Improvement in mercury cold vapour atomic techniques by resorting to organized assemblies and on-line membrane drying of vapour

Abstract

The most commonly used technique for mercury determination today is cold vapour (CV) generation with atomic absorption spectrometry (AAS), although inductively coupled plasma atomic emission spectrometry (ICP-AES) has also been used for final detection in several continuous CV-ICP-AES methodologies. In this paper improvements in Hg CV generation for both ICP-AES and AAS detection have been tried. The metal transport/excitation in the ICP-AES was improved by incorporating on-line with the ICP, a commercially available ‘membrane drier tube’. The ability of such a membrane to eliminate water vapour from the gaseous sample stream enhanced Hg signals in ICP-AES and decreased baseline drift in flames. On the other hand, earlier work on the use of organized media in atomic spectrometry has shown the ability of micelles and vesicles to improve the kinetics of volatile species generation with sodium tetrahydroborate. In this vein the effect of different organized media on Hg CV generation has been studied. Micelles studied include: cationic hexadecyltrimethyl-ammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS) and non-ionic Triton X-100. Cationic vesicles studied were didodecyldimethylammonium bromide (DDAB). The best detection limits were obtained using DDAB vesicles. The detection limits obtained when the Hg vapour was generated from conventional aqueous techniques were 0.6 and 1 ppb using ICP-AES and AAS detection, respectively. When both vesicles of DDAB and drying membrane tube were used in the CV generation system, the detection limits obtained were 0.2 ppb both for ICP-AES and AAS techniques, showing the beneficial effect of using a drier tube on-line with the plasma in ICP-AES. The proposed CVAAS method has been applied to the determination of Hg in sulfuric acid and sea-water samples.