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Evaluation of electrothermal vapourization as a method of sample introduction for the ICP-MS and determination of trace levels of titanium, gallium and indium in the central Pacific gyre Chan, Sanny

Abstract

The analysis of trace metals in the ocean is a relatively new field of study. Since the elements of interest often exist at picomolar levels, this work demands analytical methods with ultra low detection limits. The inductively coupled plasma mass spectrometer (ICP-MS), with its improved detection powers, has proven to be a valuable tool. However, problems resulting from the method of sample introduction, such as low transport efficiency, still prevail. The first objective of this work was to evaluate an electrothermal vaporization (ETV) device as a method of sample introduction for the ICP-MS. Absolute precisions obtained ranged from 2-10% RSD after modifications to improve the performance of the ETV were made. Once optimized, improvements for both sensitivity and detection limits resulting from increased transport efficiency and matrix separation were found to be more than two orders of magnitude for most of the elements studied. Several elements suffered from isobaric interferences, and therefore exhibited improvements in sensitivity and detection limits of only one order of magnitude. In general, detection limits were in the range of 0.057-72 fmol. Acceptable absolute precision was obtained for multielement determination of three isotopes. Better precision was obtained for isotope ratio measurements. Five or more isotopes may be simultaneously analysed when using isotope ratio techniques. The use of freon to form volatile metal halides was instrumental in the analysis of refractory metals. Signals obtained from refractory metals, previously erratic and poorly defined in graphite furanace atomic absorption spectroscopy (GFAAS), were well defined with large increases in sensitivity as a result of freon addition. Matrix effects were observed using the ETV-ICP-MS in the analysis of seawater samples, thus requiring the use of standard additions or isotope dilution. The second objective of this thesis was to determine trace levels of Ti, Ga and In in the central Pacific gyre. Little is known about Ti and Ga distributions in the ocean and virtually nothing is known about In behavior in seawater. Dissolved titanium exhibited elevated surface values (~100 pM), a subsurface minimum (~50 pM) and a bottom maximum (~230 pM). Enhanced Ti concentrations at 400-1000 m, correlating with the mid-depth oxygen minimum in this region were observed. This Ti distribution, in combination with the limited published data, suggests both atmospheric and bottom sources, removal via scavenging throughout the water column, and a mobilization in the O₂ minimum. The dissolved Ga distribution shows intermediate surface values (~17 pM), a subsurface increase (~20 pM), an intermediate depth minimum (5-10 pM) and a bottom maximum (~30 pM). On combining the present data with the previous data pool, the observed trends suggest that Ga has an atmospheric input. Sub-surface and intermediate water concentrations may be a result of vertical processes combining scavenging removal and regeneration, or from horizontal advection. The dissolved In distribution was similar to Ga, with intermediate surface values (~0.3 pM), a subsurface maximum (~0.45 pM), a mid-depth minimum (~0.12 pM), and higher concentrations at deep waters (~0.28 pM). Because this is the first reliable profile of indium to be produced, there are no other data to compare with. A comparison of In distribution with Ga show some similarities. The In/Ga ratio, however, changes linearly with depth suggesting that the two elements are controlled by different input and removal processes or rates in the water column. All three elements demonstrate enrichment with respect to Al by comparison with their crustal abundances. Although the Ti/Al enrichment (11 times) may be explained by preferential removal of Al, the degree of Ga/Al and In/Al enrichments (750 and 1500 times) suggest that the source may not be of crustal abundance. Work to improve the existing chelating resin, a TSK 8-hydroxyquinoline, has afforded a 30-fold improvement of resin capacity. Chelating capacity obtained for this resin is 34 ± 3 μmole Cu(II) / g resin. A new resin was synthesized by coupling 5-amino 8-hydroxyquinoline to a new solid support, Affi-Prep®, gave similar chelating capacities.

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