Mass Transfer Characteristics of Solvent Extraction into a Single Drop at the Tip of a Syringe Needle

The amount of a sample compound extracted into a 1-μL drop of n-octane suspended in a stirred aqueous solution from the tip of a microsyringe needle is measured by gas chromatography (GC) as a function of time. The observed extraction rate curve is first order and yields the overall mass transfer coefficient for the sample compound, βo. For a given compound, βo varies linearly with stirring rate. Among the four compounds malathion, 4-methylacetophenone, 4-nitrotoluene, and progesterone, at a given stirring rate, βo is linearly proportional to the diffusion coefficient of the compound (Daq). This supports the film theory of convective−diffusive mass transfer, as opposed to the penetration theory. The relative standard deviation of the GC signal for 4-methylacetophenone after a 1.00 min extraction at 1500 rpm is 1.5%, which suggests that the system exhibits excellent potential as a tool for rapid analysis by solvent extraction/GC.