Development of Methodologies for Determination of Trace-level Concentrations of Elements by Atomic Spectrometry via On-line Pretreatment Procedures Exploiting Sequential Injection (SI) Lab-on-Valve (LOV) Schemes

Sample pretreatment is often a necessary step in analysis of real samples which
contain trace/ultra-trace level concentrations of the measurand - especially when the detector used does not exhibit extremely high sensitivity - and comprise complex matrices that can interfere in the analysis. Among the many pretreatment techniques available, the use of the third generation of flow injection, the so-called sequential injection lab-on-valve, is probably the most promising approach. The present Ph.D. thesis is focused on exploitation of the versatility of the sequential injection lab-on-valve (SI-LOV) system for the development of robust on-line automatic SI-LOV pretreatment procedures employing solid phase extraction (SPE) for separation and preconcentration of trace elements in environmental samples coupled with various atomic spectrometric detection techniques. Taking advantage of the precise and reproducible timing and versatility of the SI-LOV system and that the solid-phase bead materials used can be renewed whenever called for, special attention
is placed on the intelligent exploitation of the interplay between the thermodynamics and the kinetics of reactions involved, that is, executing kinetic discrimination schemes, which in turn has resulted in the development of a number of novel concepts as illustrated by the procedures detailed in the scientific publications which have arisen from this project. Two categories of sorptive materials, that is, hydrophilic and hydrophobic ones, have been employed for the purpose of SPE in SI-LOV system. Thus, the analytical performance of the hydrophilic chelating Sepharose, containing iminodiacetate groups as functional entities on a support of cross-linked agarose, as used in an online SI-LOV system for the determination of ultra-trace levels of Cd(II), Pb(II) and Ni(II) in biological and environmental samples employing electrothermal atomic absorption spectrometry (ETAAS) detection has been investigated. The approach developed exhibits not only high preconcentration efficiency, but due to the low hydrodynamic impedance it allows also easy handling of the beads. In comparison with hydrophilic beads, the hydrophobic ones, which inherently adsorb non-charged species, potentially offer high selectivity due to the possible intelligent selection of the chelating reagent used for the formation of the neutral compounds. Hence, SI-LOV approaches using various hydrophobic beads have been developed. In an earlier study in our group, a PTFE material (Aldrich PTFE beads) was, as compared to other hydrophobic materials, shown to yield excellent performance for adsorption of neutral complexes of transition metals and chelating reagents. However, its inherent physical and morphological characteristics made this material difficult to manipulate in the SI-LOV system. Therefore, a novel PTFE material, granular Algoflon®, which is spherical and possesses higher hydrophobicity, was investigated. The operational characteristics of this sorbent, employed for the determination of Cd(II), as complexed with DDPA and using detection by ETAAS, in a SI system furnished with an external packed column reactor was evaluated and compared with a
SI-LOV system using a renewable column. In comparison with the previously used PTFE beads the Algoflon® beads exhibited much higher sensitivity, better retention efficiency and enrichment factor. Moreover, no flow resistance was encountered under the experimental conditions used. However, the aforementioned approach utilizing hydrophobic sorbents to collect on-line generated neutral compounds is not directly applicable when slow kinetics in the formation and/or adsorption of the non-charged chelates are encountered. Thus, a new concept involving the use of C₁₈-PS/DVB beads, which were preimpregnated with a selective organic metal chelating agent prior to the automatic manipulation of the beads in the microbore conduits of the LOV unit for the determination of trace metals, was conceived. By adapting this approach, the immobilization of the most suitable chelating agent can be effected irrespective of the kinetics involved, optimal reaction conditions can be employed for the immobilization procedure and for implementing the chelating reaction of the measurand with the immobilized reagent, and by using the bead renewal scheme an added degree of freedom is obtained, allowing the selection of the most favorable elution mode in order to attain the highest sensitivity. A SI-LOV-ETAAS system, using 1,5-diphenylcarbazide (DPC)-coated C₁₈-PS/DVB beads, was successfully applied to the determination of Cr(VI) in natural waters containing high levels of dissolved salts. Slow kinetics can also be encountered during the process of the generation of the chelate before its adsorption on the beads, as illustrated in the on-line formation of the non-ionic coordination compound between Ni(II) and dimethylglyoxime (DMG) and its collection on a bead material consisting of a reversed-phased copolymeric sorbent with a balanced ratio of hydrophilic and lipophilic monomers, as aimed for the determination of nickel in saline matrixes via a SI-LOV-ETAAS system. Thus, simple on-line mixing of the reactants did not result in any retention of Ni(II), indicating that the formation of the Ni(DMG)₂ chelate was slow, and therefore a delay time for its generation and subsequent adsorption had to be incorporated. Thus, to assure sufficient reaction time a reaction coil attached to one of the external ports of the LOV was employed to stack the mixture of sample and reagent for a reproducible period of time prior to the exposure to the beads. The sorbent material exhibited not only superior reversed-phase retention capacity, but also entailed a trouble-free handling in the SI-LOV micro-conduits. The proposed methodology showed high tolerance to the commonly encountered alkaline earth matrix elements in environmental water. Taking advantage of being readily able to control the kinetic conditions, a SI-LOV-ETAAS system using SPE with hydrophilic chelating Sepharose beads was further proposed for the automatic preconcentration and speciation analysis of Cr(III) and Cr(VI). Exploiting on-line reduction of Cr(VI) to Cr(III), the aspirated sample was initially divided into two portions, which were treated simultaneously. Thus, while the Cr(III) ions in the first portion were subjected to a separation/preconcentration procedure on the beads, elution and subsequent quantification by ETAAS, the Cr(VI) ions in the second portion were mixed with a reducing reagent and parked under stopped-flow conditions for a reproducible period of time in an open tubular reactor attached to one of the peripheral ports of the LOV unit. Following quantification of the native Cr(III), the Cr(III) formed from Cr(VI) plus the original Cr(III) was subjected to the same separation/preoncentration/elution procedure. The proposed method was successfully applied to the speciation and determination of trace levels of Cr(III) and Cr(VI) in environmental samples. The versatility of SI was also demonstrated by its application for in-line microcolumn soil extraction under simulated environmental scenarios and accurate monitoring of ultra-trace levels of readily bioavailable Cr(VI) in soil environments. The SI-LOV system, as attached with a specially designed soil column at one of the peripheral ports of the LOV unit, integrates dynamic leaching of the Cr(VI), on-line pH adjustment, separation/preconcentration of the Cr(VI) by a Q-Sepharose strong anion-exchanger, elution and ultimate detection by ETAAS. Finally, the LOV was proposed for the separation and preconcentration of metalloids coupled with atomic fluorescence spectrometry (AFS) detection. This was made feasible by interfacing it with a multisyringe flowing stream network for on-line post column derivatization of the eluate aimed at the generation of hydride species. The potential of this new hyphenated technique for environmental assays was ascertained via the determination of ultra-trace level concentrations of total inorganic arsenic in freashwater. Demonstrated for the assay of As, the method involved quantitative oxidation of As(III) to As(V) in the sample before loading into the LOV unit, preconcentration of As(V) on a renewable anion exchanger, pre-reductive elution, mixing of eluate with reducing reagent for hydride generation and subsequent quantification by AFS. Maximum benefit can be taken from the application of the bead renewable strategy, because the application of high concentrations of reductant and extreme pH conditions for the elution prevents the sorbent to be re-used due to
the gradual deactivation of the functional moieties. The proposed procedure featured high tolerance to metal species and interfering hydride forming elements.