## 3 Results and Discussion
### Application to Shrub Leaves  
After optimizing and validating the proposed methodology, we proceeded with the assays using shrub leaf samples, as outlined in Section 2.2.4. It is crucial to note that field sampling, especially with live species like shrubs, is always subject to availability and prevailing atmospheric conditions. During the period under study (April-May 2023), there was abundant rainfall. It is widely recognized that excessive rainwater can lead to a significant decrease in the content of essential oils in plant leaves; consequently, this affects the type and amount of BVOCs released. Therefore, it is not our intention to systematically evaluate in depth the profiles of all BVOCs, including the minor ones emitted by the shrub species under study, as this requires a much more demanding and detailed sampling program. From a different perspective, the goal of the application to real matrices is to demonstrate, in addition to the levels found, that we have an effective and alternative analytical technology that can be regularly used for monitoring the main BVOCs emitted by plants. Table 2 presents the amounts of the six target monotereponids, expressed on a DW basis, emitted by the leaves of the six Portuguese shrubs under study using the HS-BA\(\mu\)E(CN1)/GC-MS methodology, where good reproducibility is observed. Based on the results in Table 2, it is evident that _Thymus camphotatus_ exhibits the highest emissions of BVOCs among all six shrub leaves; notably, it releases significant amounts of 1,8-cineole (4136.9 \(\pm\) 6.3 \(\upmu\)g/g) as well as \(\alpha\)- and \(\upbeta\)-pinene (763.9 \(\pm\) 0.5 and 259.3 \(\pm\) 0.5 \(\upmu\)g/g, respectively). Moreover, it is the only species among the six to emit carryophylle oxide (411.4 \(\pm\) 0.3 \(\upmu\)g/g). Conversely, _Thymus villosus_ L. leaves demonstrated higher emissions of limonene (138.8 \(\pm\) 0.3 \(\upmu\)g/g) and substantial amounts of 1,8-cineole (104.1 \(\pm\) 0.3 \(\upmu\)g/g) and \(\upalpha\)-pinene (69.1 \(\pm\) 0.1 \(\upmu\)g/g). Figure 5b illustrates a total ion chromatogram profile obtained from application of the HS-BA\(\mu\)E(CN1)/GC-MS methodology to _Thymus camphotatus_ leaves. Under optimized experimental conditions, a significant response to the main BVOCs emitted by this species is observed, comparable to the total ion chromatogram obtained by the injection of the standard solution itself (Figure 5a). Additionally, other BVOCs have been identified, demonstrating the effectiveness of the proposed methodology for BVOC monitoring. _Cistus ladanifer_ L. emits moderate contents of \(\alpha\)- and \(\upbeta\)-pinene (325.1 \(\pm\) 0.2 and 148.5 \(\pm\) 0.5 \(\upmu\)g/g, respectively), whereas _Cistus nonspeliensis_ L. and _Lavandula stocchus_ L. species present lower amounts (<30 \(\upmu\)g/g) of both as well as limonene. None of the six target monotereponids evaluated was detected in _Erica scoparia_ L. leaves, and thymol was not observed in any leaves of the six species under study. This absence may be due to thymol emissions being below the LODs used for the present methodology, or the prevailing conditions may have caused a decrease in its emission. In line with the methodology proposed in this paper, [PERSON] et al. [PERSON] et al. (2017) reported BVOC amounts from tree leaves that align well with our  
Figure 4: Comparison of the response from HS-BA\(\mu\)E(CN1) against HS-SPME(PDMS/DVB) methodologies followed by GC-MS analysis obtained for three common BVOCs (\(\alpha\)-pinene, 1,8-cineole, and thymol) under similar optimized experimental conditions. findings. For example, \(\alpha\)-pinene content in _Cistus ladanifer_ L. and _Thymus camphoratus_ fell within the expected range (95.2-3183.3 \(\upmu\)g/g), as did \(\beta\)-pinene, limonene, and 1,8-cineole (10.2-3503.8, 9.2-883.0, and 110.8-7828.0 \(\upmu\)g/g, respectively). This consistency confirms the robustness and reproducibility of our methodology, as demonstrated before [7].