Speciation of aluminium in soil extracts by employing cation-exchange fast protein liquid chromatography–inductively coupled plasma atomic emission spectrometry

Abstract

A fast protein liquid chromatographic–inductively coupled plasma (ICP) atomic emission spectrometric procedure was developed for the speciation of Al over a wide pH range, from acidic to alkaline regions. Positively charged monomeric Al species in synthetic aqueous solutions were separated from polymeric, neutral and negatively charged species on a cation-exchange fast protein LC column. Linear gradient elution with aqueous NaNO₃(8 mol dm^–3) at a flow rate of 1.0 cm³ min^–1 over 10 min was employed for the separation. The separated Al species were determined ‘off-line’ by ICP atomic emission spectrometry in 0.5 cm³ fractions, which were diluted prior to analysis; yttrium was added as the internal standard. Retention times were found to be 1.5 min for Al(OH)₂(H₂O)₄⁺[Al(OH)₂⁺], 4.0 min for Al(OH)(H₂O)₅²⁺[Al(OH)²⁺] and 4.5 min for Al(H₂O)₆³⁺[Al³⁺]. Good reproducibility of measurement (relative standard deviation 1.5%) was obtained in the optimum concentration range. The limit of detection (3s) for the separated Al species was found to be 120 ng cm^–3(1.0 cm³ sample). The distribution of Al species over a wide pH range agreed with the reported calculated data and, in general, agreed also with the data from the 8-hydroxyquinoline spectrophotometric method. An investigation of the effect of inorganic and organic complexing ligands on the speciation of Al indicated that positively charged [Al(SO)₄⁺, AlF₂⁺] and negatively charged (oxalate and citrate) Al species co-eluted with Al(OH)₂⁺ and were separated from Al(OH)²⁺ and Al³⁺ species, while AlF²⁺ co-eluted with Al(OH)²⁺ species. At lower pH values (e.g., 4.0), aluminium citrate was partially co-eluted with Al³⁺ and Al(OH)²⁺ species. The same observations were found when soil extracts were analysed.