## Introduction  
Phenolic compounds are present in the environment as a result of their uses and the processes in which they are implicated. Although they can be originated naturally due to the degradation of humic substances, tannins and lignins, many industrial processes, including production of drugs, textiles, dyes, pesticides and paper, are the main source of these compounds in the environment [1; 2]. Furthermore, chlorophenols have been widely used as wood preservative agents and disinfectants during decades, so they have been released in the environmental media [3]. Although the use of pentachlorophenol is prohibited in most countries, it is still widely found in the wood of pallets, containers, crates and in cardboard, paper, etc. Wooden crates and cardboard boxes are often used to store and transport fresh fruits. Consequently, chlorophenols present in these materials may contaminate the stored fruits by migration [4]. Chlorinated phenols can also be generated from non-chlorinated phenols during drinking water chlorination [5]. Tri-, tetra- and pentachlorophenol are considered the precursors in the formation of corresponding chloroanisoles, known to be powerful odorants in corks and wine. This is one of the most critical problems in the enological industry [6]. Nitrophenols are formed photochemically in the atmosphere from vehicle exhausts [7]. More hydrophilic phenols, such as less chlorinated phenols, are easily distributed in the aquatic media, meanwhile non polar compounds, more chlorinated phenols such as pentachlorophenol, usually persist longer in environment, especially in soils and sediments. Phenols, and particularly chlorophenols, are toxic, and potentially carcinogenic, and they can affect the taste and odour of drinking water with concentrations as low as a few \(\upmu\)g\(\cdot\)L-1. As a consequence, both, the US Environmental Protection Agency (EPA) and the European Union (EU) have included some phenols, mainly chlorophenols and nitrophenols, in their lists of priority pollutants [8; 9]. The structures of eleven phenols considered priority pollutants by the EPA are shown in Figure 1. EU Directive 2455/2001/EC sets a maximum concentration of 0.5 \(\upmu\)g\(\cdot\)L-1 in drinking water and their individual concentration should not exceed 0.1 \(\upmu\)g\(\cdot\)L-1. Analytical techniques commonly used in the determination of phenols are high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) in combination with ultraviolet detection (UV), electrochemical detection or mass spectrometry detection (MS) [10-14]. Liquid chromatography of phenols is generally carried out with the addition of acids or buffers to the mobile phase. Their function is to suppress the ionisation of both, the analytes and the residual silanols of the stationary phase base material, which otherwise would either decrease retention on the analytical column or lead to interactions of the analytes and the stationary phase, resulting in lower separation efficiencies. Also, gas chromatography (GC), using several detection methods like flame ionisation detection (FID), electron-capture detection (ECD) or mass spectroscopy detection (MS), have been used, although in the case of CG, a derivatization step is needed [15-17]. The high polarity of free phenols hinders their correct chromatographic resolution because they produce broad, tailed peaks; this limitation can be circumvented by derivatising free phenols to less polar compounds, such as acetylated derivatives. They are commonly derivatised either before or after extraction, or with an on column reagent in the GC injector port. Nevertheless, the detection limits imposed by environmental quality legislation can only be achieved by using appropriate sample preparation techniques, which provide high enrichment factors of these analytes. Current official analytical methods for phenolic compounds extraction are liquid-liquid extraction (LLE) (US EPA Methods 604, 605, 8041) [18-20] for liquid samples, and Soxhlet extraction, for solid samples [21].