Illustration of important mechanisms controlling mass transfer in supercritical fluid extraction

Supercritical fluid extraction profiles are presented for various model systems. The purpose is to illustrate some mechanisms encountered in supercritical fluid extraction by using simplified systems and also to give examples of how pre-extraction steps can be utilized to improve the mass-transfer rate. Extraction of clevidipine (a calcium antagonist) from different, allegedly inert materials, showed significant differences in extraction efficiency. Extraction from stainless-steel beads gave complete recovery with pure carbon dioxide, whereas the extraction from filter paper required the addition of 4% methanol. This indicates that interactions between matrix and analyte can be strong and that a modifier can be used, not only to increase solubility in the carbon dioxide, but also to break analyte/matrix bindings. Recovery of oil applied on an inert material was constant in terms of recovery per volume of extraction fluid for different flow rates, illustrating a solubility-limited behavior. Metal rods with drilled orifices filled with lubricating oil were used in experiments designed to reveal the influence of matrix characteristics on the extraction profile, The plots obtained 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 improved by changing the matrix in a pre-extraction step thus making the analytes more accessible to the extraction fluid. Static pre-extraction improved the extraction efficiency for target compounds from fat samples. Also recovery of clevidipine from a fat containing emulsion was considerably increased when water was removed in a dynamic pre-drying step. (C) 1998 Elsevier Science B.V.