## 3 Results
### IT-SPME Coupled to Nano-LC  
The number of capillaries with suitable dimensions for coupling IT-SPME to Nano-LC is still very low. For this reason, only the extraction efficiency of TEOS-MTEOS modified with SiO\({}_{2}\) NPs and TiO\({}_{2}\) NPs could be tested and compared. Different herbicides were tested as the target compounds (Table 1). Preliminary tests showed that the desorption of the extracted analytes from the capillaries could be effected under a wide variety of eluent compositions; therefore, this study was carried out with the IT-SPME connected to the analytical column. As for Cap-LC, an increment volume had a positive effect on peak areas for a majority of the tested compounds; increasing responses were observed up to sample volumes of 500 \(\upmu\)L, although a further increment to 750 \(\upmu\)L had no a positive effect on peaks areas. This is illustrated, in Figure 3, which shows the effect of the sample volume on peak areas when processing different sample volumes with the TiO\({}_{2}\) NPs modified coating. Figure 2: Chromatograms obtained under optimized conditions for (**a**) three successive injections of a standard solution containing 2.5 \(\upmu\)g/mL of naphthalene and 0.18 \(\upmu\)g/mL of fluoranthene; (**b**) a blank (nanopure water), ditch and river water, and for the extract obtained for a soil sample, and the same soil spiked with naphthalene and fluoranthene. Volume of working solution processed, 200 \(\upmu\)L.  
Consequently, a sample volume of 500 \(\upmu\)L was selected to compare the extraction efficiency of the two capillaries tested. The results obtained are depicted in Figure 4.  
In view of the chemical structures of the tested compounds (Table 1), the retention of the analytes on the extractive phase can be considered the result of hydrophobic, polar and, in some cases, hydrogen bonding interactions. Since all the tested compounds have similar K\({}_{\mathrm{ow}}\) coefficients (see Table 1) functional groups are expected to play a key role in the extraction. In fact, the results of Figure 4 show that even compounds with very similar chemical structure exhibit quite different affinity for the two extractive phases. For example, whereas for the phenylurea fluoreturon, isoproturon and metobromuron both phases provided similar extraction rates, linuron showed much stronger affinity for the TiO\({}_{2}\) NPs functionalized phase. The main difference between the latter compound and the other members of the family is the presence of an additional substituent in the aromatic ring. The presence of two contiguous substituents with partial negative charges was suggested as the reason for the strong affinity of phthalate esters for TiO\({}_{2}\) surfaces by [PERSON] et al. (2018). The reason given by the authors was that this conformation allows the establishment of mono and bidentate interactions between the negatively charged substituents and the TiO\({}_{2}\) sites. The presence of the two electronegative halogen  
Figure 4: Comparison of the peak areas obtained with the TEOS-MTEOS coated capillaries functionalized with SiO\({}_{2}\) NPs and TiO\({}_{2}\) NPs. Concentrations assayed, 25 ng/mL for linuron and 10 ng/mL for the rest of compounds. Figure 3: Variation of the peak areas with the sample volume with the TEOS-MTEOS extractive phase reinforced with TiO\({}_{2}\) NPs. Concentration assayed, 25 ng/mL for linuron and 10 ng/mL for the rest of compounds. substituents in the linuron molecule would facilitate a similar interaction with the TEOS-MTEOS/TiO\({}_{2}\) NPs phase, thus explaining the high extraction rate found for this herbicide. On the other hand, different studies have demonstrated the strong affinity of TiO\({}_{2}\) for compounds containing phosphate groups [15; 19]. The results of Figure 4 indicate that TiO\({}_{2}\) NPs are a better option for reasoniphos and fenthion. However, the extraction efficiency for fenitrothion is superior with the SiO\({}_{2}\) NPs modified phase.