## 2 Tools for Sampling Semiochemicals to Study Insect Interactions
### Most Commonly Used Methods for Collecting (Trapping) Semichemicals
#### 2.2.1 Enclosure Techniques
The first applications of SPME involved fiber geometries [57], and in-tube devices were introduced, which are attachable to analytical platforms for direct desorption and analysis of VOCs. Regarding coating polymers, the most popular are carboxen (CAR), polydimethylsiloxane (PDMS) and divinylbenzene (DVB), both as single or combined materials in different proportions, and polyethylene glycol (PEG), with a broad range of applications due to their distinct polarity [e.g., 7-100 \(\upmu\)m PDMS (non-polar), 60 \(\upmu\)m PEG (polar) and 65 \(\upmu\)m PDMS-DVB (bipolar)] extraction mechanism [absorbents (e.g., PDMS) or adsorbents (e.g., PDMS-DVB)] [55; 58; 59]. Thus, the correct selection of the polymeric material can enhance the affinity for a particular group of semiochemicals depending on the polarity and/or the analyte size [60]. From commercial options, the main advantages are interlaboratory validation, variable geometry (fibers, film-coated bars, membranes, tapes, etc. ), biocompatibility (useful for in-vivo and in-vitro sampling) [61], and applications in diverse fields of knowledge including metabolomics. However, among the disadvantages are the low maximum operating temperature (240 to 280 \({}^{\circ}\)C) and low selectivity [55]. Headspace--Solid-Phase Micro-Extraction HS-SPME  
When the extraction of VOCs of interest is performed through SPME from a gas phase and the adsorbent is located above the emission source without direct contact with it, this is known as HS-SPME [62]. It is very useful for field or in-vivo sampling, which is a substantial advantage considering that it allows the extraction of VOCs that can only be obtained from living organisms in very specific biological or activity-related conditions. This collection can be performed during some particular times of the day or the year and, in this way, it minimizes the loss of these compounds associated with destructive sampling methods. The utilization of adsorbents in fibers designed for this purpose is very common, but recently, other approaches have been considered, such as HS with stirring-bar sorptive extraction (HS-SBSE), in which the adsorbent material is coated on a stirring bar. This technique shows better efficacy when compared with classical HS-SPME because the quantity and surface of the adsorbent are higher [63]. Once the time of capture has elapsed, the bar is washed with distilled water, dried and then submitted to direct thermal desorption in a gas chromatography device [64]. HS-SPME sampling has been a widely utilized method for collecting VOCs from different materials, such as grains at different conditions, as healthy grains [65], grains damaged by insects [66], grains infected with fungus [67; 68] as well as from nectar [69], nuts [70], intact leaves or those affected by herbivory [71; 72; 73; 74], bark [75], flowers [73; 76; 41; 78], essential oils [78], roots [79], fresh fruits [80], insect excrement [74], whole plants [81], living insects [82], and more examples which can be found in the available literature. Other Enclosure Technique Devices  
The enclosure method does not always involve the trapping of VOCs on adsorbent materials at the time of sampling, and the recipient containing the VOCs can be used for its storage until analysis. Depending on the employed configuration, VOCs can be analyzed by attaching the container directly to the instrument without further sample treatment. Nevertheless, in some cases, an additional procedure prior to analysis can be required, for example, when VOCs are collected for subsequent GC analysis, cryofocusing can be necessary. This consists of cooling the VOCs sample under ambient temperature, aiming to avoid the troubles related to injection peak band distortion [83]. Among the existing options for enclosure without adsorption, sampling bags are widely used due to their economic cost for collecting numerous types of volatile compounds, including VOCs emitted from green plants.