## 5 Analytical Methods to Assess VOCs
### VOCs Extraction Techniques and Sample Introduction  
#### 5.2.1 Liquid Extractions from a Solid Sample  
Leaching or solid-liquid extraction is the process of solute component removal from the solid sample by using a liquid solvent. The methods most often used are Soxhlet extraction [130], hydrodistillation, and maceration. The latter named method can be assisted by shaking or ultrasonication. The advantage of ultrasound waves lies in the penetration ability of the matrix material while rupturing the cell walls and driving the solvent into the matrix to extract the target components [132; 133].  
Solvents frequently used are n-hexane, alcohols (ethanol, methanol), or other solvents such as acetone or dichloromethane. Based on the aim of a study, mixtures of solvents are used either to improve extraction yield or to simulate a specific solvent (water/ethanol) in case of VOCs extraction from casks or wood chips to various alcoholic beverages. Naturally, the extraction power of different solvents should be taken into account when designing the method for a target group of compounds.  
The Soxhlet apparatus has been used in a number of studies for the extraction of various sample components, including volatile and semivolatile compounds [52; 61; 65; 89]. In principle, a repeated extraction of a solid sample is performed with condensed vapor of hot solvent in a glass apparatus. When the extraction chamber is full, then it is automatically emptied using siphon. The extracted compounds are being concentrated in a distillation flask below the extraction chamber. In the last decade, focusing on costs reduction and more environmental-friendly extraction, alternative approaches to traditional Soxhlet apparatus were introduced. A similar principle is used in the Soxtec instrument (repeated automated extraction by solvent) or the PLE (pressurized liquid extraction), which is also called ASE (accelerated solvent extraction) [134]. Based on the comparison with the traditional Soxhlet apparatus, PLE is considered as a greener option, since it has similar efficiency, is faster, and uses lower amounts of organic solvents [135; 136; 137; 138].  
Hydrodistillation is often used for essential oils extraction from various plant materials, including wood. It is also suitable for the extraction of semivolatiles' constituents. Three hydrodistillation methods are considered: (i) direct water distillation, when the material is boiled with water in a flask and a mixture of extracted compounds, and water steam is cooled down, and collected; (ii) more gentle, water-steam extraction, where the material is exposed to steam from boiling water below, preventing extracted material from making contact with the bottom of the extraction flask where overheating can occur; and (iii) direct steam extraction, when steam is generated outside of the extraction vessel, reducing the extraction time significantly [63; 135; 139; 140; 141; 142]. In hydrodistillation, the extracted material is exposed to temperatures close to 100 \({}^{\circ}\)C, which can cause the degradation of thermolabile compounds. In case of boiling with water, also an unwanted reaction between extracted compounds can take place.  
Depending on the matrix extracted, authors comparing organic solvent extractions with hydrodistillation reported similar qualitative information, while for specific compounds, the quantitative yield was better in the case of organic solvent extraction [63; 122].