1802
Anal. Chem. 1987, 5 9 , 1802-1805
(11) Thurman, E. M.; Malcolm, R. L. Environ. Sci. Techno/. 1981, 15,463. (12) Thurman, E. M.; Barber, L. B., Jr.; LeBlanc, D. R. Contam. Hydro/. 1986, 1 . 143. (13) Patt, S. L.; Shoolery, J. N. J . Magn. Reson. 1982, 46, 535. (14) Thurman. E. M.; Malcolm, R . L.; Aiken, G. R. Anal. Chem., 1978, 50, 775. (15) Bidlingmeyer, B. A.; Deming, S.N.; Price, W. P.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 186, 419. (16) Bidlingmeyer, B. A. J . Chrornatogr. Sci. 1980, 18, 525. (17) Bidlingmeyer, B. A . LC Mag. 1983, 1 , 344. (18) Kissinger, P. T. Anal. Chem. 1977, 4 9 , 883. (19) Aiken, G. R.; Thurman, E. M.; Malcolm, R . L.; Walton, H. F. Anal. Chem. 1979, 5 1 , 1799.
(20) Thurman, E. M.; Malcolm, R. L. U S . Geol. Surv. Water-Supply Pap. 1979, No. 1817-G. (21) Thurman, E. M., U S . Geol. Sum. Water-Supply Pap. 1984, No. 2262. (22) Kosugi, Y.; Yoshida, Y.; Takeuchi, T. Anal. Chern. 1979, 5 1 , 951. (23) Swisher, R. D. J . Am. Oil Chem. Soc. 1963, 40 648.
RECEIVED for review January 9, 1987. Accepted March 25, 1987. The use of brand names is for identification purposes Only and does not represent endorsement by the U.S. Geelogical Survey.
Determination of Rhenium in Marine Waters and Sediments by Graphite Furnace Atomic Absorption Spectrometry Minoru Koide, Vern Hodge, Jae S. Yang, and Edward D. Goldberg*
Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093
A graphite furnace atomic absorption method for the determination of rhenium at picomolar levels in seawater and parts-per-billion levels in marine sediments Is based upon the isolation of heptavaient rhenium specks upon anion exchange resins. All steps are followed with '*'Re as a yield tracer. A crucial part of the procedure is the separation of rhenium from molybdenum, which significantly interferes with the graphite furnace detection when the Mo/Re ratio is 2 or greater. The separation Is accomplished through an extraction of tetraphenylarsoniumperrhenate into chloroform in which the molybdenum remains in the aqueous phase.
Rhenium is one of the last stable elements discovered, one of the least abundant metals in the Earth's crust, and one of the most important sentinels of reducing aqueous environments through its abundance in sediments. Although its chemistry is fairly well circumscribed, its marine chemistry is as yet poorly developed. In addition, the understanding of rhenium's marine chemistry will provide a n entry to the understanding of the marine chemistry of technetium, an element which is just above rhenium in group VIIA (group 7 in 1985 notation) of the periodic table. Technetium has only unstable isotopes whose origins are primarily in nuclear weapon detonations and in nuclear reactor wastes. These two elements have remarkably similar chemistries. Previously , we have carried out some preliminary analyses of technetium in the marine environment ( I ) . Rhenium's solution chemistry primarily involves anionic species in the IV, V and VI11 oxidation states ( 2 ) . The oxo anion perrhenate is especially stable. There have been some analyses of rhenium is seawater (3-6) usually employing activation analysis techniques in which the rhenium is isolated from seawater before irradiation. The reported concentrations range from slightly under 3 to slightly under 11 ng/L. These values are remarkably higher than those of neighboring elements like iridium, platinum, and gold whose seawater concentrations do not exceed 0.3 ng/L (7). On the other hand, the crustal rock and deep sea sediment concentrations of rhenium are usually a t least an order of magnitude less than its periodic table neighbors (7). Thus,
Table I. Effect of Added Molybdenum upon the 5-ng Signal of Rhenium
amt of added Mo, ng 0 2 5 10 25
Re signal normalized to 100% for 0 Mo added
added Mo, ng
100 100
50 75
100 91 67
100
Re signal amt of
150 250
normalized to 100% for 0 Mo added 47 40 33 9 0
during the major weathering cycle, seawaters serve as a major reservoir for rhenium. The unusual seawater concentration is attributed to the the nonreactivity of the perrhenate ion (7). Removal of rhenium from its dissolved state in seawater to precipitated solid phases probably involves reduction to lower valence states. There have been a few analyses of rhenium in marine organisms (5,8) and in marine sediments (6). Our laboratory has made a survey of rhenium in oxic and anoxic deposits and has found remarkable enrichments of rhenium in anoxic sediments, especially hydrothermal sulfides (7). On the other hand, rhenium is usually depleted, relative to crustal abundances, in sediments that accumulate under oxidizing conditions. EXPERIMENTAL SECTION The following rhenium technique by atomic absorption spectroscopy using a graphite furnace was developed subsequent t o an initial observation that the rhenium signal was attenuated by as little as 10 ng or less of molybdenum in the isolate (Table 1). Thus, importance is placed upon molybdenum decontamination steps. In seawaters as well as in many marine sediments the Mo/Re varies about 1000 (7) (see also Table V). In addition, a clean separation of rhenium from other elements (the salt effect) is required. Otherwise, false peaks result upon atomization due to the high background generated by impurities. Seawater Collection. Open ocean seawaters were collected in 30 L, modified Go-Flo bottles (General Oceanic), coated with Teflon and suspended on a Kevlar line. The water was immediately pressure filtered through 0.4-gm Nuclepore filters and acidified with 10 mL of 6 M HC1 (G. Frederick Smith and Co.) per liter. The samples were stored in 8-L acid-cleaned poly-
0003-2700/87/0359-1802$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
ethylene bottles. S I 0 Pier seawaters were filtered through 0.45-pm Millipore filters. Some samples were acidified with HC1 and others were not acidified prior to the addition of the '%Re tracer. P r e p a r a t i o n of '%Re Yield Tracer. l%Re (half-life of 3.7 days) was prepared by a 57-h neutron irradiation (University of Missouri Reactor Facility with a flux of 1 X 1014n/(cm2/s)) of enriched rhenium (85.8atom % of lg5Recompared to the natural ; from the Oak Ridge Naabundance of 37.07 atom '70obtained tional Laboratory). The irradiated rhenium metal (0.2 mg) was dissolved in concentrated nitric acid and diluted 1:l with distilled water. An aliquot was received a t our laboratory 1-2 days later. A 100-pL aliquot of this solution contained a total of about 9.3 X lo6 Bq which was diluted with 0.5 M nitric acid (G. Frederick Smith and Co.) to 20 mL. Seawater and sediment samples were spiked with 100 pL of this tracer which contained undetectable amounts of Re as determined by our atomic absorption technique (
