Surfactant-based ordered media in analytical atomic spectrometry

Abstract

During the last 10 years or so we have witnessed an enormous growth of interest and applications of surfactant-based ordered media in analytical chemistry. However, their use in analytical atomic spectroscopy (AAS) has been rather scarce and often controversial. The utilization of surfactants in this latter field is discussed here along two main lines: one refers to the favourable manipulation of physical properties of the sample solutions (Part A) while the other, demonstrated very recently, refers to the adequate manipulation of chemical reactions and/or interactions of analytes in solution by resorting to surfactants use (Part B). The control of physical properties of sample solutions, e.g. manipulation of the surface tension, allows three main applications of surfactants in atomic methods: possible increases of nebulization/atomization efficiencies in flame-AAS, improvement of aqueous/organic solvent compatibility (emulsification applications) and enhancement of the wettability of graphitic solid surfaces. The facts and controversies existing today on this method of utilization of surfactants to enhance atomic methodologies is critically discussed. The ability of surfactant-based “ordered media” to organize reactants at the molecular level has also been applied to enhance chemical generation of volatile species ( e.g. hydride generation or cold Hg vapour generation) used in atomic methods. The analytical potential and usefulness of micelles and vesicles to improve the detection power of hydride generation ICP-AES methodologies are summarized for the determination of arsenic, lead and cadmium by plasma emission. Increases up to two-fold in the sensitivity of As and Pb have been observed by addition of organized media. A volatile Cd species is formed very easily in cationic vesicles with NaBH 4. This Cd species can be used to increase by five times the detectability of Cd by ICP-AES. Moreover, synergic combinations of liquid chromatography separations/atomic detection are possible by resorting to the use of micellar or vesicular mobile phases. The successful application of this principle to the modern problem of toxic arsenic HPLC speciation by using a vesicular solution [as mobile phase for the HPLC separation of As(III), As(V), monomethylarsonic and dimethylarsinic acids] and “on-line” surfactant-enhanced arsine generation is also described in detail and completes the whole picture of the present interface between analytical atomic spectroscopy and surfactant assemblies.