Accurate trace metal analysis in environmental samples using functionalized sorbent particles for analyte extraction and pre-concentration prior to ICP-OES measurement

Abstract

Trace metal analysis has been noticed since the hazards of their exposure to human body was realized. Several methods of analysis such atomic absorption spectrometry (AAS), atomic emission/fluorescence spectrometry (AES/AFS), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), neutron activation analysis (NAA), X-ray fluorescence (XRF), and anodic striping voltammetry (ASV) have been applied over years. Although the sort of sample and the level of target metal concentration is the confining factor to select the analysis method, ICP-MS and ICP-OES techniques are the methods of choice because these techniques enable sensitive multi-elemental analysis with high accuracy and precision. Owing to low concentration levels of target analytes in environmental or biological sample digests (usually in the range of ng L-1 or µg L-1) a pre-concentration step is required prior to measurement. Furthermore, the presence of major sample constituents (sometimes in the range of g L-1) demands isolation of the analytes from matrix components to reduce potential interferences in ICP-MS/OES analysis. In literature, several well-established techniques are reported for enrichment and/or isolation of target analytes. Among all applied methods like cloud point extraction (CPE), liquid-liquid extraction (LLE) and solid phase extraction (SPE) the latter one has been commonly used due to the ease of application, selectivity and high recovery. In solid phase extraction, the analyte-containing sample solution is passed through a column packed with a sorbent material. The analytes are trapped by the sorbent and thereby separated from sample matrix which does not interact with the sorbent. After a washing step, the retained elements are desorbed by elution with appropriate solvents. Nevertheless, this routine method suffers some deficiencies such as incomplete analyte elution, or the use of solvents not or only partially compatible with ICP analysis. Moreover, time effort and possibility of column contamination have to be considered. To solve these problems, an improved method for analyte enrichment has been developed recently at the Institute of Chemical Technologies and Analytics, which is named dispersed particle extraction (DPE). This method is based on the concept of solid phase extraction but it benefits from the use of fresh sorbent particles instead of packed columns, which results in faster reaction time, less chemical consumption and higher reproducibility. In dispersed particle extraction the aqueous solution of analyte in its matrix, which can be natural water or digested soil/plant etc. is conditioned to reach the pH required for efficient analyte trapping by the adsorbent particles. Then, the µm-sized particles (in mg level) are added to this solution, and the target analytes are retained by the sorbent material. The particles are separated from the liquid by centrifugation, re-suspended in diluted acid or dissolved by microliters of concentrated acid and directed as suspension or homogeneous solution to ICP-OES or ICP-MS for analysis. The particles used in the mentioned works were house-made functionalized silica micro-particles. In the present thesis, the idea is to further improve the extraction process and enhance accuracy and sensitivity of analyte measurement using ICP-OES. To achieve these goals, two kinds of adsorbent particles including magnetic nanoparticles and natural bioparticles were prepared. Furthermore, alternative sample introduction strategies were developed, including Electro-Thermal-Vaporization for measurement of organic bioparticles and flow-injection combined with a magnetic trap for analysis of Co-nanoparticles. Magnetic and surface functionalized Co-nanoparticles were used for effective separation of Pb from drinking water, followed by ICP-OES analysis of the enriched particles. For this purpose, cobalt nanoparticles were silica-coated through sol-gel process and finally covered with a layer of ionic liquid acting as a cation exchanger. Applicability of this approach was demonstrated by the analysis of Pb in the reference material ERM-CA022a and tap water samples collected from different districts in Vienna. In another application of dispersed particle extraction, solid sample introduction was performed using an electrothermal vaporization (ETV) system combined with ICP-OES for direct analysis of organic sorbent particles. These particles were prepared by coating the plant spores Lycopodium clavatum with ionic liquid. The method was successfully applied for determination of rare earth elements (REE) in ten different green tea samples.