Supercritical fluid extraction of free fatty acids from siliceous solids

Fatty acid distributions are routinely studied by organic geochemists as they provide insight into the origins and processes involved in the transformation of organic matter during its incorporation into sediments. This study investigates the technique of supercritical fluid extraction (SFE) for the isolation of saturated, branched, unsaturated, and hydroxy-substituted fatty acids from a model siliceous matrix (silica-fused silica boiling stones) and two clays (montmorillonite and kaolinite). Spike-recovery experiments from the model matrix were performed to determine the effects of SFE temperature (40-100$\sp\circ$C), pressure (100-400 atm), and modifier content on free fatty acid extractabilities. Threshold pressures were determined for all of the analytes, and kinetics of extraction at different temperatures were studied. Results of the spike-recovery experiments demonstrated that solubility was not a limiting factor in the extraction of saturated, branched, and unsaturated fatty acids from siliceous samples. Hydroxy fatty acids, however, were poorly soluble in supercritical CO$\sb2$, and addition of polar modifiers was necessary for their isolation. Increasing the temperature of extraction provided dramatic improvements in the recoveries of free fatty acids, with the exception of unsaturated acids, from the model matrix by facilitating analyte diffusion and providing the thermal energy necessary to overcome adsorption energies associated with hydrogen bonding and van der Waals interactions. At 100$\sp\circ$ and 400 atm, recoveries of branched and saturated fatty acids from boiling stones using pure CO$\sb2$ ranged from 80-102% with excellent RSD's (2-6%). The results of SFE experiments from clays which had been fortified with the analytes by a slurry-coating procedure demonstrated the profound influence of the matrix on the conditions required for extractability using SF's. The strength of clay-analyte hydrogen bonding interactions, van der Waals interactions, and coordination to lattice ions precluded the isolation of free fatty acids from fortified clays using pure CO$\sb2$ even at high SFE temperatures. Substantial improvements in recoveries were observed upon the addition of polar modifiers, which compete for active sorptive sites, or upon acid pre-treatment of the fortified clay prior to SFE, which presumably facilitates desorption by disruption clay-fatty acids interactions, thereby freeing the analyte for extraction.