## 3 Results and Discussion
### Comparison of Sputtered Carbon and Silicon SPME Fibers with Our New Test Mix  
One of the objectives of the current study was to develop a test mix with a diverse set of functional groups, which, in theory, will help investigators elucidate differences among SPME fiber coatings. In the text above, we compared the commercial CWR SPME fiber and our sputtered carbon SPME fibers (with and without PDMS). Despite all three fiber configurations being carbon based, we aimed to elucidate functional group affinities, which are attributable to the cauliflower morphology of our sputtered fibers and their sp\({}^{2}\) carbon (and other chemical) characteristics. We are able to confidently assert that our sputtered carbon fibers have a strong affinity for 2-picoline and potentially other bases, presumably through carboxyl groups on its surface. Beyond the aforementioned observation, it is difficult to draw any definitive conclusions on additional functional group affinities. Consequently, we compared the current sputtered carbon SPME fibers (with and without PDMS) with our previous sputtered silicon SPME fibers (with 6-phenylhexlsilane) in an attempt to draw out differences among these SPME fiber coatings using our custom test mix [19]. The results of this experiment are shown in Figure 7, where both sputtered carbon fibers outperformed the sputtered silicon fiber, except for 1-octanol, linalool, \(N\)-\(N\)-dimethylamine, dicyclohexylamine, and acenaphthylene. Similar to the previous comparisons with the commercial CWR SPME fiber, a majority of these observations may be attributable to phase thickness/volume. However, the performance of sputtered silicon SPME fibers with \(N\),\(N\)-dimethylamine and dicyclohexylamine was significantly higher (\(p\leq 0.05\)) than for the sputtered carbon SPME fibers. This trend is present despite the fact that the sputtered silicon fiber has less phase volume than the sputtered carbon fibers, i.e., 2.8 \(\upmu\)m vs. 11.5 \(\upmu\)m thickness, respectively. These results suggest a legitimate functional group affinity to the sputtered silicon phase. In particular, \(N\),\(N\)-dimethylamine and dicyclohexylamine were chosen to potentially demonstrate irreversible adsorption, which may be the case with the sputtered carbon fibers. Nevertheless, this adsorption was not entirely irreversible on the sputtered carbon fibers, i.e., there is a response for \(N\),\(N\)-dimethylamine and dicyclohexylamine. The sputtered silicon SPME fiber does not extract pentane and chloroform at all. These C5 and C1 compounds were probably not extracted due to a lack of phase thickness of the sputtered silicon relative to the sputtered carbon fibers. It is possible these compounds were never extracted, and/or they desorbed/were lost before the fiber reached the GC inlet. Another possibility is that these lower molecular weight species were replaced by higher molecular weight compounds due to competitive adsorption, which we have previously observed with our sputtered fibers [18; 19; 20]. This observation would otherwise have been missed by the [PERSON] and Grob's mixes, which only go down to C7 compounds. Overall, the sputtered carbon fiber with PDMS provided higher efficiencies over the sputtered carbon fiber without PDMS, with the exception of linalool, 2,6-dimethylphenol, and acenaphthylene. This general trend is probably due to the extra phase thickness/volume/hydrophobicity afforded by the PDMS, regardless of the functional group. However, this observation is only significant (\(p\leq 0.05\)) for half of the compounds. Finally, we note that on a per volume basis, the sputtered silicon fiber outperforms the sputtered carbon fibers for 11 of the 15 compounds in the test mix (see Figure 8). The strong performance of the sputtered silicon fiber is presumably the result of its unique chemistry (aromatic ring, alkyl chain, unfunctionalized silanol groups, and siloxane bonds) and morphology. This chemistry may be more controlled than that of the air-oxidized sputtered carbon.