Anal. Chem. 1990, 62, 2709-2714
LITERATURE CITED (1) Hebb, D. 0. The Ofganlzation of Behavior; Wiiey: New York, 1949. (2) Nilsson. N. J. Leaming &chines; W a w Hili: New York, 1965. (3) Kowalski, E. R.; Jurs, P. C.; Isenhour, T. L.; Reilley, C. N. Anal. Chem. 1969, 4 7 , 1945. (4) Preuss, D. R.: Jurs, P. C. Anal. Chem. 1974, 46, 520. (5) Llddell, R. W., 111; Jurs, P. C. Anal. Chem. 1974, 46, 2126. (6) Minsky, M.; Papert, S. Perceptrons; MIT Press: Cambridge, MA, 1969. Robb, E. W.; Munk, M. E. Mikrochim. Acta 1990, 1 , 131. Donahue, S.M.: Brown, C. W.; Kumresan, R. Neural Networks and the Interpretation of Infrared Spectra. 1990 Pittsburgh Conference On Analytlcai Chemistry and Applied Spectroscopy, New York, March 1990; paper no. 1062. (9) Long, J. R.; Gemperline, P. J. Determination of Protein in Wheat from Near-Infrared Spectra Using Artificial Neural Networks. 1990 Pittsburgh Conference On Analytical Chemistry and Applied Spectroscopy, New York, March 1990; paper no. 1063. Wythoff, E. J.; Tomellini, S. A. Anal. Chim. Acta 1989. 227, 343. Wythoff, E. J.; Tomellini. S. A. Anal. Chlm. Acta 1989, 227, 359. Wythoff, B. J.; Hong-Kui, X.: Levine, S. P.: Tomellini, S. A. Unpublished results. Grushka, E.; Monacelli, G. C. Anal. Chem. 1972, 4 4 , 484. Reich, G. Anal. Chim. Acta 1987, 207, 171. Bryant, W. F.; Trivedi, M.; Hinchman, B.. I V : Sofranko, S.: Mitacek. P. Anal. Chem. 1980, 5 2 , 38.
2109
(16) Currie, L. A. J . Res. Natl. Bur. Stand. 1985, 9 0 , 409. (17) Lippmann, R. P. I€€€ASSP Mgazine 1987, 4 , 4. (18) Rumelhart, D. E.; McClelland, J. L. Parallel DlsMbuted Processl~g;MIT Press: CambrMge, MA, 1986; Vol. 1. (19) McClelland, J. L.; Rumelhart, D. E. Explorations In Parallel Distributed Processing; MIT Press: Cambridge, MA, 1988. (20) Hong-kui, X.; Levine, S. P.: D’Arcy, J. E. Anal. Chem. 1989, 67, 2708. (21) Massart, D. L.; Kaufman, L. The Interpretation of Analytical Data by the Use of Cluster Analysis; John Wiley 8 Sons, Inc.: New York, 1983. (22) Caudill, M. AIIExpert 1988, 3, 53. (23) Helstrom, C. W. Statistical Theory of Signal Detection; Pergammon Press: New York, NY, 1968.
RECEIVED for review March 28, 1990. Accepted August 30, 1990. This research was supported by Grant 1-R010H02404-01 from the National Institute for Occuptional Safety and Health, Centers for Disease Control, and by a Dissertation Fellowship from the Graduate School of The University of New Hampshire.
Determination of Trace Lanthanides and Yttrium in Seawater by Inductively Coupled Plasma Mass Spectrometry after Preconcentration with Solvent Extraction and Back-Extraction Mohammad B. Shabani,* Tasuku Akagi, Hiroshi Shimizu, and Akimasa Masuda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo 113, Tokyo, Japan
A method for the analysis of sub parts per trillion levels of rareearth elements (REEs) and yttrium (Y) In seawater after preconcentrationwith solvent extraction and back-extraction has been developed. Almost perfect extraction and backextraction of all REE and Y were achieved by single extraction, and most of the matrix elements were removed during the extraction procedure. Even after 200-fold preconcentration, matrix problems by ICP-MS measurement were negligible. Contaminatkn from reagents and water USBd durlng the pretreatment was below 1% of the concentration of REE and Y In seawater. The standard deviatlon obtalned for triplicate separation of 100- and 1000-mL samples of the same seawater was better than 5 % for all REE. The average precision of the measurement for all REE and Y after preconcentratlon of 1000 mL of raw seawater to 5 mL of final measurement solution was calculated to be less than 2.5 % . Slnce there is no standard seawater sample for REE and Y, in order to evaluate this novel technique, the analytical results obtained by this method were compared with those obtained by Isotope dilution mass spectrometry coupled with Fe copreclpltation. The comparison Indicated the good accuracy of the present method. Sample preparation and measurement could be carrled out within 30 mln for every sample. Two internal standard elements, I n and Cd were used to check sample loss during the extradon and back-extractkbn procedures and to control the POgsiMe matrix effect and instrument fluctuation.
INTRODUCTION The chemistry of rare-earth elements (REEs) makes them particularly useful in studies of marine geochemistry. They
are an extremely coherent group so that their relative abundances can be used to deduce their sources in sedimentary deposits (1). However, Ce and Eu are uniquely interesting because anomalies can be defined easily and quantitatively by comparison with the strictly trivalent neighbors in the REE series ( 2 ) . The determination of ultra trace levels of REEs in seawater, which are extremely low and almost lower than the concentrations of REEs in ultrapure reagents available on the market and are below the detection limits of all present instruments, is pursued with great difficulty. Until now neutron activation analysis (NAA) (2-5) and isotope dilution mass spectrometry (ID-MS) (1,6-9) were the main techniques used to perform determination of REEs in seawater samples. Both techniques have some serious drawbacks. NAA uses large amount of seawater (30 L), and sample preparation is very tedious. This technique has a relatively low sample throughput; some REEs can only be measured after one month, and some REEs cannot be measured a t all. ID-MS is the most reliable technique for REE measurement. The precision and accuracy determined by running standard solutions is typically i l %(2a) or better (I). However, when this technique was applied after preconcentration of 400-2000 mL of seawater samples, the determination of some elements was difficult and precisions of i 8 % (2a) for Ce and *3% ( 2 4 for Nd were the results (9). The technique has drawbacks: it is a time-consuming, expensive method, and monoisotopic elements, which are very important for the expected tetrad effect (6), cannot be determined. Inductively coupled plasma mass spectrometry (ICP-MS) has proved to be a unique, powerful, and alternative instrument for determination of REEs in natural samples (10-15). ICP-MS determination of REEs can be carried out in most
0003-2700/90/0362-2709$02.50/00 1990 American Chemical Society
2710
ANALYTICAL CHEMISTRY, VOL. 62, NO. 24, DECEMBER 15, 1990
cases directly without preconcentration, because of its high sensitivity and low background. However, its applications to analyses of subpicogram and picogram amounts of REEs, which are below the detection limits of the instrument, are limited. So it is necessary to preconcentrate the REEs from a volume of seawater and separate them from high abundances of matrix elements by employment of any of the techniques, NAA, ID-MS, and ICP-MS. Preconcentration techniques that have been used are coprecipitation with Fe(OH), followed by cation-exchange chromatography (I, 5-9) and with Chelex 100 followed by cation- and anion-exchange chromatography ( 2 , 3 ) .Most of these isolation methods require relatively large volumes of chemicals, which can lead to high blanks and a long sample preparation time. The other drawback is analyte loss; a 100% recovery is difficult to obtain, especially for the coprecipitation method, although 100% recovery is not needed for ID-MS (7). This research presented here deals with a solvent extraction and back-extraction of REEs in seawater using a mixture of bis(2-ethylhexyl) hydrogen phosphate (HDEHP) and 2ethylhexyl dihydrogen phosphate (H,MEHP) in heptane for the ICP-MS instrument. The literature on solvent extraction of REEs gives ample evidence that HDEHP and HzMEHP are suitable and strong complexing reagents and, therefore, will ensure the extraction of REEs (16-21). Most of these studies have been done from the standpoint of pure chemistry with standard solution, and no one has used these reagents for seawater preconcentration to our best knowledge. Because of the high distribution coefficient of the REEs with a mixture of HDEHP and H,MEHP in heptane, we applied a very low ratio of organic to aqueous solutions for excellent recovery of REEs in seawater. The validity of the novel technique is checked by two methods: first, preconcentration of a spiked seawater sample and measurement with ICP-MS and, second, comparison of the results of preconcentration and determination of certain seawater samples with those of ID-MS. The purpose of the present study is to develop a rapid and accurate analytical method for REEs applicable to a large number of seawater samples in short time.
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 the Jarrell Ash cross-flow type in conjunction with a water-cooled spray chamber. Details of the operating conditions and data collection parameters are given in Table I. The ICP-MS ion lens voltages were optimized with a 10 ng/mL solution of In and Pb. The lens voltages were varied to obtain a maximum signal for ? P b , while an adequate sensitivity for l151n was maintained, because the lower concentrations of heavy REEs in seawater samples require higher sensitivity for heavy REEs. The instrument was then adjusted to obtain minimum blank counts on masses 14*Prand 175Luby small changes of the voltages of the front plate and pole bias without losing the sensitivity. Reagents. The ultrapure HC1, HNO,, heptane, and octyl alcohol were purchased from Wako Pure Chemical Industries Ltd. Water, acids, heptane, and octyl alcohol were further purified in our laboratory by subboiling distillation up to three times. Deionized-distilled water is further purified by two-stage quartz distillation and then quartz subboiling distillation. Hydrochloric and nitric acids were purified twice by subboilingdistillation, while heptane and alcohol were purified once by subboiling distillation. The di(2-ethylhexyl)hydrogen phosphate and mixture of 65% of di(2-ethylhexyl)hydrogen phosphate and 35% of 2-ethylhexyl dihydrogen phosphate were purchased from Tokyo Kasei Co. Ltd. and diluted in heptane to proper concentrations. The solution prepared prior to solvent extraction was contacted once for 1min with equal volume portions of 6 N HCl to remove REE impurities. Seawater Samples. The seawater was collected in June 1989, from latitude 35’40’22’’ N and longitude 133’3’33” E., Japan Sea, near the Shimane prefecture (depth 1 m from surface). After
Table I. Instrument Operation Conditions
Plasma Conditions forward power 1.35 kW reflected power
