Solvent microextraction as a speciation tool: determination of free progesterone in a protein solution

A 1-microL drop of n-octane suspended from the tip of a microsyringe needle in 500 mL of stirred aqueous solution is used to extract unbound progesterone in the presence of 1% (w/v) bovine serum albumin (BSA) for analysis by gas chromatography (GC). The observed extraction kinetics are in good agreement with a proposed convective-diffusive mass transfer model which accounts for diffusion of both free and bound species in the aqueous Nernst diffusion film adjacent to the interface. The equilibrium binding constant, which can be measured both at equilibrium and at nonequilibrium extraction times, is in good agreement with literature values. The very small phase ratio employed (i.e., 2 x 10(-6) mL of organic/mL of water) avoids perturbation of the aqueous solution equilibria.