## Extraction methods using organic solvents  
### Liquid samples preparation  
As mentioned above, the current official analytical methods used to extract phenolic compounds from liquid samples are based on liquid-liquid extraction (LLE), followed by gas chromatography (GC) determination with different detection methods. Although this technique offers efficient and precise results, it is relatively time-consuming, possibly harmful due the use of large volume of organic solvents (frequently toxic) and highly expensive. For these reasons, there is an increasing tendency to replace LLE by solid-phase extraction (SPE) for liquid samples. SPE was developed in the 1980s, and has emerged as a powerful tool for chemical isolation and purification. This methodology is an alternative extraction to LLE due to it reduces organic solvents consumption, the length of analysis and it can be automated [22-24]. This extraction method is based on differential migration processes, during which analytes are adsorbed in a solid sorbent and then eluted by elution solvent. Cartridges, columns and syringes are classic devices used in SPE. Elution of retained analytes into the sorbents is carried out with different organic solvents like ethyl acetate, methanol, acetonitrile or acetone [25-27]. The type of solvent chosen depends on the kind of sorbent and the polarity of each analyte. SPE technique has been developed in the off-line and on-line modes, although on-line approach is preferred due to advantages such as higher sensitivity and less manipulation of the samples. Non-polar reversed-phase sorbents with silica base were the first supplies used in SPE of phenolic compounds from water samples [28-30]. Among these types of sorbents, C18 was the most accepted. However, these types of sorbent were soon replaced by other ones due to the low efficiencies achieved in the extraction of phenols. First of all, the interactions between phenolic compounds and silica sorbent are apolar Van der Waals interactions, therefore, the pH sample must be adjusted to avoid ionisation of some phenols, such as for pentachlorophenol and dinitrophenols, but silica based materials exhibit pH instability. On the other hand, phenol and monosubstituted phenols are hydrophilic compounds, with lower affinity for non-polar sorbents, such as silica sorbents, and they present low breakthrough volume. This is the volume of analytes solution that can flow through the sorbent before breakthrough occurs and it is a measure of the sorbent extraction capacity. In SPE, the breakthrough volume is an important feature to take into account since it determines the detection limit that can be reached for each compound. [PERSON] et al. obtained recoveries about 70%, except for phenol and o-chlorophenol, for 10 \(\upmu\)g\(\cdot\)L\({}^{\text{-1}}\) samples when used C18 cartridges. The breakthrough volumes for these two compounds were 20 and 100 mL, respectively, while all other compounds of lower polarity had breakthrough volumes of over 250 mL [31]. For these reasons, there was a trend to use more stable organic polymer sorbents. In this way, new sorbents have been developed by modification of polystyrene-divinylbenzene polymeric (XAD) resins and they have been incorporated to SPE devices. They comprise a polystyrene-divinylbenzene (PS-DVB) hydrophobic structure with different particle size, superficial areas and crosslinked grades. The procedure to extract phenols from water samples with this type of sorbents is similar to the procedure used with silica sorbents. The large carbon content of these sorbents (90% for polymeric sorbents whereas 18% for C\({}_{18}\)), the higher surface area, compared to silica-based materials, and the presence of aromatic modifiers in their surface result result in greater breakthrough volumes, up to 1 L of water for some sorbents [32-34]. Among these types of cartridges, OASIS (_N_-vinylpyrrolidene-divinylbenzene based sorbent) is the most used. [PERSON] _et al_.