## 2 Results and Discussion
### Development of the Microextraction Methodologies
#### 2.2.2 Optimization of the HS-BA\(\mu\)E Methodology  
Analogously to HS-SPME, the optimization of HS-BA\(\mu\)E was also carried out, including the selection of the sorbent coating, the liquid desorption (LD) time, the solvent type, the equilibrium time and the temperature, regarding the enrichment of the five BVOCs. Starting with the sorbent phase selection, four Ps (S-CN, S-DVB, HLB and S-X) and four ACs (R, SX PLUS, CA1 and CN1) were tested. One of the great advantages of this technology is the possibility of tailoring or adapting the best coating phase for each particular type of application, using common and easily available sorbents. Figure 4a depicts the comparison of the recoveries for the five BVOCs by BA\(\mu\)E using eight different sorbents in the HS sampling mode. From the data obtained, it is possible to notice that the AC(R) sorbent presents the best recovery of all the target analytes, while the AC(SX PLUS) phase has a similar efficiency, with the exception of 1,8-cineole. The ACs are nanoporous solid materials that retain the BVOCs through electrostatic and/or dispersive interactions, according to the textural adsorptive properties, surface area, nanopore volume and dimensions, as well as the surface functional groups. Regarding the micropore network, from the studied ACs, only SX PLUS and R present both ultramicropores (\(\leq\)0.7 nm) and supermicropores (0.7-2.0 nm), while CA1 and CN1 presented merely supermicropores (0.7-2.0 nm). Since the BVOCs studied are very small molecules, this might suggest that an AC with smaller pore sizes (i.e., ultramicropores) presents a better affinity to the target BVOCs, which results in higher microextraction efficiencies. Additionally, the surface chemistry may play a relevant role; the ACs SX PLUS and R present pH\({}_{\text{PZC}}\) values of, respectively, 8.4 and 6.5, while the other ACs present clearly acidic surface chemistries (pH\({}_{\text{PZC}}\) of 2.2 and 5.1). From the AC tested, R gathers the highest volume of ultramicropores and the narrower ultramicropores, as indicated by its high BET constant value (1666 versus 362-937 for the other ACs), associated with a pH\({}_{\text{PZC}}\) value of 6.5. This combination of properties of R seems to enhance the performance of this AC for the quantification of the target BVOCs. In the case of Ps, the recoveries were all above 40%--in particular, P(S-CN) showed a similar response as AC(SX PLUS). Ps retained the target BVOCs through different mechanisms such as \(\pi\)-\(\pi\), dipole-dipole, hydrogen bonds and ionic interactions, depending also on their surface area and particle size. From the results achieved, since AC(R) showed much higher selectivity and recovery yields, it was chosen as the sorbent coating to proceed with the further assays. After selecting the most selective sorbent phase, the back-extraction optimization was carried out, including the solvent type and desorption time under ultrasonic conditions. Five solvents were tested (\(n\)-C\({}_{6}\), DCM, MeOH, \(\Lambda\)CN and MeOH:\(\Lambda\)CN (1:1, \(v/v\))) to obtain the best desorption removal. The results obtained (data not shown) prove that \(n\)-C\({}_{6}\) was the only solvent that allowed for the complete back-extraction of all five BVOCs, while the remaining solvents did not present any signal during GC-MS analysis and, therefore, was selected for further experiments. The influence of the sonication time was also studied at 15, 30, 45 and 60 min. As shown in Figure 4b, the recoveries of \(\alpha\)-pinene, \(\beta\)-pinene and myrcene were at the maximum at 45 min and 30 min for 1,8-cineole, while 60 min was needed for limonene, respectively. Since longer periods of desorption time do not decrease the efficiency of the other four BVOCs, 60 min was selected. HS-BA\(\mu\)E is also a passive approach based on the equilibrium of analytes between the HS sampling mode and the sorbent phase, in which the equilibrium time always plays a key role during the microextraction process.