## 3 Discussion  
Depending on the polarity of VOCs of an analyte, several types of fibers may be used for extracting different groups of compounds such as PDMS (a non-polar fiber used for volatile compounds), PA (a polar fiber used for polar semi-volatile compounds), CW/DVB (a polar fiber used for alcohols and volatiles), PDMS/DVB (a bipolar fiber used for volatile compounds of medium polarity, amines and nitroaroamatics), CAR/PDMS (a bipolar fiber used for low molecular weight volatile compounds) and DVB/CAR/PDMS (a bipolar fiber used for polar and non-polar volatile compounds) [34; 36]. In this study, DVB/CAR/PDMS and PDMS/DVB fiber, which were previously shown to be most suitable for untargeted HS-SPME analysis of volatiles [35], were used for the analysis of VOCs from aerial parts of _S. romana_ and _S. montana_ from Croatia. In our study, the number of extracted VOCs was higher using DVB/CAR/PDMS fiber (69 compounds) than it was by using PDMS/DVB fiber (56 compounds), which is in accordance with the work of [PERSON] et al. [37], in which DVB/CAR/PDMS fiber showed better efficiency to extract volatile compounds (50 compounds) from the samples of _Aquilegia japonica_ Nakai and H.Hara, compared to CAR/PDMS (47 compounds) and PDMS/DVB fibers (45 compounds), as well as with the work of [PERSON] et al. [38], where more volatile components were extracted using DVB/CAR/PDMS fiber (51 compounds) than with PDMS/DVB (38 compounds), PDMS (38 compounds) and CAR/PDMS (37 compounds) fibers in _Murraya koenigii_ (L). Sprengel fresh leaves. In our study, the dominant chemical classes of volatile compounds in all samples of the two investigated _Sideritis_ species extracted using DVB/CAR/PDMS and PDMS/DVB fibers were sesquiterpene hydrocarbons followed by monoterrene hydrocarbons, except for _S. romana_ M in whose extract monoterrene hydrocarbons were more abundantly present. The obtained results for _S. montana_ are in accordance with the results reported by [PERSON] et al. [23] for EO of _S. montana_ subsp. _montana_ from central Italy obtained by hydrodistillation, where sesquiterpene hydrocarbons led by germacrene D and bicyclogermacrene were observed as the most abundant EO components. According to the latter, oil-poor species from Lamioideae subfamily produce EOs rich in sesquiterpene hydrocarbons. Furthermore, our results are in accordance with the classification of _Sideritis_ species from Turkey given by [PERSON] et al. [18], based on the main components of hydrodistilled EOs, where _S. montana_ ssp. _montana_ was classified into the sesquiterpene hydrocarbons-rich group, and _S. romana_ ssp. _romana_ into the oxygenated monoterrenes-rich group. Considering the relative percentages of extracted chemical classes, in comparison to PDMS/DVB fiber, DVB/CAR/PDMS fiber extracted more oxygenated monoterrenes and other compounds (mostly non-terpenes), which were more abundantly present in investigated _S. romana_ populations. Conversely, in accordance with previously reported findings [36], PDMS/DVB fiber was observed to be more efficient in extracting sesquiterpene hydrocarbons, which were relatively higher in investigated _S. montana_ populations when comparing to the results obtained for the _S. romana_ populations included in this study. In fact, for both species, the most abundant VOCs, namely the sesquiterpene hydrocarbons germacrene D (_S. montana_) and bicyclogermacrene (_S. romana_, except for _S. romana_ M) were extracted more efficiently by PDMS/DVB fiber. The same was observed for viridiflorol, an oxygenated sesquiterpene that was found as the major characteristic compound of the chemically distinct population of _S. romana_ from Morinje Bay (_S. romana_ M). The dominant compounds of _S. romana_ (except _S.