JAAS
PAPER

Cite this: J. Anal. At. Spectrom., 2013,
28, 883

Development of an automatic pH-adjustment system
for solid phase extraction prior to the determination of
REEs in seawater by ICP-MS
Yanbei Zhu,* Akiharu Hioki and Koichi Chiba
An automatic pH adjustment instrument was developed for solid phase extraction (SPE) of rare earth
elements (REEs) using chelating resins. The automation of pH adjustment was carried out by using aspectrometer, an input/output (I/O) board, an automatic-switching valve, and a personal computer withLabVIEW programming platform. Contactless monitoring of the pH was carried out by measuring theintensity of transmitted light at 550 nm that passed through a sample solution with an added pHindicator, i.e. methyl red. A reagent for adjusting the pH, i.e. an aqueous ammonia solution, was sprayed
into the sample solution using a nebuliser, which permitted the addition of a small amount of thereagent and the precise adjustment of the pH. The pH value of the sample was monitored by measuring
the intensity of the transmitted light using the spectrometer, while the open-time of the valve to add
the reagent was automatically controlled simultaneously with the measurement of the transmitted

Received 18th October 2012
Accepted 14th March 2013

light. The precision of the pH adjustment was tested and conﬁrmed by using a seawater sample. The
blank values and detection limits of REEs were given as the analytical ﬁgures of merits. The usefulnessof the present method was conﬁrmed by analysing seawater samples from the Nikkawa Beach, the

DOI: 10.1039/c3ja30312h

Kashima Bay, and the Ishigaki Island. The results of REEs in these samples were discussed with shale

www.rsc.org/jaasnormalised REE distribution pattern.

Introduction
As dened by the International Union of Pure and Applied
Chemistry (IUPAC), rare earth metals include the een lanthanoids and other two chemically similar elements Sc and Y.1
The synonym of rare earth metals, i.e. rare earth elements(REEs) are used in the present work. The present targeting REEs
are, however, the 14 lanthanoids elements with exception of Pm
all of whose isotopes are radioactive.
Due to their similar chemical and physical properties, thegeochemical activities of REEs are generally similar to one
another, which could be attributed to the fact that the electroniccongurations of the REEs result in a particular trivalent positive oxidation state. On the other hand, fractionation of REEs
occurred during the geochemical evolution, owing to the factthat the ionic radii of the REEs decrease slightly but steadily
with the increase in atomic number for a given co-ordination
number.2 As a result, the distribution pattern of REEs is a good
tracer for the source of a sample and a sensitive indicator of theenvironment. The REEs in seawater could serve as water-masstracers to evaluate the marine geochemical cycle,3 an objective
National Metrology Institute of Japan (NMIJ), National Institute of Advanced
Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba, Ibaraki
305-8563, Japan. E-mail: yb-zhu@aist.go.jp; Fax: +81-29-861-6889; Tel: +81-29-8616889

This journal is ª The Royal Society of Chemistry 2013of elemental budgets of the ocean,4 and a bio-record for depositional environment.5 The researches on REEs in seawater werealso attractive along with Nd isotopes, which are a set of key
trace elements and isotopes (TEIs) of the global GEOTRACES
programme.6,7 On the other hand, a Gd anomaly observed basedon the distribution pattern of REEs was investigated as anindicator of anthropogenic impact on seawater and riverwater.8–15
Nowadays, inductively coupled plasma mass spectrometry(ICP-MS) is one of the most popular methods for elemental
analysis.16–19 Despite the fact that some modication of theintroduction system could permit the direct introduction of aseawater sample into an ICP-MS instrument,20–22 direct determination of REEs in most seawater samples is diﬃcult becauseof their extremely low concentrations, which are generally at
lower pg mL1 to fg mL1 level.
Solid phase extraction (SPE) techniques using chelating
resins are oen applied as the pretreatment of seawater
samples to determine REEs,23–42 with both purposes to remove
the salt contents and to enrich the REEs in the samples. In SPE
using chelating resins, the recoveries of REEs depend on the pH
condition for adsorption. In most of the cases, the adsorption of
REEs to the chelating resins was carried out at a neutral
condition close to pH 6.23,26–29,31,34–36,39,41,42 This fact could be
explained by the adsorption of REEs to the chelating functional

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Paper

groups being stable only in a narrow pH range, with neither
signicant competition between H+ and REE ions nor signicant competition between OH and chelating functionalgroups. Therefore, pH adjustment is an important step in SPE
operations to ensure the recovery of REEs and the reproducibility of the results.
To the best of the present authors' knowledge, up to date,there is not an automatic pH adjustment instrument for preconcentration of REEs or other trace elements by SPE. In thepresent work, an automatic pH adjustment instrument wasdeveloped. Contactless monitoring of the sample pH wascarried out by measuring the light intensity at the wavelength ofthe characteristic absorption of the pH indicator. The adjustment of the sample pH was performed by adding an aqueous
ammonia solution into the sample using a nebuliser, which isusually used in ICP-MS. Controlling the quantity of the aqueous
ammonia solution added into the sample was carried out by anelectromagnetic valve. The usefulness of the present instrument
was conrmed by analysing REEs in seawater samples.

A tungsten halogen light source (LS-1-LL), a spectrometer(USB2000 + VIS-NIR), and a pair of optical bres were purchasedfrom Ocean Optics, Inc. (Dunedin, FL, USA), to construct thecontactless pH monitoring system. An I/O board (NI USB-6009,
National Instruments Japan Corp., Tokyo, Japan), an electromagnetic valve (LS067A040, ASCO Japan Co. Ltd, Tokyo, Japan),and a MicroMist nebuliser (Glass Expansion, West Melbourne
Vic, Australia) were utilised to construct the aqueous ammonia
solution injection system. A digital mass ow controller (SEC500, HORIBA Ltd, Kyoto, Japan) was applied to the control ofcarrier gas for the nebuliser used in the aqueous ammonia
solution injection system. A magnetic stirrer (lab disc white,
IKA Japan K.K., Osaka, Japan) and a Teon coated stirring bar
were used to homogenise the sample solution during pH
adjustment. A pH meter (SevenEasy, METTLER TOLEDO,Tokyo, Japan) with an Inlab 410 pH electrode (glass) was used
to monitor the pH of the sample. An all-plastic pH electrode(AMANI-650, Warner Instruments, Hamden, CT, USA) was used,in comparison to the glass electrode, to check the blank valuesof REEs when an electrode was used.

Experimental
Instrumentation

Chemicals and materials

An Agilent 7700x (Agilent Technologies) ICP-MS instrument wasapplied to the measurements of the REEs. The typical operating
conditions for the ICP-MS are summarised in Table 1. Syringe
driven chelating columns (SDCCs, NOBIAS CHELATE-PB1M)
were purchased from Hitachi High-Technologies Corp. (Tokyo,
Japan) and applied to the solid phase extraction of REEs fromseawater samples. Measurements of the REEs were carried out
using an automatic column changing system29 aer adsorptionof REEs to the SDCCs, where an inert type liquid chromatography pump (LC-10Ai, Shimadzu Corp., Tokyo, Japan) wasapplied to passing 2 mol L1 HNO3 through the SDCCs to elute
REEs.

Ultrapure grade nitric acid, acetic acid and an aqueous
ammonia solution were purchased from Kanto Chemicals(Tokyo, Japan). Single REE standard solutions (1000 mg L1each, for atomic absorbance spectrochemical analysis) formaking the working calibration curves were purchased from
Wako Pure Chemicals (Osaka, Japan). A standard solution of Ba
was also purchased from Wako Pure Chemicals and was properly diluted followed by the analysis to check the polyatomic
interference with Eu. Methyl red and ethanol were purchasedfrom Wako Pure Chemicals to prepare a 1 mg mL1 methyl red
solution in ethanol, which was used as the pH indicator.
A coastal seawater sample collected near the shore of the
Nikkawa Beach (Kamisu, Ibaraki, Japan) was used for optimising the experimental conditions and to check the usefulness ofthe present instrument. Coastal seawater samples collected
respectively from the Kashima Bay (Kashima, Ibaraki, Japan)
and the Ishigaki Island (Ishigaki, Okinawa, Japan) were also
analysed as the application of the present method. The seawater
samples were ltered with a membrane lter (pore size 0.45 mm,
Nihon Millipore Kogyo, Tokyo, Japan) immediately aersampling and acidied to approximately pH 1.0 with nitric acid.
All bottles, test tubes and pipette tips used in the present
experiment were soaked in 3 mol L1 nitric acid for a week andthen rinsed three times with pure water, except for the transparent polyethylene terephthalate (PET) bottles, which wereconrmed free of REE contaminations in advance. Pure waterused throughout the present experiment was prepared by aMilli-Q purication system of model Element A-10 (Nihon
Millipore Kogyo).

Table 1

Typical operating conditions of ICP-MS instrument

Plasma conditions
Incident power
Coolant gas ow rate
Auxiliary gas ow rate
Sample gas ow rate
Make-up gas ow rate
Dilution gas mode
Collision gas ow rate

1.55 kWAr 15.0 L min1
Ar 0.90 L min1
Ar 0.80 L min1
Ar 0.40 L min1
Oﬀ
He 3.00 mL min1

Sampling conditions
Sampling depth

8 mm from load coil

Nebuliser: MicroMist
Sample uptake rate

1.0 mL min1(plunger pump)Spray chamber: Scott double path
Wall temperature

2 C

Data acquisition
Peak pattern
Data points
Integration time

Time resolved analysis
1 point per peak
20 ms

884 | J. Anal. At. Spectrom., 2013, 28, 883–889

Construction of the automatic pH adjustment system and theow chart of the controlling programme
The construction of the pH adjustment instrument is illustrated
in Fig. 1. As can be seen in Fig. 1, the sample (2) was stirred by aThis journal is ª The Royal Society of Chemistry 2013

PaperJAAS
and the valve position were automatically controlled. When thevalue of Ii was larger than Io, the counting number (n) increased
by one until the value of n reached ve and the adjustment
procedure ended. A typical adjustment time for 100 mL ofseawater in 0.1 mol L1 HNO3 was approximately ve minutes,
where 0.5 mL of acetic acid and 0.1 mL of the methyl red
solution were added into the sample prior to the adjustment.
The adjustment could be carried out in less than one minute byadding a proper amount of a 28% aqueous ammonia solution
prior to the adjustment. The present system could be applied tothe pretreatment of a sample volume from 20 mL to 100 mL.

Fig. 1 Construction of the pH adjustment instrument. (1) Light source; (2)sample; (3) stirring bar; (4) magnetic stirrer; (5) polypropylene sample bottle(transparent); (6) spectrometer; (7) transmitted light signal; (8) controllingcomputer; (9) I/O board; (10) open/close signal to the valve; (11) electromagnetic
valve; (12) a 28% aqueous ammonia solution; (13) N2 gas; 14, nebuliser; (15)nebuliser tube.

magnetic stirrer (4) with a stirring bar (3). The light passed
through the sample was monitored by the spectrometer (6).
Based on the signal intensity of the transmitted light (7), open/
close signals (10) were sent to the electromagnetic valve (11). An
aqueous ammonia solution (12) was added into the sample
through the nebuliser (14) with the assistance of N2 gas and thecontrol of the electromagnetic valve (11). Measurement of
the transmitted light and generation of open/close signals forthe electromagnetic valve were carried out with a laboratory
made programme based on LabVIEW system design soware.
The ow chart of the programme for the present pH adjustment
system is shown in Fig. 2. As seen from Fig. 2, the signal
intensity of the transmitted light (Ii) was always monitored
during the adjustment procedure. Based on the comparison of
the Ii with the objective signal intensity (Io), the waiting time (t)Fig. 2 Flow chart of the controlling programme for the pH adjustment system. Ii
and Io, the monitored and the objective signal intensities of the transmitted light.

This journal is ª The Royal Society of Chemistry 2013

Procedures of SPE for measuring REEs in seawater
The SDCCs used for SPE were preserved in pure water aer
passing through 5 mL of 1 mol L1 ammonium acetate. Prior toloading the samples, 5 mL each of 3 mol L1 HNO3 and purewater were passed through the SDCCs in succession for washing.
Aer being adjusted to pH 6.0 using the pH adjustment system,
each sample (50 mL) was loaded into one of the SDCCs using an
all-plastic syringe at the ow rate of 5 mL min1. Loading of thesample to the SDCC was carried out with the assistance of alaboratory made multi-syringe pump. Aer loading the sample,
5 mL of pure water was passed through the SDCC to remove theresidual seawater. Finally, the REEs adsorbed in the SDCC were
eluted with approximately 0.4 mL of 2 mol L1 HNO3 at the ow
rate of 1 mL min1 and measured on-line with the ICP-MS, where
the on-line measurements were carried out with the assist of an
automatic column changing system.29

Results and discussion
Absorbance characteristics of the methyl red reagent
Taking into consideration the fact that the maximum recoveries
for REEs by SPE using chelating resins were obtained aroundpH 6, the objective pH condition in the present work was set as
pH 6.0. In order to avoid the contamination by inserting the pH
electrode into the sample, monitoring of the pH was carried out
by measuring the signal intensities of the transmitted lightthrough the sample. Methyl red is a well known pH indicator,
which changes the colour from red to yellow along with the pHchange from 4 to 6. The usefulness of methyl red as the pH
indicator for the present purpose was conrmed by measuring
the signal intensity at the wavelength from 350 nm to 1000 nm
to investigate the characteristic absorbance. The tests werecarried out using 100 mL of seawater sample in 0.1 mol L1
HNO3, into which 0.5 mL of acetic acid was added.
The spectrograms of the transmitted light passed through thesamples are plotted in Fig. 3. As can be seen from Fig. 3, thespectrogram obtained with methyl red (dotted line) at alkalinecondition was close to that obtained without methyl red (solidline) with slight absorbance around 450 nm. By contrast, thespectrogram obtained with methyl red at acidic condition
showed signicant absorbance in the range of 450 nm to 600 nm
with the maximum absorbance at ca. 550 nm. These results
indicate that the variation of the pH of the sample could besensitively monitored by measuring the transmitted light at ca.

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Paper
ensured that the signal intensity of the transmitted light at
622 nm was 12 600 counts.
Operating conditions of the nebuliser and reproducibility ofthe pH obtained with the present instrument

Fig. 3 Spectrograms of the transmitted light through the sample. Solid line,
without addition of methyl red; dashed line, with methyl red, pH 1.0; dotted line,with methyl red, pH 8.0.

550 nm. Therefore, the signal intensity of the transmitted light at
550 nm was monitored for the purpose of pH adjustment.
Correlation between the signal intensity of the transmitted
light at 550 nm and the pH of the sample
The correlation between the signal intensity of the transmitted
light and the pH of the sample was investigated using 100 mL ofa seawater sample in 0.1 mol L1 HNO3, into which 0.1 mL of
1 mg mL1 methyl red and 0.5 mL of acetic acid were added.
The pH of the sample was measured by the pH meter and waselevated by gradual addition of an aqueous ammonia solution;
the signal intensity of the transmitted light at 550 nm wasrecorded for 17 points from approximately pH 0.7 to pH 8.2. The
results are plotted in Fig. 4. These results indicate that there is adenite correlation between the signal intensity of the transmitted light and the pH of sample in the range between 4.7 and6.4. Therefore, in the following experiments the objective signal
intensity of the transmitted light at 550 nm was set as 5700
counts, corresponding to pH 6.0. Furthermore, the maximum
signal intensity of the present light source was observed at
622 nm. In order to achieve a stable pH, the sample bottles were
set to the automatic pH adjustment instrument and it was

The nebuliser for the addition of the aqueous ammonia solution was working at a ow rate of approximately 0.4 mL min1
using 0.80 L min1 N2 as the carrier gas, which was optimized to
get the highest ow rate of the aqueous ammonia solution. The
time used for adjustment of the pH of a sample was recorded by
the controlling programme, so that the consumed volume of theaqueous ammonia solution could be calculated.
In order to check the reproducibility of the pH obtained by
the present instrument, 10 seawater samples were subjected tothe pH adjustment. The pH values of the samples aer
pH adjustment were 5.9 (2 samples), 6.0 (4 samples), 6.1(4 samples), respectively. These results indicate that pH
adjustment using the present instrument was reproducible,
which permitted a reproducible SPE operation for REEs'
measurements in seawater to be obtained.
Blank values in comparison with those obtained with theassist of pH electrodes
In the present work, contactless pH monitoring was carried out
by measuring the signal intensity of the transmitted light at 550
nm with methyl red as the pH indicator. Contamination frompH measurement was expected to be much lower than thatobtained with the assist of a pH electrode.
Blank tests were carried out using 0.1 mol L1 of HNO3
solutions as the test samples. The pH adjustments were carriedout by the present pH adjustment instrument, with the assist ofa glass electrode, and with the assist of a plastic electrode. The
SPE operations and the measurements were carried out in asimilar way to the seawater samples. The results of the blanktests are summarised in Table 2.
It can be seen from Table 2 that the blank values obtained by
the present method were in the range from 0.13 fg mL1 of Tm
to 34.1 fg mL1 of Ce, which are adequately low for themeasurement of REEs in natural seawater samples. By contrast,
the blank values obtained with the assist of the glass electrodeand the plastic one were in the ranges from 4.0 fg mL1 of Tm to5750 fg mL1 of Ce and 1.9 fg mL1 of Tm to 422 000 fg mL1 of
La, respectively. The blank value for each REE obtained with thepresent method was signicantly lower than those obtainedwith the assist of the pH electrodes. These results conrmedthat the contaminations of REEs were eﬀectively controlled in
the present method, which could be attributed to the fact thatthe methyl red reagent was almost free of REEs. Higher blankvalues of REEs on glass and plastic electrodes might be attributed to the electrolytes used in the electrodes.
Analytical results of REEs in a seawater samples

Fig. 4 Correlation between the signal intensity of the transmitted light at 550
nm and the pH of the sample. Sample, 100 mL seawater; methyl red, 0.1 mL; bar,standard deviation of the average value of each point, n ¼ 3.

886 | J. Anal. At. Spectrom., 2013, 28, 883–889

The present method was applied to the analysis of REEs incoastal seawater samples collected from the Nikkawa Beach, theKashima Bay, and the Ishigaki Island. The recovery values of
REEs by the present method were tested by adding a mixed

This journal is ª The Royal Society of Chemistry 2013

PaperJAAS

Table 2 Blank values obtained with the present method in comparison withthose obtained with the assist of pH electrodes

Table 3 Analytical results of REEs in seawater sample from the Nikkawa Beach,recoveries, and detection limits

Blank valuea/fg mL1b

Element

m/z

Present method

Glass electrode

Plastic electrode

La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

139
140
141
146
147
153
157
159
163
165
166
169
172
175

25.2  9.8
34.1  13.4
3.01  1.38
10.1  4.5
2.56  1.56
0.44  0.28
2.20  1.14
0.29  0.24
1.78  0.73
0.29  0.03
1.03  0.38
0.13  0.03
1.26  0.59
0.32  0.07

4220  110
5750  120
431  131180  30
53.2  4.5
12.7  1.2
143  10
11.5  0.9
70.6  2.7
17.1  0.6
57.4  4.2
4.0  0.4
56.2  0.3
9.2  0.8

422 000  2400
1020  20
28.9  1.5
94.1  0.8
21.9  3.9
9.6  1.2
82.1  3.9
4.6  0.5
23.7  0.9
5.6  0.4
17.3  0.8
1.9  0.3
15.2  1.9
3.0  0.3aMean  standard deviation, n ¼ 3. b The values given in italic fontwere lower than the respective detection limits in Table 3.

Concentration in aseawater sample/
pg mL1
Element

Observeda

Reporteda,bRecoverya/%

Detection limit/
pg mL1

La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

4.34  0.16
3.86  0.23
0.79  0.02
3.72  0.15
0.91  0.05
0.252  0.014
1.50  0.08
0.246  0.012
2.02  0.11
0.52  0.02
1.74  0.10
0.26  0.02
1.75  0.13
0.30  0.03

4.7  0.3
3.8  0.2
0.83  0.04
4.2  0.2
0.90  0.07
0.25  0.02
1.46  0.10
0.25  0.02
1.86  0.13
0.49  0.03
1.60  0.06
0.25  0.02
1.59  0.11
0.28  0.02

97.9  1.8
97.5  2.6
97.0  1.1
100.2  1.6
99.0  0.8
97.8  2.2
100.7  1.8
97.1  1.8
100.4  2.2
99.5  1.9
97.8  2.6
96.7  2.8
97.5  3.2
97.5  3.3

0.029
0.040
0.004
0.014
0.005
0.00080.00340.00070.00220.00009
0.00120.000080.00180.00022asolution of REEs to obtain a spiked sample of the Nikkawa
Beach seawater, in which the concentration of each REE wasapproximately 2-fold of that in the non-spiked sample.
The concentrations of REEs were calibrated using a 3-pointcalibration curve for each REE, which was made with thereagent blank and two standard solutions containing diﬀerent
concentrations of the REE. The correlation factor (R2) of thecalibration curve for each REE was better than 0.9999.
The standard solutions were made in 3 groups in the order of
atomic numbers: group I, from La to Sm; group II, from Eu to
Ho; group III, from Er to Lu. The polyatomic spectral interferences to each REE were investigated with the above standard
solutions along with a solution of Ba (5 ng mL1 in 2 mol L1
HNO3). The interference ratios of MO+/M+ for Ba, Pr, Nd, Sm,
Nd, Eu, Gd, and Tb were in the range from 1.2% of EuO+/Eu+ to
2.9% of PrO+/Pr+, while the ratios for the other REEs were lessthan 0.1%. Therefore, corrections of the interferences of137
Ba16O, 141Pr16O, 143Nd16O, 147Sm16O, 149Sm16O, 150Nd16O,
153
Eu16O, 156Gd16O, and 159Tb16O were carried out to the resultsof 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, and175
Lu, respectively.
The results of the REEs in the Nikkawa Beach seawater sample
are summarised in Table 3, along with the reported value,43 therecovery values, and the detection limits. The detection limits
were calculated as 3-fold of standard deviation of the blankvalues obtained by the present method, as is summarised inTable 2. It can be seen from Table 3 that the recovery of all REEs
were close to 100% with the standard deviations less than 3.3%.
All of the observed values were obtained aer blank value
corrections and recovery value corrections.
It can be seen from Table 3 that the detection limit for each
REE was low enough for the analysis of the present seawater
sample. The present seawater sample was taken from the bottle
that was previously used to determine REEs and to report theresults in ref. 43. The concentrations of REEs in the presentThis journal is ª The Royal Society of Chemistry 2013

Mean  standard deviation, n ¼ 4. b Cited from ref. 43.

sample covered a range from 0.246 pg mL1 of Tb to 4.34 pg mL1
of La, and the observed value for each REE was coincident withthe reported value taking into consideration the standard deviations of both values, which were the major sources of theuncertainties of the values. These results indicate that thepresent method is useful for the determination of REEs in anatural seawater sample.
The results of REEs in seawater samples from the Ishigaki
Island and the Kashima Bay are summarised in Table 4, alongwith the spiked seawater samples with two levels of spikes. The
results in Table 4 cover a concentration range from 0.074 pg mL1
of Eu in seawater from the Ishigaki Island to 25.5 pg mL1 of La inseawater from the Kashima Bay with Spike II. It can be seen that
all of these results were obtained with good reproducibility.
Furthermore, the result for each element in both spiked samples
was in good agreement with the sum of the concentration in the
spike and that in the non-spiked sample. These results further
conrmed that the present method is eﬀective for the determination of REEs in seawater samples.
Shale normalised REE distribution pattern in seawater
samples
Shale normalised REE distribution pattern is an eﬀective
approach to assess the features of REEs in water samples.8–15
The results of REEs in the present samples were normalised tothose in post-Archean Average Australian Shale44 ([REE]PAAS)
and are plotted in Fig. 5 against the REEs based on atomic
number.
It can be seen from Fig. 5 that all the three seawater samples
showed generally smooth REE distribution patterns with relativeenrichment of heavy REEs. The enrichment of heavy REEs inseawater samples could be attributed to the fact that the ion
species of heavy REEs are more soluble in the solution than thoseof middle and light REEs.44 Furthermore, signicant Ce

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Table 4

Paper
Analytical results of REEs in seawater samples from the Ishigaki Island and the Kashima Bay

Concentration/pg mL1
Element

Ishigaki Islanda

Kashima Baya

Spike I

Kashima Bay with Spike Ia

Spike II

Kashima Bay with Spike IIa

La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

1.51  0.03
2.84  0.05
0.349  0.008
1.53  0.05
0.370  0.008
0.074  0.004
0.500  0.023
0.081  0.003
0.562  0.023
0.141  0.008
0.488  0.018
0.082  0.004
0.532  0.024
0.085  0.005

4.20  0.20
2.73  0.14
0.70  0.03
3.33  0.12
0.78  0.04
0.195  0.018
1.99  0.10
0.207  0.010
1.55  0.06
0.413  0.012
1.35  0.05
0.185  0.007
1.13  0.05
0.202  0.008

10.7
8.44
2.10
8.43
2.09
0.51
3.12
0.51
4.18
1.04
3.17
1.05
3.17
0.52

15.1  0.4
11.4  0.3
2.84  0.07
11.8  0.2
2.89  0.09
0.73  0.04
5.13  0.16
0.714  0.016
5.65  0.12
1.43  0.05
4.47  0.09
1.22  0.03
4.30  0.10
0.741  0.015

21.0
16.5
4.10
16.5
4.09
1.00
6.09
1.00
8.17
2.03
6.19
2.05
6.20
1.02

25.5  0.6
19.6  0.5
4.87  0.10
19.8  0.3
4.90  0.15
1.24  0.03
8.13  0.20
1.20  0.04
9.57  0.14
2.40  0.04
7.45  0.09
2.21  0.03
7.32  0.12
1.25  0.02

aMean  standard deviation, n ¼ 4.

Besides, the concentrations of Ce in the seawater samples fromthe Nikkawa Beach and the Kashima Bay were relatively close tothat in the seawater sample from the Ishigaki Island, which wassignicantly diﬀerent from other REEs. This fact also supports
the above speculation about the lack of the Ce depletion in theseawater from the Ishigaki Island. The signicant Gd enrichment in the seawater sample from the Kashima Bay could be
attributed to the discharge of Gd compounds due to their
medical use as MRI contrast reagents, which had been observed
for seawaters and river waters from various countries.8–15

Conclusions

Fig. 5 Shale normalised REE distribution pattern of seawater samples from the
Nikkawa Beach, the Kashima Bay, and the Ishigaki Island. O, the Nikkawa Beach;B, the Kashima Bay; >, the Ishigaki Island; bar, standard deviation, n ¼ 4.

depletions could be observed for seawater samples from the
Nikkawa Beach and the Kashima Bay. This phenomena could be
attributed to the fact that Ce(IV) is much more stable but lesssoluble than Ce(III),44 which resulted in the sedimentation of Ce
as CeO2 and resulted in the relative depletion of Ce in seawater
samples. It is noted that the concentrations of REEs in theseawater sample from the Ishigaki Island were signicantly
lower than the other two samples, which could be attributed to
the fact that the Ishigaki Island is located in the open ocean andis far from the continents, which is one of the major sources for
REEs in seawater samples. The Ce depletion in the seawater
sample from the Ishigaki Island was not signicant, which might
indicate that the concentration of Ce in this sample wasextremely low and could not produce any more CeO2 precipitate.

888 | J. Anal. At. Spectrom., 2013, 28, 883–889

An automatic pH adjustment instrument was developed for SPE
of REEs using chelating resins. Contactless monitoring of the pHof a sample was carried out by monitoring the signal intensity oftransmitted light at 550 nm with methyl red as the pH indicator.
The adjustment of the pH of a sample was carried out by adding
an aqueous ammonia solution into the sample to elevate the pH,
where the amount of the aqueous ammonia solution was sensitively controlled by a nebuliser. The pH of the sample was precise
and reproducible aer pH adjustment using the present instrument. The blank values of REEs obtained using the present
method were signicantly lower than those obtained with theassist of pH electrodes and permitted the determination of REEs
in natural seawater. The recoveries of the REEs were close to100% with standard deviations less than 3.3%. The analyticalresults of a seawater sample conrmed that the present instrument and method are useful for the determination of REEs innatural seawater. The present method was applied to the determination of REEs in three seawater samples, which results werediscussed using shale normalised REE distribution pattern.

Acknowledgements
The present research has been partly supported by Research &Development Project for Assessing Products of Small andThis journal is ª The Royal Society of Chemistry 2013

PaperMiddle Enterprises (2009FY), Ministry of Economy, Trade andIndustry, Japan. The present authors express sincere gratitude
to Prof. Akihide Itoh, University of the Ryukyus, for providing
seawater sample from the Ishigaki Island. Application for thepatent on the present pH adjustment system was submitted to
the World Intellectual Property Organization (WPO, PCT/
JP2012/057276).

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