1998 The Japan Society for Analytical Chemistry 529 Preconcentration of Trace Cadmium from Seawater Using a Dynamically Coated Column of Quaternary Ammonium Salt on C 18 -Bonded Silica Gel and Determination by Graphite-Furnace Atomic Absorption Spectrometry Kunihiko AKATSUKA*, Yumi YOSHIDA*, Naoki NOBUYAMA*, Suwaru HOSHI*, Seiji NAKAMURA** and Takunori KATO*** *Department of Applied and Environmental Chemistry, Kitami Institute of Technology, Koen-cho, Kitami, 090 8507, Japan **Muroran Institute of Technology, Muroran, 050 8585, Japan ***Hokkaido Institute of Environmental Sciences, Sapporo, 060 0819, Japan A dynamically coated column of C 18-bonded silica gel with methyltricaprylylammonium chloride was used for the selective preconcentration of Cd from water samples under acidic conditions. The complete adsorption of Cd occurred from seawater with no additional reagent, and was found from non-saline waters containing 0.05 5 M HCl. The interfering matrix ions were not retained and pass through the column. The sorbed Cd was eluted with 4 ml of 0.1 M HNO 3 for a subsequent measurement by graphite-furnace AAS. The technique has been successfully applied to seawater and riverwater certified reference materials. The preconcentration process is simple and minimizes the contamination of blanks. The detection limit (3s) is 0.2 ng l 1 of Cd based on a 75-fold preconcentration. Keywords Cadmium, seawater, preconcentration, C 18-bonded silica gel, methyltricaprylylammonium chloride, graphitefurnace atomic absoption spectrometry The determination of trace elements in environmental waters is increasing in contamination studies, owing to the need to guarantee the good quality of water for various purposes. For the determination of Cd in water samples, graphite-furnace atomic absorption spectrometry (GF-AAS) is the chosen method of many analysts because of its relative simplicity, the possibility of automation and the low instrumental detection limits given. However, for complex samples such as seawater, there are important problems: firstly, matrix interference from the salt matrix remains, which causes a deterioration in the accuracy and precision. Secondly, a previous preconcentration of Cd is required, because of the very low concentration of cadmium in the surface seawater. 1 Thus, a different preconcentration procedure, e.g., coprecipitation 2, solvent extraction 1,3, sorption on chelating resins 3,4 or immobilized adsorbents 5-8, have been proposed to preconcentrate and separate cadmium from the seawater matrix. However, most of the procedures involve several chemical stages, including a laborious ph adjustment in order to remove any interfering matrix, thus increasing the possibility of a loss of the analyte and sample contamination. Recently, a column of a reversed-phase substrate loaded with hydrophobic molecules, such as bis(2-ethylhexyl)- hydrogen phosphate and 2-ethylhexyldihydrogen To whom correspondence should be addressed. phosphate 10, tris(2,6-dimethoxyphenyl)phosphine 11 and quaternary ammonium salts 9,12, has been prepared to develop a preconcentration method for trace elements in acidic solutions. These materials were used for the preconcentration of rare-earth elements 10 and Cd 12 in seawater by column techniques. Previously, we 13 proposed a new preparation technique of a dynamically coated column involving C 18 - bonded silica gel (C 18 ) coated with methyltricaprylylammonium chloride (Aliquat 336). The column was applied to the matrix isolation and preconcentration of platinum 13 and zinc 14 in environmental samples. This paper describes a simple and convenient method for the selective preconcentration of trace cadmium from seawater, followed by an atomic absorption spectrometric determination of the element. The precision and accuracy of the method were demonstrated by analyzing certified reference materials. The process of preconcentration is simple, easily yields a large enrichment factor, requires no buffer and results in a low contamination of the blank. Experimental Apparatus Atomic absorption analyses were performed on a Perkin-Elmer (Model 4100ZL) atomic absorption spec-
530 ANALYTICAL SCIENCES JUNE 1998, VOL. 14 trometer with a transversely heated graphite atomizer and a longitudinal Zeeman-effect background correction system. The spectrometer was equipped with a Model AS-70 autosampler. Measurements were performed at 228.8 nm, operated at a slit-width of 0.7 nm. A 175 14 mm glass column fitted with a course sintered glass frit and a Teflon stopcock was used for metal-ion separation and preconcentration. All laboratory manipulations were performed in a class-100 clean room. Reagents Concentrated nitric and hydrochloric acids were purified by sub-boiling distillation in a quartz still. De-ionized, distilled water (DDW) was used for all sample preparations. All other reagents were of reagent-grade or better purity. Acid-resistance C 18 -bonded silica gel, 40 60 micron (Wakogel LP-60C18, Wako Pure Chemical Industries Ltd., Osaka, Japan) was precleaned by sequential additions of 20 ml methanol, 50 ml 1 M HCl and 10 ml methanol gravity fed through the column. The material was then dried at room temperature under a clean laminar-flow bench. Aliquat 336 (methyltricaprylylammonium chloride) was obtained from Aldrich Chemical Co. (Milwaukee, WS, USA). A standard cadmium(ii) solution was prepared by the dilution of a solution of the pure metal (99.999% purity, Wako Pure Chemical Industries Ltd., Osaka, Japan). Serial dilutions were made with DDW in order to prepare working standards. Certified reference materials, CASS-2 (seawater) and SLRS-3 (river water), were obtained from the National Research Council of Canada, Ottawa, Ontario. A coastal seawater sample, collected at Shiretoko (Hokkaido, Japan), was used for trace-metal recovery experiments. A portion of the sample was immediately acidified to ph 1.8 with nitric acid after collection and stored in a high-density polyethylene bottle. Standard procedure The dynamically coated resin was prepared by adding 10 ml of hexane containing 3%(w/v) of Aliquat 336 to 600 mg of C 18 resin. The resin slurry was quantitatively transferred to a glass column and washed with 20 ml of 3% Aliquat 336 in hexane. The resin was removed from the column dried at room temperature and stored in a screw-cap bottle. A new column containing 600 mg of resin was precleaned by passing 20 ml of 1 M HNO 3 through the column. Prior to passage of the sample, the column was conditioned with 15 ml of 0.5 M HCl. Samples were adjusted between 0.1 and 0.5 M HCl with highpurity HCl, and passed through the column at a flow rate of 2 ml min 1. Following passage of the sample, the column was washed free of matrix ions with 5 ml of 0.1 M HCl. The adsorbed trace metals were then eluted from the column using 4 ml of 0.1 M HNO 3. The column was washed with 5 ml of 1 M HNO 3, followed by pre-conditioning with 0.5 M HCl before the next sample. Since seawater contains a large amount of chloride ions, the present method requires no additional reagent for the quantitative retention of Cd onto the column from seawater samples. Thus, for seawater analyses, seawater samples up to 300 ml were pass through the column without any previous addition of HCl to the sample. Blanks for the preconcentration procedures were determined by passing 50 ml of 0.1 M HCl through the column in place of the sample. Results and Discussion It is well known that cadmium(ii) easily forms halocomplexes which are extractable into an organic phase with amine dissolved in various diluents; 15 17 it is generally accepted that the extracted-cadmium(ii) species are tetrahalo-complexes of Cd(II). 16,17 Sawada et al. 12 reported on the application of a solvent-extraction system to the column-extraction preconcentration of cadmium. The column was prepared by mixing the support (i.e., Chromosorb W-DMCS) with a 1%(w/v) trioctylmethylammonium chloride solution in xylene, and excess xylene was expelled from the column by injecting 0.1 M HCl prior to passage of the sample. A quantitative recovery of Cd was reported from solutions of seawater acidified to 0.1 M HCl. However, the column could not be used repeatedly. For the present resin, prepared with a combination of C 18 and 3%(w/v) Aliquot 336 in hexane, a complete uptake of Cd was achieved from a solution of seawater with no additional HCl, and the column could be used repeatedly. Effect of the HCl concentration on the adsorption of Cd The effect of the hydrochloric acid concentration on the adsorption of Cd was studied using 50 ml of acidified samples. The samples were passed through a column of C 18 coated with Aliquat 336 at a flow-rate of 2 ml min 1 ; and the effluent was then analyzed by GF- AAS. As shown in Fig.1, complete uptake of a 2 µg l 1 Cd solution occurred at between 0.05 and 5.0 M HCl acid concentrations; concentrations greater than 5.0 M were not studied. The present column allowed the selective separation of cadmium from major matrices of environmental water samples. Alkali and alkaline-earth elements, Al, Mn, Cr, Co and Cu were not retained on the column, and the recovery efficiency of V, Pb and Fe were minimized from acidified solutions of 0.1 0.5 M HCl. Therefore, these elements can be eliminated by washing the column with 0.1 M HCl prior to the elution of cadmium. Capacity measurement The capacity of the dynamically coated resin (0.25 g) was determined using 50 ml of 50 mg l 1 Cd(II) in 0.5 M HCl and equilibrating for 5 d. The resin was filtered and rinsed with 0.5 M HCl. The filtrate and rinsing
531 ions interfered with the metal uptake for subsequent preconcentrations. Alternatively, DDW could be used to quantitatively elute the metals, although, after several weeks of use, the capacity of the column decreased, possibly due to a dissolution of the Aliquat 336 in the DDW. Thus, the present method precluded any passage of DDW and perchloric acid through the column. The effect of the flow-rate of a sample solution on the recovery of the metal was studied by varying the flowrate from 1 to 7 ml min 1. A quantitative recovery was obtained at a flow-rate lower than 3 ml min 1. Fig. 1 Effect of the HCl concentration on Cd adsorption with the C 18-Aliquat 336 column ( ), and with the untreated C 18 column ( ). solution were collected and diluted to 100 ml. The concentration of Cd(II) was determined. The loading capacity of the resin using a batch procedure was determined to be 20 µmol Cd per g of the resin from 0.5 M hydrochloric acid solutions. This result did not change after storage of the resin for 3 years. Elution profiles The elution profiles for Cd are given in Fig. 2, which shows the relative concentration of the meal in each 0.25-ml portion of the resulting effluent. Passage of 3.7 ml of 0.1 M HNO 3 eluting acid was sufficient to elute the metal from the column. Under harder eluting conditions, complete recovery was also obtained using 2 ml of 1 M HNO 3 eluting acid. In this study, a solution of 0.1 M HNO 3 was employed for the elution of Cd. Although attempts to use 0.05 M HClO 4 as an eluting acid were successful, the presence of perchlorate Recovery of Cd from seawater The recovery of spikes added to a seawater sample is given in Table 1. The recovery test was carried out using 100 ml of seawater with and without the addition of HCl. The percentage of Cd recovery was calculated from the amounts of metal spikes into the original sample, and those found in the eluate from which the amounts of Cd for the un-spiked sample were subtracted. The recovery of Cd was quantitative under the examined conditions. Therefore, the major elements in seawater, such as sodium, potassium, magnesium, calcium and sulfate ions, have no influence on the recovery of Cd. The seawater samples are generally acidified to around ph 1.5 with nitric acid after collection and being stored in bottles. The effect of HNO 3 on the recovery efficiency of Cd from seawater was studied using 100 ml of spiked seawater containing 0.1 M HCl. The results are shown in Fig. 3. Although nitric acid did not interfere with the retention of cadmium onto the column at concentrations up to 0.3 M, Cd recovery began to decrease as the HNO 3 concentration increased. Effect of sample volume Further recovery tests showed a complete uptake of Cd from a sample volume of up to 500 ml of non-saline solution acidified to 0.1 M hydrochloric acid. Examinations on sample volumes larger than 500 ml were not made. The present method was applied to various volumes (90 325 ml) of seawater samples, which were previously acidified to ph 1.8 with nitric acid. The process was carried out without any addition of hydrochloric acid to the seawater sample, prior to passage through Table 1 Recovery efficiencies of Cd from seawater samples with and without the addition of hydrochloric acid HCl concentration in seawater a / mol dm 3 Cd recovery b, % Fig. 2 Elution of cadmium retained on a C 18-Aliquat 336 column by nitric acid: 0.1 mol dm 3 HNO 3 ( ); 1 mol dm 3 HNO 3 ( ). 0 100±2 0.1 100±3 0.5 99±3 1.0 100±3 a. High-purity HCl was added to 100 ml of seawater sample. b. Mean and standard deviation of three determinations.
532 ANALYTICAL SCIENCES JUNE 1998, VOL. 14 Table 2 Analysis of the Coastal Seawater and the Riverine Water reference materials Sample Sample taken/ ml Determined/ mg l 1 Certified value/ mg l 1 CASS-2 a 100 (n=6) 0.019±0.001 c 0.019±0.004 d SLRS-3 b 100 (n=5) 0.0125±0.0008 0.013±0.002 a. Coastal Seawater reference material: the salinity is 2.9%. b. Riverine Water reference material. c. Blank value is subtracted. Precision expressed as the standard deviation of 5 6 replicates. d. Precision expressed as the 95% confidence interval. Fig. 3 Effect of nitric acid on the recovery of Cd from seawater containing 0.1 mol dm 3 hydrochloric acid. the column. As shown in Fig. 4, the recovery yields of the element were essentially independent of the sample volumes. The Cd concentration of the sample could be calculated from the slope of the relationship between the sample volume and the total amounts of the element if the recovery efficiency was 100%. The result obtained by this procedure was 0.047 ng ml 1 Cd. The value was in satisfactory agreement with that obtained by isotope dilution ICP mass spectrometry after preconcentration with a column of silica-immobilized 8- hydroxyquinoline 18, i.e. 0.0465±0.0013 ng ml 1. These data indicate that the recovery efficiency of the present method is 100% from seawater samples up to 325 ml. Although larger volumes than that were not tested, a quantitative recovery should be obtained at least up to 500 ml of seawater. Analytical blank and detection limit The absolute Cd blank was determined to be 0.06 ±0.02 ng based on 300 ml of DDW acidified with 5 ml of high-purity HCl. This corresponds to limits of detection of 0.2 ng l 1 for Cd, defined by three-times the standard deviation of the blank. Blanks were also prepared from 50-ml and 100-ml volumes of acidified DDW. There was no significant difference between the 50-ml and 300-ml volume of acidified DDW blanks. The impurity in HCl used for the blank test was found to be <0.2 pg ml 1 Cd by analyzing separately with isotope dilution ICP mass spectrometry; thus, the contamination of Cd from the reagent used was negligible. Application to seawater and non-saline water samples The results for analyses of coastal seawater CRM CASS-2 are given in Table 2. An analysis of the samples was performed using GFAAS and a calibration method by spiking the standard solution of Cd to an aliquot of the concentrate, thereby obtaining an exact matrix match. In order to study the accuracy and applicability of the method, the river water CRM SLRS-3 was analyzed. An aliquot of the river-water sample was adjusted to 0.1 M HCl by adding high-purity HCl; then, the samples were processed by passage through the column. The results for analyses of SLRS- 3 are also given in Table 2. As can be seen in Table 2, the results of Cd for both samples are in satisfactory agreement with the certified values. The precision of the analysis, expressed as a relative standard deviation, was 5.3% (n=6) for CASS-2 and 1.8% (n=5) for SLRS- 3 samples. The use of GF-AAS in combination with this column has provided a simple, rapid and accurate technique for the determination of trace Cd in water and seawater samples. This preconcentration method requires very few reagents, permits the isolation of the analyte from the matrix and results in a low sample blank. The authors thank Drs. J. W. McLaren and S. N. Willie (National Research Council of Canada, Ottawa) for helpful comments with the preparation of this manuscript. Fig. 4 Absolute amount of Cd found in a coastal-seawater sample collected at Shiretoko seashore (Hokkaido, Japan) as a function of the sample volume. The slope of this plot gives the concentration of Cd in the sample. References 1. M. Murozumi, Bunseki Kagaku, 30, S19 (1981).
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