Surfactant molecules can form self-aggregates on solid surfaces. They are called "admicelles" or "hemimicelles", whose interior tends to incorporate sparingly soluble organic compounds (e.g., trichloroethylene, dichlorobenzene, pentachlorophenol and toluene). 1-3 The use of admicelles in trace analysis has only recently been reported. 4 The idea developed in our laboratory is summarized in Fig. 1. The upper illustration shows reagent-impregnated admicelles. Chelating agents (e.g., dithizone and 1-nitroso-2-naphthol) were incorporated in the hydrophobic region of the admicelles, on which the desired trace metals were retained by complexation. 5,6 For example, dithizone-impregnated admicelles were formed by mixing alumina particles with an ammoniacal dithizone solution (containing SDS) and acidifying the mixture. During the acidification, the SDS was adsorbed on the alumina to form admicelles, into which water-insoluble dithizone was incorporated. Commercial SDS, however, contained appreciable amounts of metal impurities, such as copper and lead, which were complexed with dithizone and introduced into the admicelles. The purification of admicelles was therefore required before separation. 7 On the other hand, the desired trace metals were converted into water-insoluble chelates and incorporated in the admicelles (see the lower part of Fig. 1). 8 In this case, admicelles were prepared by mixing an aqueous solution of SDS and an alumina suspension at pH 2. Although the SDS solution contained metal impurities, the contaminants did not enter into admicelles because they were not converted into hydrophobic compounds (no chelating agents are present through admicelle formation). The resulting admicelles were packed into a small column and used for collecting water-insoluble chelates in aqueous solutions. Aggregates of sodium dodecyl sulfate (SDS) formed on alumina particles were employed as enrichment vehicles of hydrophobic metal-chelates from water samples. To a 50 cm 3 suspension of 1.5 g of purified alumina was added 100 mg of SDS; the mixture was then acidified to pH 2, where the anionic surfactant was strongly adsorbed on positively charged alumina surfaces to form admicelles. Nanogram quantities of heavy metals (Co, Ni, Cu, Cd and Pb) in water were complexed with APDC at pH 2 -6 and quantitatively incorporated in the admicelles. A nearly complete desorption was achieved for those metals (except for Co) in 4 mol dm -3 nitric acid with the aid of ultrasonic irradiation. A multielement determination was performed by ICP-MS. The reliability of the proposed method was evaluated by analyzing a certified reference seawater sample.

Enrichment techniques are often required prior to use of instrumental determination methods, to improve the accuracy and precision of analytical results.