ILLUSTRATION OF IMPORTANT MECHANISMS CONTROLLING MASS-TRANSFER IN SUPERCRITICAL-FLUID EXTRACTION

Supercritical fluid extraction profiles are presented for various mode l systems. The purpose is to illustrate some mechanisms encountered in supercritical fluid extraction by using simplified systems and also t o give examples of how pre-extraction steps can be utilized to improve the mass-transfer rate. Extraction of clevidipine (a calcium antagoni st) from different, allegedly inert materials, showed significant diff erences in extraction efficiency. Extraction from stainless-steel bead s gave complete recovery with pure carbon dioxide, whereas the extract ion from filter paper required the addition of 4% methanol. This indic ates that interactions between matrix and analyte can be strong and th at a modifier can be used, not only to increase solubility in the carb on dioxide, but also to break analyte/matrix bindings. Recovery of oil applied on an inert material was constant in terms of recovery per vo lume of extraction fluid for different flow rates, illustrating a solu bility-limited behavior. Metal rods with drilled orifices filled with lubricating oil were used in experiments designed to reveal the influe nce of matrix characteristics on the extraction profile, The plots obt ained showed a decreased recovery at higher flow rates, demonstrating diffusion limited behavior due to slow mass transfer through stagnant regions of the supercritical fluid. Bad recovery can sometimes be impr oved by changing the matrix in a pre-extraction step thus making the a nalytes more accessible to the extraction fluid. Static pre-extraction improved the extraction efficiency for target compounds from fat samp les. Also recovery of clevidipine from a fat containing emulsion was c onsiderably increased when water was removed in a dynamic pre-drying s tep. (C) 1998 Elsevier Science B.V.