Determination of ultra-trace amounts of cobalt in ocean water by laser-excited atomic fluorescence spectrometry in a graphite electrothermal atomizer with semi on-line flow injection preconcentration

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Abstract
A method has been developed for the determination of trace and ultra-trace amounts of cobalt in sea-water. Samples of CASS-2 Nearshore Seawater and NASS-4 Open Ocean Water reference materials from the National Research Council of Canada were employed. Laser-excited atomic fluorescence spectrometry in an electrothermal atomizer (ET-LEAFS) was used, and integrated with semi on-line flow injection microcolumn preconcentration. For cobalt, the effects of pH on the preconcentration efficiency, the concentration of the chelating agent and the distribution of cobalt in the ethanol eluate were studied. A bonded silica, with octadecyl functional groups (C18) in a 10 µl column, was employed for preconcentration of cobalt in ocean water. Ocean water volumes of 0.40 and 1.00 ml were required for the determination of cobalt in CASS-2 and NASS-4, respectively. These volumes were almost two orders of magnitude smaller than those required by inductively coupled plasma mass spectrometry and some other competitive techniques. The preconcentration factors were 5- and 12.5-fold for CASS-2 and NASS-4, respectively. The detection limits (3s), based on 12.5-fold preconcentration, were 0.08 and 1.0 ng l–1 for cobalt in aqueous standard solutions and in Ocean Water Reference Materials, respectively. Results for the determination of cobalt in CASS-2 and NASS-4 showed that there were no significant differences between the certified values and the measured values, based on Student's t-test at the 95% confidence level. The relative standard deviations for the determinations of the concentrations of cobalt in CASS-2 and NASS-4 were 9 and 13%, respectively.

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