Surface-phase extractor techniques

Silica gel-chemically immobilized-Eriochrome black T was synthesized and the surface coverage was found to be 0.38 mmol/g. The stability toward hydrolysis of this silica gel phase in various buffer solutions (pH 1.0-10.0) was studied and evaluated. The applicability of silica gel phase-immobilized-Eriochrome black T as a solid-phase extractor for Zn(II), Mg(II), and Ca(ll) was performed by the batch equilibrium technique and was found to show an order similar to the formation constant values of the three metal ions with the indicator. The separation and selectivity of this modified silica gel for these metal ions, based on a column technique, were found to afford a reasonable performance of the three studied metal ions. The structure of silica gel-chemically immobilized-Eriochrome black T is given in Scheme 20. 

The contact area between the phases can be enlarged in film extraction and column extraction. The discussion of column extraction techniques will be postponed to the section on TREF. In film extraction, a thin pol5mier film is deposited onto a support material with a large surface area. This technique can also be performed inside Sohxlet extractors, but it is also prone to several experimental difficulties (11). 

Contimious liquid extraction techniques are used when the sample volume is large, the distribution constant is small, or the rate of extraction is slow. The efficiency of extraction depends on many factors including the viscosity of the phases, the magnitude of the distribution constant, the relative phase volumes, the interfacial surface area, and the relative velocity of the phases. Numerous continuous extractors using llghter-than-water and heavier-than-water solvents vee been described [3,2 7,42,73,74]. Generally, either the ligi Pr or heavier density... 

Two principle methods of surface modification are well known they can be summarized as the physical adsorption method and the chemical immobilization technique of the organic modifier.Each method is experienced with certain advantages and disadvantages over the other. For example, the physical adsorption approach is commonly accomplished in a single-step reaction this requires less time for obtaining the final modified solid extractor, but the active donor centers or atoms in the physically modified phases may be consumed in the adsorption process. In addition, these modified phases were found to suffer from leaching or desorption processes under the influence of... 

In addition to solid-liquid collection techniques, in situ sampling methods were developed that contemporarily performed analyte collection and extraction. These systems utilize either sequential or continuous LLE. An example is provided by the aqueous phase liquid extractor (APLE) of Clement and coworkers [22] designed to sample surface water near dump sites where PCDDs and PCDFs were present at level of pg/L. The APLE was capable of extracting up to 200 L water in a single batch process. A spray-bar on the top dispersed a heavier-than-water solvent (methylene chloride) as a fine spray across the surface of the water sample, pushed into the system by a submersible pump. Efficiency in extraction is ensmed by continuous recirculation of the solvent. Devices like the one described avoid the problem of transporting to the laboratory large volumes of sample, but they often remain cumbersome and difficult to be transported. 

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