Abstract

Selenium speciation in waters is performed by ion chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS). Selenite, selenate and selenocyanate [Se(IV), Se(VI) and SeCN] are separated with a step gradient elution using dilute NaOH as the eluant. After the separation, the eluant is converted to water on-line by a membrane self-regenerating suppressor, and then introduced into the nebulizer. It is shown that the elimination of the NaOH matrix improves analytical performance and instrument stability. From a 1 mL sample, detection limits (as 3σ of the baseline noise) of 2.5, 1.6 and 2.6 ng L⁻¹ are achieved for Se(IV), Se(VI) and SeCN, respectively, if 2% methanol is added to the eluant. Reproducibility of the method was 7.6–10.5% for the three species at the 50 ng L⁻¹ level and 0.4–7.4% at the 500 ng L⁻¹ level. The method was validated with SRM 1643d (NIST). The analytical performance for the three isotopes ⁷⁷Se, ⁷⁸Se and ⁸²Se is compared under various plasma conditions, and ⁸²Se is identified as the most suitable isotope yielding the lowest detection limits under any conditions. As a demonstration of the method's detection power, it is successfully applied to determine Se speciation in an estuarine water.