ANALYTICAL SCIENCES DECEMBER 2018, VOL. 34 1445 2018 © The Japan Society for Analytical Chemistry Notes Micro-organic Ion-associate Phase Extraction/micro-volume Back-extraction for the Preconcentration and GF-AAS Determination of Cadmium, Nickel and Lead in Environmental Water Syeda Mushahida-AL-NOOR, Ryo MURASHIMA, Takuya OKAZAKI, Shigeru TAGUCHI, Hideki KURAMITZ, and Noriko HATA† Department of Environmental Biology and Chemistry, Graduate School of Science and Engineering (Sci. Div.), University of Toyama, Gofuku 3190, Toyama 930–8555, Japan Micro-organic ion-associate phase (IAP) extraction was combined with a micro-volume back-extraction (MVBE) to reduce coexisting components and viscosity in the concentrates. Heavy metals were converted into a complex with 2-(5-bromo- 2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol in a 40-mL sample solution, and were extracted into ion associates. After centrifugation and discarding the aqueous phase, trace metals were stripped from IAP into a nitric acid solution, followed by GF-AAS determination. Only one vessel was required for 400-fold enrichment. The detection limits (3σb) for Cd, Ni, and Pb were 0.6, 3.7, and 0.8 ng/L, respectively. This method was applied in recovery tests in seawater. Keywords Ion-associate phase, micro-volume back-extraction, GF-AAS (Received June 25, 2018; Accepted August 27, 2018; Advance Publication Released Online by J-STAGE September 7, 2018) river water and seawater,17 and the liquid electrode plasma Introduction emission spectrometry (LEP-AES) determination of trace metals in waste water.18 To date, the separation of analytes from other interfering In the present work, to avoid the use of organic solvent for components during a measurement and a further increase of the extraction and to reduce coexisting components in the sensitivity via enrichment are still important, despite an concentrate, IAP extraction was combined with a micro-volume improved sensitivity of analytical instruments. The value for back-extraction technique (MVBE) with acids. Unlike liquid– the Pb concentration in WHO guidelines for drinking water liquid extraction/MVBE,2–4 this preconcentration/separation quality1 is a provisional value because the minimum measurable technique does not use any organic solvent. The acids rapidly value is greater than the guideline level. stripped targeted trace metals from IAP by sonication. It was a Different preconcentration/separation methods are employed convenient combination for separating and enriching cadmium, for quantifying trace heavy metals in water, such as liquid–liquid nickel, and lead in seawater. These metals are toxic, which have extraction,2–4 coprecipitation,5 solid-phase extraction,6,7 cloud the WHO guideline value.1 Cadmium is probably carcinogenic point extraction,8,9 dispersive liquid–liquid extraction,10 to humans (Group 2A) and nickel is carcinogenic to humans thermoresponsive polymer-mediated extraction,11 homogeneous (Group 1).1 The minimum measurable values of these metals liquid–liquid extraction,12 and enrichment of the membrane are greater than the concentration of most environmental water, filter.13 The above-mentioned separation/preconcentration and the measurement of these metals is sometimes affected by techniques comprise the use of xylene,2,4 diisobutylketone,3 the matrix, such as sea salt, and then separation and lanthanum phosphate,5 dependence on equilibration temperature preconcentration are necessary. Different experimental and time,8,9 the use of trichloroethylene as an extraction parameters, such as the pH for chelate formation, the solvent,10 the application of heat,11 or the use of a fluorine-based concentration of acid, and the sonication time for MVBE were surfactant.12 The metals collected on a membrane filter were optimized. determined by energy-dispersive X-ray fluorescence (EDXRF) spectrometry.13 These methods are attainable higher enrichment. IAP extraction14–18 is a unique technique for separating, Experimental enriching, and determining traces in water. This method was already applied for the separation and spectrophotometric Reagents and chemicals determination of ammonia in river water,14 nitrite in river water Standard solutions (1000 mg/L) of Cd2+, Ni2+, and Pb2+ were and seawater,15 the HPLC determination of di(2-ethylhexyl)- purchased from Wako Pure Chemical, Japan. A tetramethyl- phthalate in river water,16 the GF-AAS determination of Cd in ammonium hydroxide (TMAH) solution was diluted with a 25% TMAH solution (TAMAPURE AA grade, Tama Chemicals, † To whom correspondence should be addressed. Japan) to 12.5% with water. A 2-(5-bromo-2-pyridylazo)-5-(N- E-mail: [email protected] propyl-N-sulfopropylamino)phenol disodium salt (5-Br-PAPS) 1446 ANALYTICAL SCIENCES DECEMBER 2018, VOL. 34 (0.4%) solution was dissolved 0.4 g of 5-Br-PAPS (Dojindo, Japan) in 100 mL of water. A phenolsulfonate (PS–, 0.5 mol/L) solution was dissolved 11.6 g of sodium p-hydroxybenzesulfonate dihydrate (Kanto Chemical, Japan) in 100 mL of water, which was then purified by the addition of a 0.1% PAN {1-(2-pyridylazo)-2-naphthol} ethanol solution, followed by solid-phase extraction with Inertsep-C18 cartridges (GL Sciences, Japan). To prepare a benzethonium (Ben+, 0.1 mol/L) solution, 4.5 g of N-benzyl-N,N-dimethyl-2-{2-[4-(2,4,4- trimethylpentan-2-yl)phenoxy]ethoxy}ethanammonium chloride (Kanto Chemical, Japan) was dissolved in 100 mL of water. An ammonium dihydrogen phosphate (0.5%) solution dissolved 0.5 g of ammonium dihydrogen phosphate (Wako, Japan) in 100 mL of nitric acid (2 mol/L). A mixed cellulose ester membrane filter (0.45 μm pore) (Advantec, Japan) was used to filter the environmental water. All reagents were of analytical Fig. 1 Effect of the pH on the relative absorbance of trace metals. grade and water was obtained by using a Direct-Q 3UV The relative absorbance was calculated from the absorbance found at different pH values divided by the highest absorbance found at (Millipore). pH ca. 9. Apparatus Measurements of trace metals were performed using a Hitachi (Japan) Model Z-5010 polarized Zeeman GF-AAS. Centrifugation was performed using a Kubota (Japan) centrifuge Results and Discussion (Type 5420). A measurement of the pH was performed using a Horiba (Japan) Type D-53 pH/ion meter. For the entire sample Chelate formation in IAP extraction/MVBE preparation process, plastic centrifuge tubes (50 mL) were used. Figure 1 shows the effect of the pH on chelate formation. A Toshiba (Japan) home-use microwave oven (Model ER-250) In this study, a pH adjustment was performed by nitric acid, 19 with 500 W output power was used. ASU-10 CLEANER (AS- TMAH, and a phenolsulfonate solution (pKa = 8.56). The ONE, Japan) was used for sonication. optimum pH for the extraction of the metal/5-Br-PAPS complex in IAP of benzethonium and phenolsulfonate ion ranged over Procedure 8.5 – 9.5. At a high alkaline pH (≥10), the relative absorbances An acidified sample solution (40 mL) containing 0.02 M of the metals gradually decreased again because the acidity for nitric acid was taken into a centrifuge tube. As a chelating MVBE became insufficient. IAP was not observed for a high reagent, 0.4 mL of a 5-Br-PAPS solution and as both a alkaline pH (≥12). The effect of the pH on chelation formation component of the pH buffer and the organic anion 1.6 mL of a is considered to be almost the same as in previous studies.17,18 phenolsulfonate solution were added to the sample. The pH of However, at pH 10 or higher, the IAP extraction/MVBE the solution was adjusted to be about 9 using the TMAH technique was affected by the alkali component, it seems that solution. Then, 0.2 mL of the benzethonium solution as the the alkaline component attached to the IAP lowered the acidity organic cation was added to the operating solution. The solution of the nitric acid solution and interfered with the back extraction. was centrifuged at 3760g-force (4500 rpm) for 15 min. After For further experiments, pH 9.0 ± 0.5 was selected. centrifugation, the aqueous phase was discarded and a highly 5-Br-PAPS was chosen as a chelating reagent, as reported viscous IAP was remained at the bottom of the tube. The IAP concerning a previous study.17,18 The effect of the chelating was dried by microwave irradiation for 20 s for one tube. For reagent on the relative intensities of the metals was investigated back-extraction from the IAP, 75 μL of 2 mol/L HNO3 was by increasing the concentration of 0.4% 5-Br-PAPS from 0.1 to added upon the dried IAP, and sonication was performed for 0.8 mL (from 0.02 to 0.15 mmol/L). The chelating reagent 30 s. After the MVBE of the IAP, 25 μL of an ammonium solutions of 0.3 mL or less were not sufficient for complex dihydrogen phosphate solution was added as a chemical formation with trace metals. Quantitative complex formation modifier. The concentrate (10 μL portion) was injected into a was obtained under 0.4 mL or more of a 5-Br-PAPS solution. cuvette and the absorption was measured by GF-AAS at 228.8, Therefore, 0.4 mL (40 mL sample) was chosen for this 232, and 283.3 nm wavelengths for cadmium, nickel, and lead, experiment because the absorbance of trace metals was high, respectively. and the chelating reagent is expensive. Environmental water samples were filtered through a membrane filter to remove any suspended solids. Seawater Micro-organic ion-associate phase (IAP) formation samples were spiked with a standard solution, which was done In the present study, as an organic anion to form IAP, the by following the procedure mentioned above. After discarding phenolsulfonate anion was recommended because it served as a the aqueous phase, an additional cleanup step was performed to pH buffer component in previous studies.17,18 As an organic eliminate any interference from sea salt by washing the ion- cation to form IAP, the effect of the benzethonium cation’s associate by the supernatant liquid obtained by the centrifugation concentration was studied ranged from 0.1 to 2 mmol/L.