## 4 Discussion  
### Comparison of the Extraction Techniques for Chemical Analysis  
The chemical composition of the sheep wool extracts varied across the collections depending on the extraction methods or types of fibers used: e.g., the hydrodistillate mainly contained thialdine, while it was not detected in HS-volatile collections. Thialdine previously was reported in the volatiles of roasted lamb fat [30; 33], and preserved duck eggs [44]. Thialdine is formed during cooking and is most familiar as a flavor component of foods [45]. Investigation using X-ray diffraction data showed that thialdine has an all _cis_-configuration [46]. 3,5-Dimethyl-1,2,4-trifluoriane isomers were also characteristic of hydrodistillate and detected by both MS methods. Hexahydrofaresyl acetone, and alcohols: 1- do-, tri-, tetra-, penta-, hexa-, and hepta-decanols, with a few other trace compounds were detected in hydrodistillate by GC-FID/GC-MS (Tables 1 and 2). GC-FID/GC-MS also found \(p\)-Cresol, hexadecane, pentadecane, 1-octanol, and 1-nonanol in hydrodistillate, and in at least one of the HS collections. For DHS extract, almost all compounds identified by GC-MS were detected by GC-FID/GC-MS, as well, except for linalool, dodecane, and decanal; while, with GC-FID/GC-MS, additionally, 2-hexanone, 3-hexanol, 2-hexanol, 1-heptanol, hexadecane, 1-nonanol, heptadecane, octadecane, nonadecane and ionol were detected (Table 2). Nonanal was the only compound found in all HS collections, detected by both MS methods. 1-Heptanol, 2-ethyl hexanol, and 1-octanol were also found in all HS collections except for SPME by PDMS. The volatiles collected using HS-SPME by PDMS/DVB, and CAR/PDMS, such as hexanal, 1-butanol, heptanal, isoamyl alcohol, 1-pentanol, 1-hexanol, (E)-2-hexenol, benzaldehyde, \(\gamma\)-pentalactone, 2-methylbutanoic acid, phenol, \(m\)-cresol, were not detected in DHS collection. While the major component of wool hydrodistillate was thialdine, the major components for HS-SPME (by PDMS/DVB, CAR/PDMS, and PDMS fibers), and DHS (by Hayesep-Q fiber) collections of row wool were alcohols, aldehydes, alkanes and sesquiterpenes. The HS extracts did not contain thialdine, since this compound is formed during cooking (in this case, hydrodistillation) [45]. It is known that the composition of extracts could greatly depend on the extraction methods used, e.g., in HS extracts the volatile collections vary according to the type of fiber coating, and its ability to absorb/adsorb volatiles; also, desorption techniques (e.g., with solvent, thermal) can play an important role on the composition of the extract. HS SPME and DHS are convenient methods for determination of volatile compounds, and both require much smaller sample amounts compared to hydrodistillation; also, HS SPME and DHS methods require no solvents to absorb/adsorb volatiles, and this is an important advantage. Hydrodistillation can extract large amounts of semi-volatile constituents, most of them might not be in sufficient amounts for headspace collection; hydrodistillation uses water steam, and sometimes this can induce chemical reactions (e.g., thialdine formation). Furthermore, when concentration of the extracted volatiles is very low, the spectrometer cannot detect compounds at concentrations that are below its detection levels. The wool material we used in this study was stored and, accordingly, the volatiles emission might not be as intensive as for fresh material. Accordingly, the volatiles composition varied greatly not only in terms of the extraction techniques used, but also by detection methods applied. Although the two methods, GC-MS and GC-FID/GC-MS, have advantages and disadvantages [47], both analytical approaches provided important insights. Thus, according to the analytical results (Tables 1 and 2), none of the methods were ideal for extraction, or detection of the volatiles, rather they complemented each other.