Development of Sedimentation Field-Flow Fractionation Coupled Online to electrothermal Atomic Absorption Spectrometry for the Characterization of Environmental Colloids

Colloidal particles (size range 0.001 – 1.0 m) play an important role in many environmental processes: they can strongly bind pollutants and thereby facilitate their transport (i.e. in river systems or in the atmosphere) or enhance their retention (i.e. in soils), depending upon the degree of mobility of the colloids. Many methods have been developed in order to characterize colloidal matter, but the sizing of material in the colloidal range and their chemical characterization is difficult to obtain by a single method. These studies require a preliminary separation step, in order to obtain relatively monodisperse fractions to present to the detection system for the chemical characterization. By using Field-Flow Fractionation (FFF) methods, different size fractions can be collected (across the sample size distribution) for subsequent observation, as for example, heavy metal load determination.1 Among FFF techniques, Sedimentation Field-Flow Fractionation (SdFFF) is an established method for the high resolution, mass based separation and sizing of colloids in the 0.05 – 1.0 m size range2,3 and has already been applied to environmental colloids.2,4 A limiting factor of this technique is the small amount of the sample that can be processed in an FFF separation run. Such a limitation means that only a very sensitive analytical method can be used to characterize the separated particle fraction, or time-consuming concentration steps are required, with the consequence of a further sample handling procedure. For this reason, the coupling of SdFFF with Inductively Coupled Plasma Mass Spectrometry (ICP-MS) has been presented5,6 and it was demonstrated that ICP-MS has a high enough sensitivity to detect even minor elements. However, two of the main drawbacks of ICP-MS are the occurrence of element interferences, which often limits the accuracy of the analysis, and the high cost of the instrumentation. For these reasons, Electrothermal Atomic Absorption Spectrometry (EAAS) equipped with a pre-concentration device, is here presented as on-line detection system for SdFFF. In situ pre-concentration of trace elements in EAAS, which is based on the direct introduction of the sample solution into the furnace by a fused silica capillary, has been previously presented.7 The hyphenation of these techniques permits the determination of trace elements at pg/g levels, on different size range colloidal fractions, in the SdFFF eluate, avoiding the concentration steps and possible sample contamination.