## 4 Discussion  
In this work, the extraction capabilities of different commercially available capillaries (TRB 35, FFAP and PS-DVB) have been tested and compared with those of two polymeric coatings reinforced with NPs, using different pollutants with aromatic rings and Cap-LC. For the highly polar compound saccharine, the TEOS-METOS phase reinforced with NPs is the best option. This is because, besides hydrophobic interactions, the analyte can establish electrostatic interaction with the coating. For the analysis of the aromatic hydrocarbons naphthalene and fluoranthene the PS-DVB coated capillary is recommended. This capillary provided extraction rates much higher than those achieved with the other capillaries; this can be, at least, partially attributed to the thickness of the coating. A relatively high acetonitrile content in the mobile-phase was necessary for the rapid desorption and transfer of the retained analytes from the capillary to the analytical column. In principle, the specificity was also suitable for the analysis of these pollutants in environmental matrices such as water and soil extracts, but further studies would be required to optimized and validate the entire method. In this sense, substantial improvement of the sensibility can be expected by increasing the sample volume (see Figure 1). However, with this extractive capillary, the eluent composition required may be a limitation for the chromatographic separation if compounds with more than four aromatic rings (highly apolar) are going to be processed.  
Although less effective for the extraction of naphthalene and fluoranthene, the TEOS-MTEOS capillary reinforced with SiO\({}_{2}\) NPs is a good alternative, particularly for compounds with functional groups capable of establishing dipole-dipole or hydrogen bonding interactions.  
The number of capillaries that can be used for IT-SPME coupled on-line to Nano-LC is even more limited. In the present study, we have tested and compared TEOS-MTEOS phases modified with SiO\({}_{2}\) or with TiO\({}_{2}\) NPs. The extraction rates were highly dependent on the chemical structure of the analytes. The extraction efficiency was generally better with the TiO\({}_{2}\) functionalized capillary for most of the tested compounds. However, the SiO\({}_{2}\) NPs showed stronger interaction with compounds having nitro groups. Both capillaries allowed the direct analysis of the tested herbicides at ng/mL, illustrating the potential of IT-SPME-Nano-LC for the analysis of organic pollutants. It has to be noted that, throughout our study, we used TEOS-MTEOS capillaries synthetized in different batches and no significant differences in their performance was found, which is in agreement with previously reported results [8; 20].  
The results of this study may help to select the extractive phase according to the characteristics of the targeted compounds. Nevertheless, extending the study to new types of substances is still necessary, particularly in Nano-LC systems.  
[PERSON] performed part of the experiments; the rest of authors designed and performed part of the experiments, analyzed the data and wrote the paper. This research was funded by EU FEDER and the Spanish Agencia Espanola de Investigacion (AEI)(project CTQ2017-90082-P), and the Generalitat Valenciana (PROMETEO 2016/109) for the financial support received. [PERSON] expresses his grateful to the EU FEDER and the Spanish AEI for his pre-doctoral grant.  
**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.