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)

t matrix, in which anion exchange columns. The eluted fractions 
the average concentration of calcium was (10 ml each) were collected with the flow rate 
about 100 mg/L (ppm) but these values of of 0.5 mL/min for the determination of Ca 
rubidium and strontium were about 1 and 2 concentration and that of 1 mL/min for Sr 
mg/L, respectively. It thus very much elution. Figures 3 and 4 below gave the 
interferes with the mutual separation of small examples of elution chromatographs on mutual 
amount rubidium from strontium in that separation of calcium from strontium at various 
sample by ion chromatography due to the concentration ratios. 
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 element in eluted solution and the result was 
were well separated from each other when the shown in Table I. 
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 Fig. 5 Elution curve of a mixed solution 
[13,14,17]. 
 As can be seen in Fig.5, the elution 
 The separation test was also carried peak of Rb appeared latter than that of Sr in 
out with a mixed solution of 100 ppm Ca, 1 the same elution condition. The present 
ppm Sr, 1 ppm Rb under the same finding is somewhat different to the former 
conditions in order to learn about the study [13] when other alkali metals were 
rubidium removal. The following figure separated from mixed elements sample at 
performs the result. The recovery was higher flow rate and lower HNO3 
calculated according to the content of each concentration. 
 Table I. Separation efficiency and recovery of elements 
 Content in eluted Content Separation 
 Total amount Recovery 
 Elem. solution (100 ml) remained in Efficiency 
 (mg/L) (%) 
 (mg/L) column (mg/L) (%) 
 Ca 100.64 93.33 10.10 92.74 102.77 
 Rb 0.999 0.054 0.95 94.58 99.16 
 Sr 0.998 0.043 0.95 95.50 98.92 
 The data in Table II show that the Mixed standard solution of rubidium 
removal of calcium from studied sample and strontium in 0.5 M HNO3 medium (1ppm 
reaches rather high efficiency after 5 fractions of each element) was loaded onto the cation 
(100 mL) of elution, and the next 4 eluted exchanger column. The elution of mutual 
fractions (80 mL) contains almost Sr and Rb, rubidium from strontium was carried out 
which will be used for the separation of Rb on under gradient HNO3 concentration (1.0 M 
cation exchanger. and 2.0 M) solutions at the rate of 0.25 and 
B. Study on the removal of rubidium from 0.5 mL/min for Rb and Sr, respectively. The 
strontium using cation exchange elution curves for these elements were shown 
chromatography in below figure. 
 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 should be applied for the quantitative 
could well be separated from strontium with removal of the isobaric interference caused 
1.0 M HNO3 eluant at the low flow rate as by Rb on the determination of strontium 
0.25 mL/min. The faster elution would isotope ratio. 
remove small amounts of strontium that However, the small amount of calcium 
cause the lower separation efficiency and remained in sample solution at this stage was 
recovery. Under this condition, the recovery also considered when using cation 
of rubidium was quantitatively over 99%. chromatography technique for removal of Rb 
On the other hand, the elution of strontium interference. The mixed solution containing 1.5 
was successful with using 2.0 M HNO3 ppm Ca, 0.5 ppm Sr and 0.25 ppm Rb was 
eluant at 0.5 mL/min flow rate and the loaded on the cation exchanger column. The 
recovery nearly completed. That is the elution was carried out under the same 
reason why the gradient conditions of eluant condition as above mentioned. The elution 
concentration and flow rate of 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 isotopic standard solutions, in which total 
almost Ca presented in mixed solution was eluted concentration of strontium was fixed as 100 
together with Sr meanwhile Rb was completely μg/L but the isotope ratio 86Sr/87Sr was 
removed within the first 7 eluted fractions. It various with the addition of a certain amount 
confirms that the small amount of Ca in sample of 86Sr isotope standard solution in to the 
solution does not interfere with the quantitative natural Sr standard solution (see Table II) in 
separation of Rb, and that the isobaric effect a matrix (100 mg/L Ca and 50 μg/L Rb). The 
caused by Rb can be completely removed. separation procedure was repeated for all 
C. Validation of separation procedure synthesized samples under the same 
 The recovery of strontium through conditions as reported above. The results 
separation procedure was studied by using Sr were given in Table II. 
 Table II. Recovery of Sr in synthesized samples 
 87Sr Total Sr
 Total Sr Spiked 86Sr 86Sr/87Sr (by Recovery 
 found found 
 (µg/L) (µg/L) theory) (%) 
 (µg/L) (µg/L) 
 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 exchanger column. Whole separation 
recovery of strontium for whole cases (from procedure was carried out for this standard 
100.89% to 104.85%) seemed reliable for Sr sample and the final elution fractions were 
analysis through the long procedure of mutual collected for the determination of 87Sr/86Sr 
separation. It is thus suitable for the application isotopic ratio on ICP-MS. Five replicates of 
of Sr isotopic ratio analysis in petroleum drill- experiment were done and the results were 
holes water samples. 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 0,71034 ± 0.71453 ± + 0.59 
 0.00026 0.00836 
standard reference material in dilute HNO3 
and a small portion of this solution The relative correctness of analyzed 
containing 100 μg/L (as total Sr value is 99.41% to that of the certified value 
concentration) was loaded on anion 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 
measurement deviation of instrument to the sample was analyzed using ICP-MS in order to 
error. That is also the reason why the more preliminary classification of the solution 
precision of the isotopic analysis can be matrix. The sample solution was diluted with 
obtained from MC-ICPMS [1]. pure water as needed before applying the 
 separation procedure, followed by the isotopic 
D. Analysis of petroleum drill-holes water 
 ratio measurement on ICP-MS. The analytical 
samples 
 data were given in Table IV together with total 
 Petroleum drill-holes water samples concentrations of Rb and Sr and relative 
were pretreated to remove the oil and standard deviation (RSD) of 87Sr/86Sr isotopic 
suspended solid particles as denoted in ratio measurement. 
 Table IV. Analysis of petroleum drill-holes water samples 
 Sample Total Rb 87Sr/86Sr Isotopic ratio 
 Total Sr (µg/L) 
 Code (µg/L) 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, 87Sr/86Sr exchange chromatography. The anion 
isotopic ratio of petroleum drill-holes water exchange resin (Bio-Rad AG1X8 200-400 
samples was various with different sample mesh) was employed for the separation of 
matrix. These data relatively agreed with those, major calcium by 0.25 M HNO3 in 95% 
which were obtained from similar study [4] of methanol with Sr recovery over 99%. The 
drill-holes water in Vietnam Petroleum mutual separation of rubidium and strontium 
Institute, where the water samples were 
 by gradient conditions of HNO3 concentration 
 87 86
pretreated and the analysis of Sr/ Sr isotopic and flow rate on cation exchanger (Bio-Rad 
was carried out by TIMS in over-sea AG50X8 200-400 mesh) was taken part with 
laboratory. 
 nearly complete Sr recovery. The validation of 
 method was also studied using isotopic 
 IV. CONCLUSIONS 
 standard solution and standard reference 
 The removal of calcium in matrix from material with relative correctness of the 
petroleum drill-holes water samples and the analyzed value about 99.41% to the certified 
elimination of rubidium isobaric interference value of NIST SRM 987 reference material. 
with strontium isotopic ratio determination The analytical procedure was then applied for 
were successfully achieved by using ion the determination of 87Sr/86Sr isotopic ratio in 
 41 
 STUDY ON THE REMOVAL OF INTERFERENCES FOR THE DETERMINATION OF 87Sr/86Sr 
petroleum drill-holes water samples using ICP- Table for Derivation of Numeric Age”, The 
MS, which would contribute to the Journal of Geology, Volume 105, p. 441–456, 
development of an analytical method to supply 1997. 
the demand of petroleum research and [6]. K. Notsu, H. Wakita, and Y. Nakamura, 
exploitation in Vietnam. “Strontium isotopic composition of oil-field 
 and gas-field waters, Japan”, Appl. 
 ACKNOWLEDGEMENT Geochemistry, Vol. 3, No.2, 173–176, 1988. 
 The authors are thankful to the [7]. Toshiro Takahashi, Yuka hirahara, Takashi 
assistance of M.Sc. Ngo Quang Huy on Miyazaki, Bogdan stefanov Vaglarov, Qing 
 Chang, Jun-ichi Kimura, Yoshiyuki Tatsumi, 
carrying out some preliminary experiments. 
 “Precise determination of Sr isotope ratios in 
The financial support under framework of a 
 igneous rock samples ans application to micro-
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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