Prog. Theor. Exp. Phys. 2017, 113H01 (11 pages)
DOI: 10.1093/ptep/ptx145

Determination of trace levels of uranium andthorium in high purity gadolinium sulfate using
the ICP-MS with solid-phase chromatographic
extraction resin
S. Ito1,∗ , Y. Takaku2 , M. Ikeda3,4 , and Y. Kishimoto3,4
1

Okayama University, Faculty of Science, Okayama 700-8530, Japan
Institute for Environmental Sciences, Department of Radioecology, Aomori 039-3212, Japan
3
Kamioka Observatory, Institute for Cosmic Ray Research, University ofTokyo, Kamioka, Gifu 506-1205, Japan
4
Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa, Chiba
277-8582, Japan∗
E-mail: s-ito@okayama-u.ac.jp
2

Received September 15, 2017; Revised October 5, 2017; Accepted October 6, 2017; Published November 23, 2017...................................................................................................................
The new Super-Kamiokande-Gadolinium (SK-Gd) project is an upgrade of the SuperKamiokande (SK) detector. In the SK-Gd project, 0.2% Gd2 (SO4 )3 is loaded into the 50 kton
water tank of the SK. One of the main purposes of the project is to discover supernova relic neutrinos. Neutrino measurements and proton decay searches will also be performed in the SK-Gd.
In order to measure solar neutrinos with a low energy threshold of ∼3.5 MeV in the SK-Gd,
the main radioactive contaminations, 238 U and 232 Th, in Gd2 (SO4 )3 ·8H2 O, should be minimized
before loading. Our maximum levels for U and Th are 5 mBq (U)/kg (Gd2 (SO4 )3 ·8H2 O) and0.05 mBq (Th)/kg (Gd2 (SO4 )3 ·8H2 O). In order to measure such low concentrations of U and
Th in Gd2 (SO4 )3 ·8H2 O, we developed the solid-phase extraction technique. Using this method,about 90% or more U and Th could be efﬁciently extracted while Gd was reduced by a factor of about 104 . This allowed these radioactivity contaminations to be measured precisely as
0.04 mBq/kg (Gd2 (SO4 )3 ·8H2 O) for U and 0.01 mBq/kg (Gd2 (SO4 )3 ·8H2 O) for Th. We measured
three pure Gd2 (SO4 )3 ·8H2 O samples using this method and estimated that the purest one contained < 0.04 mBq (U)/kg (Gd2 (SO4 )3 ·8H2 O) and 0.06 ± 0.01 mBq (Th)/kg (Gd2 (SO4 )3 ·8H2 O)
by the ICP-MS.
...................................................................................................................
Subject Index

H20

1. Introduction
1.1. The Super-Kamiokande and the new SK-Gd project
The Super-Kamiokande (SK), which is located 1000 m underground at Ikenoyama, Kamioka, Gifu,
Japan, is the world’s largest water Cherenkov detector with a mass of 50 kton (Ref. [1]). The main
physics targets for the SK are proton decay searches and measuring neutrinos from various sources
such as the Sun, the atmosphere, supernova explosions, and artiﬁcial neutrino beams. The energy
threshold is set at about 3.5 MeV to measure low energy neutrinos, such as solar neutrinos.
On 27 June 2015, the Super-Kamiokande-Gadolinium (SK-Gd) project was approved by the SK
Collaboration; this is an upgrade of the SK detector, in which 0.2% gadolinium sulfate (Gd2 (SO4 )3 )is added to the SK water tank (Ref. [2]). One of the goals of the SK-Gd is to discover supernova© The Author(s) 2017. Published by Oxford University Press on behalf of the Physical Society of Japan.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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relic neutrinos and study the star formation history of the universe (Ref. [3]). Gadolinium sulfate
octahydrate (Gd2 (SO4 )3 ·8H2 O) is used for the SK-Gd, as it has higher solubility than unhydrated
gadolinium sulfate. We aim to introduce Gd2 (SO4 )3 ·8H2 O in JFY-2019 and then start the SK-Gd
experiment.
The concentration of radioactive impurities in Gd2 (SO4 )3 ·8H2 O loaded water needs to be kept at
the same level as in the current SK water in order to continue precise measurements of the solar
neutrino spectrum (Ref. [4]). Thus, Gd2 (SO4 )3 ·8H2 O should be puriﬁed before loading into the SK
water tank to avoid interference with the solar neutrino measurements. The maximum levels for themain radioactive contaminations, U and Th, are 5 mBq (U)/kg (Gd2 (SO4 )3 ·8H2 O) and 0.05 mBq(Th)/kg (Gd2 (SO4 )3 ·8H2 O),1 respectively.

1.2.

ICP-MS measurement

Inductively coupled plasma-mass spectrometry (ICP-MS) is a highly sensitive mass spectrometry
technique that is widely used in analytical chemistry to measure elements in solution. As 238 U and
232 Th have long half-lives (4.5 ×109 and 1.4 ×1010 years, respectively), the ICP-MS has a higher
sensitivity to their radioactivity than other detectors (e.g., Ge detectors). Introducing highly concentrated solution into the ICP-MS, however, affects the sensitivity. High matrix elements interferewith the ionization of target elements, which reduces the sensitivity of the ICP-MS. This problem isgenerally called “matrix effect”.
The introduction of high matrix elements into the ICP-MS also increases the number of polyatomic
ions. If many polyatomic ions originating from high matrix elements (in our case, Gd and O in thewater, and Ar, which is used for Ar plasma in the ICP-MS) are produced, whose mass numbers are
similar to those of target elements (in our case, 238 U and 232Th) in the ICP-MS, the target elements
would be overestimated. This issue is called “spectral interference”. In our case, potential spectralinterference resulting from Gd, O, and Ar would be caused by 158 Gd40Ar2 , 158 Gd16 O5 , and so on
for 238 U, and 152 Gd40Ar2 , 160 Gd18 O4 , and so on for 232Th.
In order to remove this interference, matrix elements should be reduced at least to < 100 μg mL−1 .
It is, however, difﬁcult to measure U and Th with the high precision that we require by the dilution of
Gd2 (SO4 )3 ·8H2 O solution. Therefore, we developed a solid-phase extraction method to extract U and
Th from Gd2 (SO4 )3 ·8H2 O and measured these concentrations precisely at an order of 0.01 mBq/kg
using the ICP-MS. We report on this method in this paper.

2. Experimental equipment
2.1. ICP-MS in Kamioka
In December 2016, an ICP-MS “Agilent 7900” (Agilent Technologies, Ref. [5]) was installed in the
Kamioka clean room, a clean room in the Kamioka mine near the SK detector. Figure 1 shows apicture of the ICP-MS in the Kamioka clean room. The main characteristic of the Agilent 7900 is thehighly sensitive ICP-MS because of low dark noise. Additionally, the collision/reaction cell of the
ICP-MS is ﬁlled with helium (He) gas in order to reduce dark counts originating from polyatomic
ions. When He gas is not used, the dark counts from polyatomic ions at mass numbers of 232 (Th)and 238 (U) in 0.2 mol L−1 nitric acid (HNO3 ) blank solution are both about 10 counts per second(CPS). When using He gas, the counts of polyatomic ions could be reduced to < 1 CPS. Figure 2
Unless otherwise speciﬁed, the unit “mBq/kg” represents mBq of U or Th in kg of Gd2 (SO4 )3 ·8H2 O
powder.
1

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Fig. 1. Picture of the ICP-MS “Agilent7900” in the Kamioka clean room. The auto-sampler is fully coveredby the class 100 clean booth to reduce contamination from the outside.

Fig. 2. Calibration curves of 238 U (left) and 232 Th (right) with He gas. The horizontal axes show concentrationsof U and Th, and the vertical axes represent CPS. These points are ﬁtted to linear functions.

shows the calibration curves of U and Th for the ICP-MS. The detection limit (DL) of the ICP-MS
can be deﬁned as
DL <3σblank,a(1)

where σblank and a represent the statistical uncertainty of the CPS for the blank solution and theslope of the calibration curve with He gas, respectively. While the values of DL for U and Th wererespectively 13 fg mL−1 and 15 fg mL−1 without He gas, this improved to 3 fg mL−1 when He gas
was introduced.
Figure 3 shows a typical example of the matrix effect from various concentrations of
Gd2 (SO4 )3 ·8H2 O on the sensitivity of the ICP-MS. For this study, 10 ng mL−1 of 115 In, 205Tl,
and 209 Bi mixed solution was used. Compared with the blank solution, the CPS of all elements
dropped to 30%∼40% at 0.2 mg mL−1 Gd2 (SO4 )3 ·8H2 O solution due to the matrix effect of Gd.
The Kamioka clean room was designed as class 1000, and the auto-sampler is fully covered by
the clean booth, which is designed as class 100. This clean environment is set up to minimize
contamination from the outside.
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Fig. 3. Typical example of the matrix effect on Gd2 (SO4 )3 ·8H2 O. The horizontal axis shows the concentrationsof Gd2 (SO4 )3 ·8H2 O (mg mL−1 ) and the vertical axis represents the normalized CPS. The CPS for 115 In, 205 Tl,
and 209 Bi were normalized to the CPS for each element in the blank solution.

Fig. 4. HNO3 concentration dependence for U (red triangles) and Th (blue circles) with the UTEVA resin(Ref. [7]). The horizontal axis shows the concentration of HNO3 and the vertical axis shows the distribution
coefﬁcient (k  ).

2.2. Reagents and equipment
Chromatographic extraction resin. In this study, we used UTEVA resin, which is a commerciallyavailable product of Eichrom Technologies LLC (Ref. [6]) and has been widely used for a variety ofanalytical separations. Figure 4 shows the HNO3 concentration dependence for U and Th with the
UTEVA resin (Ref. [7]). All the tetravalent actinides and U(VI) have strong retention (distribution
coefﬁcient k  > 100, which is deﬁned as the ratio of the number of chemicals (U and Th) in
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Fig. 5. Schematic of the experimental setup.

the resin to the number of the chemicals in the solution (Refs. [6,7])) with the UTEVA resin in>5 mol L−1 HNO3 . On the other hand, Gd forms a trivalent ion so that Th and U can be extracted
from Gd2 (SO4 )3 ·8H2 O using the UTEVA resin. These features make it possible to separate U and
Th from Gd-loaded water.

Reagents. In order to produce solutions with low contamination, the ultra-pure SK water (Ref. [1])and ultra-high-purity analytical grade 68% HNO3 (TAMAPURE AA-100 made by Tama Chemicals,
Ref. [8]) were used. Calibration of the ICP-MS and estimation of the recovery for U and Th were
performed using a high-purity 29 element mixed standard solution (XSTC-331, SPEX Inc., Ref. [9]).
In order to check the magnitude of the matrix effects, In, Tl, and Bi standard solutions (Kanto
Chemical Company Inc., Ref. [10]) were used as internal standard solutions. These solutions weremixed and diluted to 10 ng mL−1 in 0.2 mol L−1 HNO3 solution. Electronic (EL) grades of 70%
HNO3 and 35% hydrochloric acid (HCl) (Wako Pure Chemical Industries Ltd., Ref. [11]) were used
to prewash the resin, column, bottles, and other equipment.

Column and equipment for solid-phase extraction. A column with a volume of about 20 mL
and an inside diameter of 9 mm (LSC-φ9, GL Sciences Inc., Ref. [12]) was used for this study.
Figure 5 shows a schematic of the experimental setup. A valve was added to the column in order toadjust the ﬂow rate. This setup was connected to a tip, and ﬁxed to a free-fall manifold. The UTEVA
resin was sandwiched between two ﬂits to ﬁx.

Bottles. All bottles used for this study were made of polypropylene. Bottles with a volume of
100 mL were used for the preservation of the sample solutions and the collection of solutions during
the solid-phase extraction procedure. Bottles with a volume of 14 mL were used for the ICP-MS
measurements to place samples on the auto-sampler.
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Fig. 6. Diagram of the whole procedure for the solid-phase extraction.

3. Study of the solid-phase extraction
3.1. Procedure
The specially puriﬁed Gd2 (SO4 )3 ·8H2 O samples were provided by various companies; we call themsamples A, B, and C in this paper.2 Figure 6 shows a diagram of the whole procedure. The details ofeach step are described in this section.
3.1.1. Prewash
The resin and equipment were suspected to be contaminated during the production process. Thus,
the following prewash procedures were performed. The UTEVA resin was soaked in 0.5 mol L−1
HNO3 solution (EL grade) for more than one night. The columns, bottles, and all other equipment
were also soaked in 1 mol L−1 of HNO3 solution (EL grade) for at least one night. All of the soaked
items were washed with the ultra-pure SK water. Then, the column was charged with 0.3 g of the
UTEVA (∼1.5 cm of the bed height) and the column, the valve, and the tip were set up as illustrated
in Figure 5. The resin and inside of the column, the valve, and the tip were washed with 20 mL of
7 mol L−1 HNO3 (EL grade) followed by 20 mL of 1 mol L−1 HCl (EL grade). Finally, this setup
was washed by loading 20 mL of ultra-pure SK water. In the prewash procedure, the valve was fully
opened (the ﬂow rate was about 5 mL/min). The temperatures of all the solutions were the same as
the temperature in the clean room, being about 22 ◦ C.
3.1.2. Procedure blank
In order to collect the procedure blank solution, 20 mL of 7 mol L−1 HNO3 solution was loaded
into the column. After all the solution passed through the column, 20 mL of 0.1 mol L−1 HNO3
solution was loaded into it. Then, the 0.1 mol L−1 HNO3 solution that passed through the column
was collected as the procedure blank solution. If the prewash procedure was skipped, more than
1 pg mL−1 of U and Th contamination was observed in the procedure blank solution. On the other
hand, the concentration of U and Th in the procedure blank solution could be reduced to about
10 fg mL−1 after the prewash procedure.
2

For the contract between companies and us, the companies’ and products’ names are blinded.
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3.1.3. Conditioning
To condition the resin, 3 mL of 7 mol L−1 HNO3 solution was loaded into the column. During thisprocedure, the valve was adjusted to set the ﬂow rate to about 0.3 mL/min in order to efﬁciently
adsorb U and Th in the resin for the next procedure.
3.1.4. Separation
For this study, 1.0 g of the new Gd2 (SO4 )3 ·8H2 O sample was dissolved in 100 mL of 7 mol L−1
HNO3 solution. In order to separate U and Th from the Gd2 (SO4 )3 ·8H2 O solution, 18 mL of thesample solution was loaded into the column with a ﬂow rate of about 0.3 mL/min. The valve was
fully opened again after all the sample solution had passed through.
3.1.5. Washing the resin
Before eluting the U and Th, the resin should be washed in order to reduce the remaining Gd containedin it. The resin was washed three times with 1.5 mL of 7 mol L−1 HNO3 . When this procedure was
not performed, the count rate of the ICP-MS decreased to about 50% due to the matrix effect fromthe remaining Gd. According to Figure 3, about 10% of the Gd remained in the resin before thisprocedure.
3.1.6. Elution
In general, U and Th in the UTEVA resin can be eluted with dilute HNO3 solution. However,
U and Th are adsorbed by several materials in low concentration acid solution (Ref. [13,14]). In
particular, Th is adsorbed to a greater extent than U and this effect is signiﬁcant for < 1 pg mL−1 levelmeasurements. In order to reduce such a problem, the elution procedure was improved as follows:
6 mL of 0.1 mol L−1 HNO3 was loaded into the column to break the chemical bonds between theresin and U/Th (more than 80% of U is eluted in this procedure). Next, 6 mL of 7 mol L−1 HNO3
was loaded to remove and collect Th on the columns or the surface of the resin. Finally, 6 mL
of 0.1 mol L−1 HNO3 was loaded to collect the remaining U and Th. These eluted solutions werecollected in the same bottle (18 mL in total) and measured using the ICP-MS.

3.2.

Estimation of recovery rate

3.2.1. Extraction of U and Th from non-Gd solution
In order to estimate the recovery rate (the U and Th recovered in this study divided by added U
and Th), 7 mol L−1 HNO3 solutions with U and Th with no Gd2 (SO4 )3 ·8H2 O were prepared. The
concentration of U and Th in the sample solutions was adjusted to 0.1, 1, and 10 pg mL−1 . Figure 7
shows the results of this study. By this method, about 90% or more of the U and Th could be recovered.
Extraction of U and Th from Gd2 (SO4 )3 ·8H2 O solution
Spectral interference. The effects of the spectral interference from Gd and the efﬁciency ofthe separation between U, Th, and Gd were studied. For these studies, 1.00 g of the three new
Gd2 (SO4 )3 ·8H2 O samples A, B, and C was dissolved in 100 mL of 7 mol L−1 HNO3 and the developed solid-phase extractions were performed. Figure 8 shows mass spectra of the three eluted
solutions for a mass range of 211 to 244. For reference, the mass spectra of Gd2 (SO4 )3 ·8H2 O
solutions are also shown. In order to the reduce matrix effect, the Gd2 (SO4 )3 ·8H2 O solutions werediluted to 100 μg mL−1 . Compared to the reference spectra, we can see that the spectral interference
was efﬁciently reduced by our separation method. Figure 9 shows mass spectra of Gd in the eluted
3.2.2.

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Fig. 7. Recovery rates of the eluted solution for U (red circles) and Th (blue squares). The horizontal axis
shows the concentrations of U and Th, and the vertical axis represents the recovery rate.

Fig. 8. Mass spectra of the eluted solution (dashed red) and 100 μg mL−1 of Gd2 (SO4 )3 ·8H2 O solutions ofsamples A (left), B (center), and C (right) in the mass range 211 to 244.

solutions and 100 μg mL−1 Gd2 (SO4 )3 ·8H2 O solutions for reference. Because the Gd spectra for
all the eluted solutions could be reduced to an order of 10−6 g mL−1 , the effects of the spectralinterference on U and Th were reduced to a negligible level.

Recovery rate. The inﬂuence of Gd2 (SO4 )3 ·8H2 O on the retention of the resin was also studied
by adding U and Th to the sample solutions with concentrations of 0.1, 1, and 10 pg mL−1 . Figure 10
shows the results of this study. We can see that the recovery rates remained > 90% for all samples
at all concentrations of U and Th. Because of U and Th impurities in the Gd2 (SO4 )3 ·8H2 O samples,
the recovery rates of U and Th at 0.1 and 1 pg mL−1 , especially sample C, highly exceed 100%. The
measured concentrations of U and Th will be described in the next section in more detail. In thisstudy, we conﬁrmed that Gd2 (SO4 )3 ·8H2 O did not inﬂuence the recovery rate.
Matrix effect. During the ICP-MS measurements, the variation in the sensitivity of this technique
was always monitored by mixing the internal standard solution (10 ng mL−1 In, Tl, and Bi) into the
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Fig. 9. CPS of all Gd isotopes for samplesA (left), B (center), and C (right) before and after chemical extraction.
The gray histograms show the 100 μg mL−1 Gd2 (SO4 )3 ·8H2 O solutions and the red histograms indicate the
CPS of Gd in the eluted solutions.

Fig. 10. Recovery rates of U (left) and Th (right) with and without Gd2 (SO4 )3 ·8H2 O samples. The horizontal
axes show the concentrations of U and Th, and the vertical axes represent the recovery rates for U and Th.
Black circles, red squares, blue triangles, and pink diamonds indicate the sample without Gd2 (SO4 )3 ·8H2 O,
and samples A, B, and C, respectively.

sample solutions. Figure 11 shows the variation in the internal standard solution for all samples.
In this study, no decrease in the sensitivity of the ICP-MS due to the matrix effect from Gd wasobserved.

4.

Results and discussion

The results of the measurements of U and Th in samples A, B, and C are listed in Table 1. For comparison, the values for the procedure blank solution, typical commercially available Gd2 (SO4 )3 ·8H2 O,
and our requirements are also given. The relationships between p g−1 (Gd2 (SO4 )3 ·8H2 O) and mBq/kg
for U and Th areand

1 mBq (U)/kg = 81.3 pg (U) g−1(2)

1 mBq (Th)/kg = 246 pg (Th) g−1 .

(3)

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Fig. 11. Variation of the count rate for the internal standard solution, In (black squares), Tl (red circles), andBi (blue triangles) for the blank and eluted solutions of samples A, B, and C. Since the mass number of In isabout half that of Tl and Bi, the count rate of In is also different to the others.

As shown in Figure 7, the recovery rates for U and Th at 0.1 pg mL−1 are (107 ± 5)% and (106 ±5)%, respectively, while the recovery rates at 1 pg mL−1 for U and Th are respectively (89 ± 2)%
and (93 ± 1)%. Therefore, 15% uncertainty was conservatively added to the conversion from thesolution to the powder.
The DLs of U and Th with this developed method were limited by the procedure blank solution
and the uncertainty in the recovery rate since the matrix effect and spectrum interference werereduced to a negligible level. The concentrations of U and Th in the procedure blank solution wereabout 0.01 pg mL−1 . By conservatively taking three times these values inﬂated by 15%, which is theuncertainty in the recovery rate, the DLs of U and Th were estimated to be 0.04 mBq (U)/kg and0.01 mBq (Th)/kg. These values are lower than our requirements, by two orders of magnitude for U
and a factor of 5 for Th.
By the efforts of various companies, U was well puriﬁed to about one or two orders of magnitude
lower than our requirement. With respect to Th contamination, sample A almost reaches our goal,
sample B should be further puriﬁed by a factor of 2.4, and sample C showed two orders of magnitudelarger than the required concentration. At present, all companies make purer Gd2 (SO4 )3 ·8H2 O. We
expect to obtain ultra-pure Gd2 (SO4 )3 ·8H2 O soon and load it into the SK in the near future.

5.

Conclusions

A new SK-Gd project that involves Gd2 (SO4 )3 ·8H2 O into the SK water tank is in preparation. In
order to measure radioactive impurities in Gd2 (SO4 )3 ·8H2 O, we developed a solid-phase extraction
method using chromatographic extraction resin. This method has a high recovery rate for U and Th ofabout 90% or more with a high efﬁciency of separation of U and Th from Gd of a factor of about 104 .
The matrix effect and spectral interference in the ICP-MS measurements were effectively reduced
and we could measure U and Th in three pure Gd2 (SO4 )3 ·8H2 O samples. The DLs of the developed
method for U and Th were estimated to be 0.04 mBq (U)/kg and 0.01 mBq (Th)/kg. These values
are sufﬁciently precision for our requirements, which are 5 mBq (U)/kg and 0.05 mBq (Th)/kg. The
concentrations of U and Th in the three puriﬁed samples were estimated to be < 0.04 mBq (U)/kg
and 0.06 ± 0.01 mBq (Th)/kg for sample A, 0.05 ± 0.01 mBq (U)/kg and 0.12 ± 0.02 mBq (Th)/kg
for sample B, and 0.64 ± 0.10 mBq (U)/kg and 1.77 ± 0.27 mBq (Th)/kg for sample C.
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Table 1. Results for samples A, B, C, and the values for the procedure blank solution, a typical commerciallyavailable product, and our requirements. The values for U and Th in the procedure blank are already subtracted
from the values for U and Th for all the samples. The value of U for sample A was estimated using 3σ witha 15% inﬂation of the ICP-MS measurements.

Sample A
Sample B
Sample C
Procedure blankCommerciala
Requirement

U (pg mL−1 )ICP-MS
measurement

U (mBq/kg)
Converted topowderTh (pg mL−1 )ICP-MS
measurement

Th (mBq/kg)
Converted topowder

0.02 ± 0.01
0.04 ± 0.01
0.52 ± 0.01
0.01
0.23
—

< 0.040.05 ± 0.01
0.64 ± 0.10
—
28±2
5

0.15 ± 0.01
0.30 ± 0.01
4.35 ± 0.18
0.01
0.47
—

0.06 ± 0.01
0.12 ± 0.02
1.77 ± 0.27—
19 ± 10.05

a

The concentrations of U and Th are sufﬁcient to measure without the solid-phase extraction method. Therefore,
100 μg mL−1 of Gd2 (SO4 )3 ·8H2 O solution was directly measured by the ICP-MS.

Acknowledgements
This work was supported by the JSPS KAKENHI Grant-in-Aid for Scientiﬁc Research on Innovative Areas
No. 26104007, Grant-in-Aid for Specially Promoted Research No. 26000003, and Grant-in-Aid for Young
Scientists No. 17K14290. We also thank the companies who provided the puriﬁed Gd2 (SO4 )3 ·8H2 O samples.

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