Anal. Chem. 1981, 53, 2337-2340
2337
Preconcenlration of Trace Elements from Seawater with Silica-Immobilized 8-H yd r oxyquino1ine R. E. Sturgeon," S. S. Berman, S. N. Wlllie, and J. A.
H. Desaulnlers
Division of Chemistty, National Research Council of Canada, Montreal Road, Ottawa, Ontario, Canada K I A OR9
8-Hydroxyquinoline immobilized on sillca gel (1-8-HOQ) was prepared and used foir the preconcentration of Cd, Pb, Zn, Cu, Fe, Mn, NI, and Co from seawater prlor to thelr determlnatlon by graphlte furnace atomic absorption spectrometry. An I8-HOQ column technique permitted large enrlchment factors to be attalned whlle provldlng rapid processing of large volume samples, quantltatlve recovery of these elements, and a matrix free concentrate suitable for Instrumental analysis. The preclsion and accuracy of this approach are demonstrated by the analysis of two samples of near shore and one open ocean seawater.
The use of chromatographic techniques (including gas chromatography, ion-exchange and reversed-phase extraction, and adsorption chromatography) for metal analyses has substantially increased in recent years (1). In particular, metal chelating resins and immobilized (adsorbed or chemically bonded) chelates have found widespread application for the concentration and/or rreparation of trace metals from a variety of matrices (1-22). lDimethylglyoxime ( 2 ) ,alkylamines, diamines, xanthates, and dithiocarbamates (3-6), propylenediamenetetraacetic acid (7), rz-butylamides (8), N-substituted hydroxylamine (9), hexylthioglycolate (IO),ferroin-type chelating agents ( I I ) , iminodiacetate (Rio-Rad Laboratories, Richmond, CA), amidoxime (12), dithizone (13, 141, and 8hydroxyquinoline (14-21) have all been immobilized on various substrates. Tailoring chemically bonded chelating agents to specific needs allows the use of selective or general concentration schemes and also permits the "recycling" of the chelating agent. Their major use lies in the preconcentration of trace metal ions from aqueous and saline media. The iminodiacetato containing resin, Chelex-100, is the rnost commonly employed chelating resin for the removal and preconcentration of trace heavy metals from seawater (22-25). However, the technique is rather time-consuming (sample flow rates 1-2 mLJmin) and suffers from the fact that removal of Ca and Mg from the resin prior to elution of trace metals requires careful washing procedures (24, 25). 8.Hydroxyquinoliine is a well-characterized reagent that reacts with over 60 metal ions to form complexes whose aqueous phase formation constants range from about lo4 (Ba2+)to (Fe3+) Advantage has been taken of the differences in formation constants between the transition metals and alkali and alkaline earth elements to effect separations of trace metals from high-purity water and lake water (3),high ionic strength samples (19, 21), and seawater (18) using an 8-hydroxyquinoline immobilized onto glass beads. The application of silica immobilized 8-hydroxyquinoline (1-8-HOQ) to the concentration and determination of Cd, Pb, Zn, Cu, Fe, Mn, Ni, and Co in seawater is reported here. EXPERIMENTAL S E C T I O N Instrumentation. A Varian Techtron atomic absorption spectrometer, Model AA-5, fitted with a Perkin-Elmer HGA-2200 graphite furnace and temperature ramp accessory was used for all atomic absorption ineasurements. Simultaneous background corrections were made by using a deuterium lamp. Sample so-
lutions were delivered to the furnace in either 10- or 20-pL volumes using a Perkin-Elmer AS-1 autosampler, and absorbance peaks were simultaneously recorded on a fast response Speed Servo I1 strip-chart recorder (Esterline Corp.) and a digital storage oscilloscope (Gould Advance). Samples were held in acid-washed polypropylene cups prior to injection. Pyrolytic graphite coated furnace tubes were used exclusively. Reagents. All reagents were purified prior to use. Concentrated nitric and hydrochloric acids were prepared by subboiling distillation in a quartz still using reagent grade feedstocks (26). A saturated solution of ammonium hydroxide (28%) was prepared by isothermal distillation according to the procedure recommended by Zief and Horvath (27). Stock standard solutions of the elements of interest were prepared by dilution of solutions of the pure metals or their salts. Serial dilutions were made with high-purity distilled, deionized water (DDW) in order to prepare working standards. 8-Hydroxyquinoline was purified by vacuum sublimation at 120 "C onto a cold finger. Silica gel (Waters Associates Inc., Milford, MA; LC Porasil B, 37-15 pm) was extensively hot leached in aqua regia, concentrated nitric acid, and concentrated hydrochloric acid for several days and washed with DDW prior to use. Triethylamine (Aldrich) and 3-aminopropyltriethoxysilane (Aldrich) were used as received. Coastal seawater samples were obtained from the Atlantic Research Laboratory of the National Research Council of Canada, Halifax, Nova Scotia. The samples had a salinity of 29.5% and were filtered through a precleaned, nominally 0.45 pm membrane filter, acidified to pH 1.6 (with "OB) and stored in precleaned polypropylene bottles. Open ocean seawater was collected at a depth of 1500 m using Go-Flo samplers and a stainless steel hydrowire. The samples were taken from a site roughly 13 km south east of Bermuda (PanulirusStation) and had a salinity of 35.0%. Two-liter aliquots of this homogeneous water sample were stored in precleaned polyethylene bottles and acidified to pH 1.6 (using high-purity "OB) for storage. Procedures. All sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cupboards providing a class 100 working environment. Porasil I-8-HOQwas prepared using Hill's procedure (19). No difficulties were encountered with the synthesis provided sufficient solvent volume was maintained throughout the multistep sequence. As noted by Jezorek and Freiser (20),this procedure yields a phenylazo-8-hydroxyquinoline. Following several cleanings in 1.0 N HC1,600 mg of the product was slurry loaded into a glass column supported by a coarse sintered glass frit. The column assembly is shown in Figure 1 and consists of a l-L polypropylene separatory funnel (serving as a sample reservoir) connected by a short piece of Bev-A-LineIV tubing (Cole Parmer Instrument Co., Chicago, IL) to a borosilicate glass column (dimensions given i s Figure 1). Prior to use, the entire assembly was carefully precleaned by gravity feed of 200 mL of a solution consisting of 1 N HCl/O.l N "OB through the column. The column bed was then washed free of excess acid using DDW. Aliquots of seawater (500 mL for coastal samples, 900 mL for open ocean samples) were adjusted to pH 8.0 i 0.2 using highpurity ammonium hydroxide and drawn through the column (using water aspirator vacuum) at a nominal flow rate of about 15 mL/min. Following passage of the sample, the column was washed free of seawater using two 10-mL aliquots of pH 7.0 DDW, allowing each to gravity drain. The sequestered trace metals were then eluted from the column using 10.0 mL of a solution consisting
0003-2700/81/0353-2337$01.25/00 1981 American Chemical Society
2338
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
VOLUME OF EFFLUENT, mL
Figure 2.
Elution characteristics of trace elements from Id-HOQ using
1 N HCVO.1 N
600 mg silicaImmobilized
Coarse sintered
Figure 1. Preconcentration apparatus.
of 2 N HCl/O.l N “0% This concentrate was stored in a 30-mL screw-capped polypropylene bottle prior t o analysis for all elements except Fe. A similar but separate sample workup at pH 3.0 was required for Fe as a result of relatively large Fe blanks obtained when preconcentration was made at pH 8.0. The column was then cleaned by further passage of 25 mL of the elution acid. Following a wash with DDW, to remove excess acid, the column was ready for the next sample aliquot. Blanks were also carried through this procedure. Initially, blank runs consisted of 20U-mL aliquots of DDW at pH 1.6 (acidified with HN03 to simulate the seawater treatment). The DDW was then adjusted to pH 8.0 (3.0 for Fe analysis) and passed through the column. The trace metals were eluted as for the seawater sample. This procedure was compared to a simpler one involving only the acid elution of a cleaned column. Analyses of both blanks yielded identical levels of all trace metals, indicating that, except for Fe, the column was not a significant source of metal contamination for sample preconcentrationat pH 8.0. Similarly, for pH 3.0 runs, no significant contaminationarose from the column, even for Fe. Subsequent blanks were therefore obtained simply by passage of 10 mL of the elution acid through the column. Three such blanks were prepared for each analytical kun. Analysis of blanks and concentrates was accomplished by calibration against a spike aliquot of concentrate, thereby effecting an identical matrix match. Furnace conditions used for the atomization of samples were those recommended by the manufacturer. RESULTS AND DISCUSSION Silica immobilized 8-hydroxyquinoline is stable over a wide pH range of pH &9 (19),is inert to chloroform,benzene, acetic acid, and acetone (191,and functions well in saline media (15, 18,21). The chelation characteristics of I-8-HOQ are similar to those obtained by using 8-hydroxyquinolinein liquid-liquid extractions (15). However, the relatively low exchange capacity exhibited by this material (9, 15,20) limits its use to separations involving trace or low level concentrations. The exchange capacity of the material synthesized for this study was determined both by batch equilibration and column “breakthrough” techniques (15,20) using Cu as the indicating species. The freshly prepared material, precleaned by acid stripping, exhibited an exchange capacity of 0.061 mmol g-l, similar in magnitude to that reported previously for this
“03.
material (9,15,20). The capacity of I-8-HOQ was rechecked after about 3 months of continuous operation during which time approximately 80-100 samples had been processed, representing the passage of 60-70 L of seawater through the column. The exchange capacity of this used material was reduced to 0.040 mmol g-l. Extended operation of the column at high pH results in hydrolysis of the silica substrate and cleavage of the bonded phase, thereby reducing exchange capacity. As a result of sighificant hydrolysis above pH 9, the bonded phase “bleeds” from the column to yield a pink effluent. In light of the extremely minute amounts of trace elementa in seawater, the reiatively small exchange capacity of the I-8-HOQ column does not present a problem, even for the processing of several liters of sample. Substantial preconcentration factors can therefore be realized, particularly if the acid concentrates are further concentrated by simple evaporation. Average levels of Na, Ca, and Mg in the sample concentrates were found to be 17 f 2, 13 f 2, and 65 f 1 pg/mL, respectively. These levels can be substantially reduced by washing the column with pH 6 DDW thereby making the concentrates essentially matrix free and suitable for analyses by a wide range of instrumental techniques (e.g., isotope dilution spark source mass spectrometry). Recovery Efficiency. Elution of the sequestered metals from the column bed was effected by using a solution consisting of 1 N HCl and 0.1 N ”OB. Nitric acid alone (0.1 N) was not effective in eluting Cu and Fe from the column. Similarly, hydrochloric acid alone (1 N) did not allow quantitative recovery of Cu. Quantitative elution of Fe, Co, and P b required a minimum of 1N HC1 in the presence of 0.1 N “OB. AU trace metals of interest could be quantitatively recovered from the column using 10 mL of the above acid mixture. Elution curves for individual elements are given in Figure 2. The first milliliter of effldent apparently represents the dead volume of the column, as no metals are detected. The recovery of spikes added to DDW and seawater samples is shown in Table I. With the exception of Co, all elements of interest were quantitatively recovered. Whereas the recovery of Fe and Cu did not change as the pH of sample loading was lowered to 3.0, that for Ni dropped to 65% and Zn dropped to 30%, and no Cd, Co, and Mn were recovered. The large formation constants for chelate formation by Fe and Cu ensure their recovery even at pH 3. Although samples were usually drawn through the column at a flow rate of approximately 15 mL m i d , recovery of spikes for all elements but Mn remained unchanged at flow rates up to 80 mL min-l. Recovery of Mn decreased monotonicallywith flow rates above 20 mL min-l, dropping to about 55% a t 80 mL mi+. Analytical Blanks. Absolute blanks are presented in Table 11. With the exception of Fe, these data refer to sample
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
Table I. Recovery of Trace Metals from Deionized Water and Seawater recovery, % seawater
element
DDW
Cd
1061 8 1001 8 98r 3 93 i: 2 98r 3
Pb Zn cu Fe Mn Ni co
94 r 921 94 r 96 t 96r
Table 111. Analyses of Near-Shore Seawater (Salinity 29.5 01,) concentration, ng/mL sample 1 sample 2 eleaccepted accepted ment I-8-HOQ value I-8-HOQ value
(I
5 10 9 12
Cd
10 115i: 5
101 i 7 9 l t 2 80t 4
0.020t 0.001 0.22r
Pb
0.01
96 i: 8
Zn
74 r 1
Mean and standard deviation of eight determinations, salinity 35.0 o/-.
blank
element
Cd Pb Zn cu
