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A new method for MC-ICPMS measurement of titanium isotopic
composition: Identification of correlated isotope anomalies in meteorites†
Junjun Zhang,*a Nicolas Dauphas,a Andrew M. Davisa and Ali Pourmandab
Received 17th June 2011, Accepted 18th July 2011
DOI: 10.1039/c1ja10181a
A new protocol is presented for precise measurements of titanium isotopes in natural samples.
Titanium was separated via ion-exchange and extraction chromatography in two stages. Tests on Ti
standard solutions show that isobaric interferences from Ca, V, and Cr can be adequately corrected, as
long as these elements are present at atomic ratios of Ca/Ti < 20, V/Ti < 2, and Cr/Ti < 0.1.
Furthermore, Zr2+ and Mo2+ have no influences on Ti+ signals when atomic ratios of Zr/Ti < 0.002 and
Mo/Ti < 0.04. Compared with these correction limits, the purified solutions have corresponding ratios
several orders of magnitude lower, indicating that the chemical separation technique is effective. This
newly developed method has been successfully applied to geostandards and a wide variety of bulk
meteorites. Our results are in good agreement with the data from Trinquier et al. (Science, 2009, 324,
374–376)1 and reveal a linear correlation between isotope anomalies of two Ti nuclides in bulk
meteorites. The correlation reflects incomplete mixing of the carrier phases for Ti isotope anomalies
before bulk meteorite formation.
Introduction
Several studies of titanium isotope anomalies (relative to the
terrestrial Ti isotopic composition) in solar system materials were
done by thermal ionization mass spectrometry (TIMS) in the
1980s.2–8 Anomalies of neutron-rich nuclide 50Ti were found in
bulk carbonaceous chondrites and calcium-, aluminum-rich
inclusions (CAIs). In those studies, TiO+ ion beams were
measured because they were more intense than Ti+ beams.
Titanium isotopic compositions were obtained after correction
for oxygen isotope contributions. In recent years, there has been
renewed interest in Ti isotopic analysis in meteorites with the
development of multicollector inductively coupled plasma mass
spectrometry (MC-ICPMS).1,9–11 With this instrument, the ionization yield of Ti exceeds 90%12 and a recent study showed that
small isotope anomalies of neutron-poor nuclide 46Ti can be also
resolved in CAIs, amoeboid olivine aggregate, chondrules, and
bulk meteorites.1 Interestingly, there is a linear relationship
between 46Ti and 50Ti isotope anomalies in solar materials when
normalizing to a fixed 49Ti/47Ti ratio.1 Despite this progress,
several critical questions remain unanswered, including the
origin of Ti isotope anomalies, what the carrier phases of Ti
a
Origins Laboratory, Department of the Geophysical Sciences, The
University of Chicago, Chicago, IL, 60637, USA. E-mail: junjunzhang@
uchicago.edu
b
Neptune Isotope Laboratory, Division of Marine Geology and Geophysics,
The University of Miami-RSMAS, 4600 Rickenbacker Causeway, Miami,
FL, 33149, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ja10181a
This journal is ª The Royal Society of Chemistry 2011
isotope anomalies are, and how these carrier phases were
distributed and preserved during the solar system evolution.
In addition, isotopic heterogeneity in the solar nebula is
a critical issue in chronology. The validity of using short-lived
radionuclides as high-resolution chronometers of early solar
system events is based on the assumption that they were wellmixed and homogeneously distributed in the solar system.13–16
However, previous studies indicated that Ti nucleosynthetic
anomalies varied even at macroscopic scales in Allende
samples.1,10 Among a variety of elements with isotope anomalies
at a planetary scale,1,10,17–19 titanium is well suited to test the
mixing level of nucleosynthetic anomalies in the initial solar
nebula for two reasons: (1) it is highly refractory and was
condensed into the earliest solids in the solar system; (2) it is
immobile and its isotopic composition was not easily modified
during secondary alteration processes. The efficiency of initial
mixing of the Ti isotopes could therefore provide better
constraints on dynamical models of early solar system evolution.
In order to understand the origin and distribution of Ti
isotopic anomalies through isotopic analysis of natural materials, we have developed a new separation procedure. In contrast
to previous procedures,1,3,4,9 (Table S1, ESI†), we aim to separate
multiple elements apart from Ti, that could be analyzed to gain
further insights of nucleosynthetic origins and preservation
during the solar system evolution. The advantages of this new
procedure are as follows: (1) elements that can form isobaric
interferences (Ca, V, Cr, Zr, and Mo) were removed efficiently;
(2) titanium was well separated from Zr and Hf during the first
stage of ion-exchange chromatography, which forms the basis
for Zr and Hf separation (details will be provided in
J. Anal. At. Spectrom., 2011, 26, 2197–2205 | 2197
a forthcoming paper). It allows us to measure isotope compositions of Ti, Zr, and Hf, three elements with similar cosmochemical behavior, but with diverse nucleosynthetic origins, in
the same aliquot of sample solution; (3) the first stage of this new
procedure provides a basis for Ca separation by starting the
chemistry without any hydrofluoric acid, which allows us to
investigate whether or not there is a relationship of isotope
anomalies between two neutron-rich nuclides 48Ca and 50Ti in the
same aliquot of sample solution (details will be provided in
another forthcoming paper).
Materials and methods
(a) Reagents and analytical materials
Nitric (HNO3) and hydrochloric (HCl) acids were doubledistilled in subboiling quartz followed by Teflon distillation
units. Optima grade hydrofluoric (HF) and perchloric (HClO4)
acids and reagent grade hydrogen peroxide (H2O2) were used as
supplied without further purification. Water purified by ion
exchange (resistivity > 18 MU cm1) was used for acid dilutions
and chromatography. AG1-X8 resin (200–400 mesh, chloride
form) and prepacked, 2-mL cartridges containing TODGA resin
(0.8 cm diameter 4 cm length, particle size of 50–100 mm) were
purchased from Bio-Rad and Eichrom, respectively.
(b) Dissolution of geostandards and meteorites
Sample handling and all chemical treatments were performed
under clean laboratory conditions at the Origins Lab. Bulk
meteorites were ground into powder in an agate mortar.
Approximately 20–50 mg of geostandard powder was weighed
into a clean 6 mL PFA Savillex vial. Four mL of concentrated
HNO3 and HF in a volume ratio of 1 : 3 were added and the
mixture was heated on a hotplate overnight then evaporated to
dryness. Subsequently, concentrated HNO3 and HCl in a volume
ratio of 1 : 3 with 3–5 drops of HClO4 were added and the
mixture was evaporated. Five to ten mL of 12 M HNO3 with 60–
70 mg of boric acid (H3BO3) were added as the final acid medium
prior to extraction chromatography. The addition of boric acid
was to complex fluoride as the presence of this anion could
hamper purification of high field-strength elements on TODGA
resin. For bulk meteorite samples, approximately 100–150 mg of
powder was weighed into a clean 6 mL PFA Savillex vial and an
alternative digestion method with a high pressure Parr bomb was
used. Four mL of 1 : 3 concentrated HNO3 : HF was added and
the vial was then heated inside a 45 mL PTFE Parr bomb at 160–
170 C for 5 days. The mixture was evaporated and HClO4 was
added to convert insoluble fluorides to soluble compounds. The
residue was dissolved with concentrated HNO3 and heated in the
bomb for another 5 days. Similar to the geostandards, the bulk
meteorite samples were finally dissolved in 5 to 10 mL of 12 M
HNO3 with 60–70 mg of boric acid prior to extraction chromatography. More details are provided elsewhere.20
(c)
Titanium separation
A new procedure of Ti separation was developed via a two-stage
procedure using TODGA and AG1-X8 resins. In contrast to
previous methods1,3,4,9 (Table S1, ESI†), we used the first stage to
2198 | J. Anal. At. Spectrom., 2011, 26, 2197–2205
obtain the Ti cut as well as the Ca cut (with matrix elements)
using acids without any addition of HF, and then collected a Zr–
Hf cut (Table 1). For column calibration purposes, a standard
mixture of 21 elements in 10 mL of 12 M HNO3 was prepared
from single-element standard solutions at an amount of 50 mg
each. The standard mixture was loaded onto a 2-mL TODGA
cartridge, which had been cleaned and preconditioned with
a sequence of acids (Table 1). Matrix elements including alkali,
alkaline earth and some trace elements (e.g., Cr and V) were
removed during the loading and during a subsequent rinsing step
with another 10 mL of 12 M HNO3 (Fig. 1a). This scheme is
based on a comprehensive study of TODGA/acid solution
partition coefficients of 60 elements.20 Following matrix removal,
Ti was stripped from the column with 10 mL of 12 M HNO3 + 1
wt% H2O2 together with Mo and a small fraction of Nb, Ta and
W, while Zr, Hf and the lanthanides remained on the TODGA
resin. The total Ti yield for the standard mixture exceeded 95%.
The second separation scheme was modified from an earlier
technique for Zr purification.21 It was used to remove major
matrix elements independently from the first stage and to efficiently separate Ti from Mo, Nb, Ta and W. Elution tests were
done using a standard mixture with 17 elements at amounts of 50
mg each. The mixture was dried down and then dissolved in 2.5
mL of 4 M HF. After centrifuging and decanting, this supernatant was loaded onto a preconditioned 0.8-mL column (0.32 cm
diameter 10 cm length) filled with AG1-X8 resin (Table 1). The
matrix elements, including alkali, alkaline earth and some trace
elements (e.g., Cr and some of the V) were eluted with 10 mL of 4
M HF (Fig. 1b). Vanadium was further removed with 10 mL of
0.4 M HCl–1 M HF. Titanium was stripped from the column
with 9 M HCl–0.01 M HF. The Ti yield for an elution volume of
2.5 mL was about 98%. Tungsten, Mo, Nb, and Ta were retained
on the resin even after 6 mL of 9 M HCl–0.01 M HF was passed
through the column.
(d)
Titanium recovery in natural samples
In order to assess the efficiency of Ti separation and recovery in
natural samples, 14 mg of USGS BCR-2 basalt (containing 184
mg of Ti) and 121 mg of bulk carbonaceous chondrite Allende
powder (119 mg of Ti) were digested using the protocols
described above. After complete dissolution, sample solutions
were passed through the TODGA resin. The Ti yields for BCR-2
and bulk Allende samples from the first column exceeded 98%.
Titanium solution from BCR-2 was then loaded to the second
column filled with AG1-X8 resin. For the same elution volume of
9 M HCl–0.01 M HF, the Ti yield for BCR-2 basalt was lower
than that of the standard mixture due to slight tailing of Ti.
Specifically, the yield was 90% with 2.5 mL of 9 M HCl–0.01 M
HF and it reached 97% with 5 mL. That is because the BCR-2
basalt contained much more Ti than the standard mixture.
Another similar test was then done on the second column
using 350 mg of Ti elemental standard. The Ti yields were 81%
and 97% with 2.5 mL and 5 mL of 9 M HCl–0.01 M HF,
respectively. Therefore, in order to obtain a high Ti yield for
natural samples, an elution volume of 5 mL of 9 M HCl–0.01 M
HF was adopted as the elution cut for the second column (Table
1). To ensure a good separation of Ti, all natural samples were
passed through the first column once and the second one twice.
This journal is ª The Royal Society of Chemistry 2011
Table 1 Titanium ion-exchange chromatography
Step
Volume (mL)
Acid
Step
Volume (mL)
Acid
Column 1 (2-mL TODGA; 0.8 cm diameter 4 cm length)
Column 2 2 (0.8-mL AG1-X8, 200–400 mesh,
chloride form; 0.32 cm diameter 10 cm length)
Clean
Clean
Precondition
Load
Rinse matrix (Ca)
Elute Ti
Fe
Zr and Hf
10
10
4
15
10
10
10
10
20
3 M HNO3
3 M HNO3–1 wt % H2O2
H2O
12 M HNO3
12 M HNO3
12 M HNO3
12 M HNO3–1 wt% H2O2
3 M HNO3
3 M HNO3–0.3 M HF
The eluted Ti solution was dried down and redissolved in the acid
medium of 0.3 M HNO3–0.0014 M HF for isotope measurements. This process was repeated twice to remove trace HCl. The
total procedural blank varied from 4 to 20 ng, with an average of
12.1 ng.
(e)
Multi-collection ICPMS analysis
Titanium isotope measurements were performed using a samplestandard bracketing technique on a Thermo Scientific Neptune
MC-ICPMS. A detailed description of the instrument was
provided in Wieser and Schwieters.22 Aridus I and Aridus II
desolvation inlet systems were preferred over using an ESI SIS
spray chamber and ESI Apex-Q. This is because the latter two
systems are both composed of quartz and introduce significant
molecular interferences from 28Si19F on 47Ti, and probably 29Si19F
and 30Si19F on 48Ti and 49Ti, respectively (Table 2). These
potential interferences were not mentioned in previous studies.
The positions of the Faraday cups for Ti isotopes and isobaric
interferences that were monitored correspond to the species
44
Ca+, 46Ti+, 47Ti+ (axial mass), 48Ti+, 49Ti+ and 50Ti+ (sequence 1)
and 48Ti+, 49Ti+, 51V+, and 53Cr+ (sequence 2) with integration
times per cycle of 8.4 and 4.2 s, respectively (Table 2). Calcium-44
Precondition
Load
Rinse matrix
Rinse V
Elute Ti
10
2
6
5
5
6
2.5
10
10
5
3 M HNO3
H2O
0.4 M HCl–1 M HF
9 M HCl–0.01 M HF
H2O
4 M HF
4 M HF
4 M HF
0.4 M HCl–1 M HF
9 M HCl–0.01 M HF
was measured to monitor the interferences from 46Ca+ (0.004%,
in atom %) and 48Ca+ (0.187%). Vanadium-51 and 53Cr+ were
measured to monitor the interferences from 50V+ (0.2497%) and
50
Cr+ (4.3452%). All measurements were performed at high mass
resolution, m/(m0.95–m0.05) 11 000, in order to resolve mass
spectrometric interferences by polyatomic ions 36Ar14N+ on 50Ti+
and 22Ne2+ on 44Ca+ as indicated in Trinquier et al.1 as well as
40
Ar13C+ on 53Cr+. The interferences from molecular 35Cl14N+ on
49
Ti+, 35Cl15N+ on 50Ti+, 36Cl15N+ and 35Cl16O+ on 51V+, 37Cl16O+
and 35Cl18O+ on 53Cr+ can be excluded in our measurements
(details are shown in results section (b)). The ion intensities in 0.3
M HNO3–0.0014 M HF acid medium were measured at the
beginning of each sequence and were subtracted from all subsequent sample and standard measurements online. We ran 3–4
ppm solutions. The sample uptake rate was 100 mL min1 and
uptake time was 90 s, while the wash time between consecutive
sample and standard measurements was 120 s. Data were
collected in one block of 10–20 cycles and the total time for one
standard-sample-standard run was approximately 30 min. Our
bracketing standard was Alfa Aesar Ti solution (AATS). The
average sensitivity was 2 and 4 Volts ppm1 for 48Ti at high
resolution using Aridus I and Aridus II desolvation inlet systems,
respectively.
Fig. 1 (a) Elution curves for standard mixtures of 21 elements on a 2-mL cartridge of TODGA resin. Matrix elements were removed during the load of
10 mL of 12 M HNO3, followed by rinse solutions in another 10 mL of 12 M HNO3. Titanium was eluted by using 12 M HNO3 with 1 wt% H2O2
together with Mo, and some minor Nb, Ta, and W. Zirconium and Hf were stripped with 3 M HNO3–0.3 M HF at 65 C. (b) Elution curves for standard
mixtures of 17 elements on 0.8-mL column filled with AG1-X8 resin (200–400 mesh, chloride form). Matrix elements were removed with 4 M HF.
Vanadium was further removed with 10 mL of 0.4 M HCl–1 M HF. Titanium was stripped with 9 M HCl–0.01 M HF.
This journal is ª The Royal Society of Chemistry 2011
J. Anal. At. Spectrom., 2011, 26, 2197–2205 | 2199
Table 2 Faraday collector configurations and possible interferences on Ti isotope measurement using MC-ICPMS in high resolution
Faraday cup positions
L4
Ca+
L2
46
Ti+
48
Ti+
44
Sequence 1
Sequence 2
L1
47
Ti+
49
Ti+
Axial
48
Ti+
50
Ti+
H1
49
Ti+
51 +
V
H2
50
Ti+
53
46
46
Ca+
47
48
48
Ca+
49
50
50
51
53
36
40
Ca/Ti 20
V/Ti 2
Cr/Ti 0.1
Zr/Ti 0.002
Mo/Ti 0.04
+
V
50
Cr+
Double charged ion
92
92
Polyatomic ion
Cr+
Upper limits
(atomic ratios)
Interferences
Mass
Single charged ion
H3
Zr++
Mo++
94
96
94
Zr++
Mo++
28
Si19F+
96
Zr++
Mo++
98
35
Mo++
Cl14N+
100
36
Mo++
Ar14N+, 35Cl15N+
Recently, there has been a major upgrade of the Neptune MCICPMS using a large interface pump, jet sample cone and Xskimmer cone from Thermo Fisher Scientific. As a result, the
typical intensity for 48Ti is around 25 Volts ppm1 at high resolution using Aridus II desolvation inlet system. We then
remeasured samples Allende-2 and Juvinas at a Ti concentration
of 2 ppm. The bracketing standard for these two repeat
measurements was Alfa Aesar Ti metal wire with the source
being Ti ore from Utah (Utah Ti, 99.99% pure). This is because
the AATS in Origins Lab of the University of Chicago was
almost all consumed and we started to use Utah Ti as a long-term
bracketing standard.
(f) Correction of interferences and internal normalization
The interferences from 46Ca+, 48Ca+, 50V+ and 50Cr+ on 46Ti+, 48Ti+,
and 50Ti+, respectively, were corrected using the following
procedure. First, instrumental mass bias coefficients (b) were
calculated by normalizing measured 49Ti/47Ti ratios in the sample
and bracketing standards to a fixed 49Ti/47Ti ratio of 0.749766
using the exponential mass fractionation law,23,24
r ¼ R(1 + Dm/m)b
(1)
where r is the measured isotopic ratio, R is the true ratio, Dm/m is
the relative mass difference of the isotopes, and b represents the
instrumental mass bias and is determined empirically. Instrumental mass bias was calculated assuming bCa ¼ bV ¼ bCr ¼ bTi
and using established ratios 46Ca/44Ca ¼ 0.0019175, 48Ca/44Ca ¼
0.0896453, 50V/51V ¼ 0.002506, and 50Cr/53Cr ¼ 0.45732.25
Contributions from isobaric interferences (46Ca+, 48Ca+, 50V+, and
50
Cr+) were subtracted from the intensities of 46Ti+, 48Ti+, and
50
Ti+. Given the possibility that bCa s bV s bCr s bTi, more
accurate corrections were made by obtaining fractionated ratios
of 46Ca/44Ca, 48Ca/44Ca, 50V/51V, and 50Cr/53Cr through manually
modifying these ratios to eliminate spurious Ti isotope anomalies
for a Ti standard solution doped with Ca, V, or Cr standard
solutions. This is a practical way of accounting for the fact that
bCa s bV s bCr s bTi. After corrections for isobaric interferences, Ti mass-dependent fractionations (MDF) can be
expressed in d notation as:
2200 | J. Anal. At. Spectrom., 2011, 26, 2197–2205
Cl15N+, 35Cl16O+
d Ti ¼
i
Ar13C+, 37Cl16O+, 35Cl18O+
"
ði Ti=47 TiÞsample
ði Ti=47 TiÞAATS
#
1 1000
(2)
where i represents 46, 48, 49, and 50 and AATS standards for our
bracketing standard Alfa Aesar Ti solution. Titanium non-massdependent fractionations (non-MDF) represent deviations from
the composition of bracketing standard after internal normalization using the exponential law. Three methods of normalization have been used in the literature so far: (1) the earliest studies
with TIMS in the 1980s normalized the Ti data to 46Ti/48Ti ¼
0.108548;2–5,7,8 (2) Niederer et al.6 later obtained absolute Ti
compositions using the double spike technique and normalized
their data to 48Ti alone; (3) recent studies with MC-ICPMS have
normalized to 49Ti/47Ti ¼ 0.749766.1,9–11 In the current study we
have adopted the third normalization for direct comparison with
recent studies. However, we report our results using both
methods (1) and (3) to illustrate how each normalization method
affects the linear correlation between Ti isotope anomalies of two
Ti nuclides. Titanium isotope anomalies are reported in 3 notation (parts per ten thousand) to resolve small variations in the
samples:
"
#
*
ði Ti=j TiÞsample
3i Ti ¼
1
10; 000
(3)
ði Ti=j TiÞ*AATS
where the ratios marked with * have been corrected for MDF by
internal normalization and j is 47 or 48 when normalizing to
49
Ti/47Ti or 46Ti/48Ti, respectively. Analytical uncertainties for Ti
isotope anomalies (2s) were 95% confidence intervals calculated
from n replicate analyses during a single session.
Results and discussion
(a)
Titanium standards
Three Ti standards were analyzed in this study: NIST SRM3162a
solution, Alfa Aesar Ti solution (AATS), and Alfa Aesar Ti
metal wire with the source being Ti ore from Utah (Utah Ti,
99.99%). The Utah Ti was dissolved in a concentrated mixture of
HNO3 and HF at a ratio of 3 : 1. All three standards were dried
down and dissolved in 0.3 M HNO3–0.0014 M HF prior to
isotopic measurements. The results show that Utah Ti and
This journal is ª The Royal Society of Chemistry 2011
Fig. 2 Titanium isotopic compositions for three titanium standards: NIST SRM3162a, Alfa Aesar titanium solution (AATS), and Alfa Aesar Ti metal
wire (Utah Ti). (a) Titanium mass-dependent fractionation part (MDF). The theoretically calculated exponential, Rayleigh, equilibrium, and linear
fractionation lines are shown for comparison. Errors are 95% confidence intervals. (b) Theoretical calculations on artifact anomalies of 350Ti caused by
inadequate MDF corrections using inappropriate mass-fractionation laws.
SRM3162a standards have different Ti MDFs relative to that
of AATS (Fig. 2a). Utah Ti shows slight MDF, with d50Ti and
d46Ti values of 0.27 0.11& and 0.11 0.04&, respectively,
while SRM3162a is more fractionated, with d50Ti and d46Ti
values of 1.38 0.02& and 0.47 0.02&, respectively
(Table 3). The Ti MDFs of these standards plot along
a straight line with a slope of –2.87 0.07 for d50Ti vs. d46Ti
(Fig. 2a). This slope is similar to those of 2.87 and 2.81
using exponential and Rayleigh fractionation laws, respectively,
but different from the slopes of 2.76 and 2.99 using equilibrium and linear fractionation laws, respectively. Small deviations from the exponential law would cause inadequate MDF
corrections by assuming the exponential law. Any inadequate
MDF corrections would then introduce artifact Ti isotope
anomalies (e.g., 350Ti values) (Fig. 2b). Theoretical calculation
shows that the degree of 350Ti offset depends on two factors:
(1) MDF values (e.g., d46Ti) and (2) the chemical mass fractionation laws. This calculation provides a possible reason to
explain why SRM3162a shows small, yet resolvable, deficit in
50
Ti, with an 350Ti value of 0.28 0.17, while Utah Ti has
normal Ti isotopic values within analytical uncertainties (Table
3). It is not known how the Ti in SRM3162a was purified and
the process may have been governed by a different mass fractionation law from the exponential law we used. Geostandards
BCR-2 and AGV-2 show Ti MDFs close to those of AATS
and Utah Ti, with d50Ti values of 0.11 0.10& and 0.18 0.04&, respectively, relative to AATS. Therefore, AATS and
Utah Ti are both suitable as bracketing standards, whereas
SRM3162a is significantly mass fractionated and is not an
appropriate standard for the study of Ti isotope anomaly
measurements in natural samples.
(b)
Influence of isobaric interferences
It is important to pay attention to interferences for isotopic
measurements using MC-ICPMS. Isobaric interferences from
single charged ions Ca+, V+, Cr+ and doubled charged ions Zr2+
and Mo2+ can directly affect the accuracy and precision of Ti
isotopic analyses. Here we have examined their influence by
doping a Ti standard solution with various levels of Ca, V, Cr
(Fig. 3), and Zr or Mo (Fig. 4). The Ca, V, and Cr interferences
can be accurately corrected up to a ratio of 0.0002 for 46Ca/46Ti,
0.0001 for 48Ca/48Ti, 0.001 for 50V/50Ti, and 0.01 for 50Cr/50Ti
(Fig. 3, 4), if corrections were made using natural 46Ca/44Ca,
48
Ca/44Ca, 50V/51V, and 50Cr/53Cr ratios of 0.0019175, 0.0896453,
0.002506, and 0.45732, respectively.25 Better corrections can be
obtained by artificially modifying the 46Ca/44Ca, 48Ca/44Ca,
50
V/51V, and 50Cr/53Cr ratios to 0.001522, 0.089765, 0.002309, and
0.45791, respectively, to eliminate isotopic anomalies to a greater
extent. The correction limits are shown in vertical dash lines in
Fig. 3. These modifications of 48Ca/44Ca and 50Cr/53Cr ratios
correspond to reasonable changes from 1.97 to 1.95 for b48Ca/44Ca
and from 1.86 to 1.88 for bCr. However, b46Ca/44Ca, and bV would
have to change from 1.97 to 7.17 and from 1.86 to 6.04,
respectively, which may be due to inaccuracies in the recommended natural 46Ca/44Ca and 50V/51V ratios.25 Niederer and
Papanastassiou26 reported a value of 0.001518 0.000002 for
the 46Ca/44Ca ratio, close to the modified ratio of 0.001522. The
Table 3 Isotopic compositions for SRM3162a Ti solution and Ti metal wire (Utah Ti), relative to Alfa Aesar Ti solution (AATS)a
Standards
SRM3162a
Utah Ti
a
Non-mass-dependent fractionations normalized
to 49Ti/47Ti ¼ 0.749766
Mass-dependent fractionations
d46Ti
d48Ti
d49Ti
d50Ti
346Ti
348Ti
350Ti
0.47 0.02
0.11 0.04
0.45 0.02
0.10 0.04
0.91 0.02
0.19 0.07
1.38 0.02
0.27 0.11
0.04 0.21
0.08 0.23
0.10 0.10
0.01 0.06
0.28 0.17
0.13 0.12
n
20
15
Errors are 95% confidence intervals.
This journal is ª The Royal Society of Chemistry 2011
J. Anal. At. Spectrom., 2011, 26, 2197–2205 | 2201
Fig. 3 Titanium isotopic compositions after interference corrections for Alfa Aesar Ti standard solution doped with Ca (a, b), V (c), and Cr (d), using
natural (blue filled diamonds) and adjusted ratios (red filled squares). Errors are 95% confidence intervals. The Ca/Ti, V/Ti, and Cr/Ti ratios of our
sample solutions after purification (red vertical lines) are much smaller than the correction limits (black vertical dash lines) using adjusted ratios.
Ca/Ti, V/Ti, and Cr/Ti ratios of our sample solutions after
purification are always sufficiently low that it does not matter
what isotopic ratios are adopted for the interfering elements
(Fig. 3).
Doubly-charged Zr2+ can interfere with three Ti isotopes
46
( Ti+, 47Ti+, and 48Ti+), while doubly-charged Mo2+ can interfere
with all five Ti isotopes. Apparent Ti isotope anomalies due to Zr
interferences are not observable within analytical uncertainties
when the 94Zr/47Ti ratio is lower than 0.005 (Zr/Ti ¼ 0.002)
(Fig. 4a). The effects on Ti isotope anomalies from doublecharged Zr2+ interferences are linearly correlated (Fig. 4b). For
example, there is a linear relationship between 350Ti and 346Ti
values, with a slope of 0.80 0.03, comparable with the
expected slope of 0.84 based on literature Zr isotopic composition and the assumption that bZr ¼ bTi. For the Mo doping
tests, no apparent Ti isotope anomaly is detected when the
96
Mo/48Ti ratio is lower than 0.01 (Mo/Ti ¼ 0.04, Fig. 4c). Above
that value, a linear relationship is also found between Ti isotopic
anomalies. For example, the slope of the 350Ti vs. 348Ti correlation is 1.53 0.30 (Fig. 4d). The Zr/Ti and Mo/Ti ratios after
purification are always much smaller than the limits where Zr
and Mo can affect Ti isotope measurements (Fig. 4).
We have examined the influence of chlorides, which can create
interferences on Ti isotopes (49Ti+ and 50Ti+), 51V+, and 53Cr+
(Table 2), by doping a Ti standard solution in 0.3 M HNO3–
0.0014 M HF with various molarities of HCl (Fig. 4e). No Ti
isotope anomaly is found when the molarity of doped HCl is up
2+
2202 | J. Anal. At. Spectrom., 2011, 26, 2197–2205
to 0.01 M. This limit is nearly two orders of magnitude higher
than the maximum estimation of 9 104 M in our sample
solutions, assuming there was 1 mL of 9 M HCl–0.01 M HF
remaining even after we twice nearly completely evaporated the
eluted Ti solution to 1 mL and then diluted the tiny droplet in at
least 10 mL of 0.3 M HNO3–0.0014 M HF.
(c)
Isotope effects from chemical separation
The potential influence of chemical separation on MDF and
successful MDF correction using the exponential law were
investigated. Titanium standard solutions with Ti amounts of
350 mg were passed through the first and second column and Ti
was collected in 5 steps totaling 10 mL and 6 steps totaling 5 mL
for the first and the second columns, respectively (Fig. 5a).
Titanium isotopic compositions were measured for the solution
from each step (Fig. 5b).
For the first column with TODGA resin, Ti MDF is observed
during chemical separation, with d50Ti values from 2.61 0.08&
in the first step to 2.52 0.05& in the fifth step (Fig. 5a). No
spurious isotopic anomalies are detected during this elution,
except for small effects in steps 3 and 5, which cannot be fully
resolved within uncertainties. The second column with AG1-X8
resin shows d50Ti values from 0.90 0.05& in the first step to
1.22 0.04& in the sixth step (Fig. 5b). Titanium isotopic
compositions are normal in solutions from step 1 to step 5.
Again, only a small offset is detected for the isotopic anomaly
This journal is ª The Royal Society of Chemistry 2011
Fig. 4 Titanium isotopic compositions for Ti standard solution in 0.3 M HNO3–0.0014 M HF doped with Zr (a, b), Mo (c, d) and HCl in various
molarity (e). Errors are 95% confidence intervals. The Zr/Ti and Mo/Ti atomic ratios of our sample solutions after purification (red vertical line) are
much smaller than the limits (vertical dash line) where Zr and Mo can affect Ti isotope measurements. The maximum molarity of HCl remaining in our
sample solution (red vertical line) cannot introduce Ti isotope anomalies within the uncertainties.
Fig. 5 Titanium mass-dependent fractionation (MDF, in red dots) and non-mass-dependent fractionation (non-MDF, in blue squares) effects
introduced by the chemical separation through: (a) the first column with TODGA resin; (b) the second column with AG1-X8 resin. Errors are 95%
confidence intervals. The errors of Ti MDF are mostly smaller than the symbol size. The pink and the light blue areas show the weighted-average Ti
MDF and non-MDF, respectively.
This journal is ª The Royal Society of Chemistry 2011
J. Anal. At. Spectrom., 2011, 26, 2197–2205 | 2203
Table 4 Titanium isotopic compositions in geostandards and bulk meteorites
Norm. 49Ti/47Ti ¼ 0.749766
Type
Samplea
Geostandards
AGV-2-1
AGV-2-2
BCR-2b
Allende-1
Allende-2b
Allende-2b,c
Murchison
St. Severin-1
St. Severin-2
Juvinasb
Juvinasb,c
Carbonaceous chondrites
Ordinary chondrites
Eucrite
346Ti
0.02 0.15
0.11 0.09
0.03 0.10
0.68 0.11
0.66 0.10
0.62 0.07
0.47 0.15
0.14 0.16
0.19 0.19
0.21 0.09
0.32 0.06
348Ti
0.00 0.06 0.01 0.10 0.06 0.08 0.06 0.01 0.03 0.02 0.04 0.10
0.09
0.07
0.08
0.05
0.09
0.09
0.10
0.08
0.05
0.12
Norm. 46Ti/48Ti ¼ 0.108548
n
350Ti
347Ti
349Ti
350Ti
0.03 0.24
0.03 0.22
0.08 0.21
3.68 0.25
3.41 0.09
3.37 0.07
2.83 0.19
0.78 0.22
0.79 0.34
1.33 0.14
1.29 0.05
0.01 0.08
0.09 0.06
0.01 0.06
0.28 0.05
0.30 0.06
0.35 0.06
0.20 0.08
0.08 0.10
0.11 0.08
0.11 0.06
0.18 0.06
0.01 0.17
0.04 0.14
0.02 0.12
0.48 0.14
0.41 0.08
0.18 0.14
0.32 0.16
0.06 0.15
0.04 0.18
0.07 0.07
0.10 0.18
0.00 0.40
0.05 0.34
0.12 0.16
4.53 0.38
4.16 0.10
3.80 0.18
3.40 0.34
0.90 0.37
0.90 0.59
1.49 0.17
1.52 0.25
20
20
20
20
20
14
20
20
20
20
16
a
Sample numbers indicate separate dissolutions. Errors are 95% confidence intervals. b Aridus II desolvation inlet system was used, while Aridus I was
used for other sample measurements. c Repeat measurements of the same sample solution using Utah Ti as the bracketing standard after the instrument
update, while other measurements were done using AATS as the bracketing standard before the instrument update.
during this elution for step 6. The weighted average of Ti isotopic
compositions, including both MDF and non-MDF, are calculated and shown in Fig. 5. The fact that minimal mass fractionation and no anomalies are detected for weighted-average
values shows that there is no artifact effect on Ti isotope
anomalies from chemical separation of Ti.
(d) Correlated isotopic anomalies in bulk meteorites
The reliability of the method has been tested by analyzing geostandards (USGS AGV-2 andesite and USGS BCR-2 basalt) and
bulk meteorites. For all geostandards, Ti isotopes show normal
values within analytical uncertainties (Table 4), verifying that
our measurements are accurate. Furthermore, for bulk meteorites, we have analyzed three carbonaceous chondrites (two
Allende and one Murchison), one ordinary chondrite (St.Severin), and one eucrite (Juvinas). After normalization to
a 49Ti/47Ti ratio of 0.749766, carbonaceous chondrites show
positive anomalies on 46Ti and 50Ti. One Allende sample
(Allende-1) has an 346Ti value of 0.68 0.11 and an 350Ti value of
3.68 0.25, while 346Ti ¼ 0.66 0.10 and 350Ti ¼ 3.41 0.09 for
another Allende sample (Allende-2). The Murchison sample has
346Ti and 350Ti values of 0.47 0.15 and 2.83 0.19, respectively.
Two analyses of ordinary chondrite St.-Severin show negative
anomalies on 50Ti, with 350Ti values of 0.78 0.22 and 0.79 0.34, while other Ti isotopes for St.-Severin samples are normal.
One eucrite (Juvinas) has negative anomalies on both 46Ti and
50
Ti, with 346Ti and 350Ti values of 0.21 0.09 and 1.33 0.14, respectively. After the upgrade of the MC-ICPMS, we
remeasured the sample solutions of Allende-2 and Juvinas and
the results are consistent with previous measurements. The
precisions of 346Ti and 350Ti have been improved after internal
normalization to 49Ti/47Ti (Table 4). All the results are in good
agreement with the data from Trinquier et al.,1 except for those
of the two Allende samples. The observed discrepancies may be
associated with sample heterogeneity. The Allende samples that
we measured are from the Smithsonian reference powder, which
was homogenized from 4 kg of meteorite.27 All samples show
significant correlated variations in 346Ti and 350Ti when normalizing to a 49Ti/47Ti ratio of 0.749766, with a slope of 5.12 0.38
and an intercept of 0.17 0.16 (2s, n ¼ 11). They are in good
agreement with the slope of 5.48 0.27 and the intercept of
0.04 0.20 (2s, n ¼ 39) from Trinquier et al.1 (Fig. 6a). 347Ti
and 350Ti are linearly correlated when normalizing to a 46Ti/48Ti
Fig. 6 (a) 350Ti vs. 346Ti after normalization to a fixed ratio of 49Ti/47Ti for geostandards (BCR-2 and AGV-2) and bulk meteorites (Allende, Murchison,
St.-Severin, and Juvinas). The linear slope defined by this study (350Ti ¼ (5.12 0.38) 346Ti + (0.17 0.16), in black) is in good agreement with the line
provided by Trinquier et al.1 (350Ti ¼ (5.48 0.27) 346Ti (0.04 0.20), in gray); (b) 350Ti vs. 347Ti after normalization to a fixed ratio of 46Ti/48Ti.
2204 | J. Anal. At. Spectrom., 2011, 26, 2197–2205
This journal is ª The Royal Society of Chemistry 2011
ratio of 0.108548, with a slope of 12.9 2.7 and an intercept of
0.37 0.49 (2s, n ¼ 11) (Fig. 6b). Additional measurements of
a more diverse suite of samples are required to better constrain
this correlation.
Conclusions
A new method is presented for high-precision Ti isotopic
measurements in natural samples. Titanium is separated via
a two-stage procedure, followed by Ti isotope analyses using
MC-ICPMS. NIST SRM3162a shows a large mass-dependent
fractionation relative to natural specimens and should not be
used to study Ti isotope anomalies. In contrast, AATS and Utah
Ti can be both used in Ti isotopic studies as they show minimal
mass-dependent fractionations relative to terrestrial rocks and
meteorites. Tests performed on Ti standard doped with various
levels of isobaric interferences show that the abundances of
interferences in the purified solutions are several orders of
magnitude lower than the correction limits. This demonstrates
that the Ti separation is very effective. Furthermore, tests for Ti
collections in several elution steps indicate that there is no artifact on Ti isotope measurements.
This newly developed protocol has been successfully applied to
geostandards and different meteorite groups ranging from
carbonaceous chondrites to differentiated meteorites such as
eucrite. Normal Ti isotopic compositions were observed in geostandards, while 46Ti and 50Ti isotope anomalies could be
resolved among different meteorite groups. Linear correlations
between isotope anomalies of different Ti nucleus are found, with
350Ti ¼ (5.12 0.38) 346Ti +(0.17 0.16) and 350Ti ¼ (12.9 2.7) 347Ti +(0.37 0.49), when normalizing to 49Ti/47Ti and
46
Ti/48Ti, respectively.
Acknowledgements
We thank the Chicago Center for Cosmochemistry and the Field
Museum of Natural History for providing bulk meteorite
samples (Murchison, St.-Severin, and Juvinas). We are grateful
to Paul Ryan Craddock, Thomas Ireland, and Haolan Tang for
technical assistance and discussions. This study was supported
by NASA through grants NNX09AG39G (AMD),
NNX09AG59G (ND), and by a fellowship from the David and
Lucille Packard Foundation (ND).
This journal is ª The Royal Society of Chemistry 2011
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