## 3 Results and Discussion  
### Arrow Method Optimization  
A variety of sorbent materials are now commercially available as part of the PAL SPME Arrow product line. As such, it was necessary to verify suitability for biological applications, specifically for the range of volatiles expected from microbial sources as described for this study. The results of fiber comparison for the SPME 1.1 mm Arrow range are shown in Figure 1a. The CWR/PDMS fiber recovered the highest overall abundance of combined compounds. This was largely attributed to an above average recovery of analytes across all compound classes, whereas other fibers exhibited low recoveries within specific compound groups. For example, the PA fiber had high abundance for >C\({}_{12}\) n-alkanes; however, short chain alkanes (<C\({}_{12}\)) as well as branched and cyclic alkanes--such as 2-methylpentane, 3-methyl-pentane, 3-methylhexane, and cyclohexane--could not be detected from the PA fiber. The high total peak area of the PA fiber is largely attributed to high recovery of >C\({}_{13}\) alkanes. The DVB/PDMS and PDMS fibers exhibited lower recoveries than CWR/PDMS and polyacrylamide for aromatics, aldehydes, sulfides, and alcohols. Overall, the CWR/PDMS fiber was found to exhibit the highest efficacy for compounds expected within biological matrices such as the headspace of bacterial cultures. The longest n-alkane recovered by the CWR/PDMS fiber was n-heptadecane at a retention time of approximately 41.5 min; therefore, it is unlikely that compounds with a retention index above 1700 would be recovered if the GC-MS conditions employed herein are applied in future work. The GC-MS method extended beyond this time to 52 min, however, no larger alkanes were detected. Further testing of the CWR/PDMS Arrow demonstrated that exposure time peaked in performance after 5 min (Figure 1b). Though fluctuations in response occurred with longer exposure times, extending the exposure time beyond 5 min did not result in any substantial increase in VOC recovery. Additionally, a saturated CWR/PDMS Arrow was found not to require a lengthy desorption time in the inlet for maximum desorption (Figure 1c). Sufficient desorption occurred between 40-60 s; however, as the manufacturer recommended a 3 min desorption to ensure conditioning of the Arrow prior to next use, a 3 min desorption was chosen. Carryover testing also confirmed effective conditioning of the fiber prior to next usage. A chromatogram displaying the results of the final SPME Arrow parameters is displayed in Figure 2.  
Figure 1: (**a**) Arrow fiber performance based on total summed peak area of all compounds in the volatiles reference mix (VRM) 1 and the commercial saturated alkanes mix (SAM\({}_{\text{C}}\)). (**b**) Total peak area measured for carbon wide range (CWR)/polydimethylsiloxane (PDMS) Arrow at different exposure times to VRM 2 and SAM 2. (**c**) Total peak area measured for CWR/PDMS Arrow at 5 min exposure time and varied desorption times after saturation in VRM 1 and SAM 1. Figure 2: Total ion current chromatogram of the volatiles reference mix (VRM) 1 and the saturated alkanes mix (SAM) 1 using carbon wide range (CWR)/polydimethylsiloxane (PDMS) solid-phase microextraction (SPME) Arrow. The comparison of CWR/PDMS Arrows to the CWR/PDMS traditional SPME fibers is displayed in Figure 3. In this case, the same method (5 min exposure time, 3 min desorption time) was used. For all compounds except for benzene, naphthalene, and >C\({}_{13}\) alkanes, the SPME Arrow provided significant increase in recovery (\(p<0.05\) for Student's \(t\)-test). Often, the SPME Arrow recovered more than double the total peak area detected with the traditional SPME fiber. This demonstrates the higher capacity of sorbent present on the SPME Arrow fibers, specifically for the range of compounds expected in biological matrices. In addition, it was originally hypothesized that larger recovered peak areas may result in a difference in the reproducibility of the SPME Arrows compared to the traditional SPME fibers.