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Anal. Chem. 1002, 64, 737-743 (3) Mottola, H. A. Kinetlc Aspects of Analyticel Chemistry; Wiley: New York, 1988. (4) Mark, H. B., Jr.; Rechnltz, 0.A. KineNcs in Analyticel Chemistry; Intersclence: New York, 1966. (5) Rklder, G. M.; Margerum, D. W. Anal, Chem. 1977, 49, 2090. (6) Brown, P. B.; Lewis, K. 0. Ann. Clin. Biochem. 1980, 77, 192. (7) Weiser, W. E.; Pardue, H. L. Anal. Chem. 1986, 58, 2523. (8) Harner, R. S.; Pardue, H. L. Anal. Chlm. Acta 1881, 127, 23. (9) Abe, S.; alto, T.; Suda, M. Anal. Chim. Acfa 1986, 787, 203. (10) Ballesteros, L.; Perez-Bendlto, D. Anal. Chlm. Acta 1986, 782, 213. (11) Eaton, D. F. Pure Appl. Chem., 1980. 62, 1631. (12) Drew, J.; Szabo, A. 0.; Morand, P.; Smith, T. A.; Ohiggino, K. P. J. Chem. Sm., Faraday Trans. 2 , 1900, 86, 3853. (13) Bajzer, Z.;Sharp, J. C.; Sedarous, S. S.; Prendergast, F. G. Eur. Biophys. J. 1900, 78, 101. (14) Schechter. I. Anal. Chem. 1991, 63. 1303. (15) Larsson, J. A.; Pardue, H. L. Anal. Chlm. Acta 1889, 224. 289.
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RECEIVED for review October 21,1991. Accepted December 31, 1991.
Preconcentration of Trace Rare-Earth Elements in Seawater by Complexation with Bis(2-ethylhexyl) Hydrogen Phosphate and 2-Ethylhexyl Dihydrogen Phosphate Adsorbed on a C,8 Cartridge and Determination by Inductively Coupled Plasma Mass Spectrometry Mohammad B. Shabani,* Tasuku Akagi, and Akimasa Masuda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan
A rapld and rellable sample preconcentratlon method with mlnknum tkne-consumptlon and labor has been developed to separate rare-earth elements (REEs) from seawater for the measurement by lnductlvely coupled plasma mass spectrometry (ICPMS). The method Involves adsorptlon of a mlxture of bls(2-ethylhexyl) hydrogen phosphate (HDEHP) and 2ethylhexyl dlhydrogen phosphate (H2MEHP) onto a C,8 cartrldge. By passage of seawater samples through the C, cartrldge, quantltatlve complexatlon of REEs wlth adsorbed complexlng agent took place and almost all matrb elements passed through to the draln. The REEs are then removed hwnthe adsorbed coy>lexlng agent wfth an ackl eluent. Thls technlque used an elght-channel perlstattlc pump for sample procedng. Elght 1-L samples of seawater at flow rates of 20 mUmln were preconcentrated wlthln 1 h. Thls method permitted large enrlchment factors of 200-1000-fold to be attalned whlk provldlng almost matrlx-free concentrate sulta b for ICPMS analyds. The total contamlnatlon detennlned for the whok process was much less than 1% of the amount of REEs In 1 L of seawater. The average preclslons for all REEs after four replkate preconcentratbns of 1000 and 5000 mL of the same seawater to flnal measurement solutlons of 5 mL were calculated to b 2.72% and 1.04%, respectively. The parameters whlch were evaluated for the optlmum experlmental condltlons Include pH of the sample, amount of complexlng agent loaded on the C, cartrklge, flow rate of the sample, volume of seawater, and concentration and volume of the acld for elution. Slnce there Is no standard seawater sample for REEs, In order to evaluate the preclslon and accuracy of thls novel technlque, the analytkal results obtalned by thls method were compared wlth those obtalned by ICPMS coupled wlth solvent extractlon concentrations and neutron actlvatlon analysls (NAA) after Chelex I00 concerb tratlons.
INTRODUCTION Inductively coupled plasma mass spectrometry (ICPMS) has been used successfully for direct determination of rareearth elements (REEs) in geological materials.'-' However, application to the determination of REEs in seawater remains limited. The major restriction comes from the extremely low levels of these elements which are below the detection limits of the instrument. Another limitation is due to the high matrix concentrations of approximately 3.5%, present in seawater which is not compatible with the sampling system of ICPMS. Therefore, the analysis of REEs in seawater requires preconcentration and matrix separation in order to eliminate matrix elements and increase the concentration factor above the instrument detection limits prior to analysis by ICPMS. Preconcentration techniques are currently used for REE determination in seawater before isotope dilution mass spectrometry (IDMS) and neutron activation analysis (NAA)."16 By far the most widely used precoconcentration technique is based upon the coprecipitation of REEs with Fe followed by cation-exchange chromatography and determination with IDMS.&14 The coprecipitation technique suffers from several limitations, which include nonquantitative recovery (quantitative recovery can be only established by the addition of spike isotopes) and additional necessary procedures for removal of Mg, Ca, and Fe. This will increase sample manipulation and contamination of blanks. In addition, the coprecipitation technique cannot be easily automated. Another technique recently used for REE preconcentration in seawater is chelating ion-exchange resin, Chelex-100.'"18 Although this technique has been successfully employed in seawater analysis by NAA,14J5it also has serious drawbacks. These include time-consumption (the recovery of REEs is strongly limited by flow rates of 1-3 mL/min) and removal of calcium and magnesium prior to elution of the REEs, requiring careful washing with ammonium acetate1gor additional
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cation- and anion-exchange chromatography before instrumental determination.14J5 All these will increase sample manipulation and contamination of blanks. Coprecipitation and Chelex-100 techniques are not suitable preconcentration techniques for ICPMS determination of REEs in seawater. A reliable method capable of preconcentration of a large number of samples with minimum time-consumption and labor and compatible with high sample throughput of the ICPMS system is strongly desired by the many researchers in the field of oceanography dealing with REE determination. Recently, we developed a successful technique based on the solvent extraction and back-extraction preconcentration of REEs in seawater and determination with ICPMS,*O where REEs were concentrated 200 times with almost all matrix elements separated. Although this method has several advantages over conventional methods in simplicity, rapidity, and small blank, there are still some problems with this technique. These problems are time-consuming procedures of the prewashing of the solvent extraction apparatus and purification of the reagents and solvents and limited preconcentration factor. In another study we developed an online two-stage solvent extraction and back-extraction and the method made possible rapid preconcentration of REEs in geological samples and synthetic seawater.21 With this method, under optimum conditions, an enrichment factor of 10 was obtained for synthetic seawater. However, for application to real seawater samples, higher enrichment factors were found to be necessary because of the extremely low levels of REEs present in seawater (below the detection limits of the instrument). In the last several years, a number of researchers have developed preconcentration methods based upon the reversed-phase adsorption of complexed metals onto a small or immobilization of complexing agents on substrate for concentration of metal ions on it.s31 These methods have been applied to several determinations of transition elements in seawater and other natural samples. None of the above mentioned methods have been used for the preconcentration of REEs in seawater. In an attempt to overcome the several limitations of the other preconcentration techniques, we have developed a new method which involved adsorption of a mixture of bis(2ethylhexyl) hydrogen phosphate (HDEHP) and 2-ethylhexyl dihydrogen phosphate (H,MEHP) onto a Sep-Pak C18 cartridge (C1& These complexing agents offer several advantages and have proven to be a particularly useful material for sample preparation, matrix isolation, and preconcentration of trace REEs in seawater by solvent extraction techniques.20 Because of the high molecular weight of HDEHP and its water insolubility, it strongly adsorbs onto hydrophobic substrates like C18 and no leaching of the complexing agent or metal complexes occurs under preconcentration conditions. The process of preconcentration is simple, requires very few reagents, and minimizes the contamination of blanks. Unlike Chelex-100 and 8-hydroxyquinoline no buffer or additional reagents are necessary and preconcentration is possible in acidic solutions. This study has attained rapid chemical separation, less contamination of blanks, and improvement of the enrichment factor necessary for high-precision measurements. EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the VG Plasma Quad (VG Elemental, Winsford, U.K.). The nebulizer used was standard concentric glass in conjunction with a water-cooled spray chamber. Details of the operating conditions and data collection parameters are given in Table I. A Gilson eight-channel peristaltic pump (Gilson Minipulse 2) was used for sample pretreatment. Reagents. The Sep-Pak CIScartridges (Part No. 51910) were purchased from the Waters Division of Millipore Corp., and all
Table I. Instrument Operation Conditions Plasma Conditions 1.35 kW
