Study on the removal of interferences for the determination of ⁸⁷Sr/⁸⁶Sr isotopic ratio in petroleum drill-Hole water samples using isotope dilution – inductively coupled plasma mass spectrometry (ID – ICP-MS)
Nuclear Science and Technology, Vol.8, No. 4 (2018), pp. 34-43
Study on the removal of interferences for the determination of
87Sr/86Sr isotopic ratio in petroleum drill-hole water samples
using isotope dilution – inductively coupled plasma mass
spectrometry (ID – ICP-MS)
Nguyen Thi Kim Dung, Thai Thi Thu Thuy*
Center for Analytical Chemistry, Institute for Technology of Radioactive and Rare elements (ITRRE),
48 Lang Ha, Hanoi, Vietnam
*Faculty of Chemistry, VNU University of Science, 19 Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam
(Received 16 November 2018, accepted 11 March 2019)
Abstract: 87Sr/86Sr isotopic ratio is one of the useful tools that can authenticate the original source of
the natural products from the earth-created and/or geological processes. However, the effect of
interferences in petroleum drill-holes water sample such as thickness of sample matrix or isotopic
signal of 87Rb might cause the low precision of 87Sr/86Sr determination using quadrupole inductively
coupled plasma spectrometry (ICP-MS). The elimination of these mentioned effects was thus studied
by using the ion - exchange chromatography. Calcium in sample matrix was separated on anionite
column (Bio-Rad AG1-X8 resin) in methanol medium with the high efficiency while rubidium was
removed from strontium on cation exchange resin (Bio-Rad AG50-X8) with strontium recovery over
99%. The isotope dilution technique with 86Sr - enriched isotopic standard solution was used for the
control of separation process. The 87Sr/86Sr isotopic ratio was thus determined using ICP-MS with the
signal correction by a strontium isotopic ratio standard reference material (NIST SRM 987).
Keywords: 87Sr/86Sr isotope ratio, petroleum drill-hole water, ion exchange chromatography, ID-
ICP-MS.
strontium content in seawater is about 8 ppm
around the oceans [4] whereas 87Sr/86Sr
isotopic ratio in the minerals and rocks spans
the range 0.7-4.0 [5]. This isotopic ratio in the
ocean water is constant of 0.7092 but it is in
the range of 0.707-0.730 in the oil-field water
[3]. From the co-relation between 87Sr/86Sr
ratio and concentration of total strontium
dissolved in seawater, the original source of
ancient water in the oil-fields would be
authenticated [4,6]. It thus supports the
evaluation of petroleum potential [4] in the oil-
fields.
I. INTRODUCTION
Natural occurring strontium has four
stable isotopes, 84Sr , 86Sr , 87Sr , and 88Sr with
relatively natural abundance variations of 0.55-
0.58%, 9.75-9.99%, 6.94-7.14% and 82.29-
82.75%, respectively, according to IUPAC [1].
The 87Sr isotope is generated by β decay of
87Rb: 87Rb = 87Sr + β– (haft life: 4.88×1010
years) [2]. In the rocks and in water contacted
with
rocks/minerals
for
long
time,
concentration of 87Rb is rather high which
consequently makes the radiogenic 87Sr isotope
87
to be comparable [3]. Sr/86Sr isotopic ratio of
87
The Sr/86Sr isotopic ratio is commonly
ancient water is thus higher than that from
pumped surface seawater to the drill-holes to
exploit the oil [4]. The average value of total
determined by thermal ionization mass
spectrometry (TIMS) due to the high precision
©2018 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute
NGUYEN THI KIM DUNG, THAI THI THU THUY
of method (from 0,01 to 1%) [2,7]. However, it
earth group with similar physical and chemical
properties, the quantitative separation of each
other is rather difficult even using ion
exchange technique, except for the use of
special resin to separate 90Sr trace from
seawater [18].
requires the high cost equipment with
sufficient skill of technician to prepare the
sample and it is also the time-consuming
method. Inductively couple plasma mass
spectrometry (ICP-MS) also provides the
capability to determine the isotopic
composition of several elements in the periodic
table together with their contents at high
sensitivity and precision. ICP-MS (quadrupole
type) and multi-collector (MC-ICP-MS) can
measure many samples with high repeatability
within short period of time, and the analyte
price is rather inexpensive. ICP-MS is thus
always a choice of researchers in the world to
study stable isotope compositions [1-3,6,8,9].
In this study, the removal of matrix and
isobaric interferences for the quantitative
determination of 87Sr/86Sr isotopic ratio using
ICP-MS was focused. The matrix effect
caused by calcium at high concentration in
petroleum drill-holes water samples was
eliminated by using anion exchanger (Bio-
Rad AG1x8) in HNO3-methanol (95%)
medium, meanwhile the isobaric effect due to
rubidium presented in sample was removed by
cation resin Bio-Rad AG50x8. The isotope
dilution technique (ID) with 86Sr enriched
isotope standard solution was applied for the
separation control and the NIST SRM 987
standard reference material was used for the
signal correction of 87Sr/86Sr isotopic ratio
measurement on ICP-MS.
Despite of its obvious advantages, ICP-
MS still shows
a
disadvantage when
determining the 87Sr/86Sr isotopic ratio due to
87
87
isobaric overlap of Rb signal at Sr isotope,
which needs to eliminate. There exists several
resolutions to correct the signal value for these
two isotopes by using 85Rb and 88Sr [10,11] but
the complete removal of Rb from Sr in
analyzed sample is preferable with the use of
ion-exchange chromatography [1,3,7,9-12]. In
case of sample containing thick matrix due to
high concentration of calcium and other
alkaline earth elements such as rock or soil, it
is necessary to remove these interfered major
elements before the separation of strontium and
rubidium [7,10-17]. The removal of calcium
from sample matrices, especially from
seawater was carried out with variety of
reagents using solvent extraction or fractional
precipitation or ion exchange [6, 8, 15-17].
Among them, the ion exchange technique was
recently more applicable to separate calcium
from different sample matrices [13-17] with
high recovery of strontium. Since calcium and
strontium are adjacent elements in alkaline
II. EXPERIMENTAL
A. Chemicals and reagents
All chemicals were of analytical grade:
HNO3 (d=1.4 g/mL),
methanol (d=
0.972g/mL) and standard stock solutions of Rb,
Ca and Sr (1000 mg.L-1) for ICP-MS (Merck,
Germany); Cation exchange resin (Bio-Rad
AG50X8, 200-400 mesh) and anion exchange
resin (Bio-Rad AG1X8, 200-400 mesh)
supplied by Bio-rad Co., USA; Commercially
available isotope enriched 86Sr (10.009 ± 0.073
mg.L-1) standard solutions (Inorganic Venture-
USA) and NIST SRM 987 (SrCO3) 87Sr/86Sr
isotopic ratio standard reference material
(National Institute of Standards and
Technology, Gaithersburg, MD, USA) were
used for isotope dilution analysis and signal
35
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 87Sr/86Sr…
correction on isotopic ratio measurement,
respectively. Purified water (18 MΩ.cm–1) was
used for preparation of aqueous solutions. High
pure argon gas (99,999%, Messer) was used
for ICP-MS measurement.
solution for the control of separation
efficiency. Each type of ion exchanger (Bio-
Rad AG50X8, Bio-Rad AG1X8) was pre-
conditioned by immersing 5 grams in pure
water for completely swelled up, then loading
them on a column. Cation exchanger (Bio-Rad
AG50X8, H+ type) was equilibrated with 0.5M
HNO3 solution and this column was ready for
separation experiment. Anion exchanger (Bio-
Rad AG1X8, Cl- type) was washed with dilute
HNO3 at flow rate of 1mL/min until the Cl- trace
was not detected in eluate by AgNO3 solution.
The excess of acid was then washed out with
pure water for next step of sample loading. The
separation procedure on both resins can be
summarized in the following figures.
B. Apparatus
The Agilent (USA) Model 7500a ICP-
MS, controlled by Chemstation software, was
used for the measurements. The optimized
instrumental operating conditions are as
follows: RF power, 1450 W; RF matching,
1.45 V; sample uptake time, 90 s; sample
uptake rate, 0.4 rps; sample depth, 6.4 mm; Ar
coolant flow rate, 15 L min–1; carrier gas, 1.2 L
min–1; auxiliary gas, 0.9 L min–1; water RF/TP
flow rate, 2.4 L min–1; water RF/TP, T = 293
K; analyzer pressure, 3 × 10–4 to 2 × 10–3 Pa.
The concentration of all studied
elements was determined by external
calibrations on ICP-MS under the optimized
operating conditions. The certified value of
87Sr/86Sr isotopic ratio in NIST SRM 987
standard reference material as 0.71034 +
0.00026 was applied for the control of isotopic
ratio measurement on ICP-MS.
Quart-glass chromatography columns
are loaded resin (internal diameter of 12mm,
400mm height), which connected the peristaltic
pump
(Masterflex®
L/S,
Cole-Parmer
Instrument Company, USA) to control flow
rate and volume of fractions during loading
and elution.
C. Procedures
The experiments on chemical separation
were performed at room temperature.
Petroleum drill-holes water samples
received from Vietnam petroleum institute,
which contained some oil and solid residue
were filtered and stored in plastic bottle at 4oC
for further treatment. A certain volume of this
water sample was taken into a glass beaker,
2mL of concentrated HNO3 was then added
and this mixture was gently evaporated on a
hot plate to dryness. This residue was dissolved
in a mixture of 0.25 M HNO3 in 95% methanol
and made up a volume of 10 mL for separation
experiment. A small amount of 86Sr enriched
isotope standard was added in to sample
Fig.1 Separation of Ca from Sr and Rb on anion exchanger
36
NGUYEN THI KIM DUNG, THAI THI THU THUY
close distribution constants of calcium and
strontium on cation exchange resin, which
might cause the overlap on elution peaks of
rubidium and strontium.
Previous report in literature [17] showed
that the distribution constant of calcium on
anionic exchanger much lower than that of
strontium in alcoholic medium. Several cations
can adsorb on strong base anion exchange
resins, which contain the quaternary
ammonium group with nitrate as counter-ion
from solutions of nitric acid in alcohol [14] and
the order of adsorption of these cations
depends on the distribution coefficients [13,
14]. The phenomenon of adsorbed cations on
anionic exchangers can only be obtained in
solutions with highly polarized alcohol [17].
Hence, the mixture of 0.25 M HNO3 in 95%
methanol was selected for calcium separation
on anion exchanger (Bio-rad AG1-X8) in our
study. Mixed standard solutions with different
ratios between Ca and Sr (1:1; 1:10; 1:100 and
1:1000) were respectively loaded onto the
anion exchange columns. The eluted fractions
(10 ml each) were collected with the flow rate
of 0.5 mL/min for the determination of Ca
concentration and that of 1 mL/min for Sr
elution. Figures 3 and 4 below gave the
examples of elution chromatographs on mutual
separation of calcium from strontium at various
concentration ratios.
Fig.2 Separation of Rb from Sr on cation exchanger
III. RESULTS AND DISCUSSION
A. Study on the removal of calcium from
sample solution using anion exchange
chromatography
The analysis of petroleum drill-holes
water samples showed that chemical
composition of this sample type was rather
complicated with high salt matrix, in which
the average concentration of calcium was
about 100 mg/L (ppm) but these values of
rubidium and strontium were about 1 and 2
mg/L, respectively. It thus very much
interferes with the mutual separation of small
amount rubidium from strontium in that
sample by ion chromatography due to the
Fig.3 Elution curve for mutual separation of Ca from Sr Fig.4 Elution curve for mutual separation of Ca from Sr
(Concentration Ratio between Ca and Sr = 10:1)
(Concentration Ratio between Ca and Sr = 1000:1)
37
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 87Sr/86Sr…
Calcium and strontium in all four cases
were well separated from each other when the
elution was taken part with 0.25 M HNO3 in
95% methanol at gradient flow rate (see Fig.1).
However, a small amount of calcium in peak
tailing exists in some early Sr eluted fractions
at the cases of high ratios between Ca and Sr
such as 100:1 and 1000:1. With 1:1 and 10:1
ratios, calcium was completely separated from
strontium after 10 fractions of elution (100
mL). The recovery of strontium in all cases
was about 98%. This result is similar to other
study with different anion exchangers
[13,14,17].
element in eluted solution and the result was
shown in Table I.
Fig. 5 Elution curve of a mixed solution
As can be seen in Fig.5, the elution
peak of Rb appeared latter than that of Sr in
the same elution condition. The present
finding is somewhat different to the former
study [13] when other alkali metals were
separated from mixed elements sample at
higher flow rate and lower HNO3
concentration.
The separation test was also carried
out with a mixed solution of 100 ppm Ca, 1
ppm Sr, 1 ppm Rb under the same
conditions in order to learn about the
rubidium removal. The following figure
performs the result. The recovery was
calculated according to the content of each
Table I. Separation efficiency and recovery of elements
Content in eluted
solution (100 ml)
(mg/L)
Content
remained in
column (mg/L)
Separation
Efficiency
(%)
Total amount
(mg/L)
Recovery
(%)
Elem.
Ca
Rb
Sr
100.64
0.999
0.998
93.33
0.054
0.043
10.10
0.95
0.95
92.74
94.58
95.50
102.77
99.16
98.92
The data in Table II show that the
removal of calcium from studied sample
reaches rather high efficiency after 5 fractions
(100 mL) of elution, and the next 4 eluted
fractions (80 mL) contains almost Sr and Rb,
which will be used for the separation of Rb on
cation exchanger.
Mixed standard solution of rubidium
and strontium in 0.5 M HNO3 medium (1ppm
of each element) was loaded onto the cation
exchanger column. The elution of mutual
rubidium from strontium was carried out
under gradient HNO3 concentration (1.0 M
and 2.0 M) solutions at the rate of 0.25 and
0.5 mL/min for Rb and Sr, respectively. The
elution curves for these elements were shown
in below figure.
B. Study on the removal of rubidium from
strontium
using
cation
exchange
chromatography
38
NGUYEN THI KIM DUNG, THAI THI THU THUY
Fig. 6 Elution curves of Rb, Sr under gradient conditions (1M HNO3 within first 7 fractions, 2M HNO3 for
next 7 fractions)
As can be seen from Fig.6, rubidium
could well be separated from strontium with
1.0 M HNO3 eluant at the low flow rate as
0.25 mL/min. The faster elution would
remove small amounts of strontium that
cause the lower separation efficiency and
recovery. Under this condition, the recovery
of rubidium was quantitatively over 99%.
On the other hand, the elution of strontium
was successful with using 2.0 M HNO3
eluant at 0.5 mL/min flow rate and the
recovery nearly completed. That is the
reason why the gradient conditions of eluant
concentration and flow rate of elution
should be applied for the quantitative
removal of the isobaric interference caused
by Rb on the determination of strontium
isotope ratio.
However, the small amount of calcium
remained in sample solution at this stage was
also
considered
when
using
cation
chromatography technique for removal of Rb
interference. The mixed solution containing 1.5
ppm Ca, 0.5 ppm Sr and 0.25 ppm Rb was
loaded on the cation exchanger column. The
elution was carried out under the same
condition as above mentioned. The elution
curve is shown in Fig. 7.
Fig.7 Elution curves of Rb, Ca and Sr under gradient conditions
39
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 87Sr/86Sr…
The elution peaks from Fig.7 depict that
almost Ca presented in mixed solution was eluted
together with Sr meanwhile Rb was completely
removed within the first 7 eluted fractions. It
confirms that the small amount of Ca in sample
solution does not interfere with the quantitative
separation of Rb, and that the isobaric effect
caused by Rb can be completely removed.
isotopic standard solutions, in which total
concentration of strontium was fixed as 100
μg/L but the isotope ratio 86Sr/87Sr was
various with the addition of a certain amount
of 86Sr isotope standard solution in to the
natural Sr standard solution (see Table II) in
a matrix (100 mg/L Ca and 50 μg/L Rb). The
separation procedure was repeated for all
synthesized samples under the same
conditions as reported above. The results
were given in Table II.
C. Validation of separation procedure
The recovery of strontium through
separation procedure was studied by using Sr
Table II. Recovery of Sr in synthesized samples
87Sr
found
(µg/L)
Total Sr
found
(µg/L)
Total Sr Spiked 86Sr 86Sr/87Sr (by
Recovery
(%)
(µg/L)
(µg/L)
theory)
100
90
0
1.40857
2.98187
4.90220
10.4921
7.80047
6.35607
5.72470
4.40357
103.605
90.801
81.781
62.908
103.61
100.89
102.22
104.85
10
20
40
80
60
40
60
21.9123
2.92630
41.804
104.51
The data in Table II show that the
recovery of strontium for whole cases (from
100.89% to 104.85%) seemed reliable for Sr
analysis through the long procedure of mutual
separation. It is thus suitable for the application
of Sr isotopic ratio analysis in petroleum drill-
holes water samples.
exchanger column. Whole separation
procedure was carried out for this standard
sample and the final elution fractions were
collected for the determination of 87Sr/86Sr
isotopic ratio on ICP-MS. Five replicates of
experiment were done and the results were
given in Table III.
The accuracy of separation method
Table III. Analysis of the standard sample SRM 987
was studied by the use of NIST SRM 987
87Sr/86Sr certified 87Sr/86Sr analyzed Absolute
(SrCO3)
isotopic
standard
reference
value
value
Error (%)
material. The isotopic standard solution was
prepared by dissolving a certain amount of
standard reference material in dilute HNO3
and a small portion of this solution
containing 100 μg/L (as total Sr
concentration) was loaded on anion
0,71034 ±
0.00026
0.71453 ±
0.00836
+ 0.59
The relative correctness of analyzed
value is 99.41% to that of the certified value
for NIST SRM 987, which seems reasonable in
40
NGUYEN THI KIM DUNG, THAI THI THU THUY
this study due to the contribution of signal
procedure. The chemical composition of
sample was analyzed using ICP-MS in order to
preliminary classification of the solution
matrix. The sample solution was diluted with
pure water as needed before applying the
separation procedure, followed by the isotopic
ratio measurement on ICP-MS. The analytical
data were given in Table IV together with total
concentrations of Rb and Sr and relative
standard deviation (RSD) of 87Sr/86Sr isotopic
ratio measurement.
measurement deviation of instrument to the
error. That is also the reason why the more
precision of the isotopic analysis can be
obtained from MC-ICPMS [1].
D. Analysis of petroleum drill-holes water
samples
Petroleum drill-holes water samples
were pretreated to remove the oil and
suspended solid particles as denoted in
Table IV. Analysis of petroleum drill-holes water samples
87Sr/86Sr Isotopic ratio
Sample
Code
Total Rb
(µg/L)
Total Sr (µg/L)
Value
RSD (%)
EW02
EW05
EW17
PW03
PW04
PW14
36.6
45.3
34.5
80.0
60.2
120.0
989.8
3502.5
1240.1
824.0
0.70715
1.17
2.78
1.52
1.45
1.63
2.46
0.70734
0.70699
0.70686
0.70639
0.70674
423.5
1980.0
87
Analytical results showed that, Sr/86Sr
isotopic ratio of petroleum drill-holes water
samples was various with different sample
matrix. These data relatively agreed with those,
which were obtained from similar study [4] of
drill-holes water in Vietnam Petroleum
Institute, where the water samples were
exchange
chromatography.
The
anion
exchange resin (Bio-Rad AG1X8 200-400
mesh) was employed for the separation of
major calcium by 0.25 M HNO3 in 95%
methanol with Sr recovery over 99%. The
mutual separation of rubidium and strontium
by gradient conditions of HNO3 concentration
and flow rate on cation exchanger (Bio-Rad
AG50X8 200-400 mesh) was taken part with
nearly complete Sr recovery. The validation of
method was also studied using isotopic
standard solution and standard reference
material with relative correctness of the
analyzed value about 99.41% to the certified
value of NIST SRM 987 reference material.
The analytical procedure was then applied for
87
pretreated and the analysis of Sr/86Sr isotopic
was carried out by TIMS in over-sea
laboratory.
IV. CONCLUSIONS
The removal of calcium in matrix from
petroleum drill-holes water samples and the
elimination of rubidium isobaric interference
with strontium isotopic ratio determination
were successfully achieved by using ion
87
the determination of Sr/86Sr isotopic ratio in
41
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 87Sr/86Sr…
Table for Derivation of Numeric Age”, The
Journal of Geology, Volume 105, p. 441–456,
1997.
petroleum drill-holes water samples using ICP-
MS, which would contribute to the
development of an analytical method to supply
the demand of petroleum research and
exploitation in Vietnam.
[6]. K. Notsu, H. Wakita, and Y. Nakamura,
“Strontium isotopic composition of oil-field
and gas-field waters, Japan”,
Appl.
ACKNOWLEDGEMENT
Geochemistry, Vol. 3, No.2, 173–176, 1988.
[7]. Toshiro Takahashi, Yuka hirahara, Takashi
Miyazaki, Bogdan stefanov Vaglarov, Qing
Chang, Jun-ichi Kimura, Yoshiyuki Tatsumi,
“Precise determination of Sr isotope ratios in
igneous rock samples ans application to micro-
The authors are thankful to the
assistance of M.Sc. Ngo Quang Huy on
carrying out some preliminary experiments.
The financial support under framework of a
VINATOM
project
encoded
analysis
of
plagioclase
phenocrysts”,
DTCB.09/18/VCNXH was highly appreciated.
JAMSTEC-R IFREE Special Issue, 59-64,
Nov. 2009.
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