Trace enrichment of metals using a supported liquid membrane technique

The supported liquid membrane technique offers new possibilities for analytical enrichment, field sampling and speciation. The possibility of using the technique for the enrichment of metals in a flow system with off-line atomic absorption spectrometry in the final analysis step is outlined. Five metals (Cu, Cd, Co, Ni and Zn) have been used as model substances and different approaches to transport metals across the membrane are demonstrated. In one system 8-hydroxyquinoline was used as complexing agent in the sample (donor) solution. In a second system potassium thiocyanate was used in the donor solution and an extradant (Aliquat-336) in the membrane liquid. In a third system 8-hydroxyquinoline was used in the donor solution and Aliquat-336 in the membrane liquid. The parameters important for analytical enrichment are considered. These include factors for determining the extraction efficiency, the possibility of carry-over effects, matrix dependence and long-term membrane stability. The memory effects were below 2% and high concentrations of humic acids or salts did not interfere in the analysis. The long-term stability using di-n-hexyl ether in the membrane was 1-2 weeks and with the extractant Aliquat-336 incorporated in the membrane liquid it was 4 d. Determination of metals at levels <1 ppb was possible. The limiting factor for the determination at lower concentrations at present is the metal contamination in the reagents used.