Electrodeposition and electrodeposited modifiers in electrothermal atomic absorption spectrometry.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The work within this thesis is directed towards optimising and expanding the application of electrodeposition-coupled-electrothermal atomic absorption spectrometry (ED-ETAAS). The technique involves modifying the surface of the graphite furnace by in-situ electrodeposition of a noble metal. The analyte is electrodeposited onto the modifier and the spent sample matrix is aspirated from the furnace, thus separating the analyte from sample matrix components which can interfere with ETAAS analysis. The electrodeposited analyte is then determined by ETAAS. Parameters for electrodeposition of modifier and analytes were optimised. It was shown that ED-ETAAS could be used to determine lead, cadmium, and copper in 0.5 M NaCl media with sensitivity and detection limits similar to conventional ETAAS. The technique was used to determine copper and cadmium in seawater. Different noble metal modifiers were compared. Palladium was shown to provide better sensitivity and thermal stabilisation than iridium or rhodium for lead determination. ED-ETAAS.was used for the determination of inorganic mercury. The detection limit for the technique, using a 20 µL sample volume, was ca. 18 ppb (corresponding to ca. 380 pg), with a characteristic mass of 91 pg. The electrodeposited palladium modifier provided greater analyte stabilisation and sensitivity than gold or ammonium sulphide modifiers. The technique was compared with cold vapour-ETAAS, for which a detection limit of ca. 1.7 ppb was determined using an 8.4 mL sample volume. The electrodeposited palladium modifier was shown to provide superior sensitivity to palladium chloride modifier. ED-ETAAS was examined as a technique for differentiating between free metal ions and those bound in inert and/or stable complexes. This involved selective deposition for fractionation of Bi3+, Pb2+, Ni2+ and Cu2+ in the presence of varying concentrations of EDTA. Fractionation of bismuth was possible (Bi-EDTA- is inert) but in labile systems the quantitative deposition process disturbed the solution equilibria resulting in an overestimation of the free metal concentration. In an attempted application to natural waters, problems were encountered with adsorption of fulvic acid on the furnace surface. Thus EDETAAS is not recommended as a method for fractionating metal-ligand species in natural systems. The ED-ETAAS technique was successfully used for fractionation of arsenic species. The detection limit for arsenite determination from nitric acid media was 0.58 ppb (corresponding to 22 pg in a 39 µL sample), with a characteristic mass (peak absorbance) of 7.5 pg. Total arsenic was determined from media containing L-cysteine, with a detection limit of 1.3 ppb (51 pg) and a characteristic mass of 6.6 pg. The technique was used to determine AsIII and total arsenic in two natural waters. The ED-ETAAS technique gave the same results as two comparative techniques (ETAAS and hydride generation ETAAS) provided that peak absorbance measurements were used.