New materials preparation and miniaturised devices for solid phase extraction as sample preparation techniques for the analysis of transition metals
The subject of this thesis is the development of solid phase extraction (SPE) materials and miniaturised apparatus applicable for sample preparation in the analysis of transition elements from environmental matrices. It consists of five main chapters and a general conclusion. The first chapter is a general introduction. Considerable attention is given first to underlining the role of SPE as an efficient sample preparation approach in trace analysis. Subsequently the materials and techniques most frequently exploited for the synthesis of SPE materials specific for metals are discussed with the aid of various demonstrative examples from recent literature. Then the current research trends towards downscaled analytical systems and the efforts to integrate SPE apparatus within miniaturised devices are pointed out. Particular attention is paid to the fabrication techniques predominantly exploited to construct microfluidic devices, i.e., from glass and polymers. Given that the developed SPE systems in this thesis were coupled with various analytical detectors/instruments including atomic spectroscopy (i.e., ICP-OES, ICP-MS), optical absorption spectroscopy (in UV-Vis range) and electrochemical (amperometry) monitoring. the essential fundamentals of these detection techniques are presented. In the second chapter, the development of a rapid and an environmentally friendly chemical transformation, consisting of minimum steps in comparison with the traditional method, to immobilise oxins (i.e., 8-hydorxyquinoline (8-HQ)) on silica surfaces is reported. The produced chelating resin shows excellent performance as SPE materials for on-line sample preparation (preconcentration and matrix elimination) of some transition metals prior to their determination by ICP-OES. The applicability of this SPE material was tested by analysing Cu, Co, Zn, Ni and Pb in the range of 50-300 ng ml-1 from a synthesis matrices simulating sediment. The recovery values were ranged from 100% for Zn to 70% for Ni. The work in this chapter also presented a method to mask the environmentally abundant (major) transition metals (i.e., Fe, Al and Mn). The system used at its optimised parameters to analyse the studied ions from different sediments reference materials. The results show good agreement with the certified values. Chapter three describes methods to fabricate miniaturised SPE columns from monolithic materials. Short monolithic columns were fabricated from silica materials using a simple sol-gel method relaying on the hydrolysis of potassium silicate (21 % Si02, 9% K20) using formamide and/or acetamide, then functionalised with 8-HQ and L-cystiene via two different in situ procedures. The functionalised monolithic microcolumns encapsulated inside a house made connector, and thus easily incorporated within a FI manifold coupled with UV/Vis spectrometer allowing the eluted metals to be derivatised, with chromogenic reagents (i.e., PAR and ferrozine), and monitored on-line. The system has been characterised for Co, Cu and Fe (II). The microcolumns functionalised with L-cystiene operating at flow rate of 0.3 ml min-l for 4 minutes, the linear range for the Co, Cu and Fe (II) ions were 20-240, 10-200 and 5-180 ng ml-1 respectively. For those functionalised with 8-HQ, the linear range were 15-300, 10-250 and 5-250 ng ml-1 in the same order. In chapter four, downscaled SPE apparatus applicable for sample preparation prior to ICP-MS monitoring, have been constructed making use of the lab on a chip concept. Standard photolithography and wet etching were used to fabricate glass microfluidic devices accommodating three microchannels, each of them incorporating a defined section that could be packed with SPE resin. The microdevice interfaced with the ICP-MS instrument throughout a low flow rate concentric nebuliser using a Teflon connector, and coupled with FI delivering sample and reagents via a splitting valve. The feasibility of this miniaturised system to perform SPE of trace metals was proved by analysing trace metals, Cd, Co, and Ni, in seawater reference materials. Chapter five reports two designs to integrate microfluidic devices with electrochemical detection. In the first one, a microfabricated glass microfluidic device incorporating single microchannel with a packed section was coupled with a specially designed miniaturised electrochemical cell in a configuration that permits the working electrode to be mounted opposite to the channel outlet to facilitate end channel amperometric detection. The miniaturised electrochemical cell was made from three pieces of glassy carbon, silver and platinum rods of 2 cm length as working, reference and auxiliary electrodes respectively. These rods were assembled in a miniaturised Perspex block, stabilised with insulating epoxy resin and their ends were polished to mirror like discs. In the other design, the microfluidic devices were fabricated from PDMS by a simple casting and moulding techniques permitting the construction of three dimensional (3D) microchannels. The elastic characteristics of PDMS offer a great degree of flexibility for the placement of the microelectrodes inside the microchannel; thus, the monitoring is performed in-channel. To minimise variation in background current due to the pH change, the SPE process was carried out in buffer media i.e., the metals were loaded in acetate buffer of pH 4.8 and eluted with buffered solution of 50 nM Pyridine-2,6-dicarboxylic acid (PDCA) containing 50 mM of KCI as a supporting electrolyte to maintain a constant conductivity. The system shows good performance in the SPE and monitor Cu ion from standards solution in the range 100-400 ng ml-1 with LOD at 52 ng ml-1 In chapter six a general conclusion and a concise prospective for further work are presented.