016
0.030
0.024
0.036
0.019
0.028
0.032
0.021
0.023
0.020
0.018
0.018
0.017
0.015
0.020
0.019
0.030
0.022
0.023
0.021
Ti wire
2sd
Weighted mean
Av. HR
0.142
0.143
0.004
0.006
0.021
0.023
Av. MR
0.142
0.007
0.021
0.099
0.003
0.023
Weighted mean
This journal is © The Royal Society of Chemistry 2014
95% c.i.
J. Anal. At. Spectrom., 2014, 29, 1444–1458 | 1451
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Table 3
Paper
(Contd. )
#38
#39
#40
#41
#42
#43
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Std bracketing LR
Std bracketing MR
d49Ti
95% c.i.
d49Ti
95% c.i.
2sd
0.111
0.115
0.109
0.106
0.126
0.109
—
—
0.025
0.022
0.027
0.020
0.021
0.021
—
—
0.105
0.109
0.030
0.013
0.079
0.029
Data for repeated measurements of two pure Ti solutions. Both solutions show distinct stable isotope composition to the OL-Ti standard.
Reproducibility of d49Ti measurements, calculated as the 2sd of each dataset, is ca. 0.020& in each case. Error bars on single data points are
internal errors for each measurement once errors on bracketing standards are propagated.
Fig. 5
Difference between the b factors of samples and bracketing standards (Db). The Db parameter is a proxy for the amount of stable isotope
fractionation occurring during sample processing. Rock samples and a single OL-Ti standard passed through chemistry after addition of double
spike consistently record higher b factors. Despite having a Db ¼ 0.0066 0.0002 (95% c.i.), a column processed OL-Ti standard displays a d49Ti
of 0.002 0.012& (95% c.i.). This indicates that Ti stable isotope fractionation occurs during chemical purification but that it is reliably
accounted for by the double spike method.
Fig. 6
1452 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
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Table 4 Ti stable isotope data for a column processed OL-Ti standard, basaltic reference materials (BHVO-2, BCR-2, W2, BIR-1 and JB2) as well
as a suite of subduction zone basalts (New Britain Island Arc Basalt Reference Suite). All weighted means and 95% confidence intervals calculated
using Isoplot (Ludwig, 2003)
d49Ti
95% c.i.
#1
#2
#3
0.000
0.006
0.001
0.021
0.020
0.021
#A1
#A2
#A3
#B1
#B2
#B3
#B4
0.025
0.035
0.019
0.013
0.008
0.023
0.001
0.025
0.024
0.025
0.019
0.025
0.034
0.035
#A1
#A2
#B1
#B2
#B3
#C1
#C2
#C3
#C4
0.025
0.024
0.017
0.020
0.007
0.027
0.015
0.031
0.041
0.024
0.032
0.028
0.027
0.016
0.017
0.021
0.040
0.025
#A1
#A2
#A3
0.069
0.063
0.066
0.035
0.036
0.040
#A1
#A2
#A3
#B1
#B2
#B3
#B4
0.028
0.050
0.032
0.050
0.040
0.022
0.026
0.026
0.023
0.026
0.017
0.032
0.046
0.027
#A1
#A2
#A3
#A4
0.041
0.032
0.049
0.048
0.022
0.032
0.024
0.024
#1
#2
#3
0.062
0.037
0.051
0.025
0.022
0.029
#1
#2
#3
0.031
0.030
0.025
0.023
0.020
0.025
#1
#2
#3
0.012
0.010
0.019
0.016
0.020
0.030
#1
#2
#3
0.018
0.023
0.014
0.016
0.029
0.033
Sample
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Column processed OL-Ti
Reference materials
BCR-2
BHVO2
BIR1
W2
JB2
New Britain basalts
116852-1
116852-2
116852-3
116852-4
This journal is © The Royal Society of Chemistry 2014
d49Ti
95% c.i.
2sd
TiO2 wt%
MgO wt%
Weighted mean
0.002
0.012
0.007
—
—
Weighted mean
0.018
0.010
0.024
2.26
3.59
Weighted mean
0.021
0.008
0.020
2.73
7.23
Weighted mean
0.066
0.021
0.006
0.97
9.70
Weighted mean
0.040
0.009
0.023
1.06
6.37
Weighted mean
0.044
0.013
0.016
1.19
4.62
Weighted mean
0.049
0.014
0.025
0.30
9.20
Weighted mean
0.029
0.013
0.006
0.76
4.69
Weighted mean
0.000
0.012
0.032
0.49
5.60
Weighted mean
0.014
0.013
0.040
0.52
6.90
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Paper
Table 4 (Contd. )
d49Ti
95% c.i.
#1
#2
#3
0.023
0.003
0.014
0.033
0.036
0.020
#1
#2
#3
0.006
0.018
0.003
0.029
0.028
0.035
#1
#2
#3
0.015
0.001
0.011
0.019
0.030
0.022
#1
#2
#3
0.009
0.001
0.026
0.036
0.039
0.021
Sample
116852-5
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116852-6
116852-7
116852-12
6.2
d49Ti
95% c.i.
2sd
TiO2 wt%
MgO wt%
Weighted mean
0.014
0.016
0.021
0.52
10.1
Weighted mean
0.009
0.017
0.021
0.80
4.45
Weighted mean
0.011
0.013
0.015
1.43
5.30
Weighted mean
0.018
0.016
0.025
0.95
5.80
Robustness of double spike measurements
Sample processing can fractionate the isotope composition of
samples before mass spectrometry. This mainly occurs during
ion chromatography stages through incomplete recovery of the
puried analytes. Zhang et al.24 estimated the overall yields of
their protocol to be ca. 85–90%. Although the double spike
method employed here has the ability to account for any
potential isotope fractionation occurring to the sample aer it is
equilibrated with the spike, it is necessary to evaluate the effect
of sample processing on the stable isotope composition of
double-spiked samples in order to certify the robustness of the
method.
In eqn (1), b is labelled as the instrumental fractionation
factor. This denition applies to pure analyte solutions whereas
for rock powders, b also includes isotopic fractionation
produced in the laboratory by sample processing aer equilibration of sample and double spike. In the present case, since
double spike is added to rock samples aer digestion, the b
factor obtained during double spike calculation encompasses
stable isotope fractionation related to both mass spectrometry
and chemical purication. By comparing the b factors of
samples run as unknown to that of bracketing reference standards (not passed through chemistry) it is therefore possible to
assess the potential effect of sample processing on Ti stable
isotope ratios. Fig. 6 shows the difference between b factors of
samples and bracketing standards (Db ¼ bsample bbracketing
standard) for all reference materials and natural samples run
during the course of this study. All samples, regardless of their
nature, consistently display b factors higher than that of the
bracketing standards (Db > 0). The values of Db range from
0.004 to 0.011 with most of the samples plotting around a value
0.008. Each samples was measured 3 or 4 times and the error
associated with each data point can be attributed to small
secular variation in the instrumental mass bias. In comparison,
the Db calculated for pure Ti solutions run as unknowns (OL-Ti
wafers, Ti Wire and Claritas Ti) are identical to that of
their respective bracketing standards (Db ¼ 0.0000 0.0009,
1454 | J. Anal. At. Spectrom., 2014, 29, 1444–1458
2sd, n ¼ 91). This indicates that detectable stable isotope fractionation occurs during sample processing, most likely related
to the incomplete recovery of Ti elution peak tails that are
preferentially enriched in light isotopes.
Instrumental b factors for bracketing Ti standards on the
Origins laboratory Neptune MC-ICPMS are typically around 1.4.
Adding a Db of 0.008 to this value would create an average offset
of ca. 0.33& on the d49Ti value, much larger than the analytical
uncertainty. This value can be compared with the d49Ti of a OLTi standard solution processed as a sample (HF–HNO3 digestion, spiking, chemical purication). The processed OL-Ti
displays a d49Ti of 0.002 0.012 (95% c.i., n ¼ 3, Table 4),
despite having a Db of 0.0066 0.0002 (95% c.i.), thus
demonstrating the ability of double spike technique to reliably
account for signicant stable isotope fractionation occurring
aer equilibration of sample and double spike.
Furthermore, this shows that high-precision accurate titanium stable isotope measurements can only be generated by
double spike methodology. A way to generate accurate Ti stable
isotope data by sample-standard bracketing is to use a Ti
solution passed through chemistry as a bracketing standard.26,27
The large range in observed Db values (Fig. 6) implies that
reproducibility would be up to an order of magnitude worse
than that attainable by double spike.
6.3 Data for international reference material and terrestrial
basalts
During the course of this study, we have measured ve different
basaltic reference materials (BCR-2, BHVO-2, BIR-1, W2 and JB2) as well as eight subduction zone basalts from New Britain
Island Arc Basalt Reference Suite.56 All data can be found in
Table 4.
Repeated digestion and measurements of BCR-2, BHVO-2
and W2 powders allow us to evaluate to the external reproducibility of our method (Fig. 7). All data obtained for a single rock
powder are within 0.025& (2sd) of their respective weighted
mean. These values are similar to the reproducibility of pure
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Fig. 7 Data for repeated digestion and measurements of 3 basaltic reference materials digested and measured multiple times (BCR-2 in red, W2
in blue, BHVO-2 in green). Individual measurements are represented as coloured symbols and are pooled by digestion. Averages of single
solution (prepared from a single digestion) are represented as black squares. All data reproduces within 0.020 to 0.024& (2sd).
Fig. 8 Data for all basaltic samples measured during the course of this study. Five basaltic reference materials show small but detectable Ti stable
isotope variation (d49Ti range: 0.066& to +0.040&) whereas the New Britain basalt sample suite shows little to no variations. This relative
homogeneity hints that the d49Ti of the Earth's upper mantle is +0.004 0.062& (2sd).
unprocessed solutions. This indicates that no Ti stable isotope
fractionation occurs before samples are aliquoted and mixed
with the double spike (i.e., during powder digestion).
The d49Ti values determined for the ve basaltic rock standards appear to be slightly different at the 95% c.i. level (Fig. 8),
with W2 and BHVO-2 showing slightly heavy compositions
relative to the OL-Ti standard (+0.040 0.009& and +0.021 0.008& respectively) whereas BCR-2, BIR-1 and JB-2 all display
stable isotope composition enriched in light isotopes (0.018 0.010&, 0.066 0.021& and 0.044 0.013& respectively).
It does not appear that these differences are related to the type
of basalt (OIB, Continental basalt, or IAB) or the chemistry of
the lavas (Mg# or TiO2 content, not shown).
This journal is © The Royal Society of Chemistry 2014
In order to investigate this matter further, we measured eight
subduction zone basalts from New Britain (Fig. 8) for which
large trace element as well as radiogenic and stable isotope
variation have been recorded.57–59 These variations are well
correlated and have been related to both partial melting and
source effects. Contrary to these large variations, Ti stable
isotopes do not vary in New Britain basalts and display a Ti
isotope composition similar or marginally heavier than OL-Ti,
similar to BHVO-2, W2 or BCR-2 rock standards.
The partition coefficient of Ti during mantle melting to vary
between 0.09 for a garnet peridotite (containing 4% garnet) and
0.13 for a spinel peridotite (with 3% spinel).60 Consequently,
most of the Ti contained in a mantle source will be extracted
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during partial melting. Mass balance considerations would
therefore require that (i) Ti stable isotope composition of
basalts will record that of their mantle source and (ii) if any Ti
stable isotope fractionation occurs during partial melting, it will
be recorded by the melting residue. This observation is of
signicant importance as it could lead to interesting applications of Ti stable isotopes in igneous geochemistry. Assuming
that all samples analysed during the course of this study are
representative of the Earth's upper mantle, it is possible to
calculate its Ti stable isotope composition. The calculated d49Ti
value of the upper mantle (+0.004 0.062&, 2sd), although
only preliminary, suggests that the OL-Ti standard is a particularly adequate reference for Ti isotope investigations of
terrestrial and extra-terrestrial material.
7. Conclusion
We have developed a new method to measure Ti stable isotope
ratios for the rst time. The double spike is a 50–50% mixture of
47
Ti and 49Ti. This spike is mixed in 0.48 : 0.52 proportions with
the samples. A range of tests demonstrates the accuracy and
robustness of the method and that precision of ca. 0.020& is
attainable for d49Ti. All data are reported relative to a new
isotope reference standard called OL-Ti. The standard is
demonstrated to be homogeneous and is available upon request
to either of the authors.
Data for a range of basaltic reference materials as well as a
suite of subduction zone basalts are reported. Only limited
variations are revealed across this data set, despite samples
showing variations in provenance, chemical composition,
radiogenic isotope ratios and even Fe stable isotope ratios. This
hints that the titanium stable isotope composition of basalts
may primarily record that of their respective mantle source and
lead us to propose a preliminary value for the d49Ti of the upper
mantle of +0.004 0.062& (2sd).
Acknowledgements
N.D. thanks the NSF Petrology and Geochemistry (EAR1144429)
and NASA Cosmochemistry (NNX12AH60G) programs for
support. MAM thanks F. Tissot for his invaluable help in getting
to grasp with Mathematica®. Sarah Farley is gratefully
acknowledged for editorial handling as well as two anonymous
reviewers for their comments that improved the manuscript.
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