## 3 Results
### IT-SPME Coupled to Cap-LC
#### 3.1.2 Study of the Extraction Efficiency  
The extraction efficiency for the different capillaries tested was studied by processing increasing volumes of standard solutions of the analytes (50-400 \(\upmu\)L), under the elution conditions indicated above and without the chromatographic column. Although the absolute recoveries were not obtained, the efficiency of the different capillaries tested was compared though the measurement of the peak areas obtained for the target compounds with them [5; 6; 7; 8]. The peak areas obtained for the three compounds tested are depicted in Figure 1.  
As observed from the above figure, the results obtained for saccharine were significantly different to those observed for naphthalene and fluoranthene. In all of the extractive phases tested, increasing the sample volume from 50 \(\upmu\)L to 100 \(\upmu\)L led to an increment of the peak areas of saccharine, but a further increment of the sample volume did not increase the responses. In contrast, the peak areas registered for naphthalene and fluoranthene increased as the volume of sample processed was increased within the tested interval. For the two compounds, the increment of peak areas was particularly marked with the capillaries that provided the highest analyte responses, PS-DVB and TEOS-MTEOS modified with SiO\({}_{2}\) NPs; with these two extractive phases a nearly linear relationship between the sample volume and the peak areas was observed. For the rest of the capillaries, the increment was much more modest.  
For a given sample volume, highest peak areas were obtained for saccharine with the TEOS-MTEOS capillaries modified with SiO\({}_{2}\) and TiO\({}_{2}\) NPs; no significant differences between these two capillaries were found. This suggests that the hydrogen bonding and dipole-dipole interactions that can be established between the NPs and the amino groups of the analyte are the predominant mechanisms of interaction between the extractive coatings and the analyte molecules. Among the phases with aromatic rings, the FFAP coated capillary provided the highest responses. This can be explained by the electrostatic interactions that can be established between the amino group of the analyte and the nitro groups of the nitroterephthalatic acid coating (with positive charge on the nitrogen and negative on the oxygen atoms).  
Figure 1: Effect of the sample volume (or mass of analyte introduced into the system) on the responses obtained with the different extractive capillary coatings tested for (**a**) saccharine (125 \(\upmu\)g/mL), (**b**) naphthalene (20 \(\upmu\)g/mL) and (**c**) fluoranthene (1 \(\upmu\)g/mL).  
Naphthalene and fluoranthene are expected to interact with the TEOS-MTEOS extractive phase mainly by hydrophobic interactions, and also by \(\pi\)-\(\pi\) interactions with the TRB, FFAP and PS-DVB phases. As observed from Figure 1, the PS-DVB provided much higher extraction efficiencies than the TRB and FFAP capillaries. This can be explained by differences on the film thickness, which was much higher for the PS-DVB. In fact, even the TEOS-MTEOS SiO\({}_{2}\) NPs reinforced polymer coating provided higher responses than the TRB and FFAP coated capillaries. As regards the two types of particles used for the reinforcement of the TEOS-MTEOS polymer, the SiO\({}_{2}\) NPs provided higher responses, which suggested that the resulting composite had a higher specific area for interaction, as neither naphthalene nor fluoranthene have functional groups capable of establishing interactions other than hydrophobic interactions.