Determination of 129I/127I isotope ratios in liquid solutions and environmental soil samples by ICP-MS with hexapole collision cell

Abstract

The determination of ¹²⁹I in environmental samples at ultratrace levels is very difficult by ICP-MS due to a high noise caused by Xe impurities in argon plasma gas (interference of ¹²⁹Xe⁺), possible ¹²⁷IH₂⁺ interference and an insufficient abundance ratio sensitivity of the ICP mass spectrometer for ¹²⁹I/¹²⁷I isotope ratio measurement. A sensitive, powerful and fast analytical technique for iodine isotope ratio measurements in aqueous solutions and contaminated soil samples directly without sample preparation using ICP-MS with a hexapole collision cell (ICP-CC-QMS) was developed. Oxygen is used as reaction and carrier gas for iodine thermal desorption via the gas phase from solid environmental material in the sample introduction device coupled on-line to ICP-CC-QMS. A mixture of oxygen and helium as reaction gases in the hexapole collision cell was applied for reducing disturbing background intensity of ¹²⁹Xe⁺. After optimization of measurement procedures the detection limit for ¹²⁹I⁺ in aqueous solution was 8 × 10⁻¹³ g ml⁻¹, which is better by about two orders of magnitude in comparison to the detection limit for ¹²⁹I⁺ in sector field ICP-MS. The detection limit for direct ¹²⁹I⁺ determination in contaminated environmental (soil) samples via gas-phase desorption without any additional sample preparation was 3 × 10⁻¹¹ g g⁻¹ (30 ppt). Furthermore, the results of the determination of ¹²⁹I/¹²⁷I isotope ratios at the 10⁻⁵–10⁻⁶ level in synthetic laboratory standards and environmental soil samples from contaminated areas are given.