Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 59-62, 2007

Development of On-line Solvent Extraction System with Microchips for Heavy Element
ChemistryK. Ooe,a Y. Tashiro,a D. Saika,a Y. Kitamoto,a K. Matsuo,a T. Takabe,a T. Kuribayashi,aN. Takahashi,a T. Yoshimura,a W. Sato,a K. Takahisa,b and A. Shinohara*,aaGraduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japanb

Received: January 31, 2007; In Final Form: May 9, 2007
A technique of microchip chemistry was introduced into an on-line solvent extraction system for studies of chemical behaviors of heavy elements, and the performance of the system was investigated with some rare earth elements as their homologues. It was confirmed that the extraction equilibria of the elements between the aqueous(acetic acid-sodium acetate buffer solution) and organic (2-thenoyltrifluoroacetone-toluene solution) phases wereattained in the microchip within about one second. Using the system, on-line solvent extraction experiments were
performed for the short-lived 150,151Dy as model experiments of heavy actinides at the AVF cyclotron facility of the
Research Center for Nuclear Physics (RCNP), Osaka University. All the chemical stages from the RI production totheir detections were completed within about three minutes, signifying that this system is applicable to studies ofheavy actinides having half-lives of a few minutes.

1. Introduction

2. Experiments

Investigation of the chemical properties of the heavy elements with atomic numbers ≥ 101 is an attractive subject
because the chemical properties of the heavy elements may
deviate from the periodicity of their lighter homologues in the
Periodic Table as theoretically predicted as to the strong relativistic effects on their valence electrons.1,2 The heavy elements
can be produced only in heavy-ion induced nuclear reactions
using an accelerator. Because of their low production rates andshort half-lives, the chemistry of these elements has to be performed on a single atom basis (one-atom-at-a-time chemistry3,4), and on-line rapid chemical operations are required.1,2
For these purposes, much effort has been made in the researchand development, and some rapid chemical operation systems
have already been in practical use: a liquid chromatographic
separation system called ARCA5 and a solvent extraction system called SISAK,6 for example. As has been performed inexperiments with SISAK, the solvent extraction method is useful for investigation on the chemical properties of heavy elements because we can obtain the equilibrium constants ofcomplexation reactions, which provide information about thechemical bonding nature of the complexes.
In the present work, we incorporated microchips in a solvent
extraction system for their application to heavy element chemistry as a simple method. Microchips7–9 are glass plates utilized for chemical reactions, having microscale ditches oftypically 1–100 µm in width and depth. Because of a large
specific interfacial area and a short diffusion length of solutes,
chemical equilibria are rapidly achieved in the microspace,which is very much suited to heavy element chemistry. Using
the present system, we examined solvent extraction behaviorsof the rare earth elements: Y, Gd, Tm, and Dy; as test experiments for the homologues of heavy actinides. In this paper, theperformance and applicability of the present on-line solvent
extraction system are shown.

2.1. Experimental apparatus. The on-line solvent extraction system was installed at the AVF cyclotron facility of the
Resea rch Center for Nuclea r Physics (RCN P), Osa ka
University. As schematically depicted in Figure 1, the system
consists of two different parts: a production chamber for heavy
elements and a solvent extraction apparatus with microchips.
They are connected with a gas-jet transport system. The specifications of each part of the system are shown in the following
subsections.
2.1.1. Production chamber for heavy elements. We newly
installed a production chamber for heavy element chemistry at
the K-course of the AVF cyclotron of RCNP. The structure of
the production chamber is similar to that used in the Japan
Atomic Energy Agency (JAEA),10 having a size of 55 mm i.d. ×
49 mm. The size of reaction targets is 9 mm in diameter. The
heavy-ion beam passes through several different materials*Corresponding author. E-mail: shino@chem.sci.osaka-u.ac.jp.
Fax: +81-6-6850-5418.

K-course
RCNP
AVF cyclotron

Chemistry laboratory
70 m

Production chamber
coupled with a gas-jet
transport system

Capillary tube

He/KCl gas-jet

Syringe pump (aqueous phase)Collection plate

Elution position

Syringe pump (organic phase)
Aqueous phase
IN

OUT
Organic phase

Sample tubes
Microchip apparatus

Figure 1. Schematic view of the on-line solvent extraction systeminstalled at the AVF cyclotron facility of RCNP. The system consistsof a production chamber for heavy elements and a solvent extraction
apparatus with microchips. They are connected with a gas-jet transport system. The structure of the production chamber is similar tothat used in the Japan Atomic Energy Agency (JAEA).10© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007(a)
Aqueous phase

Organic phase
100 µm(b)

Aqueous phase

Organic phase

100 µmFigure 2. Photographs of the liquid-liquid interface and phase separation (a) at the inlet junction and (b) at the outlet junction of the
microchannel in a microchip.

before the target for nuclear reactions: a beam current monitor
consisting of three aluminum foils (4.8 mg cm–2 in total) coatedwith gold (0.81 mg cm –2 in total), a HAVAR vacuum window(4.4 mg cm–2), helium cooling gas (0.41 mg cm–2), two HAVAR
vacuum windows (8.8 mg cm –2 in total), helium cooling gas(0.06 mg cm–2), and a target backing. Reaction products recoiling out of the target are thermalized in a helium gas flow,
attached to KCl aerosols generated by sublimation of KCl powder, and then transported through a Teflon capillary (2 mm i.d.
× 70 m) to the on-line solvent extraction apparatus placed in achemistry laboratory. The transport efficiency of reaction
products is approximately 35%.
2.1.2. On-line solvent extraction apparatus. Nuclear reaction products transported by the gas-jet are kept deposited on acollection plate. The collected products are mechanically
moved to the elution position and then dissolved in an aqueous
solution pumped by a syringe pump (HARVARD, PHD2000).
The yield by the dissolution is approximately 60%. The dissolved products are fed into a microchip apparatus through apolyether ether ketone (PEEK) capillary (0.26 mm i.d., 0.5 mmo.d.). Simultaneously, the organic phase is fed from the otherinlet of the microchip (Institute of Microchemical Technology
Co. Ltd., ICC-DY15). The sectional view of the flow channel
in the microchip (called a microchannel hereafter) is a half circle of 100 µm in width and 40 µm in depth. The microchannel
is 12 cm long. The wall of the microchannel for the organic
phase is chemically modified so as to be hydrophobic.11 In the
microchannel of the microchip, a parallel two-phase flow witha liquid-liquid interface is formed. At the end-junction of the
microchannel, two phases are separated without mixing witheach other as shown in Figure 2. The aqueous and organic
effluents coming out of the microchip are collected separately
in sample tubes. Because the volume of the solution treated ina single microchip is minute, we employed a multi-track micro-

Ooechip system composed of three microchips arranged in parallelin order to increase the volume of the solution treated per unit
time in the microchip apparatus. In this apparatus, an aqueous
solution of about 50 µL is needed to dissolve all collectedradioactivities when the flow rate of the solution is 50 µL min–1.
The volume of PEEK capillaries connected to the multi-track
microchip is about 50 µL and the total volume of microchannels for either phase is about 1 µL.
2.2. Extraction equilibrium in a microchip. It is important to investigate the reaction time required to attain theextraction equilibrium in the microchip. The contact time of
the aqueous and organic phases in a microchip corresponds tothe shaking time for the ordinary batch method. Therefore, we
examined the contact time of the liquid-liquid interface
required to attain the equilibrium by varying the flow rates ofboth the solutions. In addition, batch experiments were performed to examine if the extraction percentage in the microchip experiments would agree with that in the ordinary solvent
extraction experiments.
The rare earth tracers as the homologues of heavy actinides
were prepared by different nuclear reactions. The 88Y tracer
was obtained from the EC decay of 88 Zr produced by the89
Y(p,2n) 88Zr reaction and the 153Gd tracer by the 153Eu(p,n)153Gd
reaction with the AVF cyclotron of RCNP. The 170 Tm tracer
was produced by neutron irradiation of Tm 2 O 3 at Kyoto
University Reactor. The aqueous phase used was 0.1 M acetic
acid-sodium acetate buffer (pH 4.5) containing the tracers, andfor the organic phase 0.04 M 2-thenoyltrif luoroacetone(HTTA)-toluene solution was employed.12 The aqueous andorganic phases were simultaneously fed into each of the inlets
of a microchip with syringe pumps. The flow rate was variedin the range from 3 µL min –1 to 120 µL min –1. The aqueousand organic effluents from the microchip were collected separately in sample tubes, and were subjected to γ-ray spectrometry with a Ge detector.
The batch experiments were performed by the following
procedure. In a polypropylene tube, 1 mL of 0.1 M acetic acidsodium acetate buffer (pH 4.5) containing the above tracers
was mixed with an equal volume of 0.04 M HTTA-toluene
solution. The mixture was shaken for 20 min. After centrifuging, the aliquots of the aqueous and organic phases were recovered separately in sample tubes, and subjected to γ -rayspectrometry.
2.3. On-line experiments using the solvent extraction system. The short-lived dysprosium isotopes, 150 Dy (T1/2 = 7.17min) and 151 Dy (T1 / 2 = 17.9 min) were produced by the140
Ce(16O,6n)150Dy and 140Ce(16O,5n)151Dy reactions13 with a 102
MeV 16O beam delivered from the K course of the AVF cyclotron of RCNP. The natCe target of a thickness of 211 µg cm –2
was in advance prepared by electrodeposition on a 2.7 mg cm–2
aluminium backing foil. The beam intensity was approximately 160 particle nA. Reaction products recoiling out of thetarget were thermalized in a helium gas flow (2.5 L min–1, 178
kPa in the production chamber), attached to KCl aerosols generated by sublimation of KCl powder at 620–640 °C, and thentransported to the chemistry laboratory. The transported products were kept deposited on the collection plate for five minutes, dissolved in dilute hydrochloric acids (pH 3.01, 3.14, 3.23,3.40) pumped by a syringe pump at a flow rate of 50 µL min–1,
and subsequently fed into the multi-track microchips. A toluene solution of 0.1 M HTTA was fed from the other inlets witha syringe pump at a flow rate of 90 µL min–1. The aqueous andorganic effluents from the microchip were collected separately
on Ta discs. After evaporation to dryness on ceramic heaters,
the samples were subjected to α spectrometry with Si detectors. The distribution ratio was obtained from the activities ofthese two phases.

Development of On-line Solvent Extraction System with Microchips

described by the following reaction:

where A is the radioactivity, V the volume, v the flow rate, and t
the collection time of the aqueous and organic effluents. The
subscripts a and o denote the aqueous and organic phases,
respectively. The contact time is calculated from the flow rateof a faster phase and the volume of the microchannel where
two phases contact. One can see in Figure 3 that the extraction
percentages of the rare earth elements increase with the contact
time, and reach the values obtained by the batch method after
one second. These results evidently show that the microchip isapplicable to rapid chemistry for the heavy elements having
half-lives of a few seconds.
3.2. On-line solvent extraction of dysprosium. The distribution ratio (D) is defined as
AoVa Aovat Aova==.
AaVo Aavot Aavo

(2)

90
80
70
60
50
40
30
20
10
0

40

(a)
151Dy

30

20

150Dy

10

0
3700 3800 3900 4000 4100 4200 4300
α-Energy / keV

70

(b)

60

151Dy

50
40

150Dy

30

20

10

0
3700 3800 3900 4000 4100 4200 4300
α-Energy / keV
Figure 4. Alpha-particle spectra, obtained by the on-line experimentof 150Dy and 151Dy, (a) for the aqueous phase (hydrochloric acid of pH
3.14) and (b) for the organic phase (0.1 M HTTA-toluene solution).

0.6
0.4
0.2
0
log D

Extraction percentage / %Here, Aa and Ao were obtained from the α counts of 150Dy (E α =4233 keV) and 151Dy (E α = 4067 keV). An example of α-particle spectra of 150Dy and 151Dy obtained from this experiment(pH 3.14) is shown in Figure 4. The α-particle energy resolution in the organic phase was about 40 keV in FWHM, whereas
for the aqueous phase, the resolution was poor because of the
KCl residues from the gas-jet. This was not a serious problem
with determining the quantities of Dy because no events otherthan Dy were observed in these experiments. The dependenceof the distribution ratio of Dy on pH is shown in Figure 5. The
extraction of trivalent lanthanides (Ln3+) by HTTA is usually(3)

The extraction equilibrium constant (K) is expressed for the

Conuts / 10 keV

100AoVa
100Aovat
100Aova==, (1)AoVa + AaVo
Aovat + Aavot
Aova + Aavo

→ Ln(TTA)3(o) + 3H+ .
Ln3+ + 3HTTA(o) ←Conuts / 10 keV

3.1. Extraction percentage. The dependences of the extraction percentages of the rare earth elements on the contact timeof the liquid-liquid interface in the microchip are shown in
Figure 3. The three different lines stand for the extraction percentages of each rare earth element obtained in the batch
experiments. The extraction percentage (P) is defined as

D=

61

14

3. Results and Discussion

P=

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

-0.2
-0.4

Tm
Y
Gd

-0.6

4

-0.8

Contact time of liquid-liquid interface / s

-1

0

1

2

3

Figure 3. Dependences of the extraction percentages of rare earth
elements on the contact time of the liquid-liquid interface in the
microchip. For the aqueous and organic phases, 0.1 M acetic acidsodium acetate buffer (pH 4.5) and 0.04 M HTTA-toluene solution
were used, respectively. The closed square, the open circle, and theopen triangle symbolize Tm, Y, and Gd, respectively. The lines show
the extraction percentages of each rare earth element obtained by thebatch experiments. The dashed line, the dotted line, and the dasheddotted line correspond to Tm, Y, and Gd, respectively.

2.9

3

3.1

3.3

3.2

3.4

3.5

3.6

pH
Figure 5. Distribution ratio of Dy as a function of pH taken with theon-line solvent extraction system. For the organic phase, 0.1 M
HTTA-toluene solution was used. The solid line of a slope of 2.78 ±0.20 was obtained from a weighted least-squares fit to the experimental values. The dashed line shows the result of another least-squares
fit with the slope fixed to 3. See the text for detailed explanations.

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Ooe

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

reaction as:
K=[Ln(TTA)3](o)[H+]3.
[Ln3+][HTTA](o)3(4)

half-lives having a few seconds. On-line solvent extraction
experiments of Dy using the system were successfully performed. The total time taken for the on-line experiments was
about three minutes. This system can thus be used for the solvent extraction experiments of the heavy actinides having halflives of a few minutes.

The distribution ratio is also defined as
D=[Ln(TTA)3](o). (5)[Ln3+] + [Ln(TTA)2+] + [Ln(TTA)2+] + [Ln(TTA)3]

Neglecting the metal complexes in the aqueous phase([Ln(TTA)2+], [Ln(TTA)2+], and [Ln(TTA)3]), we can simplify
eq 4 as
K=D

[H+]3
.
[HTTA](o)3

Acknowledgements. We are grateful to Profs. H. Toki and
K. Hatanaka for their interest and supports. We are indebted to
Dr. H. Hisamoto for his valuable advices to introduce a microchip technique. We also thank the operating staff of RCNP for
providing the heavy ion beams. This work was supported inpart by Research Grants in the Natural Sciences from the
Mitsubishi Foundation and the Grant-in-Aid from the Ministryof Education, Culture, Sports, Science, and Technology ofJapan.

(6)

Equation 6 means that log D vs. pH plots give a linear plot witha slope of 3, which is confirmed by solvent extraction experiments of rare earth elements with HTTA.14 As shown in Figure
5, the slope was estimated to be 2.78 ± 0.20 by a weighted
least-squares fit to the experimental values. Taking into
account the uncertainty, this result can be regarded as reproducing the above report.14 The reproducibility is also confirmed
from the fact that another least-squares fitting process with theslope fixed to 3 passes the chi square test with a confidencecoefficient of 95% (see the dashed line in Figure 5). We therefore concluded that this system functioned well in the extraction of Dy.
It took only about three minutes to complete the sample
preparations for α -particle measurements starting from the
beginning of dissolution: one minute for the dissolution, one
minute for the elution from the apparatus, and one minute forthe sample preparations for the α-particle measurement. It washence found that this system can be applied to the solvent
extraction experiments of the heavy actinides having half-livesof a few minutes.
To utilize the short reaction time of one second in the microchip, the following improvements of the apparatus are needed:(1) introduction of a PEEK capillary with a smaller inner diameter for the decrease of the dead volume in the apparatus and(2) additional parallelization of microchips for the increase of
the volume of the solution treated per unit time in the microchip apparatus. Furthermore, automation of the experimental
procedure from the solvent extraction to the radiation detection
is essential for repetitive experiments in the chemistry of transactinide elements.
4. Conclusion
An on-line solvent extraction system with microchips forheavy element chemistry was installed in the AVF cyclotron
facility of RCNP. The extraction equilibria of the rare earth
elements, Y, Gd, and Tm, between acetic acid-sodium acetate
buffer solution and HTTA-toluene solution were attained
within about one second in the microchip. The extraction percentages of the rare earth elements at the equilibria obtainedby the microchip method agreed with those obtained by thebatch method. These results evidently show that the microchip
is applicable to rapid chemistry for the short-lived elements ofReferences(1) The Chemistry of Superheavy Elements, Ed. M. Schädel,Kluwer Academic Publishers, Dordrecht (2003).
(2) M. Schädel, Angew. Chem. Int. Ed. 45, 368 (2006).
(3) R. Guillaumont, J. P. Adloff, and A. Peneloux, Radiochim.
Acta 46, 169 (1989).
(4) R. Guillaumont, J. P. Adloff, A. Peneloux, and P.
Delamoye, Radiochim. Acta 54, 1 (1991).
(5) M. Schädel, Radiochim. Acta 89, 721 (2001).
(6) J. P. Omtvedt, J. Alstad, H. Breivik, J. E. Dyve, K.
Eberhardt, C. M. Folden III, T. Ginter, K. E. Gregorich, E.
A. Hult, M. Johansson, U. W. Kirbach, D. M. Lee, M.
Mendel, A. Nähler, V. Ninov, L. A. Omtvedt, J. B. Patin, G.
Skarnemark, L. Stavsetra, R. Sudowe, N. Wiehl, B.
Wierczinski, P. A. Wilk, P. M. Zielinski, J. V. Kratz, N.
Trautmann, H. Nitsche, and D. C. Hoffman, J. Nucl.
Radiochem. Sci. 3, 121 (2002).
(7) K. Sato, A. Hibara, M. Tokeshi, H. Hisamoto, and T.
Kitamori, Anal. Sci. 19, 15 (2003).
(8) T. Maruyama, H. Matsushita, J. Uchida, F. Kubota, N.
Kamiya, and M. Goto, Anal. Chem. 76, 4495 (2004).
(9) H. Xiao, D. Liang, G. Liu, M. Guo, W. Xing, and J. Cheng,
Lab on a Chip 6, 1067 (2006).
(10) T. K. Sato, K. Tsukada, M. Asai, K. Akiyama, H. Haba, A.
Toyoshima, S. Ono, T. Hirai, S. Goto, S. Ichikawa, Y.
Nagame, and H. Kudo, J. Nucl. Radiochem. Sci. 6, N1(2005).
(11) A. Hibara, M. Nonaka, H. Hisamoto, K. Uchiyama, Y.
Kikutani, M. Tokeshi, and T. Kitamori, Anal. Chem. 74,
1724 (2002).
(12) 2-Thenoyltrifluoroacetone is an extractant belonging to βdiketones. We are planning to perform solvent extraction
experiments of heavy actinides with β-diketones to investigate the chemical-bonding nature of heavy actinides.
Because the electron-donating ability of the β-diketones
varies according to the substituent groups, it is expectedthat information about the coordination of β-diketones tometal ions is obtained by studying the dependence of theextraction percentage on the substituent groups of β-diketones.
(13) J. M. Alexander and G. N. Simonoff, Phy. Rev. 133(1B),
93 (1964).
(14) J. Alstad, J. H. Augustson, and L. Farbu, J. Inorg. Nucl.
Chem. 36, 899 (1974).