## Extraction methods using organic solvents
### Liquid samples preparation
This is probably due to its simplicity, inexpensive extraction apparatus and its use in the US EPA official methods, such as 3540 B [21]. Polar solvents, such as methanol, acetone or acetonitrile, give high extraction efficiencies. However, they also extract other undesirable polar compounds present in the samples. With apolar solvents, such as n-hexane or dichloromethane, the extraction of phenols requires a previous acidic digestion of the analytes. Satisfactory recoveries of phenols from soils and sediments have been reported with this technique using, in most cases, mixtures of polar and apolar organic solvents [46, 60-63]. [PERSON] _et al_. developed an analytical protocol for the determination of priority phenolic compounds in soil samples using a solvent mixture, methanol-water (4:1), both containing 2% triethylamine, to enhance the extraction of more chlorinated phenols [64]. Recoveries varied in the range from 67 to 97% with a standard deviation between 8 and 14%. Despite the results obtained with this methodology, Soxhlet extraction makes the analysis procedure excessive time consuming. Moreover, it requires large amount of hazardous organic solvents. Ultrasonic extraction is another conventional technique to extract analytes from solid samples. Although sonication is faster than Soxhlet extraction, it also requires large volumes of toxic and expensive organic solvents. [PERSON] _et al_. applied this technique followed by SPME and GC-FID to extract chlorophenols from soil samples [65]. Recoveries ranged from 81 to 99 % and RSD were lower than 7%. In this last decade, microwave energy has been investigated and widely applied in analytical chemistry to accelerate sample digestion, to extract analytes from matrices and in chemical reactions. Microwave energy is a non-ionizing radiation that causes molecular motion by migration of ions and rotation of dipoles, without changing the molecular structures if the temperature is not too high. Nonpolar solvents, such as hexane and toluene, are not affected by microwave energy and, therefore, it is necessary to add polar additives. Microwave-assisted extraction (MAE) is an efficient extraction technique for solid samples which is applicable to thermally stable compounds. Since its development, MAE has became a viable alternative to conventional methodologies due to it has many substantial improvements over other sample preparation techniques such as shorter extraction time, lower amount of solvent and multiple samples analysed at the same time. In the last few years, the number of papers using this extraction method has increased considerably [66-70]. Many of these publications have evaluated the variables for the optimisation of the MAE procedure and most of them made use of the experimental design approach for the parameters optimisation. The most commonly studied are temperature, extraction time and power, solvent volume and concentration of different solvent mixtures. In the case of phenolic compounds, solvent mixtures such as acetone-hexane and acetone-methanol are usually employed. Among the parameters that have showed a greatest influence on the extraction efficiency are the microwave temperature and the percentage of acetone in the solvent mixture. This fact is probably due to the differential microwave energy absorption by the solid sample and/or the solvent. When the total solvent volume is high compared with the solid sample volume and contains high proportion of a microwave non-transparent cosolvent, such as acetone, energy should be mostly absorbed by the supernatant solvent rather by the solid sample material. Therefore, heating of the sample is produced not only by direct interaction with microwaves but mainly by convective heating from the top hot solvent mixture layer, thus showing slower extraction kinetics.