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2020 © The Japan Society for Analytical Chemistry

Notes

Development of an Automatic pH Adjustment Instrument for the Preparationof Analytical Samples Prior to Solid Phase Extraction
Yanbei ZHU,*† Sayaka NISHIGORI,** Nobuaki SHIMURA,** Tomio NARA,** and Eiji FUJIMORI***
			 *National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science andTechnology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305–8563, Japan
		 **System Instruments Co., Ltd., 776-2 Komiyamachi, Hachioji, Tokyo 192–0031, Japan***National Environmental Research and Training Institute, 3-3 Namiki, Tokorozawa, Saitama 359–0042, Japan

An automatic pH adjustment instrument was developed for the preparation of analytical samples prior to solid phaseextraction, which is widely used as a pretreatment technique for the separation of sample matrixes and preconcentrationof elements for analysis. Real-time monitoring of the sample pH condition was performed by observing the light signal
intensity of the pH-sensitive wavelength of the pH indicating reagent. A light of pH-insensitive wavelength was selected
as the reference light to cancel the signal intensity variation of the pH-sensitive light due to the difference of pH indicating
reagent concentration, possible difference in transparency of sample vessels, and minute fluctuation of the light source.
The pH condition was elevated by automatic addition of ammonia solution using a nebulizer in the flow line of which an
electromagnetic valve was equipped. Open and close operation of the electromagnetic valve was controlled based on the
difference between the real-time pH condition and the target pH condition. The effectiveness of the instrument was
confirmed by using various pH indicating reagents and by analyzing trace elements in a seawater certified referencematerial.
Keywords Automatic pH adjustment instrument, solid phase extraction, pH indicating reagent, spectrometer, nebulization(Received December 27, 2019; Accepted February 10, 2020; Advance Publication Released Online by J-STAGE February 21, 2020)

Introduction
Solid phase extraction (SPE) is an effective approach for the
separation of sample matrix and the preconcentration of theanalytes in chemical analysis. SPE techniques using chelating
resins have been widely investigated as sample pretreatment
methods for trace elements prior to their measurement.1–39 Dueto the mechanism of SPE with chelating resin, i.e. adsorption of
trace elements to the resin at weak acidic to neutral pH
conditions and desorption from the resin in strong acidic
conditions, the optimum pH conditions reported for adsorption
were usually 5,1–8 5.5,9–14 5.7,15 6,16–27 6.13,28 6.5,29,30 and 7.31–33
There have also been several works reporting multiple pH
conditions, e.g. 4.0 – 7.0,34 5.0 and 6.0,35 5.5, 6.5, and 8.5,36 6, 7,
and 8,37 6.0 – 7.0,38 and 6.0 – 7.5.39 Such optimum conditions
depend on the chemical property of the target element and thesample matrix as well as the functional group and the chemical
structure of the chelating resin. Nevertheless, the optimum pH
conditions were most often chosen in the pH range from 5.0 to
7.0 as indicated by the aforementioned reports.
The precise adjustment of a solution to neutral pH condition
requires the precise controlling of H+ concentration at the 10–6 to10–7 mol L–1 level. Therefore, great attention should be paid for
manual adjustment of pH for SPE with chelating resins.
A commercially available preconcentration system, i.e. seaFAST
To whom correspondence should be addressed.
E-mail: yb-zhu@aist.go.jp†(Elemental Scientific), provids an approach for pH adjustment
by diluting the sample with an ammonium acetate buffer.40–45
Due to the limited naturalization capability of ammonium
acetate buffer, the samples after pH adjustment usually had highconcentrations of ammonium ions and acetate ions. They maycompete with the cations of trace elements and the functionalgroups of the chelating resin, respectively, during the SPE
operations. Apparent lower recoveries for some elements
reported might be attributed to the competition with such cations
and anions.41,44,45
As an alternative approach for precise pH adjustment in SPE
operations with chelating resin, the authors developed an
automatic pH adjustment instrument and applied it to themeasurement of rare earth elements (REEs) in seawater
samples.46 In the automatic pH adjustment instrument, the pHof a sample solution with pH indicating reagent (methyl-red)
was monitored by measuring the transmitted light, based on apre-established correlation between the pH and the transmitted
light-intensity. Such non-contact monitoring of the pH conditionof the sample avoided possible contamination from a pHelectrode. The addition of ammonia solution for elevating thepH from acidic to neutral condition was precisely controlled bya nebulizer (usually used in inductively coupled plasma
techniques) and an electro-magnetic valve, permitting a precise
addition of ammonia solution at less than 0.1 μL. Real-timemonitoring of the pH condition based on transmitted light andprecise controlling of ammonia solution permitted the precise
pH adjustment for SPE operations with chelating resins. The
instrument was applied to the analysis of REEs in seawater

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ANALYTICAL SCIENCES MAY 2020, VOL. 36

samples. However, it was found later that the transmitted lightbased pH monitoring required high reproducibility of thecontainer light-transparency as well as a precise concentrationof pH indicating reagent in the sample.
In the present work, the automatic pH adjustment instrument
was improved to achieve a precise pH adjustment independentof the concentration of pH indicating reagent in the sample
solution. An application of the present automatic pH adjustment
instrument had been preliminary reported and referred to as“electrodeless pH controller”.47 Technical details about the
automatic pH adjustment instrument are presented in the present
work along with the application to the determination of multiple
trace elements in a seawater certified reference material (CRM).

Experimental
Instruments
An ICP-MS instrument (Agilent 8800s type) was applied forthe analysis of trace elements. The operating conditions of the
ICP-MS were optimized daily to achieve relatively high signal
intensities for the elements of interest. Typical operating
conditions of the ICP-MS can be found in Supporting
Information (Table S1). Hydrogen gas was used as the reaction
gas for effective removal of polyatomic interferences, e.g.
40Ar16O+ with the measurement of 56Fe+, and 94Mo16O+ with themeasurement of 110Cd+.
Reagents and chemicals
Metrological traceable single element standard solutions werepurchased from Kanto Chemical Co., Inc. (Tokyo, Japan). The
traceability of the standard solutions were guaranteed by theJapan Calibration Service System (JCSS). Ultrapur® HNO3,
ammonia solution, acetic acid and reagent grade ethanol werealso purchased from Kanto Chemical Co., Inc., and used for
preparing the solutions in the present experiment. Reagent
grade bromophenol-blue, bromocresol-green, methyl-red, andphenol-red were also purchased from Kanto Chemical Co. Inc.
to prepare solutions of pH indicating reagents.
A seawater CRM was purchased from the National ResearchCouncil of Canada. Chelating resin columns, NOBIAS ChelatePA1 (M-size), were purchased from Hitachi High-Tech Fielding
Corp. (Tokyo, Japan).
Construction of the automatic adjustment instrument
The structure of the automatic pH adjustment instrument is
illustrated in Fig. 1. As can be seen from Fig. 1, the light fromlight source (1) transmitted through the sample (2), which was
mixed with a magnetic rotor (3) driven by a magnetic stirrer (4)at 100 revolutions per minute. The transmitted light wasmeasured by a spectrometer (5), which sent a signal to the I/Oboard (6) and then to the controlling computer (7). Based on
the correlation between the signal intensity ratio of a pHsensitive light to a pH-insensitive light and the pH condition, thecontrolling computer sent open/close commands to the I/Oboard (6) and then to the electro-magnetic valve (8). The open/
close operation of the electro-magnetic valve (8) controlled the
supply of ammonia solution (9) through the nebulizer (10) to thesample solution (2), where the addition of ammonia solution (2)through the nebulizer (10) was driven by the air (11) with a flowrate of approximately 1.0 L min–1 and resulting in an uptake rateof ammonia solution at 0.5 mL min–1, where an in-line filter
was equipped to prevent contamination from the atmosphere.

Fig. 1 Structure of the automatic pH adjustment instrument. 1,
Light source; 2, sample; 3, rotor; 4, magnetic stirrer; 5, spectrometer;
6, IO board; 7, computer; 8, electro-magnetic valve; 9, ammonia
solution; 10, nebulizer; 11, air.

Flow chart for the automatic controlling program
The typical operation of the automatic controlling program forthe instrument is illustrated with a flow chart in Fig. 2. As canbe seen from Fig. 2, the valve was closed and the target pH
value (At) was read after starting the adjustment. The signal
intensities of pH-sensitive light (aIi) and pH-insensitive light (rIi)were read from the spectrometer. Then, the current pH value(Ai) was calculated from the signal intensity ratio of (aIi/rIi). The
difference between the values of Ai and At was calculated as ΔA,
which was compared with 0.0 so as to evaluate whether thetarget pH was achieved. It is notable that water samples fortrace elements analysis are usually stored at relatively strong
acidic conditions with pH lower than 1.0. The purpose foradjustment of pH condition for SPE operation is usually toelevate the pH value, i.e. “Ai < At” in the beginning of the pH
adjustment.
When the target pH was not achieved, i.e. “ΔA = 0.0” was notsatisfied, the value of ΔA was further compared with 0.6, 0.4,
0.3, 0.2, and 0.1 in turn. When the value of ΔA was over 0.6,
the valve was kept open, permitting introduction of ammonia
solution into the sample to elevate the pH value until the next
round of calculations of Ai and evaluation of whether “ΔA =0.0”. When the condition of “0.6 > = ΔA > 0.4”, “0.4 > = ΔA
> 0.3”, “0.3 > = ΔA > 0.2”, “0.2 > = ΔA > 0.1”, and “0.1 > =ΔA > 0.0 ” was respectively satisfied, the valve open/close times(unit, ms) were respectively set to 3000/2000, 2000/2000,
1000/1500, 500/2000, and 150/2500 until the next round of
calculations of Ai and evaluation of whether “ΔA = 0.0”. The
open/close time sets were optimized to permit the sample tobecome completely mixed after each round of ammonia solution
addition, as well as to avoid over addition of ammonia solution.
When the target pH was achieved, i.e. “ΔA = 0.0” wassatisfied, the valve was closed and the program of pH adjustment
ended.
Procedure for SPE preconcentration with chelating resin
The typical procedure for SPE preconcentration with chelating
resin used in the present work was as follows. The chelating
resin columns (NOBIAS CHELATE-PA1) were washed with
5 mL of 3 mol L–1 nitric acid and 5 mL of pure water, followedby conditioning with 5 mL of 0.1 mol L–1 ammonia acetate.
Then, 50 mL of the sample (with 100 μL of 0.4 mg mL–1

ANALYTICAL SCIENCES MAY 2020, VOL. 36

623

Fig. 2 Flow-chart for automatic pH adjustment. ∗, aIi and rIi, indicate signal intensities of the
pH-sensitive light and the pH-insensitive light, respectively.

methyl-red in ethanol as pH indicating reagent) adjusted topH 6.0 was loaded to the chelating resin column at a flow rateof approximately 10 mL min–1. After sample loading, 10 mL of
0.1 mol L–1 ammonia acetate and 5 mL of pure water werepassed through the chelating resin column for washing. Finally,
trace elements adsorbed on the resin was eluted with 5 mL of
2 mol L–1 nitric acid and then subjected to the measurement by
ICP-MS.
The samples for blank test and recovery test were also
subjected to the same procedure for SPE preconcentration.
Quantitation of trace elements was carried out based on a
gravimetric-based standard addition method following theprocedure described in a previously reported work.48 An yttrium
standard solution was spiked to each sample in the present workto achieve a concentration of 1 ng mL–1 yttrium in the sample
solution prior to pH adjustment. The signal of 89Y+ wasmeasured as the internal standard.

Results and Discussion
Effectiveness of a reference light for ensuring the stability of pH
adjustment
In the present work, a light of pH-insensitive wavelength wasused as the reference light to cancel the variation of brightnessof the sample solution, probably due to the minute variations in
the concentration of pH indicating reagent and the transparencyof sample vessels.
The effectiveness of a reference light for ensuring the stabilityof pH adjustment was confirmed by using methyl-red as the pH
indicating reagent and by setting the target pH value to 6.0.
Based on the absorption properties of methyl-red as a pH
indicating reagent shown in Fig. 3, the pH-sensitive wavelengthand the pH-insensitive wavelength were chosen at 530 and
620 nm, respectively.
The calibrating curve for calculating the pH value from thesignal intensities at the pH-sensitive wavelength and thepH-insensitive wavelength were obtained by addition of 100 μL
methyl-red (0.4 mg mL–1 in ethanol) to each 50 mL of coastal

Fig. 3 Absorption properties of methyl-red as a pH indicating
reagent. Red line profile, pH 1.0; blue line profile, pH 8.0; dotted line,
pH-sensitive wavelength; dashed line, pH-insensitive wavelength.
Obtained with a 50-mL coastal seawater sample with 100 μL methylred (0.4 mg mL–1 in ethanol).

seawater sample. After that, test samples (50 mL each) with 80,
100, and 120 μL of methyl-red were respectively prepared andsubjected to the automatic pH adjustment. The relative
difference in signal intensity of pH-sensitive wavelength at
530 nm in these samples was over 30%. After pH adjustment
with the present automatic instrument, the pH values in thesesamples were all in the range of 5.98 to 6.00, which were
sufficiently precise for SPE operations with chelating resins.
When the reference light was not used, the difference in volume
of methyl-red by 40 μL may result in the difference of pH valueof approximately 0.3 to 0.5. These results indicate that use of areference light was effective for ensuring the stability of pH
adjustment. Therefore, a reference light was also used for pH
adjustment with other pH indicating reagents.

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Table 1ANALYTICAL SCIENCES MAY 2020, VOL. 36
Typical pH indicating reagents confirmed in the present worka

pH indicating
reagent

pH range
and color

Concentrationand solvent

Volume added(μL per 50 mL sample)

pH-sensitive
wavelength/nm

pH-insensitive
wavelength/nm

Target pH
range

Bromophenol-blue

pH 3.0 – 4.6(yellow ↔ purple)
pH 3.8 – 5.4(yellow ↔ blue)
pH 4.4 – 6.2(red ↔ yellow)
pH 6.4 – 8.2(yellow ↔ red)

0.4 mg mL–1 inammonia solution
0.4 mg mL–1 inammonia solution
0.4 mg mL–1 inethanol
0.4 mg mL–1 inammonia solution

100

571

650

3.0 to 4.0

100

573

507

4.0 to 5.0

100

530

620

5.0 to 6.0

100

560

610

6.0 to 8.0

Bromocresol-green
Methyl-red
Phenol-red

a. Typical calibration curves for these pH indicating reagents are provided as Supporting Information (Figs. S1 to S4).

Table 2 Blank values and ADLs obtained with ICP-MS after
SPE preconcentration
Elementm/zBlank valuea/ng mL–1

ADLb/ng mL–1

Fe
Co
Ni
Cu
Zn
Cd
Pb

56
59
60
63
66
110
208

0.11 ± 0.05
0.00019 ± 0.00008
0.0035 ± 0.0009
0.022 ± 0.006
0.087 ± 0.018
0.00045 ± 0.00014
0.0021 ± 0.0008

0.14
0.00025
0.00260.00170.053
0.000430.0023a. Mean ± standard deviation, n = 5.
b. Concentration corresponding to 3-fold of the standard deviation ofblank value.
Fig. 4 Results of automatic pH adjustment. (pH indicating reagents:
red circle, bromophenol-blue; blue circle, bromocresol-green; green
circle, methyl-red; purple circle, phenol-red; bar in the circle, standard
deviation of 3 sub-samples.)Typical pH indicating reagents and applicable pH range with thepresent instrument
Multiple pH indicating reagents were investigated in thepresent work to ensure the possibility of automatic and precise
pH adjustment in a relatively wide pH range from 3.0 to 8.0.
The characteristics of four pH indicating reagents are
summarized in Table 1.
The pH indicating reagents were dissolved in ammonia
solution (bromophenol-blue, bromocresol-green, and phenol-red)or ethanol (methyl-red), to obtain a concentration of 0.4 mg mL–1.
Typically, 100 μL of the pH indicating reagent solution wasadded into each 50 mL of water sample for pH monitoring. As
summarized in Table 1, the characteristic lights (unit, nm)of the pH-sensitive and the pH-insensitive wavelengths forbromophenol-blue, bromocresol-green, methyl-red, and phenolred were 580/650, 570/515, 530/620, and 560/610, respectively.
Application of the pH indicating reagents permitted thecoverage of a target pH range from 3.0 to 8.0 by the present
automatic pH adjustment instrument. The results of automaticpH adjustment to 3.0 to 8.0 are plotted in Fig. 4 against thetarget pH value assigned. As can be seen from Fig. 4,
the obtained pH values were identical to the target values. The
results show that the present instrument was capable for accurate
and precise pH adjustment.
Analytical figures of merits
Blank values and analytical detection limits (ADLs) were

evaluated as the analytical figures of merits for trace elements
obtained by ICP-MS after SPE preconcentration with chelating
resin and with the present automatic pH adjustment instrument
for sample preparation.
Blank values were obtained with 50 mL of 0.1 mol L–1 nitricacid as test samples. The test samples were subjected to theautomatic pH adjustment and SPE preconcentration withchelating resin in the same way as that for the seawater samples,in which the concentration of nitric acid was also 0.1 mol L–1.
The final solution for analysis after SPE preconcentration was
5 mL, providing a preconcentration factor of 10-fold. The
results for blank tests are summarized in Table 2, along with
ADLs obtained as the concentrations corresponding to 3-fold of
the standard deviation of blank values.
As can be seen from Table 2, the blank values for traceelements were in the range from 0.00019 ng mL–1 of Co to
0.11 ng mL–1 of Fe, while the ADLs were in the range from
0.00025 ng mL–1 of Co to 0.14 ng mL–1 of Fe. These values
were sufficiently low for the analysis of water samples taking
into consideration the regulated concentrations for these traceelements were not lower than 3 ng mL–1.49 A preliminary
experiment in the present work showed that the major factor
contributing to the blank values was the impurities in the solid
phase of the chelating resin, while the contribution of pH
adjustment was negligible.
The typical time required for pH adjustment of 50 mL of thesesamples was approximately 15 min. The time for automatic pH
adjustment could be shorten to less than 5 min by partial
neutralization of nitric acid in the sample, i.e. addition of someammonia solution prior to the automatic pH adjustment.
The present instrument provided an approach for rapid andautomatic pH adjustment independent to the skill of the operator.

ANALYTICAL SCIENCES MAY 2020, VOL. 36
Table 3 Analytical results and recoveries of trace elements inseawater CRM, NRC CRM CASS-5
Element m/z
Fe
Co
Ni
Cu
Zn
Cd
Pb

Observed value /ng mL–1

Certified value /ng mL–1

Spike
recoverya, %1.35 ± 0.15
0.093 ± 0.005
0.351 ± 0.032
0.375 ± 0.021
0.75 ± 0.05
0.0221 ± 0.0005
0.013 ± 0.003

1.44 ± 0.11
0.095c0.330 ± 0.023
0.380 ± 0.028
0.719 ± 0.068
0.0215 ± 0.0018
0.011 ± 0.002

98.8 ± 1.5
97.3 ± 1.8
98.7 ± 1.3
100.2 ± 0.8
101.3 ± 1.5
100.4 ± 1.4
98.7 ± 1.2

56
59
60
63
66
110
208

a

b

a. Mean ± standard deviation, n = 4.
b. Mean ± expanded uncertainty.
c. Information value.

Manual adjustment of a sample to pH 6.0 depended on the skillof the operator, probably taking 30 min or even over 60 min toachieve a pH value in the range of 5.9 to 6.1.
Analytical results for trace elements in a seawater CRM
Trace elements in a seawater CRM, NCR CRM CASS-5, wereanalyzed to confirm the reliability of the present automatic pH
adjustment instrument and the present analytical method. The
analytical results, after subtracting the blank values given in
Table 2, are summarized in Table 3 in comparison to thecertified values.
As can be seen in Table 3, the observed values agreed with thecertified values in the range of their expanded uncertainties. It
is notable that the concentrations of these elements were at least
one order of magnitude higher than the blank values and theADLs given in Table 2, permitting accurate and precise analysisof these elements.
The spike recovery values given in Table 3 were obtained byadding a mixture of elemental standard solution so as to achievea spiking concentration of 2 ng mL–1 for each element in the
spiked seawater CRM sample. It can be seen from Table 3 thatthe recovery values were quite close to 100% with good
reproducibility, i.e. standard deviation less than 2%.
These results indicate that the present automatic pH adjustment
instrument and the present analytical method are effective forthe analysis of trace elements in environmental samples
including seawater.

Conclusions
An automatic pH adjustment instrument was developed for SPE
preconcentration of trace elements with chelating resin. The
automatic and real-time monitoring of the sample pH condition
was obtained by monitoring the light signal intensity of the pHsensitive wavelength for the pH indicating reagent. Stable pH
monitoring was improved by using a reference light at the pHinsensitive wavelength for the pH indicating reagent. Automaticand precise addition of ammonia solution to the sample toelevate the pH condition to the target value was achieved byusing a nebulizer, for which the flow line was controlled with anelectromagnetic valve. Open and close operation of theelectromagnetic valve was controlled based on the difference
between the real-time pH value and the target pH value. The
results of trace elements in NRC CRM CASS-5 indicate that thepresent instrument was effective for automatic pH adjustment
for SPE preconcentration with chelating resin.

625

Supporting Information
Table S1, operating conditions for ICP-MS. Figures S1 to S4,
typical calibration curves for pH adjustment using different pH
indicating reagents.

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