Quantification of selenium species in petroleum refinery wastewaters using ion chromatography coupled to post-column isotope dilution analysis ICP-MS

Abstract

A post-column Isotope Dilution Analysis (IDA) methodology has been used to carry out the quantitative speciation of selenium (Se) in industrial wastewaters by ion chromatography coupled to ICP-MS with an octapole reaction system (ORS). The interfering argon dimers on the 78Se and 80Se isotopes were suppressed by pressurizing the octapole chamber with 4 mL min-1 of hydrogen. The inorganic Se species selenite [Se (IV)], selenate [Se (VI)] and selenocyanate [SeCN-] were separated by ion chromatography (IC) using isocratic elution with 20 mmol L-1 Na2CO3 / 2 mmol L-1 NaHCO3 as eluent. After the separation, the eluent carbonates were converted to CO2 on-line by a membrane self-regenerating suppressor, in order to avoid clogging problems in the nebulizer or in the cones of the ICP-MS interface. Detection limits (as 3σ of the baseline noise) of 0.18, 0.13 and 1.0 ng g-1 were achieved for Se (IV), Se (VI) and SeCN- respectively. Quantification of selenium in each separated species was performed using post-column isotope dilution analysis by continuous mixing of an enriched 77Se spike solution with the effluent from the suppressor column. Finally, the 78Se/77Se or 80Se/77Se isotope ratios were monitored and the concentration of the corresponding selenium species was evaluated. The proposed methodology has been successfully applied to study the quantitative speciation of selenium in petroleum refinery wastewaters.