Nuclear Science and Technology - Volume 8, Number 4, December 2018

obaric 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 
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 
87
Sr/
86
Sr 
40 
 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. 
C. Validation of separation procedure 
 The recovery of strontium through 
separation procedure was studied by using Sr 
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 
86
Sr 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. 
Table II. Recovery of Sr in synthesized samples 
Total Sr 
(µg/L) 
Spiked 
86
Sr 
(µg/L) 
86
Sr/
87
Sr (by 
theory) 
87
Sr 
found 
(µg/L) 
Total Sr 
found 
(µg/L) 
Recovery 
(%) 
100 0 1.40857 7.80047 103.605 103.61 
90 10 2.98187 6.35607 90.801 100.89 
80 20 4.90220 5.72470 81.781 102.22 
60 40 10.4921 4.40357 62.908 104.85 
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. 
 The accuracy of separation method 
was studied by the use of NIST SRM 987 
(SrCO3) isotopic standard reference 
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 
exchanger column. Whole separation 
procedure was carried out for this standard 
sample and the final elution fractions were 
collected for the determination of 
87
Sr/
86
Sr 
isotopic ratio on ICP-MS. Five replicates of 
experiment were done and the results were 
given in Table III. 
Table III. Analysis of the standard sample SRM 987 
87
Sr/
86
Sr certified 
value 
87
Sr/
86
Sr analyzed 
value 
Absolute 
Error (%) 
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 
NGUYEN THI KIM DUNG, THAI THI THU THUY 
41 
this study due to the contribution of signal 
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 
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 
87
Sr/
86
Sr isotopic 
ratio measurement. 
Table IV. Analysis of petroleum drill-holes water samples 
Sample 
Code 
Total Rb 
(µg/L) 
Total Sr (µg/L) 
87
Sr/
86
Sr Isotopic ratio 
Value RSD (%) 
EW02 36.6 989.8 0.70715 1.17 
EW05 45.3 3502.5 0.70734 2.78 
EW17 34.5 1240.1 0.70699 1.52 
PW03 80.0 824.0 0.70686 1.45 
PW04 60.2 423.5 0.70639 1.63 
PW14 120.0 1980.0 0.70674 2.46 
 Analytical results showed that, 
87
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 
pretreated and the analysis of 
87
Sr/
86
Sr 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 
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 
the determination of 
87
Sr/86Sr isotopic ratio in 
STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 
87
Sr/
86
Sr 
42 
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. 
ACKNOWLEDGEMENT 
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 
DTCB.09/18/VCNXH was highly appreciated. 
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