Anal. Chem. 2008, 80, 2357-2364
High-Energy Polarized-Beam Energy-Dispersive X-ray Fluorescence Analysis Combined with Activated Thin Layers for Cadmium Determination at Trace Levels in Complex Environmental Liquid Samples Eva Marguı´,† Cla`udia Fonta`s,† Katleen Van Meel,‡ Rene´ Van Grieken,‡ Ignasi Queralt,§ and Manuela Hidalgo*,†
Department of Chemistry, University of Girona, Campus Montilivi, 17071 Girona, Spain, Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium, and Laboratory of X-Ray Analytical Applications, Institute of Earth Sciences “Jaume Almera”, CSIC, Sole´ Sabarı´s s/n, 08028 Barcelona, Spain
In this paper, we describe a new method for trace level Cd determination in complex environmental liquid samples. Thin layers activated with the extractant Aliquat 336 were prepared either by direct impregnation of commercial polymeric supports or by physical inclusion in a cellulose triacetate matrix, and both were effectively used to collect Cd present at low concentration in different aqueous matrixes. Quantitation of Cd contained in the thin layers was performed by high-energy polarized-beam energydispersive X-ray fluorescence. The effects of various experimental parameters such as layer composition, equilibration time, and instrumental conditions have been investigated. The analysis of different impregnated layers contacted with solutions ranging from 5 to 8000 µg L-1 Cd showed a linear response between the Cd concentration in the aqueous solutions and the metal present in the thin layer, with a detection limit of 0.7 µg L-1. The accuracy of the proposed method was confirmed by analyzing spiked seawater samples and a synthetic water sample containing, besides Cd, high amounts of other metal pollutants such as Ni, Cu, and Pb. The attained results were comparable to those obtained by anodic stripping voltammetry or inductively coupled plasma spectrometry. Cadmium is one of the most ecotoxic metals as it exhibits highly adverse effects on soil biological activity, plant metabolism, and on the health of humans and animals. Although it is not very abundant in the earth’s crust and its content in nonpolluted soil is usually in the range of 0.1-2 mg kg-1 and mostly below 1 mg kg-1, its concentration is increased due to atmospheric deposition derived from anthropogenic activities such as mining, smelting, fuel combustion, phosphate fertilizers, and sewage sludges.1 Considering its hazards, health organizations have established * Corresponding author. E-mail:
[email protected]. † University of Girona. ‡ University of Antwerp. § Institute of Earth Sciences “Jaume Almera”, CSIC. 10.1021/ac7018427 CCC: $40.75 Published on Web 03/08/2008
© 2008 American Chemical Society
permissible limits for Cd in food commodities, including drinking water. The World Health Organization (WHO) established as 3 µg L-1 the maximum permissible in drinking water,2 whereas 5 µg L-1 is the limit established by the U.S. Environmental Protection Agency (EPA).3 Usually, the presence of trace amounts of toxic elements in environmental samples is determined by using spectrometric techniques including electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and inductively coupled plasma mass spectrometry (ICPMS). However, the direct analysis of some complex environmental samples like seawater presents some difficulties due to the high salinity of the matrix and the relatively high amount of organic compounds typically found in these matrixes.4 Therefore, in such cases, a typical dilution of the sample may be necessary before the analysis5 or a preliminary separation and/or preconcentration step may be required to eliminate interferences and/or to improve detection limits for metals in the low microgram per liter range.6 Moreover, when the analysis is performed by using solid sorbents followed by spectrophotometric techniques, an additional elution step after the preconcentration procedure is necessary to recover the species in an appropriate medium. A promising alternative is a combination of preconcentration and X-ray fluorescence (XRF) analysis. XRF spectrometry is a popular method for the direct determination of major and minor (1) Kabata-Pendias, A. Trace Elements in Soils and Plants; CRC Press: Boca Raton, FL, 2001. (2) World Health Organization. Guidelines for Drinking Water Quality, 2nd ed.; Health Criteria and Other Supporting Information; World Health Organization: Geneva, Switzerland, 1998; Vol. 2 (accessed February 15, 2008). (3) U.S. Environmental Protection Agency. Drinking Water Standards and Health Advisories, 2nd ed.; EPA 822-R-02-38; U.S. Government Printing Office: Washington, DC, 2002. http://www.epa.gov/waterscience/drinking/standards.html. (4) Bravo-Sa´nchez, L. R; San Vicente de la Riva, B.; Costa-Ferna´ndez, J. M.; Pereiro, R.; Sanz-Medel, A. Talanta 2001, 55, 1071-1078. (5) Montes, M.; Garcı´a, J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 1998, 13, 1027. (6) Ferna´ndez, F. M.; Stripeikis, J. D.; Tudino, M. B.; Troccoli, O. E. Analyst 1997, 122, 679-684.
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elements in mineralogical and environmental solid samples. With the use of this technique, the direct quantitation of metal species held in solid materials is possible, and therefore, the elution step can be avoided, leading to a reduction of sample handling. Several sorbents can be found in the literature to preconcentrate metals prior to their determination by XRF techniques, such as ion-exchange materials,7 Nafion membranes containing complexing agents,8 and polyurethane foam9 among others.10 Another possibility widely used for preconcentration purposes is the formation of insoluble precipitates of the metal of interest by adding pyrrolidinedithiocarbamates and the collection of the precipitate on cellulose filters.11,12 Recently, we have reported the determination of Cr(VI) in electroplating waters by energy-dispersive X-ray fluorescence spectrometry (EDXRF) after membrane preconcentration using a simple instrument with small-spot X-ray beam.13 However, in the case of Cd, the use of conventional EDXRF spectrometers is restricted due to the low sensitivity and spectral interferences inherent to the choice of LR lines for the determination of heavy metals. In the present work, the use of new equipment based on highenergy polarized-beam energy-dispersive X-ray fluorescence (HPEDXRF) is proposed. In this instrument, the combined use of a high-voltage gadolinium X-ray tube and a high-energy Ge semiconductor detector allows performing EDXRF analysis using K lines of high atomic number elements such as cadmium and, thus, allowing the improvement of both selectivity and limits of detection. Taking into account these facts, the use of HP-EDXRF in combination with activated thin layers containing Aliquat 336 to preconcentrate the metal has been evaluated as a possible alternative for Cd determination in liquid samples at low microgram per liter levels. Analytical figures of merit such as precision, accuracy, and limits of detection have been determined, and the feasibility of the proposed methodology has been evaluated analyzing different Cd-spiked water samples. EXPERIMENTAL SECTION Reagents and Solutions. Standard Cd solutions were prepared by dilution of the stock solution (1000 mg L-1, Pure Chemistry, Romil, Cambridge) and adding the corresponding amount of concentrated hydrochloric acid (Trace Select, Fluka, Germany) to be 2 M HCl. The extractant, tricaprylylmethylammonium chloride (Aliquat 336), the polymer, cellulose triacetate (CTA), and the plasticizer, 2-nitrophenyl octyl ether (NPOE), were purchased from Fluka Chemie. Decaline (decahydronaphtalene, cis + trans) (Aldrich, Germany) and chloroform (Panreac, Spain) were used as received. Spiked seawater was prepared by adding the appropriate amount of Cd stock solution to seawater collected from the (7) Mene´ndez-Alonso, E.; Hill, S. J.; Foulkes, M. E.; Crighton, J. S. J. Anal. At. Spectrom. 1999, 14, 187-192. (8) Koulouridakis, P. E.; Kallithrakas-Kontos, N. G. Anal. Chem. 2004, 76, 4315-4319. (9) Carvalho, M. S.; Domingues, M. L. F.; Mantovano, J. L.; Filho, E. Q. S. Spectrochim. Acta, Part B 1998, 53, 1945-1949. (10) Van Grieken, R. Anal. Chim. Acta 1982, 143, 3-34. (11) Costa, M. M.; Barreiros, M. A.; Carvalho, M. L.; Queralt, I. X-Ray Spectrom. 1999, 28, 410-413. (12) Montero Alvarez, A.; Este´vez Alvarez, J. R.; Padilla Alvarez, R. J. Radioanal. Nucl. Chem. 2000, 245, 485-489. (13) Fonta`s, C.; Queralt, I.; Hidalgo, M. Spectrochim. Acta, Part B 2006, 61, 407-413.
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Mediterranean Sea (pH 8.2 and conductivity 62 mS). No adjustment of pH, chloride, or ionic strength was carried out. All solutions were prepared using analytical reagent grade chemicals and distilled water, purified through a MilliQ Plus system (Millipore). Safety Considerations. Chloroform used to prepare CTA layers presents a moderate risk to health by inhalation, ingestion, or skin absorption. Preparation of the Activated Thin Layers. Two types of thin layers were investigated to preconcentrate Cd: a commercial polymeric support impregnated by an organic solution of the anion-exchanger extractant Aliquat 336, and CTA layers, prepared by physical inclusion of the extractant in the polymeric CTA matrix. In the first case, a polyvinylidene difluoride film (Durapore, Millipore; average pore size of 0.2 µm, porosity of 75%, and average thickness of 125 µm) was used to contain the extractant solution. For this, the polymeric support was soaked in a 500 mM solution of Aliquat 336 in decaline for 10 min and then taken out of the organic solution and wiped with a piece of filter paper. Polymer inclusion layers were prepared by the physical inclusion of Aliquat 336 in the matrix formed by the polymer CTA and the plasticizer NPOE. For this, a chloroform solution of CTA (200 mg in 20 mL), containing 200 mg of the extractant and 300 mg of plasticizer, was poured into an 8.8 cm diameter glass bottom Petri dish. The dish was covered loosely, and the solvent was allowed to evaporate overnight at room temperature. After evaporation of the chloroform, the film was carefully peeled off from the Petri dish and was ready to use. Cadmium Concentration Experiments. To check the effectiveness of both types of layers to concentrate Cd, a first set of experiments were carried out by contacting 1 cm2 of each activated layer with 50 mL of Cd solutions with concentrations ranging from 0.5 to 15 mg L-1, at room temperature. The length of the experiment was 6 h, and during this time, the solution was magnetically stirred and several samples were withdrawn and analyzed by ICP-AES in order to follow the extraction of Cd. The amount of metal collected in the layers was obtained from the difference of the initial Cd concentration in the aqueous solution and the concentration measured at the end of the experiment. The collection of Cd on the thin layers used as standards and also the samples for XRF analysis was carried out by using a cell, where the activated films were placed in a circular window, 3.8 cm diameter, and facing the source solution consisting of 190 mL of Cd solution in 2 M HCl stirred at 800 rpm for 150 min. Several samples of the feed solution were withdrawn to follow the metal extraction and analyzed by ICP-AES or ICPMS. At the end of the experiments, layers containing the extracted Cd were removed from the cell, washed with deionized water several times, and let to dry at room temperature before XRF analysis. Instrumental and Operating Conditions. The voltammetric equipment (anodic stripping voltammetry, ASV) used to analyze the spiked seawater samples consisted of a Stand (Metrohm, Switzerland) polarograph equipped with a hanging mercury drop electrode, a Ag/AgCl reference electrode, and a Pt counter electrode. Measurements were made in acetate buffer by the
Table 1. Instrumental Parameters and Measurement Conditions Anodic Stripping Voltammetry stirrer speed 2000 rpm mode DP purge time 300 s deposition potential -1150 V deposition time 10 s pulse amplitude 50 mV start potential -1150 mV end potential +100 mV voltage step 6 mV voltage step time 0.1 s Varian Liberty RL ICP-AES Spectrometer Cd wavelength 214.438 nm generator 40 MHz, free running rf power 1000 W plasma gas flow rate 12 L min-1 auxiliary gas flow rate 1.5 L min-1 nebulizer V-groove Agilent 7500c ICPMS Spectrometer rf power 1500 W plasma gas flow rate 15 L min-1 nebulizer gas flow rate 1.12 L min-1 sampling cone Ni, 1 mm aperture diameter skimmer cone Ni, 0.4 mm aperture diameter integration time for each isotope 0.1 s readings per replicate 3 signal measurement mode three points per peak 111Cd, 103Rh (as internal standard) isotopes monitored Epsilon 5 HP-EDXRF Spectrometer Gd 25-100 kV, 0.5-24 mA, maximum power 600 W detector energy range 0.7-100 keV window 8 µm Be cooling liquid N2 cooled resolution
