## 3 Results and Discussion
### Comparison of Commercial CWR and Sputtered Carbon SPME Fibers with Our New Test Mix  
Figure 5 shows a comparison of our 11.5 um sputtered carbon SPME fibers (with and without PDMS) to the commercial CWR SPME fiber using our new test mix. Notably, this comparison was made in direct immersion mode. Sulfolane in our new test mix was not detected for any of the fibers, which suggests that using sulfolane in the custom test mix was successful in demonstrating that carbon-based SPME fibers lack the appropriate polarity and strength to overcome sulfolane's strong affinity for water. This observation may prove useful in the future when evaluating more polar stationary phases, which are virtually absent from the commercial SPME market. When considering the raw data for the remaining fourteen probes, with the exception of 2-picoline and docosane, the commercial CWR fiber outperforms the sputtered carbon fibers. The 2-picoline observation (\(p\leq 0.05\)) suggests that the sputtered carbon SPME fiber has a particularly strong affinity for bases, presumably through carboxyl groups on the carbon surface. It is not clear why the other two bases (dicyclohexylamine and \(N\),\(N\)-dimethylaniline) did not exhibit the same affinity for our sputtered carbon fiber as picoline. However, we theorize that any base affinity associated with these compounds may have been preferentially overcome by stronger pi-pi interactions for this aromatic species, which is to be expected of the graphene structures anticipated on both traditional carbon SPME fibers and sputtered carbon SPME fibers. Notably, a direct comparison between the commercial CWR fiber and sputtered carbon fibers is difficult and may not be appropriate because the CWR fiber is considerably thicker, i.e., 95 um vs. 11.5 um. These results suggest that phase thickness/capacity may dominate any functional group selectivity, which is not unexpected. The fact that the custom test mix did not reveal any significant differences in functional group affinities, except for 2-picoline, is consistent with the fact that all three fibers are composed of carbon. It is important to note that the ratios of the signals from the commercial CWR fiber (95 um) to our 11.5 um carbon fiber were greater than or equal to 8:1 for pentane, chloroform, MIBK, linalool, and 2,6-dimethylphenol, where this ratio is approximately that of the carbon thicknesses on the fibers. However, the ratios of the signals from the remaining nine compounds were 3:1 or less, suggesting that the morphology/chemistry of the sputtered carbon fibers was able to overcome the phase thickness discrepancy for the majority of compounds evaluated. It is difficult to draw any definitive conclusions about these results without comparable phase thicknesses. Nevertheless, while phase thickness is an important consideration for extraction/desorption efficiencies and capacity, our previous work comparing 100 um PDMS traditional SPME fibers to 100 um SPME Arrows, indicates that phase volume must be considered [43]. For example, the SPME Arrow has ca. 6 times the phase volume of a traditional SPME fiber (with the same 100 um phase thickness), and it produced ca. 4\(\times\) the response, on average, for 92 VOCs evaluated [43]. Albeit a rather intuitive observation, these previous conclusions highlight the importance of phase volume and therefore suggest that correcting the current results for phase volume may provide more insightful comparisons on the strength/extraction capacity of our sputtered carbon fibers. The 95 um commercial CWR fiber and the sputtered carbon fibers have phase volumes of ca. 0.60 and 0.05 uL, respectively. Figure 6 shows a comparison of our sputtered carbon fibers, with and without PDMS, with the commercial CWR fiber, where the analytical response has been normalized to phase volume, i.e., analytical response/phase volume. These results show that on a per volume basis, our fiber is competitive with the commercial fiber, often outperforming it.