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Determination of Nd Isotopes in Water: A Chemical Separation Technique for Extracting Nd from Seawater Using a Chelating Resin Per-Olov Persson,*,†,‡ Per S. Andersson,‡ Jing Zhang,§ and Don Porcelli|| †
Department of Applied Environmental Science (ITM), Stockholm University, SE-106 91 Stockholm, Sweden Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden § Environmental Biology and Chemistry Faculty of Science, Toyama University, Gofuku, 3190, Toyama, Japan Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, U.K.
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‡
ABSTRACT: A new preconcentration technique for the determination of the concentration and isotopic composition of neodymium in aqueous samples is presented. The method uses a resin, Nobias PA1 from Hitachi High-Technologies, which has a hydrophilic methacrylate polymer backbone where the functional groups ethylenediaminetriacetic and iminodiacetic acids are immobilized. The function of the resin has been tested by preconcentrating 110-350 pmol of Nd from test solutions as well as from natural brackish water and seawater samples with different salinities and Nd concentrations. Samples were loaded onto the resin after the pH was adjusted, and the Nd fraction was eluted using 3 M HNO3. The method shows yields of about 90% or higher at pH 6 when the samples were buffered using ammonium acetate. Without the addition of buffer the yield decreased to below 80%. The isotopic composition of Nd in samples preconcentrated using Nobias PA1 agree within error with published data or data obtained by other methods. The total blank, including contributions from preconcentration, separation, and mass spectrometry, is estimated to be 0.2-0.4 pmol (30-60 pg) of Nd. The described preconcentration method, which can be used in the field, is easy, fast (about 8 h for a 3.6 kg sample), and reliable for preconcentration of Nd from a seawater matrix.
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d is increasingly used as a tracer of modern oceanic currents and as a proxy for paleoceanographic information.1,2 The decay of 147Sm (t1/2 = 1.061011 year) to 143Nd produces variations in 143Nd/144Nd ratios in rocks and so in rivers supplying Nd to the oceans. This creates differences in the isotopic composition of Nd in seawater both between and within ocean basins. These heterogeneities are not destroyed by global mixing because the average oceanic Nd residence time in seawater of about 500 years3 is shorter than the global mixing time of the oceans of about 1500 years.4 The distribution of these variations in isotopic composition between different water masses can be used to extract information on global circulation patterns. Further, past oceanic circulation patterns can be reconstructed from the isotopic compositions of Nd preserved in sedimentary records by understanding how the present distribution of Nd isotopes reflect present patterns. The importance of increasing the knowledge of Nd in the marine environment is also emphasized within the international GEOTRACES project,5 which is documenting the behavior and distribution of oceanic trace elements. The Nd concentration (CNd) in seawater is low and ranges between 5 and 45 pmol/kg. For determination of the 143Nd/144Nd ratio, typically 30-70 pmol is required. Because of its low concentration in seawater, Nd must be preconcentrated from up to over 5 L of seawater. After preconcentration from the seawater matrix, chemical separation is required to separate the Nd fraction from other constituents that can interfere with isotopic r 2011 American Chemical Society
analysis. Isotopic determination is done using thermal ionization mass spectrometry (TIMS) or multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS). Currently, one of the most widely used methods to preconcentrate Nd for isotopic analysis involves coprecipitation with ferric hydroxide.6-10 Although the method is time-consuming, it is proven to yield low blanks and to have good reproducibility. Other techniques to preconcentrate Nd include using chelating adsorbents that incorporate HDEHP/H2MEHP.11,12 In this article we study a new type of resin, Nobias PA1 from Hitachi High-Technologies, for preconcentration of Nd from a seawater matrix. Nobias PA1 has been used successfully for preconcentration of trace metals in seawater samples for concentration determinations using ICPMS.13,14 However, the Nobias PA1 resin has so far not been used for preconcentrating Nd from the larger samples that are needed to determine the isotopic composition of Nd. In this study we evaluate the use of Nobias PA1 for preconcentration of Nd in seawater samples and develop a method that is easily used on a ship-board laboratory or in the field. There are several advantages of preconcentrating onboard a ship or in the field, including reduction of sample size for logistical purposes. Received: September 28, 2010 Accepted: December 20, 2010 Published: January 13, 2011 1336
dx.doi.org/10.1021/ac102559k | Anal. Chem. 2011, 83, 1336–1341
Analytical Chemistry Nobias PA1 has two functional groups, ethylenediaminetriacetic and iminodiacetic acids, that are immobilized on a porous hydrophilic methacrylate polymer. These functional groups have been used in ion exchange chemistry for a long time. However, the functioning of a resin is dependent not only on the functional groups present but also on the physical properties of the material on which they are immobilized. Most chelating resins have styrene-divinylbenzene copolymers which have a hydrophobic character.15 The unique characteristic of the Nobias resin is its hydrophilic methacrylate polymer. The hydrophilic character of the polymer increases the accessibility of the functional groups to the metal ions, which improves the efficiency of both adsorption and elution of the metals. In order to evaluate the possibility of using the Nobias PA1 resin for preconcentration of Nd, we have determined the overall recovery (yield) for extraction of Nd from large volume samples at different pH and using a variety of buffer concentrations and varying amounts of resin. We have also determined the isotopic composition of Nd in a standard diluted in ultra pure (UP) water, preconcentrated using Nobias PA1, and the results are compared to the reported composition of the standard. To evaluate the yield and possible artifacts affecting the isotopic determination in seawater, we have used previously measured seawater samples for preconcentration using Nobias PA1 and the results between different methods are compared. Speciation can affect the binding of trace elements to resins. Binding is pH-dependent and previous studies using Nobias PA1 have shown high recoveries of several trace elements at pH 6.13 At higher pH, the affinity to the resin of alkaline earth metals that must be subsequently separated from Nd increases. Therefore, it is advantageous to control the pH in a water sample as it passes through the resin, which can be done by adding a buffer solution to stabilize the pH at a constant level to the sample. However, changing the pH and adding a buffer will also affect the speciation of Nd in the seawater sample, and the effect must be evaluated.
’ EXPERIMENTAL SECTION Material and Chemicals. Given the low concentrations of Nd in seawater, all laboratory work was performed in a tracemetal clean chemistry laboratory. All chemicals that were used for preconcentration are of ultrapure grade (HNO3 and HCl acids and ammonia solution are Seastar baseline products). An enriched 147Sm-150Nd spike with 12.114 pmol of 150Nd was used for isotope dilution for concentration calculations. All laboratory material, including bottles, pipet tips, and stopcocks, were cleaned in ∼1 M HNO3 (analytical grade) and in ultrapure (UP) Millipore Milli-Q H2O, each for 3 days. Tubing was cleaned in weak HNO3 and UP H2O. Teflon vials were cleaned for 3 days in each of ∼6 M HCl (analytical grade), ∼6 M HNO3 (analytical grade), UP H2O, weak HNO3 (Seastar), and finally UP H2O. Ammonium acetate (AcNH4) was added to samples as a pH buffer, and the pH was adjusted with an ammonia solution. A bulk solution of the buffer was prepared by mixing UP water with 17 M acetic acid (Fluka TraceSELECT ultra) and 11 M ammonia solution (Seastar) in the proportions 1:1.1:1.75. The prepacked Nobias PA1 resin was washed, as recommended by Hitachi High-Technologies, with 5 mL of acetone, 10 mL of UP H2O, and 10 mL of 3 M HNO3 (Seastar) prior to use. Buffer solutions with the same buffer concentration as the sample was used to condition the resin. During the preconcentration
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experiments, prepacked columns with luer male/female connectors of two different sizes were used, PA1M (190 mg) and PA1L (300 mg). The capacity of the resin was determined in ref 13 to be 0.16 ( 0.01 mmol/g using Cu2þ. Test Solutions and Samples. All samples used in the experiments had been filtered earlier either through 0.2 μm Osmonics filters or 0.22 μm Millipore filters and acidified using HCl (Seastar) to pH
