Determination of bismuth at ultra-trace levels in sea-water by inductively coupled plasma mass spectrometry after preconcentration with solvent extraction and back-extraction

Abstract

A method for the determination of sub-parts per trillion (10¹²) levels of bismuth in sea-water after preconcentration with solvent extraction and back-extraction has been developed. The method utilizes 20 cm³ of a 0.1 mol dm^–3 solution of triisooctylamine (TIOA) in heptane and 1000 cm³ of sea-water at pH 1.5. Bismuth is back-extracted from the organic phase by 1 × 10 cm³ and 2 × 5 cm³ portions of 14 mol dm^–3 nitric acid. The optimum conditions for quantitative extraction and back-extraction of bismuth from 1000 cm³ of sea-water were examined, including concentration of TIOA in heptane, pH of sea-water, ratio of aqueous to organic phase and concentration and volume of nitric acid for back-extraction. Even after 1000-fold preconcentration, matrix problems in inductively coupled plasma mass spectrometric (ICP-MS) measurement were negligible. The precision of triplicate preconcentration of 1000 cm³ of the same sea-water sample was 1.6%. The relative standard deviation after preconcentration of 1000 cm³ of sea-water to 1 cm³ of final measurement solution was better than 2.2%. The total contamination blank from the reagents and water used during the preconcentration was below 1% of the concentration of bismuth in sea-water. Sample preconcentration and measurement could be carried out within 30 min for every sample. As platinum could be preconcentrated under the same conditions with bismuth, it was therefore used as an internal standard to check on sample loss during the solvent extraction and back-extraction procedures and to control the possible matrix effects in the ICP-MS determination.