Admicelle-mediated collection followed by flotation for the preconcentration of trace metals in fresh waters

Analytica Chimica Acta 588 (2007) 82–87

Admicelle-mediated collection followed by flotation for the preconcentration of trace metals in fresh waters Hiroaki Matsumiya ∗ , Yosuke Yatsuya, Masataka Hiraide Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 23 October 2006; received in revised form 26 January 2007; accepted 26 January 2007 Available online 4 February 2007

Abstract Dithizone-impregnated admicelles were prepared by mixing silica particles with dithizone and cetyltrimethylammonium chloride in 0.1 mol L−1 aqueous ammonia. The resulting admicelles were added to 1000 mL of sample solution and dispersed by stirring for 15 min. Traces of Ni(II), Cu(II), Ga(III), Cd(II), Pb(II) and Bi(III) in the solution were simultaneously incorporated into the admicelles at pH 7.5–9. With the aid of a rising stream of numerous tiny bubbles, the admicelles were floated on the solution surface and collected in a small sampling vessel by suction. The metals were desorbed from the admicelles with dilute nitric acid and determined by inductively coupled plasma-mass spectrometry. The proposed method offered a 100-fold multielement preconcentration and it was applicable to the analysis of river and pond waters. © 2007 Elsevier B.V. All rights reserved. Keywords: Admicelle; Flotation; Preconcentration; Trace metal; Fresh water; Inductively coupled plasma-mass spectrometry

1. Introduction In spite of great advances in modern analytical instrumentation, preconcentration procedures are still often required for the precise and accurate determination of trace metals in natural waters. Many techniques, e.g. solvent and solid-phase extractions, volatilization, electrodeposition, ion-exchange and coprecipitation, have been combined with instrumental analytical methods so far [1,2]. On the other hand, the analytical use of admicelles, surfactant aggregates formed on solid surfaces [3–5], has been studied in our laboratory with a view to developing novel separation and preconcentration vehicles [6–9]. Just like normal micelles, admicelles can incorporate water-insoluble compounds into their hydrophobic region [5,8]. In addition, simple mixing of an appropriate solid support with aqueous surfactant solutions affords admicelles, thus allowing a facile preparation of a wide variety of functionalized sorbents. For example, an admicellar sorbent for trace metals can be readily prepared by just mixing alumina particles with sodium dodecylsulfate (SDS) and water-

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Corresponding author. Tel.: +81 52 789 3591; fax: +81 52 789 3241. E-mail address: [email protected] (H. Matsumiya).

0003-2670/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2007.01.068

insoluble chelating agents (e.g. dithizone) in an acidic aqueous solution [6]. Because the surfactant aggregates are supported on a solid material, the admicelle-mediated collection is free from difficulties encountered in separating small micelles from the bulk aqueous phase and handling viscous surfactant solutions, which is a great advantage over other micelle-mediated collections (e.g. cloud-point extraction and micellar-enhanced ultrafiltration). Recently, one of the authors has briefly communicated a new combination of admicelle-mediated collection and flotation [10]. In this preliminary study, a few kinds of chlorophenols were recovered from 50 mL of pure water using admicelles composed of silica and cetyltrimethylammonium chloride (CTAC). This result indicates that the adsorbed surfactant molecules provide the solid particles not only with a collection medium for waterinsoluble compounds but also with hydrophobic surfaces for flotation. The present study was undertaken to develop an improved and practical admicelle–flotation method for the multielement preconcentration of trace metals. For this purpose, dithizone was employed as a chelating agent. Although dithizone exhibits its high complexation ability under neutral to alkaline pH conditions [11], SDS–alumina admicelles are unstable under such conditions [6,8]. To overcome this problem, the

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dithizone-impregnated admicelles were prepared from CTAC and silica, instead of SDS and alumina. Owing to the high acidity of silanol groups, silica surfaces are negatively charged in neutral or alkaline solutions and thus retain cationic surfactants strongly [3,4]. Although flotation is a powerful technique for collecting trace constituents in large volumes of water samples [12], the admicelle–flotation was performed only with 50 mL of pure water in the previous study [10]. The practical applicability to trace analysis was also not demonstrated. In the present study, the sample volume was increased to 1000 mL for the practical application to fresh water analysis. The optimized admicelle–flotation system offered the facile 100-fold preconcentration of trace metals in river and pond waters, allowing their determination by inductively coupled plasma-mass spectrometry (ICP-MS). 2. Experimental 2.1. Apparatus The flotation apparatus was basically the same as that described in the literature [13]. As shown in Fig. 1, a 1000 mL graduated cylinder was used as a flotation cell. The bubbler was equipped with a sintered-glass disk (5–10 ␮m pore size), through which nitrogen gas was introduced into sample solution for generating a rising stream of numerous tiny bubbles. The sampling vessel was a Millipore (Billerica, MA, USA) filter holder (15 mm i.d. × 100 mm high) equipped with an Omnipore hydrophilic PTFE membrane filter (10 ␮m pore size, Millipore). A silicone-rubber stopper with a tapered bent glass tube was fitted on the top of the sampling vessel. The lower end of the vessel was inserted into a waste reservoir, which was connected to a vacuum pump for the suctioning of the floated admicelles. A Seiko (Chiba, Japan) SPQ-6500 ICP-mass spectrometer was used for the determination of trace metals under the fol-

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lowing plasma conditions: RF power, 1.2 kW; sampling depth, 12 mm; argon flow rates (in L min−1 ), 18 for outer, 0.8 for intermediate and 1.0 for carrier. The determination was performed at the following m/z values: 60 for Ni, 63 for Cu, 71 for Ga, 114 for Cd, 208 for Pb and 209 for Bi. A Tokyo Rika (Tokyo, Japan) NTS-1300 mechanical shaker was used for the preparation of admicelles and for the batch extraction experiments at a shaking rate of 70–80 strokes min−1 . A Tokyo Rika AU-60C ultrasonic cleaning-bath (28 kHz, 210 W) was used for the dissolution of dithizone and for the purification of silica. A Jasco (Tokyo, Japan) V-550 UV–vis double-beam spectrophotometer was used with 1 cm quartz cells for the determination of CTAC by the Orange II method [14]. 2.2. Reagents Silica gel (chromatographic grade, 40–50 ␮m in particle size) and CTAC (extra-pure reagent grade) were purchased from Kanto Kagaku (Tokyo, Japan). Dithizone was purchased from Wako Jun-yaku (Osaka, Japan). A dithizone-CTAC solution (dithizone 0.10 mg mL−1 , CTAC 7.5 mg mL−1 ) was prepared as follows. A 20 mg amount of dithizone was dissolved in 100 mL of 0.2 M aqueous ammonia with the aid of ultrasonic irradiation (1 M ≡ 1 mol L−1 ). The solution was mixed with 30 mL of 50 mg mL−1 CTAC solution and diluted to 200 mL with water. Standard solutions of metals (1.0 ␮g mL−1 in 0.1 M nitric acid) were prepared from commercial standard solutions (Nacalai Tesque, Kyoto, Japan). Artificial river water was prepared by dissolving 5.0 mg of sodium chloride, 10 mg of sodium sulfate, 2.0 mg of potassium chloride, 20 mg of calcium nitrate tetrahydrate and 15 mg of magnesium chloride hexahydrate in 1000 mL of water (Na 5.2, K 1.0, Mg 1.8, Ca 4.6, Cl 9.2, SO4 6.8 and NO3 7.1 ␮g mL−1 ). Humic acid (extracted from peat soil, Nacalai Tesque) was also added to this solution to give a concentration of 1.0 ␮g mL−1 [15]. The pH buffers used were as follows: formic acid–tetramethylammonium hydroxide (TMAH) for pH 3.0–4.2, acetic acid–TMAH for pH 4.2–5.5, 2morpholinoethanesulfonic acid (MES)–TMAH for pH 5.5–6.5, 3-morpholinopropanesulfonic acid (MOPS)–TMAH for pH 6.5–7.7 and tris(hydroxymethyl)aminomethane (Tris)–nitric acid for pH 7.7–9.3. An Orange II solution (1.0 mg mL−1 ) was prepared by dissolving sodium 4-(2-hydroxy-1-naphthylazo)benzenesulfonate (Aldrich, Milwaukee, WI, USA) in water. A 1.0 M sodium citrate–hydrochloric acid buffer solution (pH 3.5) was used for the Orange II method. All reagents used were of analytical reagent grade, unless otherwise stated. Water was purified with a Millipore Milli-Q Gradient A-10 system. 2.3. Preparation of admicelles

Fig. 1. Apparatus for flotation.

A 1.0 g amount of silica particles, placed in a 50 mL centrifuge tube, was ultrasonically washed in 5 M nitric acid for 5 min and rinsed with water until the pH of the supernatant solution became 5–6; the solutions were removed by centrifugation at 3000 rpm for 1 min. To the remaining particles, 40 mL

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of dithizone–CTAC solution was added and gently shaken for 5 min. The supernatant solution was removed by centrifugation and the resulting admicelles were washed twice with 7 mL each of water before use. 2.4. Determination of CTAC by the Orange II method [14] To a 15 mL test tube were added 1–10 mL of sample solution (containing <0.08 mg of CTAC, preliminarily adjusted to pH > 2), 1.0 mL of Orange II solution, 0.10 mL of citrate buffer solution and 1.0 mL of chloroform. After vigorously shaking for 3 min, the chloroform phase was collected in a 5 mL volumetric flask. The extraction was repeated twice more with 1.0 mL each of chloroform. The combined solution was diluted to the mark with chloroform and the absorbance at 485 nm was measured. The absorptivity was 62 cm2 mg−1 for CTAC in chloroform. 2.5. Small-scale batch extraction of metals A 40 mL volume of sample solution (containing 1.0 ␮g each of trace metals and 0.20 mmol of an appropriate pH buffer) was poured into a 50 mL centrifuge tube in which 0.25 g (as silica) of the admicelles had been placed. The suspension was gently shaken for 30 min and centrifuged at 3000 rpm for 5 min. The supernatant solution was analyzed by ICP-MS for trace metals. 2.6. Recommended procedure A 1000 mL volume of sample solution was placed in a flotation cell and gently stirred with a magnetic stirrer. The sample pH was adjusted to 8–8.5 by adding a 0.50 M Tris–HNO3 buffer solution. While gently stirring, 2.0 g (as silica) of the admicelles was added to the solution and dispersed uniformly for 15 min. After adding 5.0 mL of ethanol, nitrogen gas was introduced into the solution at a flow rate of 25 mL min−1 . The floated admicelles were collected into the sampling vessel by suction. The silicone-rubber stopper was removed and the admicelles were washed with 5.0 mL of water. After the waste reservoir was removed, the sampling vessel was fitted on a bell jar for suction filtration. A 1.0 mL volume of 5 M nitric acid was added to the admicelles, left to stand for 5 min and sucked into a 10 mL volumetric flask. The desorption was repeated once more with 1.0 mL of 5 M nitric acid. After the admicelles were washed with 2.0 mL each of 1 M nitric acid and water, the combined solution was diluted to the mark with water and analyzed by ICP-MS for trace metals. Calibration graphs were constructed using 1.2 M nitric acid containing metals of interest at pg mL−1 to ng mL−1 levels.

Fig. 2. Adsorption of CTAC on 0.50 g of silica particles in 20 mL of 0.1 M aqueous ammonia.

Different amounts of CTAC were mixed with 0.50 g of silica in 20 mL of 0.1 M aqueous ammonia. After shaking for 50 h to attain adsorption equilibria (vide infra), the CTAC remaining in the supernatant solution was determined by the Orange II method. As shown in Fig. 2, the adsorption of CTAC increased in proportion to the amount of CTAC added, and then leveled off at 290–310 mg. This indicates the saturation of the silica surfaces with CTAC (ca. 600 mg g−1 ). In the present study, the amount of CTAC loaded on silica particles was fixed to 300 mg g−1 (the reason will be described in Section 3.3). Fig. 3 shows the effect of the solution pH on the adsorption of CTAC. The complete adsorption occurred over a wide pH range of 4–10, whereas the adsorption steeply decreased below pH 4. The decrease in the adsorption can be primarily ascribed to the decrease in the negative charges on the silica surfaces, because the isoelectric point of silica occurs at approximately pH 2 [16]. Although the surface charge seems to be of great significance, the tendency for surfactant molecules to self-assemble together can also be responsible in part for the abrupt change in the adsorption. The solution pH also affected the adsorption rate. For example, the complete adsorption required a 30 h shaking at pH 3.9, whereas it took only 5 min at pH 8.8 (Fig. 4). In the present study, therefore, the admicelles were prepared around pH 9. In the presence of 0.10 mg mL−1 of dithizone, the reddish supernatant solution became colorless upon the formation of

3. Results and discussion 3.1. Adsorption of CTAC on silica particles In the previous communication [10], the adsorption behavior of CTAC on silica particles was examined only at pH 7 or 9. In the present study, therefore, a detailed investigation was carried out. First, the adsorption capacity was examined as follows.

Fig. 3. Effect of the solution pH on the adsorption of 150 mg of CTAC on 0.50 g of silica particles.

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Table 1 Flotation of admicelles from 1000 mL of artificial river watera CTAC loaded (mg)

CTAC released (mg)

Admicelles recovered (%)

50 125 150 250 290

1.2 1.1 1.5 9.9 30

69 95 97 99 98

a

Fig. 4. Time course of the adsorption of 150 mg of CTAC on 0.50 g of silica particles at different pH’s.

the admicelles; the white silica particles in turn took on a red tinge. This indicates that dithizone was almost completely incorporated into the admicelles. 3.2. Extractability of trace metals To survey the extractability of trace metals to the dithizoneimpregnated admicelles, batch extraction experiments were carried out on a small scale. At various pH’s, 0.25 g (as silica) of the admicelles were mixed with 40 mL of sample solution containing Ni(II), Cu(II), Ga(III), Cd(II), Pb(II) and Bi(III) at the 25 ng mL−1 level. After gently shaking for 30 min, the supernatant solution was analyzed by ICP-MS to determine the extraction yield. As shown in Fig. 5, Cu(II), Ga(III), Cd(II) and Bi(III) were simultaneously extracted in >90% yields over pH 5–9. The nearly quantitative extraction of Ni(II) and Pb(II) was achieved at pH 7.5–9 and pH 6.5–9, respectively. The relative narrow range for the extraction of Ni(II) and Pb(II) may be explained by the rather week complexation ability of dithizone toward

Fig. 5. Effect of the solution pH on the collection of Ni(II), Cu(II), Ga(III), Cd(II), Pb(II) and Bi(III) at the 25 ng mL−1 level.

0.50 g of silica was used.

Ni(II) and Pb(II) [11]. In the present study, the sample pH was adjusted to 8–8.5 to ensure the simultaneous collection of the metals. 3.3. Flotability of admicelles A 0.50 g amount of silica was loaded with 25–280 mg of CTAC and then dispersed into 1000 mL of pure water. A small amount of ethanol was added to the solution for the effective generation of numerous tiny bubbles [12]. While bare silica particles were negligibly floated even by a several minute bubbling, the admicelles were floated within 1 min and supported on the solution surface by a stable foam layer. The cationic head groups of the CTAC molecules would be oriented toward the negatively charged silica surfaces, which turns the hydrophilic surfaces to hydrophobic. Such surface modification causes the particles to be readily adsorbed onto the gas–water interfaces, leading to the successful flotation of the admicelles. Next, the flotation was carried out using artificial river water. Different amounts of CTAC were loaded on 0.50 g of silica. The resulting admicelles were added to 1000 mL of artificial river water and floated by bubbling; the solution foamed rather sparingly, compared with the case of pure water. Table 1 summarizes the recoveries of the admicelles, which were determined by gravimetry. The satisfactory flotation was achieved by loading 125–290 mg of CTAC. The recovery decreased to 69% when 50 mg of CTAC was loaded, probably because the surface hydrophobicity was not sufficient for sticking to the bubbles. Table 1 also gives the amounts of CTAC released from the admicelles into the solution, which were determined by the Orange II method. The CTAC released during the flotation was 1.2–30 mg, which served for the formation of the supporting foam layer. The visual observation showed that dithizone was firmly retained on the admicelles; the concentrations of the released CTAC in the solution were much lower than the critical micellar concentration (420 mg L−1 ) [17]. For the facile and rapid collection of the floated admicelles, the recommended amount of CTAC was 300 mg g−1 (150 mg in Table 1). Further loading led to clogging of the sampling vessel with the bulky flocculent admicelles. In addition, the flotation was tried in the presence of large amounts of coexisting ions (Na 12, Mg 1.3 and Cl 23 mg mL−1 ) to examine the applicability to seawater analysis. However, complete flotation was not achieved. Therefore, the application of the admicelle–flotation method would be limited to fresh water samples.

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Table 2 Recovery of 1.0 ng mL−1 each of metals from 1000 mL of artificial river watera Admicelles added (g, as SiO2 )

0.50 1.0 1.5 2.0 a b

Metals recoveredb (%) Ni(II)

Cu(II)

Ga(III)

66, 74 80, 85 91, 96 101, 101

46, 49 62, 64 83, 84 94, 95

83, 86 87, 90 93, 99 102, 104

Cd(II) 56, 59 74, 77 82, 85 95, .97

Pb(II)

Bi(III)

54, 61 66, 71 91, 96 101, 101

62, 68 82, 88 85, 90 96, 97

Concentrations (␮g mL−1 ): Na 5.2, K 1.0, Mg 1.8, Ca 4.6, Cl 9.2, SO4 6.8, NO3 7.1, humic acid 1.0. Stirring time: 15 min.

3.4. Collection and determination of trace metals by the admicelle–flotation Different amounts of the admicelles were added to 1000 mL of artificial river water (preliminarily adjusted to pH 8–8.5) containing Ni(II), Cu(II), Ga(III), Cd(II), Pb(II) and Bi(III) at the 1.0 ng mL−1 level. After stirring for a given time, the admicelles were recovered by flotation and then treated with 5 M nitric acid to desorb the collected metals. After washing the admicelles with 1 M nitric acid and water, the recoveries of the metals were determined by ICP-MS. The CTAC found in the final solution was 49–74 ␮g, which did not interfere with ICP-MS. As shown in Table 2, the simultaneous and complete recovery of six metals was achieved with 2.0 g (as silica) of the admicelles. When the desorption was carried out with 1 and 3 M nitric acid, the recoveries decreased appreciably (e.g. 76 and 86% for Cd, respectively). The stirring time required for the metal collection was 5 min for Ga(III), 10 min for Pb(II) and 15 min for the others. Under the optimal conditions (admicelles 2.0 g, stirring time 15 min and pH 8–8.5), the metals of interest were recovered almost quantitatively from artificial river water as well as from pure water. This suggests that major inorganic ions (e.g. Na, K and Mg) and humic acid at the natural abundance levels in river water (see Section 2.2 or the footnote in Table 2) caused no interference in the metal collection. In addition, the interference of some minor metals, such as Al(III), Mn(II), Fe(III) and Zn(II), widely found in surface waters at relatively high concentrations was examined. Even in the presence of 0.10 ␮g mL−1 each of these metals, 1.0 ng mL−1 each of the desired metals, viz. Ni(II), Cu(II), Ga(III), Cd(II), Pb(II) and Bi(III), were simultaneously recovered in >90% yields.

For controlling contamination, the purification of silica with nitric acid (see Section 2.3) was effective. Without the purification, the blank values through the whole procedure were 650–1000 ng for Ni, 240–440 ng for Cu, 10–50 ng for Ga, <1 ng for Cd, 75–170 ng for Pb and <0.8 ng for Bi. On the other hand, the blanks were reduced to 54 ± 7 ng for Ni, 57 ± 18 ng for Cu, 2.0 ± 0.2 ng for Ga and 23 ± 2 ng for Pb (mean ± standard deviation, n = 4) by the purification. Considering the detection power of the operated ICP-MS and the 3σ fluctuation in the blanks, the detection limits of the proposed method (in ng mL−1 ) were 0.02 for Ni, 0.05 for Cu, 0.0007 for Ga, 0.001 for Cd, 0.005 for Pb and 0.0008 for Bi. 3.5. Application to fresh water analysis Previously, a powerful multielement preconcentration method, in which coprecipitation and flotation are performed with indium hydroxide, was developed in our laboratory [13]. Although this method can be combined with ICP-atomic emission spectrometry due to the simple emission spectrum of indium, difficulties lie in the combination with ICP-MS, e.g. the indium at the mg mL−1 levels in the final solution would cause serious contamination of the mass analyzer. In contrast, the proposed method gave a “clean” final solution, hence being suitable for ICP-MS. In addition, the proposed method allowed the metal collection under more moderate pH conditions (pH 8–8.5) than the coprecipitation with indium hydroxide (pH 9.5). Therefore, the proposed method was applied to the fresh water analysis by ICP-MS. A water sample (river or pond water) was filtered with 0.45 ␮m Millipore membrane filters. A 1000 mL aliquot of the filtrate was taken for the 100-fold preconcentration by the

Table 3 Analytical results for fresh water samples Filtered samplea

Metals foundb (ng mL−1 ) Ni

Cu

Ga

Cd

Pb

Bi

River water River water IIc

0.25 ± 0.03 0.29 ± 0.01

0.35 ± 0.03 0.53 ± 0.04

0.005 ± 0.001 0.024 ± 0.001

0.058 ± 0.003 0.019 ± 0.001

0.020 ± 0.013 0.073 ± 0.010

0.013 ± 0.004 0.005 ± 0.004

Pond water

0.64 ± 0.03

0.98 ± 0.07

0.024 ± 0.001

0.016 ± 0.002

0.015 ± 0.004

N.D.d

Ic

a b c d

With 0.45 ␮m Millipore membrane filters. Mean ± standard deviation, based on four separate runs. Sampled on separate days. Not detected.

H. Matsumiya et al. / Analytica Chimica Acta 588 (2007) 82–87

admicelle–flotation, followed by ICP-MS, as described in Section 2.6. The filtration and analysis were performed immediately after sampling. As given in Table 3, Ni, Cu, Ga, Cd, Pb and Bi at the fractional ng mL−1 to low pg mL−1 levels were successfully determined. The relative standard deviations (R.S.D.’s) were generally less than 15%. The high R.S.D. values for Pb (river water I) and Bi (river water II) were ascribable to the large fluctuation in the blanks and to the low existing level close to the instrumental detection limit, respectively. When 1.0 ng mL−1 each of the analyte elements was added to the filtrated samples, they were recovered in nearly quantitative yields: >97% for Ni, >89% for Cu, >95% for Ga, >89% for Cd, >94% for Pb and >94% for Bi. This validates the analytical results obtained by the proposed method. The time required for a determination (including admicelle-mediated collection, flotation, desorption and ICP-MS for six elements) was about 40 min. 4. Conclusion The present study showed that the dithizone-impregnated CTAC–silica admicelles were stable over a wide pH region, capable of multielement collection of trace metals and readily floated by bubbling. In addition, the collected metals were desorbed from the admicelles with dilute nitric acid, which allowed the direct combination of the proposed method with ICP-MS. The proposed admicelle–flotation method was suitable for the rapid preconcentration of trace metals in large volume water samples, providing a high preconcentration factor of 100. The practical applicability was demonstrated by the successful determination of trace metals in river and pond waters.

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