Sol−Gel Coating Technology for the Preparation of Solid-Phase Microextraction Fibers of Enhanced Thermal Stability

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Abstract
A novel sol−gel method is described for the preparation of solid-phase microextraction (SPME) fibers. The protective polyimide coating was removed from a 1-cm end segment of a 200 μm o.d. fused-silica fiber, and the exposed outer surface was coated with a bonded sol−gel layer of poly(dimethylsiloxane) (PDMS). The chemistry behind this coating technique is presented. Efficient SPME-GC analyses of polycyclic aromatic hydrocarbons, alkanes, aniline derivatives, alcohols, and phenolic compounds in dilute aqueous solutions were achieved using sol−gel-coated PDMS fibers. The extracted analytes were transferred to a GC injector using an in-house-designed SPME syringe that also allowed for easy change of SPME fibers. Electron microscopy experiments suggested a porous structure for the sol−gel coating with a thickness of ∼10 μm. The coating porosity provided higher surface area and allowed for the use of thinner coatings (compared with 100-μm-thick coatings for conventional SPME fibers) to achieve acceptable stationary-phase loadings and sample capacities. Enhanced surface area of sol−gel coatings, in turn, provided efficient analyte extraction rates from solution. Experimental results on thermal stability of sol−gel PDMS fibers were compared with those for commercial 100-μm PDMS fibers. Our findings suggest that sol−gel PDMS fibers possess significantly higher thermal stability (>320 °C) than conventionally coated PDMS fibers that often start bleeding at 200 °C. This is due, in part, to the strong chemical bonding between the sol−gel-generated organic−inorganic composite coating and the silica surface. Enhanced thermal stability allowed the use of higher injection port temperatures for efficient desorption of less-volatile analytes and should translate into extended range of analytes that can be handled by SPME-GC techniques. Experimental evidence is provided that supports the operational advantages of sol−gel coatings in SPME-GC analysis.