Method development for the speciation of metals with atomic absorption spectrometry and flow injection analysis techniques (Doctoral thesis)

Παλαιολόγος, Ευάγγελος-Νεκτάριος Κ.

Chapter 1: A relatively simple, sensitive, selective, automatic fluorimetric method for the simultaneous determination of chromium(III) and chromium(VI) by flow injection analysis has been developed. The method is based on the selective oxidation of the non-fluorescing reagent 2-(a-pyridyl)thioquinaldinamide (PTQA) which with Cr(VI) yields an intensely fluorescent product (λεχ=360ηιη; Xem=500nm). Cr(III) is oxidized, on line, to Cr(VI) with sodium-meta-periodate and subsequently treated with PTQA. Fluorescence due to the sum of Cr(III)-Cr(VI) is measured and therefore Cr(III) is determined from the difference in fluorescence values. The various analytical parameters, such as effect of acidity, flow rate, sample volume, temperature, reagent concentration and interfering species were studied. Kinetic studies using both the Stopped Flow Technique and the FIA procedure were utilised in order to study and optimise the oxidation conditions for Cr(III) on the basis of its oxidation efficiency. The calibration graphs were rectilinear in the ranges of 0.1 to 10 pg mL'1 for Cr(VT) and 0.1 to 1.0 μg mL'1 for Cr(III), respectively. The method was successfully verified by performing recovery experiments of Cr in several Standard Reference Materials(peach leaves, sediments and tea), and applied for speciation analysis of Cr(III)-Cr(VI) to some environmental waters(mineral, tap and distilled water), food sample (tomato juice) and in synthetic mixtures. Up to 30 samples per hour can be analyzed with a relative precision of ca. 0.1 - 2%. Chapter 2: Aqueous solutions of most non-ionic surfactant micelles when heated above a temperature known as cloud point, become turbid. Above this temperature the solution separates into two distinct phases with the very small one composed almost totally of the surfactant including a small volume of water. This approach is applied herein to perform metal speciation, after the studied metallic forms react with suitable ligands for the formation of hydrophobic complexes which subsequently entrapped in the surfactant micelles. As an analytical demonstration, trace concentrations of Cr(III) and Cr(VI) were conveniently detected in samples with complex matrix, such as sea water without any laborious and expensive treatment using flame atomic absorption spectrometry. The speciation of Cr is performed using as surfactant Triton X-114 and as chelating agents, ammonium pyrrolidinedithiocarbamate for Cr(VI) and 8-Hydroxyquinoline for Cr(III). Analytical curves were rectilinear up to concentrations of 130 μg Γ 1 for Cr(III) and 85 μg Γ1 for Cr(IV) with detection limits of 1.4 μg Γ1 and 0.65 μg Γ1, respectively. The method affords recoveries in the range 96-104 % and relative standard deviation lower than 2.2 %. It is proved that the method can be successfully employed as an alternative to the commonly used preconcentration and speciation analytical techniques. By selecting the appropriate complexing agent(s) other metal speciations are feasible provided that some detailed study should precede the application. Chapter 3: This article outlines an analytical method towards the analysis of the species of Cr employing suitable chelating agent and the cloud point phenomenon for Cr(VI) and total Cr analysis. The method encompasses preconcentration of metal chelates followed by flame atomic absorption spectrometry analysis. The chelating agent chosen for this task is the ammonium-pyrrolidine-dithiocarbamate which reacts with either Cr(VI) or total Cr under specific experimental conditions. The condensed surfactant phase with the metal chelate(s) is introduced into a flame atomic absorption spectrometer, whereby discrimination of Cr species is feasible by calculating Cr(III) concentration from the difference between total Cr and that of Cr(VI). A multivariate design was employed to study the variables affecting the overall analytical performance for total Cr assay. Analytical curves are rectilinear up to 0-100 μg Γ1 for both oxidation states of the metal. The limits of detection are 0.6 μg Γ1 and the relative standard deviation at concentrations of 30 μg Γ1 for both species lies around 2.0 %. Ultimately, the method was validated by testing a pertinent reference material certified for both Cr species. Appreciably high recoveries were attained, in the range of 96-107 %, for the environmental and biological samples tested. Chapter 4: A new method based on the cloud point extraction(CPE) technique was applied for the differentiation and the selective determination of Cr species. Cr(III) reacts with 8-Hydroxyquinoline(8-HQ) in a surfactant solution yielding a hydrophobic complex, which is entrapped ‘in situ’ in the surfactant micelles. The Cr(VI) assay is based on its reduction to Cr(III) by sulfite which subsequently reacts with 8-HQ in a similar manner. In order to increase the sample throughput a flow injection analysis (FIA) system allied to fluorometric detection was employed. Using a simple FIA configuration the greatest sensitivity was obtained, which under the optimum working conditions, allows the reliable determination of Cr(III) and/or Cr(VI) at levels as low as 0.2 μg Γ1, in various samples even in those with complex matrix (sea water pharmaceuticals). The whole procedure is straightforward, without any laborious, expensive and hazardous treatment and was proved to tolerate interferences among the cations and anions tested. Moreover, the analytical approach offers distinct advantages over the commonly used analytical methods especially for large number of samples with complex matrixes, which would unavoidably require arduous and delicate pre-treatment. Chapter 5: A sensitive and cost-effective method for the speciation analysis of chromium(III) and chromium(VI) was developed combining the cloud point extraction-preconcentration procedure with chemiluminescence detection in a flow injection assembly. Differentiation of the two species was achieved through their different reaction conditions with ammonium pyrolidine-dithiocarbamate(APDC) while separation, along with a 50-fold preconcentration, was performed with the CPE. The parameters of the CPE and the luminescence reaction were optimized and adjusted to a FIA manifold. Furthermore enhancement of the signal was achieved in the presence of bromide ions while interference by several ions was easily alleviated by the use of EDTA. Several analysis patterns were investigated and they are presented shortly as alternatives to the overall scheme. The detection limit was 3 ng Γ1 with a respective quantitation limit of 10 ng Γ1. Precision of determination even at the low ng Γ1 level was around 2.2%. The method was applied for the speciation of chromium in river, sea, and wastewater with good results.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
Subject classification: Μεταλλικά στοιχεία
Keywords: Ειδοταυτοποίηση,Ατομική απορρόφηση,Έγχυση δείγματος σε ροή,Μεταλλικά στοιχεία,Χρώμιο,Speciation,Atomic absorption,Metals,Chromium,F.I.A.
Appears in Collections:Διδακτορικές Διατριβές

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