## 4 Methodologies Used for the Analysis of VOCs in WBPs
### Other Methods  
As an alternative to the standardized methodologies described above, several other techniques allow the extraction of VOCs either directly from wood and WBP samples or by sampling the volatiles they emit. Classic extraction methods rely on solid-liquid extraction, aiming to isolate compounds from a solid matrix through a liquid solvent. The efficiency of extraction depends primarily on the solvent's polarity and the chemical nature of the target compounds. Therefore, solvent selection is a critical step. Commonly used solvents include methanol, ethanol, acetone, dichloromethane, and n-hexane, which are often applied as mixtures or with varying percentages of water to improve yield [54]. Other variables, such as the temperature, pressure, extraction time, and the characteristics of the solid sample also influence the outcome of the extraction [113]. Among the classic extraction procedures most frequently used are Soxhet, hydrodistillation, and steam distillation. In 2007, [PERSON] et al. [114] employed the use of accelerated solvent extraction (ASE) coupled with GC-MS to extract and determine volatile and semi-volatile compounds in oak wood chips, having detected the presence of over 90 compounds and identified most of these. Similarly, [PERSON] et al. (2020) [115] determined several natural volatile compounds from Norway spruce, such as terpen and aromatic compounds, in extracts obtained by Soxhet extraction and analyzed by GC-MS. [PERSON] et al. (2017) [116] reported the extraction of volatile terpen from spruce (_Picea abies_), fir (_Abies alba_), and maritime pine (_Pinus pinaster_) by different extraction methods, namely Soxhet, ASE, steam distillation and supercritical CO2 extraction. For the conditions used, Soxhet extraction resulted in a higher concentration of terpen. A widely used alternative technique for capturing VOCs emitted directly from solid samples is solid-phase microextraction (SPME). Originally developed by [PERSON] [117], SPME is a sensitive, solvent-free, and non-exhaustive technique based on establishing equilibrium between the sample and the extraction phases. It has been applied to analyze volatile and semi-volatile compounds from various sample matrices, such as wines [118; 119], wood [53; 69], soil [120; 121; 122], and water [121; 123]. The SPME device consists of a syringe-like unit containing a fused silica fiber coated with a polymer or solid stationary phase which will hold the analytes from the sample by sorption. For air sampling, the fiber is exposed to the headspace of the extraction flask for a set period, after which it will be injected into the GC analytical instrument for thermal desorption and analysis. The selectivity of this technique depends heavily on the choice of the fiber coating, as factors such as polarity, stability, and thickness of the stationary phase will affect the results [124]. Fibers coated with polydimethylsiloxane (PDMS), carboxen (CAR), divinylbenzene (DVB) and polyacrylate, as well as combinations such as CAR/PDMS, PDMS/DVB and DVB/CAR/PDMS, are commercially available and often used for a variety of purposes [54; 124]. These coatings allow for the analysis of a wide range of compounds, including aromatic hydrocarbons (benzene, toluene, xylenes), phenols, alcohols, ketones, and nitroaromatics. Derivatization of the SPME fiber may also enhance selectivity, sensitivity, and chromatographic separation [124; 125]. [PERSON] et al. (2006) [126] studied VOC emissions from two different species of oak wood (_Quercus pyrenaa_ and _Quercus petraea_) used in cooperative using SPME with a DVB/CAR/PDMS coated fiber and results showed that hexanal, furfural, \(\alpha\)-pinene, d-limonene, and \(\alpha\)-terpineol were detected by direct analysis from solid oak wood samples. Similarly, [PERSON] et al. (2019) [127] used a headspace-SPME with a DVB/CAR/PDMS coated fiber to characterize the volatile profile of stem wood in seven different species and identified 16 groups of VOCs.