Microabsorptiometry of Chemically-Induced Single Picoliter Droplet/Water Extraction

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We have developed a new microabsorption technique combined with microcapillary manipulation and microelectrode methods, by which a single picoliter microdroplet or microparticle can be injected into an aqueous solution and extraction of a solute from water to the single droplet (solvent extraction) or particle (solid phase extraction) is measured. In the oil droplet/water system, ion-pair extraction between methylene blue (MB) or (ferrocenylmethyl)trimethylammonium (Fc) and PF_6^- was so fast, comparable with the diffusion-limited rate of the solute in water. On the other hand, ion-pair extraction between MB and a dodecylsulfate anion (SDS) was very slow and the time dependence of the extraction was analyzed on the basis of a consecutive reaction model. Furthermore, the rate constants were highly dependent on the droplet size. The results indicate that the extraction is governed by adsorption of SDS and/or the SDS-MB ion-pair at the droplet/water interface. In the microparticel/water system, sorption and desorption processes of a cationic dye in single silica gel particle were kinetically analyzed. A rate-determining step of the sorption/desorption processes of rhodamine 6G was the diffusion of the dye in the microparticle, while the mass transfer processes of MB or acridine orange were expected to be governed by the intraparticel diffusion and desorption of the dye on the solid/water interface. It is concluded that microcapillary injection/manipulation, microabsorptiometry and microelectrochemistry techniques of a single microdroplet or microparticle is sufficient to quantitatively analyze the extraction processes. Since the extraction rate depends on the droplet or particle size, the present measurements are absolutely important for the detailed analysis of the extraction processes.