738
Anal. Chem. 1981, 53, 738-739
Preconcentration of Trace Elements in Seawater by Complexation with 8-Hydroxyquinoline and Adsorption on C18 Bonded Silica Gel Hiroto Watanabe Department of Applied Chemistry, Muroran Institute of Technology, Muroran, Japan 050
Katsumi Goto and Shigeru Taguchi Faculty of Science, Toyama University, Toyama, Japan 930
J. W. McLaren,” S. S. Berman, and D. S. Russell Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
Attention has recently been focused on the application of inductively coupled plasma atomic emission spectrometry (ICP-AES) to seawater analysis ( I ) . A chemical separation of the trace elements prior to analysis serves the dual purpose of increasing their concentrations to levels at which they can be reliably determined and of removing them from the sea salt matrix. One of the most popular preconcentration methods involves the extraction of metal ions with pyrrolidine N-carbodithioate (APDC) into methyl isobutyl ketone (MIBK) (2). Another is the use of a chelating ion exchange resin, Chelex-100 ( I , 3). Although both techniques have been successfully employed in seawater analysis by ICP-AES, each has serious limitations. Concentration factors conveniently attainable by the APDC-MIBK method (20- to 30-fold) are insufficient for the ICP-AES determination of several metals of great environmental interest, particularly cadmium and lead. Although the Chelex-100 separation allows higher concentration factors than does APDC-MIBK extraction, the method is rather time consuming and the removal of calcium and magnesium prior to elution of the trace metals requires careful washing procedures. In an attempt to overcome these limitations we have developed a new method which involves complexation of the metal ions with 8-hydroxyquinoline followed by adsorption on c18 chemically bonded silica gel. Trace metal preconcentration from aqueous solution by a combination of chelation by 8-hydroxyquinoline with subsequent adsorption on activated carbon has been reported ( 4 ) . Although cl8 chemically bonded silica has been used previously for separation of metal chelates (5, 6), the present application to preconcentration of trace metals from aqueous solution is novel. Rapid and quantitative recovery of several metals from 1-L seawater samples has been demonstrated. This new method appears to offer a faster and more effective route to seawater analysis by ICP-AES. EXPERIMENTAL SECTION Reagents. C18 chemically bonded silica gel (Bondapak Porasil B, Waters Associates, Milford,MA) was used without futher purification. All other reagents were purified as previously reported (2, 7). The coastal Atlantic seawater sample had been filtered through a 0.45-fim membrane filter, acidified to pH 1.6 with nitric acid, and stored in a polypropylene bottle cleaned as previously described (7). Apparatus. A Varian AA 775 atomic absorption spectrometer and a custom ICP-echellespectrometer were employed. The latter has been fully described in previous publications ( I , 8). Procedure. Sample preparation was carried out in a clean laboratory equipped with laminar flow benches and fume cupboards, providing a class 100 working environment. A borosilicate glass column (1cm i.d.) fitted with a fine glass frit was slurry loaded with 0.8 mL of the C18bonded silica gel. A 5-mL portion of methanol was passed through the column which was then washed with water. A 1-L seawater sample, to which 5 mL of 1% 8-hydroxyquinolinesolution was added, was adjusted 0003-2700/81/0353-0738$01.25/0
Table I. Effect of Sample Volume and Flow Rate on the Recoveries of Copper and Manganese from Spiked Seawater flow rate,
sample element cu
Mn
v01,
mL
bed height, min-’ mm mL
pH
500 500
8.0
500 500
8.0 8.0
1000
8.0
1000 500 500 1000
8.9 8.9 8.9 8.9
8.0
7 14 20 40 7 26 7 26 26
%
recovery
15 10
102
8
103 101
5 15
99 109
8
110
15
99 98
8 8
102
to pH 8.9 by the addition of 2 M ammonium hydroxide solution and passed through the column at a rate of 10-40 mL mi&. (During these preliminary investigations, a gravity-feed system was used and the flow rate controlled by the column stopcock.) The column was then washed with 25-50 mL of a washing solution prepared as described below. The metal chelates of 8-hydroxyquinoline were then eluted from the column with 5 mL of methanol purified by sub-boiling distillation. The eluate was collected in a Vycor glass crucible containing 0.5 mL of 10 M hydrochloric acid and was evaporated to dryness. The residue was decomposed by a second evaporation after addition of nitric acid (0.5 mL) and perchloric acid (0.5 mL). The residue was dissolved and diluted to 5 mL with 0.1 M nitric acid. This procedure provides a theoretical concentration factor of 200. The washing solution was prepared by adding 5 mL of 1% 8-hydroxyquinolineto 1 L of deionized distilled water, adjusting its pH to 8.9, and passing it through the column. Subsequent elution of the column with methanol and treatment of the residue as described above for seawater yielded the blank. RESULTS AND DISCUSSION Recovery of Copper(I1) a n d Manganese(I1) from Seawater. Before attempting analysis of an unspiked seawater sample, a series of experiments was performed to assess the recovery of copper and manganese at about 10-20 times higher than normal concentrations in coastal seawater as a function of experimental conditions. Copper and manganese were chosen because they represent a spectrum of metals complexed by 8-hydroxyquinoline, copper forming a very strong complex (log KIKz = 25.4) and manganese the weakest of the elements of interest (log KIKz = 17.5). Copper and manganese spikes (10 and 9 pg, respectively) were added to 500- or 1000-mL seawater samples. After the eluate had been evaporated to dryness, the residue was dissolved in 0.5 mL of 10 M hydrochloric acid and diluted to 10 mL for flame atomic absorption spectrometric analysis. The recoveries of copper and manganese under various conditions of sample preparation were estimated by comparing the sample absorbances to those of copper and manganese standard solutions. 0 1981 American Chemical Society
Anal. Chem. 1981, 53, 739-740
Table 11. Analysis of a Coastal Seawater Sample (Results in bg L - l ) this method -
Cd Zn Pb Ni Mn Fe Cu
blank
seawater
