Solid-phase Spectrophotometry for On-site Analysis of Trace Elements in Natural Water.

ANALYTICAL SCIENCES JUNE 2013, VOL. 29

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2013 © The Japan Society for Analytical Chemistry

Notes

Solid-phase Spectrophotometry for On-site Analysis of Trace Elements in
Natural Water
Sarenqiqige,*1† Sulistyo SAPUTRO,*2 Satsuki KAI,*1 Makoto SATODA,*3 Shiro MATSUOKA,*4 and
Kazuhisa YOSHIMURA*1
*1 Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashi, Fukuoka 812–8581,
Japan
*2 Department of Mathematics and Natural Science Education, Faculty of Teacher Training and Education
Science, Sebelas Maret University, Surakara-57126, Indonesia
*3 Satoda Science, Hitonose, Ogaki, Etajima, Hiroshima 737–2101, Japan
*4 Department of Environmental Science, Faculty of Science, Niigata University, Ikarashi, Niigata 950–2181,
Japan

A new, simple and sensitive solid-phase spectrometry (SPS) that is easily applicable to the on-site analysis of targeted

chemical components in water at µg dm–3 or sub-µg dm–3 levels is proposed in this study. The main features of the SPS
are the simplicity of operation, high sensitivity and applicability to real samples without the need for any pretreatment
procedures. A portable spectrophotometer, consisting of an LED light source, a grating and a CCD, was used for the
solid-phase light measurements. The universal applicability of the proposed system to on-site analysis was evaluated by
determining phosphate, chromium(VI) and iron(II) in natural water.
(Received February 15, 2013; Accepted April 23, 2013; Published June 10, 2013)

Introduction
Some unstable chemical elements and/or species are very
susceptible to environmental conditions, and a change in the
valence state or chemical species may occur in a short time after
sample collection. A low-cost method for real-time monitoring
of some trace elements is needed in many industries. Therefore,
the development of an on-site analysis method is very important.
In solid-phase spectrometry (SPS),1,2 because the analyte in
the solid phase is spectrometrically determined without
desorption, dilution of the analyte does not occur and thus
higher sensitivity can be achieved. SPS is very useful for the
determination of trace chemical components present at µg dm–3
levels in water, however, SPS has not been widely accepted.

One of the reasons may be the need to improve the
spectrophotometers. Because the use of a smaller volume of the
solid phase effectively enhances the sensitivity of the SPS,3
black microcells having a smaller inside volume and 1-cm
light-path are suitable for our purpose.3,4 On the other hand, the
use of this type of black microcell causes a large background
light attenuance by the cell itself as well as by the solid phase,
because the area of the light-path portion of this type of black
cell is usually narrower than that of the incident light beam.3
In order to overcome these problems, a new SPS method that
is easy and simple enough in operation to be applicable to
on-site analyses was proposed in this study. A commercially
available portable spectrophotometer was optimized for the
solid-phase light measurements. LEDs were used as the light
†

To whom correspondence should be addressed.
E-mail: sarna2010@yahoo.co.jp

source and powered by rechargeable battery to use as portable

equipment on-site. The main characteristics of this improved
SPS method are reported as well as the analytical application for
the determination of phosphate, chromium(VI) and iron(II)
concentrations at µg dm–3 levels in natural water samples.

Experimental
Reagents and chemicals
All reagents were of analytical grade. Highly purified water
was prepared by a Milli-Q Advantage A10 system (Millipore,
Molsheim, France).
The standard phosphate solution (1000 mg dm–3) was obtained
from Wako, Osaka, Japan. A combined reagent solution to form
molybdenum blue species was prepared as described in a
previous paper.4 The Sephadex G-25 gel (Medium) was
purchased from GE Healthcare Bio-Sciences, Uppsala, Sweden.
The standard chromium(VI) solution (1000 mg dm–3) for the
AAS was obtained from Kishida, Osaka, Japan. All reagent
solutions for the color development of chromium(VI) were
prepared as described in a previous paper.3 The Muromac 50WX2 cation exchanger (100 – 200 mesh) was purchased from
Muromachi, Tokyo, Japan.

The standard iron(II) stock solution (1000 mg dm–3) was
prepared by dissolving 0.498 g FeSO4·7H2O in 11.0 g of a 36%
HCl solution and diluted to 100 cm3 with water. The combined
solution of chromogenic reagent and buffer solution for iron(II)
was prepared by dissolving 0.2 g of 1,10-phenanthroline and
7.7 g of ammonium acetate in 5.74 cm3 of 99.7% acetic acid
and diluted to 100 cm3. A 1 mol dm–3 hydrochloric acid was
prepared by diluting 8.6 cm3 of 36% HCl to 100 cm3 with water.

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Cell holder
Ball lens
To the
PC

Detector

Battery
Light
source

Fig. 2 Simple spectrophotometer for SPS. Light source, LED.
Fig. 1 Black microcell. A, Polypropylene tip; B, black microcell
(light-path length, 10 mm; width, 2 mm); C, acrylic resin plate painted
black to adjust the position. Parts A, B and C were fixed by an epoxy
resin adhesive. For practical use, the cell was masked by black paint
to make a window of 1-mm height at the bottom of the cell.

complex between phosphate and molybdate, and this equation
was used for the determination of phosphate.
∆A = log(IR(λ1)/I(λ1)) – log(IR(λ2)/I(λ2))

A 1 mol dm–3 ammonia solution was prepared by diluting
6.8 cm3 of 28% ammonia to 100 cm3 with water.
Devices and equipment
Commercial models of a daylight color LED (LPH508H238WC, 24-31 cd) and pink color LED (LP-5034FWCWR, 5-8 cd) were obtained from the Peace Corp., Koshigaya,
Japan. A black cell (Fig. 1) having a light-path length of 10 mm

and a 2-mm width (M20-B-2, GL Sciences, Tokyo, Japan) was
used. Battery-driven magnetic stirrers were used for the on-site
analysis. A constant volume of the resin beads was taken using
an aliquotting device: A PTFE tube (1.0-mm i.d. and 7-cm long)
was fitted on the side with a PP resin filter tip and connected to
a 10-cm3 disposable syringe.3,4
A portable single-beam type spectrophotometer for the SPS,
Model SP-101 (W250 × D100 × H50 mm), was purchased from
Satoda Science, Hiroshima, Japan. It contained a halogen
tungsten lamp or LED as the light source, a grating (600 lines/
mm) as the monochromator, a CCD array (2048 bit) as the light
detector (measurable wavelength: 400 – 850 nm, resolution:
2.5 nm), and a laptop PC as the data processor. In this study,
the LEDs were used as the light source. A 1.2-mm thick PTFE
film, which showed a constant light scattering behavior similar
to that of solid beads packed in the cell, was used as a reference.
Procedure for the determination of phosphate, chromium(VI) and
iron(II)
All of the water samples were filtered through a 0.20-µm
PTFE membrane filter (Advantec, Tokyo) at the site where the

samples were collected.
Phosphate. First, 3.2 cm3 of the combined reagent and 0.06 cm3
of the gel beads were added to a 20-cm3 water sample containing
1 – 10 µg dm–3 of phosphate-P in a polyethylene bottle. The
contents were mixed for 30 min to concentrate the target analyte
into the gel phase as the colored species. After a short time to
allow the resin beads to settle to the bottom of the reaction
container, the supernatant solution was removed, and an
approximate 1-cm3 volume of the resin beads and reaction
solution mixture was transferred into the cell (Fig. 1) using a
pipette. The light intensity at 640 nm (strong-absorption
wavelength, λ1), IR(λ1), and that at 450 nm (weak-absorption
wavelength, λ2), IR(λ2), of the PTFE film were first measured as
the reference, and the sample cell was then inserted in the cell
holder to record the light intensities at the two wavelengths, I(λ1)
and I(λ2). In Eq. (1), ∆A corresponds to the amounts of the blue

(1)

After the measurement, the solid beads were removed from the

cell for the next measurement.
Chromium(VI). To a 20-cm3 water sample containing 1.0 cm3 of
a 2000 mg dm–3 Ca2+ solution, 1.0 cm3 of a 0.5 mol dm–3 H2SO4
solution and 0.5 cm3 of a coloring agent solution, 0.06 cm3 of
the ion exchanger was added, then the mixture was stirred for
30 min at 25°
C. After transferring the resin beads to the cell in
the same way as described for the determination of phosphate,
the light attenuances at λ1 = 540 nm and λ2 = 680 nm were
measured and ∆A was used for the determination of
chromium(VI).
Iron(II). Natural water samples were collected through a PTFE
tube (3 mm i.d.) using a plastic syringe (50 cm3) attached to a
three-way stopcock under a nitrogen atmosphere, filtered
through 0.20-µm PTFE membrane filters (Advantec) and
immediately acidified to pH 2 with HCl under a nitrogen
atmosphere.
To a 25 cm3 sample, 0.5 cm3 of the mixed coloring reagent
and buffer solution, 0.25 cm3 of a 1 mol dm–3 NH3 solution and
0.06 cm3 of Muromac 50W-X2 resin were added. After a

30-min stirring, the solid beads were transferred into the
microblack cell and light attenuances at λ1 = 512 nm and λ2 =
650 nm were measured. Then, ∆A was used for the determination
of iron(II).

Results and Discussion
Schematic outline of the newly developed system
For the absorbance measurement of the solid phase, it is fairly
difficult to maintain the same packing condition for all samples,
which affects the light attenuance at the absorption maximum
wavelength. To minimize the error caused by the packing state
difference of the solid beads in the cell for each measurement,
the absorbances of the solid phase were measured at the
maximum-absorption wavelength (λ1 nm) and weak or
no-absorption wavelength (λ2 nm), and then the difference in
the attenuances (∆A = A(λ1 nm) – A(λ2 nm)) was used for the
quantitative analysis of the target analyte.1,2 The portable
spectrophotometer is shown in Fig. 2. The CCD array together
with the gratings made it possible to continuously and
simultaneously monitor the ∆A.

A black cell (shown in Fig. 1) was used to pack the small
volume of solid particles on which the target color species was
concentrated. A part of a 5-cm3 polypropylene tip was attached
to the cell for easier introduction of the solid slurry. If the light

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679
8000
（A ）
31°C

Intensity

21°C
6000

a

4000

b

2000
0.2

Fig. 3 Light intensity profiles. A, Pink LED; B, white LED; C,
daylight LED; D, halogen tungsten lamp. Solid phase, Sephadex
G-25; cell length, 10 mm.

intensity of the LED was weak, a ball lens (1-cm diameter)
between the light source and the cell was effective to collect the
incident light beam at the entrance of the light path portion of
the cell (Fig. 2). A Ni-Cd battery (12 V) was used to power the
LED.
Selection of LEDs
The light intensity profiles of the LEDs and halogen tungsten
lamp are shown in Fig. 3. The strong light intensity wavelengths
of the LEDs were different from one another, and narrower than
the wavelength range of the halogen tungsten lamp.5 Therefore,
the LEDs should be selected according to the absorption
wavelength of the target complexes. The daylight LED with a
stronger light intensity in the longer wavelength region was
better than the white LED for the determination of chromium(VI)
and iron(II), because the light intensity at λ2 = 680 nm or
650 nm had to be also measured. The pink LED, which has a
strong light intensity around 450 and 640 nm, was used for the
determination of phosphate.
Stability of the light intensity of LED
The spectrophotometer used was a single beam type, and
therefore, the stability of the light intensity was critical for high
precision measurements. For the tungsten halogen lamp, the
light intensities in the visible region were stable enough to allow
a quantitative spectrophotometric determination. However, for
the LEDs, the light intensity might change with the time and
temperature.6,7
At a constant temperature, the light intensities decreased in
the beginning just after turning on the light, but became almost
constant after 20 min. We determined that the light measurements
should be started then. The light intensity was almost constant
for at least 12 h.
To examine the effect of temperature, the light intensities of a
PTFE film were measured instead of a sample beads layer
because the solid beads layer was not so stable due to air bubbles
generated for a long period experiment. Figure 4(A) shows that
the light intensities at two wavelengths decreased when the
C, however, ∆A in Eq. (1)
temperature increased from 21 to 31°
remained almost constant (Fig. 4(B)).
The reference
measurements just before every sample measurements could
reduce the errors caused by the light intensity fluctuation.

∆A

（B）

0.1
21°C

31°C

0.0
0

50

100
Time / min

Fig. 4 Temperature dependence of light intensity and absorbance of
LED and ∆A. (A) Light intensity (a, 650 nm; b, 520 nm), (B) ∆A:
LED, daylight; solid phase, 1.2 mm PTFE film; temperature,
C; 60 – 120 min, 31°
C. For ∆A, the reference
0 – 60 min, 21°
measurements were taken just before every sample measurement.

Procedure to pack the solid beads into the cell
As already mentioned, the smaller the volume of the solid
beads, the higher the sensitivity. The minimum amount for
complete packing in the black cell was examined and found to
be 0.06 cm3.4 This small volume of the gel could be taken and
transferred to a water sample using a gel aliquotting device.3,4
After the adsorption of the colored species, the mixture was
stored for a short moment to settle the solid beads, and then the
solid beads were transferred to the cell (Fig. 1) with a small
amount of solution using a pipette.
Calibration curves and detection limits
Phosphate. Using the SPS phosphate determination method,
which has already been well established,4 the performances of
this new instrument and the cell were examined. When using
the pink LED as the light source, sensitivity was somewhat low
because the molar absorptivity of the molybdenum blue species
at 640 nm was lower than that at 700 nm (the calibration curve:
∆A = 0.0278 P (µg dm–3) – 0.192 (R2 = 0.990)). The detection
limit corresponding to the 3σ of the blank test ∆A was 0.3 µg P
dm–3. A natural water sample collected from the Nanatsugama
limestone cave was analyzed.
The dissolved phosphate
concentration was 2.3 ± 0.1 µg dm–3 (n = 3) and the relative
standard deviation was around 5%. The results of the recovery
test of the same water sample are shown in Table 1, and almost
all of the added phosphate was recovered.
Chromium(VI). The daylight LED was used as the light source
for the chromium(VI) determination, and the calibration curve
could be expressed as ∆A = 0.0676 Cr (µg dm–3) + 0.060 (R2 =
0.992). The detection limit was 0.6 µg Cr dm–3. A well water

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Table 1 Recovery test by the standard-addition method using
natural water sample

Table 3 Recovery test by the standard-addition method using
natural water sample

P added/µg dm–3

P found/µg dm–3

Recovery, %

Fe(II) added/µg dm–3

Fe(II) found/µg dm–3

Recovery, %

0
1.99
2.03
3.00
3.03
4.00
4.01

2.29 ± 0.11
4.24
4.28
5.36
5.32
6.76
6.80

(100)
99
100
101
101
107
108

0
0.93

2.97 ± 0.08
3.96
3.83
3.93
4.55
4.70
4.63
5.21
5.29
5.42

(100)
107
93
103
85
93
89
81
84
88

Light source: pink LED.
Sample: Groundwater emerging from Nanatsugama limestone cave.

Table 2 Recovery test by the standard-addition method using
well water sample
Cr(VI) added/µg dm–3

Cr(VI) found/µg dm–3

Recovery, %

0
1.01a
1.07a
2.00a
1.98a
2.96a
3.04a

4.33 ± 0.15
3.29
3.29
4.34
4.31
4.82
4.92

(100)
112
106
109
108
90
91

Light source: daylight LED.
Sample: Well water sample from Hakozaki campus, Kyushu University,
Fufuoka.
a. The sample solution was diluted twice with water.

sample from the Hakozaki campus, Kyushu University, Fukuoka,
was quantitatively analyzed. The dissolved chromium(VI)
concentration was 4.3 ± 0.2 µg dm–3 (n = 4). The results of the
recovery test of chromium(VI) are shown in Table 2, and ± 10%
error may be included in the results of the on-site analysis.
Iron(II). For the iron(II), 1,10-phenanthrolone was used as the
coloring reagent.8 The same daylight LED was used as the light
source. The calibration curve was ∆A = 0.053Fe (µg dm–3) –
0.039 (R2 = 0.992). The detection limit was 0.3 µg Fe dm–3. A
water sample collected from a small unpolluted stream from
Kawachi, Fukuoka, was quantitatively analyzed, and the
dissolved Fe(II) concentration was 3.0 ± 0.1 µg dm–3 (n = 4),
suggesting that the Fe(II) may be supplied from the bottom of
the stream under local anoxic conditions. The results of the
recovery test of Fe(II) are shown in Table 3. In a previous study
using the spectrophotometer with an LED light source, the
results of the recovery tests of Fe(II) (1,10-phenantroline
method) were shown, and the recovery ranged from 93 to 105%
at mg dm–3 levels.5 Our results are acceptable for the on-site
analysis of Fe(II) at µg dm–3 levels.

Conclusions
The use of LED as a light source for the simple portable
spectrophotometer allows us to use a battery as the power source
for our equipment, and therefore on-site analyses can be
realized. Although the light intensity of the LEDs changed with
the temperature, the errors could be suppressed by the
measurement of the reference just before every sample

1.87

2.77

Light source: daylight LED.
Sample: Small stream water sample from Kawachi, Fukuoka.

measurement.
This system also made it possible to reduce the consumption
of reagents and waste generation. It made the analytical
methods more environmentally friendly, and therefore, the SPS
significantly contributed to Green Analytical Chemistry.9 Many
spectrophotometric methods for various trace elements and/or
substances present at µg dm–3 levels in water, especially of
unstable species, such as Fe(II) under oxic conditions, can be
applied to on-site analyses using our method and equipment. At
present, over 10% errors may be included in the analytical
results when using the LED as the light source for the µg dm–3
level analyte. However, further study on LEDs and light
measurements of the solid beads layer should improve the
reproducibility of the measurements.

Acknowledgements
This work was partially supported by Grant-in-Aids for
Exploratory Research, No. 24651012 (2012 – 2014) from the
Japan Society for the Promotion of Science.

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