A poly(dimethylsiloxane) (PDMS) coated sapphire fiber has been investigated as a sensor for hydrocarbons (HCs) in the mid-infrared region around 3000 cm<sup>-1</sup>. In order to optimize and predict sensor response, the diffusion behavior of the analytes into the PDMS preconcentration medium has been examined. A diffusion model based on Fickian diffusion was used to quantify diffusion. The model incorporated such factors as film thickness, refractive index of the polymer and the fiber core, and principal wavelength at which the analyte absorbs. A range of hydrocarbons, from hexane to pentadecane, was analyzed at 2930 cm<sup>-1</sup> using both fiber-coupled Fourier transform infrared spectroscopy and a modular prototype system. Diffusion coefficients were determined for these compounds and diffusion behavior examined and related to factors such as analyte polarity and molecular size. The diffusion coefficients were found to range from 6.41 × 10<sup>-11</sup> ± 5 × 10<sup>-12</sup> to 5.25 × 10<sup>-11</sup> ± 9 × 10<sup>-13</sup> cm<sup>2</sup> s<sup>-1</sup> for hexane and pentadecane into a 2.9 μm PDMS film, respectively. The diffusion model was also used to examine the effect of changing system parameters such as film thickness in order to characterize sensor response.

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