## 3 Results and Discussion
### Comparison of SPME Fibers Using TITPS7414 - (-)-Germacrene D Synthase
R**i} \\ \hline NI & 1.0 & 10.48 & 1331 & & & & & & & & & & & & & & & \\ \hline NI & & & & & & & & & & & & & & & & & & & \\ \hline NI & & & & & & & & & & & & & & & & & & & \\ \hline NI & & & & & & & & & & & & & & & & & & & & \\ \hline NI & & 1.7 & 10.64 & 1340 & & & & & & & & & & & & & & & \\ \hline NI & & & & & & & & & & & & & & & & & & & \\ \hline NI & & & 1.6 & 11.20 & 1370 & 2.0 & 11.17 & 1369 & 1.9 & 11.18 & 1369 & & & & & & & & \\ \hline
**9** \(\alpha\)-Copane & 3.9 & 11.29 & 1375 & 4.6 & 11.26 & 1373 & 5.7 & 11.26 & 1373 & 1.0 & 11.24 & 1372 & 1376 & & & & & & \\ \hline NI & & 1.0 & 11.48 & 1385 & & & & & & & & & & & & & \\ \hline
**6** (–)–\(\beta\)-elemene & 1.0 & 11.56 & 1389 & & & & & 1.4 & 11.53 & 1388 & 1.1 & 11.54 & 1389 & 1391 & \\ \hline NI & & 1.6 & 11.66 & 1395 & 1.2 & 11.63 & 1393 & 1.1 & 11.64 & 1393 & & & & & & & \\ \hline NI & & 3.2 & 12.06 & 1417 & 2.3 & 12.03 & 1415 & 2.9 & 12.03 & 1416 & 8.3 & 12.03 & 1415 & & & & & \\ \hline NI & & 3.3 & 12.24 &Compared with previous publications, these rearrangement products not only show the expected heat-induced rearrangement (Cope rearrangement) products, but also products that are caused by acid induced cyclizations (Figure 4) [13, 37]. The ability of the DVB and CAR material to _ad_sorb H\({}_{2}\)O to the surface and the presence of H\({}_{2}\)O in the carrier gas H\({}_{2}\) create a weak acidic environment that leads to the acid-induced rearrangements observed. (+)-\(\delta\)-cadinene (**5**) offers the most energy favorable composition of all the described acid-induced rearrangement products, thus **5** is chemically favored over the others and consequently is produced at the highest level. Thus, occurrence of **5** indicates acid-induced cyclizations, whereas the occurrence of \(\beta\)-elemene (**6**) indicates heat-induced Cope rearrangements. Using the PDMS material resulted in the lowest amount of rearrangement products at all tested temperatures (_i.e._, 110 \({}^{\circ}\)C, 160 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C) of the four fibers analyzed. PDMS is an _ab_sorption-based coating with a fused silica core. The PDMS fiber was the only fiber where **1** was obtained as the major product at all the tested injection port temperatures (Figure 3a). Multiple products were detected at all tested temperatures, although at 160 \({}^{\circ}\)C and 110 \({}^{\circ}\)C the relative amount of **1** was \(\sim\)90%. At an injection port temperature of 250 \({}^{\circ}\)C the PDMS fiber showed **1** as the major compound, despite reports that germacrene D is only stable up to 180 \({}^{\circ}\)C [13]. The short time in the injection port did not allow for full rearrangement of **1**; thus, when high amounts of **1** are injected, it is still found in the chromatograms, as seen in Figure 3. The rearrangement profile detected with the PDMS fiber is mainly related to heat-induced rearrangements. Acid-induced rearrangement can possibly occur, but not to the same levels as seen for the _ad_sorbent materials. PDMS is very hydrophobic and H\({}_{2}\)O does not _ab_sorb to  
Figure 4: Rearrangement products of germacrene D (**1**). The left side shows the identified heat-induced rearrangement products. The right side shows the identified acid-induced rearrangement products. the material. This is one of the main arguments for the use of this fiber. At 160 \({}^{\circ}\)C the levels of rearrangement products are low and the detected levels of compounds are still high. Of the four fiber materials and different temperatures tested here, the results clearly show that PDMS fiber and an injection port temperature of 160 \({}^{\circ}\)C provide the best conditions for analysis of germacrene D synthases (Table 4).