Single drop microextraction: Principles and application

An analytical technique has been developed using a very small volume of organic solvent to selectively extract and concentrate semi-volatile organic compounds from aqueous samples for analysis by gas chromatography (GC). Two techniques are described in which the single drop of water-immiscible organic solvent (n-octane) is contained either at the end of a Teflon rod or directly on the tip of the GC microsyringe needle in a stirred aqueous sample solution. After stirring for a presrribed time, part or all of the organic phase is injected into the GC for quantification. An internal standard such as n-dodecane is present in the drop at a fixed concentration to correct for post-extraction solvent evaporation and variable GC injection volumes. The methodology has been shown to be reproducible ({dollar}sim{dollar}2% relative standard deviation), fast (analysis times of 1-10 minutes), simple to perform, and inexpensive.; Both equilibrium and kinetic aspects of the extraction process have been characterized extensively. Extraction rate curves (plots of concentration in the drop versus stirring time) are used to obtain both equilibrium and kinetic data. The dependence of the mass transfer rate on stirring rate has been elucidated for both the Teflon rod system and the microsyringe needle system. It is linear for the latter system. The extraction kinetics have been interpreted in terms of two mass transfer models, film theory and penetration theory. It has been found experimentally that the aqueous phase mass transfer coefficient is directly proportional to the aqueous phase diffusion coefficient using model compounds 4-nitrotoluene, 4-methylacetophenone, malathion, and progesterone. This is consistent with the film theory of convective-diffusive mass transfer. Both theory and experiment show that it is not necessary to reach equilibrium in an analytical application, thus greatly reducing the analysis time.; The solvent microextraction technique has been successfully applied to speciation studies of the steroid hormone progesterone in an aqueous protein (bovine serum albumin) matrix, where only the unbound species of progesterone extracts. The binding of progesterone by protein has some interesting implications for both the equilibrium and kinetic aspects of the extraction process. A mass transfer model which incorporates diffusion of both free and bound forms of progesterone in the aqueous Nernst diffusion film explains the increased rate constant for extraction with protein present, and agrees quantitatively with experimental data. At a protein concentration of 1% (w/v), 20% of the progesterone is in the free (unbound) form. The measured binding constant ({dollar}2.5times 10sp4{dollar} L/mol) is in agreement with other values reported in the literature using more traditional approaches, such as equilibrium dialysis.; The method described here shows great potential as a tool for rapid analysis by solvent extraction/GC. The advantages of a liquid-liquid method combined with the simplicity of the single drop technique make this method a promising alternative to other sample cleanup/preconcentration approaches.