Speciation of linear and branched hydrocarbons by a fluorinated polyimide film based surface acoustic wave sensor

We report a surface acoustic wave (SAW) device-based sensor that uses a fluorinated polyimide film as the chemically sensitive interface to preferentially detect linear over branched alkane isomers. Linear hydrocarbons penetrate the film, producing a change in mass that is over 100 times larger than for comparable concentrations of the branched isomer of the same alkane, consistent with very limited penetration by the latter. Results were confirmed using real-time FTIR external-reflection spectroscopy (ERS), which also shows that the penetration of normal alkanes results in conformational changes to the polymer; no similar effects are observed as a result of exposure to branched alkanes. We believe that the speciation results because the chains of this polymer pack together unusually closely and uniformly, with access to the interconnected `free volume` of the polymer film being through openings large enough to pass methyl or methylene groups, but too small for a CH-CH{sub 3} unit to go through `sideways`. The range of hydrocarbons to which this effect extends, the dependence of separation efficiency on film preparation techniques, and models of polymer chain packing to explain these results are under investigation. 12 refs., 2 figs.