Application NoteGeology

Measurement of 87Sr/86Sr Isotope
Ratios in Rocks by ICP-QQQ in
Mass-Shift Mode
The direct Agilent 8900 ICP-QQQ method removes
need for chromatographic separation of 87Sr and 87Rb

IntroductionAuthors
Xiaoming Liu
Yahui Yue1
Yibo Yang1
Shuofei Dong2
1

Institute of Tibetan Plateau
Research, Chinese Academy ofSciences, Beijing 100085, China
1

Agilent Technologies Co., Ltd.
(China), Beijing 100102, China2

Triple quadrupole ICP-MS (ICP-QQQ) instrumentation is increasingly used forisotope ratio (IR) analysis studies (1–5). Geologists use radioactive isotopes thathave long half-lives to date rocks and sediments, often with the choice of dating
method dependent on the type of samples under investigation. For example,rubidium (Rb) is abundant in many potassium-containing rocks, so the
Rb-strontium (Sr) β-decay scheme is used in geochronology studies of igneous,and sometimes metamorphic rocks and minerals (5, 6).
Strontium (Sr) has four naturally occurring isotopes: 84 Sr (0.56% abundance), 86Sr(9.86% abundance), 87Sr (7.00% abundance), and 88Sr (82.58% abundance). Whilethese are the four stable isotopes of Sr, the abundance of 87Sr varies over long
periods of time due to the formation of radiogenic 87Sr following β-decay of 87Rb.

The level of 87Sr compared to 86Sr is dependent on the original
concentration of Rb in the environmental or geologicalsystem. Therefore, 87Sr/86Sr IR measurements can also serve
as a tracer of ecosystem processes (7).
Traditional IR measurement techniques such as thermal
ionization mass spectrometry (TIMS) or multicollector
inductively coupled plasma mass spectrometry (MC-ICP-MS)
offer excellent accuracy and high precision for most IRstudies. However, they are less efficient for the measurementof β decay product isotopes, due to isobaric interferences ofthe daughter isotope by their respective parent isotope. For
example, 87Sr must be separated from 87Rb before analysis by
TIMS or MC-ICP-MS to avoid the isobaric spectral overlap atmass 87. This process is labor-intensive and time consuming,
especially for large sample numbers that are typical ofgeological and environmental survey studies. By contrast, noRb/Sr separation is needed before analysis using ICP-MS/MS
methodology, simplifying the analysis and improving sample
throughput. ICP-MS/MS methods use “chemical resolution”
to resolve isobaric interferences, as demonstrated in previous
studies using Agilent ICP-QQQ instrumentation (1, 5).
In those studies, reactive gases were used in the collision/
reaction cell (CRC) of the ICP-QQQ to separate theisobaric overlap from 87Rb on 87Sr, avoiding the need forchromatographic separation. Isobaric ion interferences canbe separated using ICP-QQQ when the cell gas reacts quickly
with one of the elements to form a product ion, while theother element does not react (or reacts slowly). Although
reaction cell gases such as CH3F, N2O, and SF6 provide a higher
reaction rate with Sr+ than O2, O2 is more widely available inlaboratories and is more suited to routine applications.
The aim of this study was to develop a fast, high throughput,routine method suitable for the accurate determination of87
Sr/86Sr IR with precision below 0.1% RSD. To achieve theobjective, an Agilent 8900 ICP-QQQ was operated in MS/MS
mass-shift mode using O2 as the cell gas.
87

Sr/86Sr IR analysis by ICP-QQQ in O2 mass-shift mode

The Agilent 8900 Triple Quadrupole ICP-MS (ICP-QQQ) uses
two identical, full-sized quadrupoles (Q1 and Q2), one before
and one after the ORS 4 reaction cell. Like the analyzer massfilter, Q2, Q1 operates in a high vacuum region, so it provides
excellent resolution and abundance sensitivity. This allows
precise selection of which ions enter the cell, ensuring thereactions used to resolve the analyte ions from their on-massinterferences are consistent and predictable. As shown in2

Figure 1, by setting Q1 to m/z 87, only Rb-87 and Sr-87 enter
the cell that is pressurized with O2. In the cell, 87Sr+ reacts
with O2 to form 87Sr16O+ product ion, while 87Rb+ does not react.
The second quadrupole (Q2) is set to m/z=103, which allows
87
Sr16O+ to pass to the detector, free of interference.
Reaction cellgas (O2)

Q1 (m/z 87) –rejects all ions
except m/z 87

ORS 4
Q2 (m/z 103) –Rb+ + O2 no reaction rejects all ions(or very slow)
except m/z 103Figure 1. Determination of 87Sr as 87Sr16O+ product ion using ICP-QQQ
operating in mass-shift mode with O2 cell gas.

Efficiency of using mass-shift mode to separate Sr from Rb
When Sr+ ions react with O2 in the cell, not all Sr+ ionscomplete the reaction to form SrO+ ions during the shorttransit time in the cell. To optimize the O2 flow rate, counts
per second (cps) of both 86Sr+ (at m/z 86) and 86Sr16O+ (atm/z 102) were monitored. For the calculation, the measured
cps of Q1 (m/z = 86)  Q2 (m/z = 102) was divided by the cpsof Q1 (m/z = 86)  Q2 (m/z = 86). The mass-shift efficiencyof Sr in O2 reaction mode ranged between the expected rangeof 10 and 14% (8). The variation was mainly due to instrument
conditions on the day of measurement.
To evaluate whether 87Rb16O+ formed in the cell sufficiently toaffect the measurement of 87Sr16O+, 85Rb was used as a proxyfor 87Rb. The signals at m/z 85 (for 85Rb+) and m/z 101 (for85
Rb16O+) were monitored during method development. First,
Q1 and Q2 were both set to m/z = 85. The measured cps of85
Rb+ in the reference materials (RMs) and rock samples were
between 6×105 and 4×106 (the cps of 85Rb+ in the solution
blank was typically below 1). Then Q2 was set to m/z 101 andthe measured cps range was below 1, suggesting minimal
formation of 85Rb16O+ in the cell. Using the natural abundanceof 85Rb (72.17%) and 87Rb (27.83%), we calculated that theconversion of 87Rb+ to 87Rb16O+ would lead to a measured cps
for 87Rb16O+ at m/z = 103 below 0.4 cps.
In this study, the Sr concentration of the RMs and rock samples
measured by ICP-QQQ were mostly around 100 ng/mL. This
concentration is equivalent to over 1 × 105 cps for 87Sr+ as
87
Sr16O+ (m/z = 103). Therefore, the interference effect of 87Rb16O+(m/z = 103) on 87Sr16O+ (m/z = 103) is negligible in this study.

Experimental
Standards and sample preparation
To determine the accuracy and precision of the method,
three US Geological Survey (USGS) RMs were used:
Basalt, Columbia River (BCR-2), Basalt, Hawaiian VolcanicObservatory (BHVO-2), and Andesite (AGV-2). Twentythree rock samples were analyzed in the study, including
basalt (Rb/Sr concentration ratio 0.02–0.12), dolerite (Rb/Srconcentration ratio 0.04–0.10), andesite (Rb/Sr concentration
ratio 0.76–0.87), and rhyolite (Rb/Sr concentration ratio 5.6–9.1).
All RMs and samples were prepared at the Institute of Tibetan
Plateau Research, Chinese Academy of Sciences (ITP-CAS)
using a classic pressurized acid digestion method for traceelement measurements. Approximately 50 mg of eachsample was digested using 1 mL each of purified HNO3 and
HF in a closed vessel oven, heated at 190 °C for 36 hours.
Following the addition of 0.25 mL of HClO4, the sample wasevaporated to incipient dryness on a hot plate at 130 °C. This
procedure was repeated with the addition of 0.5 mL of HNO3.
The sample was further heated at 170 °C until white fumes
were seen. 0.5 mL of 40% HNO3 was added, and the sample
was oven-heated at 130 °C for 3 h. After cooling, 49.5 mL ofde-ionized water was added to the sample, ready for analysisby ICP-QQQ.

Instrumentation
An Agilent 8900 ICP-QQQ was used. The sample introduction
system consisted of a quartz torch with 2.5 mm i.d. injector,
a quartz spray chamber, glass concentric nebulizer, andnickel-tipped interface cones. Samples were introduced into
the ICP-QQQ using a standard peristaltic pump and Agilent
SPS 4 autosampler.
Sr and 86Sr were determined in mass-shift mode using
O2 as the cell gas. The isobaric interference on 87Sr by 87Rbwas avoided by measuring the Sr product ions 87Sr16O+
and 86Sr16O+ at m/z=103 and m/z=102, respectively. Typicalinstrument operating parameters are listed in Table 1.
87

Throughout the analytical workflow, a BCR-2 RM sample
solution containing 100 ng/mL Sr was measured after
every two or three samples to ensure that the mass bias
calibration was up to date. Instrument mass bias occurs
when ions of different element masses transmit through theICP-MS with different efficiencies, resulting in inaccurate IR
measurements. The Agilent ICP-MS MassHunter software
automatically updates the mass bias.

Table 1. ICP-QQQ operating parameters and acquisition settings for87
Sr/86Sr IR analysis.
ParameterSetting

RF Power (W)1550

Sampling Depth (mm)8.0

Nebulizer Gas Flow Rate (L/min)

1.15

KED (V)–6.0

Cell Gas Flow Rate (mL/min)

0.45 (30% of full scale)

Axial Acceleration (V)1.0

Q1Q2 Masses (m/z)

86 10287 103

Q2 Peak Pattern

1 point

Replicates

7Sweeps

1000

Integration Time of Each Mass (s)9Results and discussion

Determination of 87Sr/86Sr IRs in the RMs
Sr/86Sr IRs were measured in BCR-2 and BHVO-2 RMs using
the ICP-QQQ mass-shift method with O2 cell gas. To confirm
that chromatographic separation of Sr wasn’t needed forsuccessful ICP-QQQ analysis, data was acquired with andwithout the Sr purification stage. Each RM was measured
seven times in different batches or on different days, withseven replicate measurements.
87The 87Sr/86Sr ratio results in Figure 2 show that there was no
significant difference in the results for BCR-2 and BHVO-2
with and without Sr purification. Both sets of measured dataare within the literature reported values that were obtainedusing TIMS and MC-ICP-MS (9). The ICP-QQQ method
is accurate enough to study 87Sr/86Sr ratios in natural
samples/systems with a wide range of 87Sr/86Sr distributions.
Also, the sample preparation procedure doesn’t affect theprecision of the ICP-QQQ measurements.
The error bars show two times the standard deviation (SD)of the average of seven separate measurements of the twoRMs, which range between 0.0003 and 0.0005 (n=7) or
between 0.02 and 0.04% RSD. This deviation is significantly
higher than the RSD of the MC-ICP-MS method. However, theresults show that ICP-QQQ can distinguish 87Sr/86Sr ratios ofmaterials within 0.001 (i.e. 0.1%) precision.

30.7060

BCR-2
BCR-2WithwithSrSrpurification
purification
BCR-2
BCR-2without
withoutSrSrpurification
purification
BHVO-2
BHVO-2withwithSrSrpurification
purification

0.7055

BHVO-2
BHVO-2without
withoutSrSrpurification
purification
Avg.
Avg.BCR-2
BCR-2With
withSrSrpurification
purification

0.7050

Avg.
Avg.BCR-2
BCR-2Without
withoutSrSrpurification
purification

87Sr/86Sr ratio

Avg.
Avg.BHVO-2
BHVO-2withwithSrSrpurification
purification

0.7045withoutSrSrpurification
purification
Avg.BHVO-2
BHVO-2withoutAvg.

0.70400.70350.7030Figure 2. Comparison of the measured 87Sr/86Sr ratios of BCR-2 and BHVO-2
0.7025
with and without Sr purification column chemistry by ICP-QQQ with literature
values (9) represented by the shaded bands. Circle symbols: average valueof each measurement, the error bar is 1SD of the seven replicates of eachmeasurement. Square symbols: the average value of the seven measurementsof BCR-2 and BHVO-2, the error bar represents 2SD of the seven measurements.

The effect of the Sr concentration in the samples
To optimize the level of sample dilution of the sample digests,
we investigated the effects of the concentration of Sr in
the rock samples on the analysis. Three fully digested rocksamples (andesite, dolerite, and basalt) were diluted to givea Sr concentration between 60 and 350 ng/mL. A BCR-2
solution containing 100 ng/mL of Sr was used for online
instrumental mass bias correction.

Analysis of Sr IRs of rock samples by ICP-QQQ
The 23 rock samples were analyzed in triplicate (n=3) using
the ICP-QQQ mass-shift method, without Sr purification. As
shown in Figure 3, the samples cover a large 87Sr/86Sr ratio
range from 0.705 to 0.748. This range is typical of commonrocks (Figure 3), as well as minerals, waters, and plants (4).
The accuracy of the 87Sr/86Sr ratio measurements obtained by
ICP-QQQ was compared with data obtained by MC-ICP-MS.
There was good agreement between the two methods (4),with a variation of between 0.00001 and 0.0016 of the meanof the 87Sr/86Sr ratio. The variation wasn’t dependent on the87Sr/86Sr ratio range.
The precision of the 87Sr/86Sr ratio measured by ICP-QQQ
varied between 0.0001 and 0.0019 (2SD, n=3) or between
0.01 and 0.13% (RSD, n=3). While this precision is lower thanthe precision of the MC-ICP-MS method, it is sufficient forapplications with large 87Sr/86Sr ratio differences, such as theidentification of 87Sr/86Sr ratio in between certain rock types.
Andesite, rhyolite, and dolerite rock samples have significant
87
Sr/86Sr ratio variations, as shown in Figure 3. However,distinguishing rock types within a narrow range of 87Sr/86Srratios, e.g., low 87Sr/86Sr basalts, could be challenging.
ICP-QQQ method

The results in Table 2 show that, within the measured
concentration range, the Sr concentration doesn’t
significantly affect the 87Sr/86Sr ratios of each type ofsample. Therefore, the digested samples could potentially bediluted into a certain range, such as from 60 to 350 ng/mL,
according to a standard operation procedure, simplifying theanalysis.
Table 2. Measured 87Sr/86Sr IRs of three different rock samples (one sampleof each type of rock) with four different Sr concentrations.
SampleBasaltDolerite

Andesite

4Sr Concentration(ng/mL)

Sr/86Sr Ratio

2SD (n=3)700.70360.00091000.70550.0002175

0.70430.0008350

0.70600.000987700.70350.0015100

0.70470.00091700.70390.0016340

0.70460.000860

0.71140.00131000.7126

0.00061400.71200.0015280

0.71260.0014MC-ICP-MS method

BasaltDolerite

Rhyolite
Andesite
0.70000.71000.72000.73000.74000.75000.760087Sr/86Sr ratio

Figure 3. 87Sr/86Sr IR distribution for typical rocks. Triangle symbol: 87Sr/86Sr
IR of ICP-QQQ method, error bar in 2SD (n=3). Circle symbol: 87Sr/86Sr IR ofMC-ICP-MS method, error bar in 2SD (n=3) which falls within the circle mark.

Conclusion

References

The study has shown that the Agilent 8900 ICP-QQQ can beused for the accurate measurement of 87Sr/86Sr isotope ratios
in a range of rock samples. The isobaric interference of 87Rbon 87Sr was resolved using a mass-shift method with O2 cellgas. Because Sr reacts with oxygen at a much faster rate
than Rb, 87Sr was measured as 87Sr16O+, free from interference
from 87Rb. This method is simpler, faster, and less costly thanTIMS or MC-ICP-MS methods, as both TIMS and MC-ICP-MS
require chromatographic separation of Sr from Rb before
analysis. The ICP-QQQ mass-shift method is also suited tononspecialist laboratories that aren’t equipped with TIMS orMC-ICP-MS.

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E. Bolea-Fernandez, L. Balcaen, M. Resano, F. Vanhaecke,
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Glenn Woods, Resolution of 176Yb and 176Lu interferenceson 176Hf to enable accurate 176Hf/177Hf isotope ratio
analysis using an Agilent 8800 ICP-QQQ with MS/MS,
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3.

Glenn Woods, Lead isotope analysis: Removal of 204Hg
isobaric interference from 204Pb using ICP-QQQ in
MS/MS mode, Agilent publication, 5991- 5270EN

4.

X. Liu, S. Dong, Y. Yue, et al. 87Sr/86Sr isotope ratios inrocks determined by ICP‐MS/MS in O2 mode without
prior Sr purification. Rapid Commun Mass Spectrom.,
2020, accessed May 2020, https://doi.org/10.1002/
rcm.8690

5.

T. Zack, K. J. Hogmalm, Laser ablation Rb/Sr datingby online chemical separation of Rb and Sr in anoxygen-filled reaction cell. Chem Geol. 2016, 437, 120–133

6.

G. Faure, J. L. Powell, Strontium Isotope Geology.
Springer-Verlag, New York, 1972

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R. C. Capo, B. W. Stewart, O. A. Chadwick, Strontium
isotopes as tracers of ecosystem processes: theory andmethods. Geoderma. 1998, 82, 197-225

8.

Naoki Sugiyama and Kazumi Nakano, Reaction data for
70 elements using O2, NH3 and H2 gases with the Agilent
8800 Triple Quadrupole ICP-MS; Agilent publication,5991-4585EN

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K.P. Jochum, U. Nohl, K. Herwig, E. Lammel, B. Stoll, A.
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The distribution of 87Sr/86Sr IRs of natural samples isgenerally between 0.70 and 0.77, although it can range
from 0.700 to 0.943 (4). The accuracy and precision of theICP-MS/MS method is sufficient to differentiate between
various sources when the 87Sr/86Sr ratio varies by more than
0.001. Therefore, this method has the potential to be used
in applications that require source identification, such asenvironmental pollutant source tracing, large-scale geologicalsurveys, and agricultural product authenticity studies.

More information
For detailed information, please see Liu, X, Dong, S,Yue, Y, et al. 87Sr/86Sr isotope ratios in rocks determined
by ICP‐MS/MS in O2 mode without prior Srpurification. Rapid Commun Mass Spectrom. 2020,
https://doi.org/10.1002/rcm.8690

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