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Determination of trace metals in water by Zeeman Graphite AAS after cloud point extraction
Author
Xu, Huimin
Supervisor
Teo, Khay Chuan
Abstract
This thesis is a follow up of the research on Determination of Trace Metals in Water Samples By Atomic Spectrometry Using Cloud Point Extraction (Jianrong Chen, 2002). The academic exercise concerns the application of the preconcentration technique in separation of trace metals present in water samples. The technique applied in the study is Cloud Point Extraction (CPE). This was followed by the quantitative determination of the trace metals by using Zeeman Graphite Furnace Atomic Absorption Spectrometer (GFAAS). The metal ions of interest in the study were nickel and cobalt.
Cloud Point Extraction works on the fundamentals of conventional liquid-liquid extraction (LLE). It is a technique that uses a non-ionic surfactant as an extractant so that trace metals can be extracted from a larger volume of aqueous phase to a much smaller volume of surfactant phase. The concentration of the surfactant must be at Critical Micellar Concentration (c.m.c) so that micelles can be formed to solubilised the trace metal in the form of metal complexes. 0.05% of Triton X-1 14 was the surfactant used. The metal complexes were formed from the reaction of chelating agent (l-2-thiazolylazo-2-naphthol; TAN at 20 μmolll) with the metal ions. This was an equilibrium reaction which was pH sensitive. The pH was maintained at 7 with phosphate buffer. When higher temperature (42 'C) was applied, the surfactant phase containing the trace metals was separated from the aqueous phase. This temperature was known as the Cloud Point (CP). This stage was facilitated by centrifugation, where small volume of surfactant phase will settle to the bottom. Estimation of chelating affinity of TAN with nickel and cobalt was also conducted. It was found that TAN had an equal affinity for both ions at pH 7.
The resultant surfactant phase was very viscous and could not be injected into the furnace for further analyses. Thus, the viscosity was being reduced by the addition of acidified methanol. Further dilution was done by the addition of 2% (vlv) nitric acid to preserve the trace metals in the surfactant phase. This also reduced the volatility of the surfactant phase so that analytes will not be lost when waiting to be analysed on the auto-sampler plate. The furnace programme was optimised to match the analyses of the samples. Suitable pyrolysis and atomisation temperatures were chosen to give the best signal. The optimal pyrolysis temperatures were 1100°C and 1300°C for nickel and cobalt respectively. An atomisation temperature of 2300°C was used for both ions during atomisation. Sensitivity checks were conducted on both ions so as to check reliability of the instrument.
Concentration factor, taken to be the ratio of the volume of aqueous phase over that of surfactant phase was found to fall within the range of 57 to 80 for both nickel and cobalt. Enhancement factor, which was taken as the slope of the plot of concentration of ions before CPE and after CPE had been determined. Enhancement factors of 50.6 and 57 were obtained for nickel and cobalt respectively.
The validity of the method had been tested by conducting interference and spiked recovery tests. Over 80% of recovery had been achieved, which proved that CPE is valid as a preconcentration and separation method. Tap, pond and seawater samples were also subjected to CPE and further analysed by GFAAS for quantitative analyses.
Cloud Point Extraction works on the fundamentals of conventional liquid-liquid extraction (LLE). It is a technique that uses a non-ionic surfactant as an extractant so that trace metals can be extracted from a larger volume of aqueous phase to a much smaller volume of surfactant phase. The concentration of the surfactant must be at Critical Micellar Concentration (c.m.c) so that micelles can be formed to solubilised the trace metal in the form of metal complexes. 0.05% of Triton X-1 14 was the surfactant used. The metal complexes were formed from the reaction of chelating agent (l-2-thiazolylazo-2-naphthol; TAN at 20 μmolll) with the metal ions. This was an equilibrium reaction which was pH sensitive. The pH was maintained at 7 with phosphate buffer. When higher temperature (42 'C) was applied, the surfactant phase containing the trace metals was separated from the aqueous phase. This temperature was known as the Cloud Point (CP). This stage was facilitated by centrifugation, where small volume of surfactant phase will settle to the bottom. Estimation of chelating affinity of TAN with nickel and cobalt was also conducted. It was found that TAN had an equal affinity for both ions at pH 7.
The resultant surfactant phase was very viscous and could not be injected into the furnace for further analyses. Thus, the viscosity was being reduced by the addition of acidified methanol. Further dilution was done by the addition of 2% (vlv) nitric acid to preserve the trace metals in the surfactant phase. This also reduced the volatility of the surfactant phase so that analytes will not be lost when waiting to be analysed on the auto-sampler plate. The furnace programme was optimised to match the analyses of the samples. Suitable pyrolysis and atomisation temperatures were chosen to give the best signal. The optimal pyrolysis temperatures were 1100°C and 1300°C for nickel and cobalt respectively. An atomisation temperature of 2300°C was used for both ions during atomisation. Sensitivity checks were conducted on both ions so as to check reliability of the instrument.
Concentration factor, taken to be the ratio of the volume of aqueous phase over that of surfactant phase was found to fall within the range of 57 to 80 for both nickel and cobalt. Enhancement factor, which was taken as the slope of the plot of concentration of ions before CPE and after CPE had been determined. Enhancement factors of 50.6 and 57 were obtained for nickel and cobalt respectively.
The validity of the method had been tested by conducting interference and spiked recovery tests. Over 80% of recovery had been achieved, which proved that CPE is valid as a preconcentration and separation method. Tap, pond and seawater samples were also subjected to CPE and further analysed by GFAAS for quantitative analyses.
Date Issued
2005
Call Number
QD142 Xu
Date Submitted
2005