DYNAMICS OF ORGANIC-COMPOUND EXTRACTION FROM WATER USING LIQUID-COATED FUSED-SILICA FIBERS

Solid-phase microextraction (SPME) is an inexpensive, rapid, and solvent-free extraction method for the isolation of organic compounds from an aqueous sample. The technique uses a few centimeters of poly(dimethylsiloxane)-coated fused silica optical fiber which is mounted for convenience and ruggedness into a microsyringe. The organic contaminants are extracted into the coating and transferred for thermal desorption and analysis into the injector of a gas chromatograph. Mathematical descriptions of the absorption and desorption processes were developed and compared with experimental results. One model assumes a perfectly agitated solution which results in extraction times dependent only on diffusion of analyte in the coating. The second model considers extraction from a static solution. In this case extraction times are determined by diffusion of analyte in water. These models facilitate a better understanding of the extraction process. Results indicate that when standard stirring equipment is used as a means of agitation, the dynamics of the extraction process is controlled by diffusion of analyte through the thin static aqueous layer located around the fiber. Extraction times for benzene, toluene, and p-xylene using a coating thickness of 55-mu-m are under 10 min and can be shortened substantially when more efficient agitation methods are used. The technique allows sub-ppb determination of organics in water with flame ionization detection. The linear range of the method is over several orders of magnitude. The sensitivity of the technique is dependent on the coating volume and the coating/water distribution constant. The relative precision of the method is a few percent and Is dependent on the thickness of the coating. Using this technique, poly(dimethylsiloxane)/water distribution constants were determined for benzene (125), toluene (294), and p-xylene (831) and are similar to corresponding values for octanol/water distribution constants.