Visual fluorimetry of trace aluminium by specific immobilization with o,o′-dihydroxyazobenzene on an octadecylsilanized silica thin layer

Abstract

A new, highly selective and sensitive visual method for the determination of trace aluminium with o,o′-dihydroxyazobenzene (dhab) has been developed. In this method, which uses an octadecylsilanized (ODS) silica thin layer, a positively charged fluorescent 1∶1 aluminium(III) chelate ([Al(dhab)]⁺) is retained at a spotting position when ascending development with acetone is performed. Conversely, other metal chelates and excess reagent, which are neutral or negatively charged, are eluted close to the solvent front. The charge difference plays an important role in the chromatographic separation. The specific immobilization of [Al(dhab)]⁺ is attributed to the electrostatic interaction between it and the anionic charge of silanol groups on the ODS silica plate. Insolubility of the chelate in organic solvents also contributes to its immobilization: [Al(dhab)]⁺ is more hydrophilic than other metal–dhab chelates because the chelate with the tridentate ligand dhab²⁻ has water molecules in its coordination sphere. When 10 μl of ethanol–aqueous (1 + 4) sample solution containing metal–dhab chelates is applied to the hydrophobic surface of the ODS silica plate, the sample retains a spherical shape rather than spreading as in conventional thin layer chromatography. The drying step results in effective enrichment of the analyte into a fleck of 3 mm in diameter prior to the chromatographic separation. When the resulting fleck is viewed under ultraviolet light in the dark, the visual detection limit is 1 μg l⁻¹. This method has been successfully applied to the determination of ppb levels of aluminium ion in tap water, river water and tea samples.