X-ray fluorescence trace metal analysis of environmental liquid samples after membrane preconcentration

Articles

01 Feb 2008

Eva Marguí,^a* Clàudia Fontàs,^a Katleen Van Meel,^b Manuela Hidalgo^a and Ignasi Queralt^c
^aDepartment of Chemistry, University of Girona, Campus Montilivi, 17071 Girona, Spain
^bDepartment of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium
^cLaboratory of X-Ray Analytical Applications, Institute of Earth Sciences “Jaume Almera”, CSIC, Solé Sabarís s/n, 08028 Barcelona, Spain

Introduction

One of the dangerous kinds of pollution in aquatic systems is due to the dumping of materials containing heavy metals. Hence, the monitoring of heavy metals in aqueous samples is becoming increasingly important.

Normally, metal concentrations in water are in the ng L^–1 range, and the analytical procedures used for their determination are usually based on Anodic Stripping Voltametry (ASV) and Atomic Spectrometry, including Electrothermal Atomic Absorption Spectrometry (ETAAS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

However, the direct analysis of some complex environmental samples like seawater presents some difficulties, mainly due to the high salinity of the matrix. Therefore, in such cases, a dilution of the sample may be necessary before the analysis, 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 µg L^–1 range. 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 elements in mineralogical and environmental solid samples. Using 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 have been reported in the literature to preconcentrate metals prior to their determination by XRF techniques,1 such as ion-exchange materials and polyurethane foam. 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. Nevertheless, these XRF methods involve the addition of reagents, adjusting the pH of test samples, laborious filtrations and drying stages, with the possibility of contamination of samples during the sample preparation. Furthermore, sometimes it is difficult to obtain homogeneous specimens with satisfactory working surfaces after water evaporation or concentration of trace elements by precipitation or coprecipitation methods.

In this article, a method is outlined for the determination of trace amounts of metal contaminants in saline and acidic solutions based on a preconcentration with membranes followed by XRF detection. The method is based on the principle of metal preconcentration using membranes containing the commercial anion-exchanger tricaprylylmethylammonium chloride (Aliquat 336). This is a particularly convenient means of separation for the XRF method because the membrane can be mounted directly in the spectrometer for analysis, thereby offering a novel and easy alternative to the complicated preconcentration methods described above when dealing with this type of matrix. Moreover, with the preconcentration procedure proposed, the problems of reduced sensitivity inherent to the choice of Lα lines for the measurement of high atomic number elements (commonly used in conventional XRF instrumentation) are reduced and the determination of some important pollutant elements is possible in the µg L^–1 range.

In particular, the feasibility of the combined use of this simple and inexpensive membrane preconcentration procedure with different configurations of XRF spectrometers [energy dispersive XRF (EDXRF), wavelength dispersive XRF (WDXRF), high-energy-polarised-energy dispersive XRF (HE-P-EDXRF)] for the determination of trace amounts of some metallic pollutants (Cd, Cr, Pd and Pt) in complex liquid samlpes is outlined in this article. Detailed information and results about this topic are described in full in previously published papers.

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