Determination of halogens by low-pressure helium ICP-MS

Abstract

A low-pressure ICP formed with helium has been utilized as an ion source for MS, which is capable of providing information about traces of iron, arsenic and selenium without any interference from ArO⁺, ArCl⁺ and Ar₂⁺, respectively. The present paper describes the application of a novel ICP for the determination of fluorine, chlorine, bromine and iodine, because the ionization of these halogens is highly suppressed in the case of conventional argon ICP (e.g., 9×10⁻⁴% for fluorine). A 5 μl aliquot of an aqueous sample was placed on a looped tungsten filament and heated electrothermally at ca. 2500°C . The resulting sample vapor was transported to the ICP with a stream of helium. The analytes (¹⁹F⁺, ³⁵Cl⁺, ⁷⁹Br⁺ and ¹²⁷I⁺) were then determined by a quadrupole mass analyzer. The detection limits (ng ml⁻¹, based on 3σ) were 23 (F), 2.4 (Cl), 0.13 (Br) and 0.05 (I), which were much better than those of argon ICP. The absolute detection limits using 5 μl of a sample reached to the sub-picogram range. Especially, the detectability for fluorine was significantly improved, which allowed a direct determination of fluorine at the ng ml⁻¹ level in water.