## 2 Tools for Sampling Semiochemicals to Study Insect Interactions
### Most Commonly Used Methods for Collecting (Trapping) Semichemicals
#### 2.2.1 Enclosure Techniques
Other carbon-based adsorbents are graphitized carbon blacks (Carbotrap(r) or Carbopak B(r)) and carbon molecular sieves (e.g., Carboxen(r) or Carbosieve(r)) [49]. HayeSep(r) porous polymers are considered second-generation materials, with minimal shrinkage and monomer bleed, and are interchangeable with the Porapak(r) series for the collection of volatiles [50]. Each material has its own distinctive fabrication and physicochemical characteristics [51]. For instance, Tenax and Porapak (or HayeSep) sorbents share low affinity for polar and low-molecular weight organic compounds and a high affinity for lipophilic to medium-polar compounds and medium-molecular weight organic compounds, but the thermal stability is different (350 \({}^{\circ}\)C and 250 \({}^{\circ}\)C, respectively), Tenax being more appropriate for thermal desorption [52]. Additionally, the amount of sorbent material is another important factor to take into consideration for VOCs collection. Such a load depends on the chemical features of the VOCs to be collected, adsorbing capacity of the sorbent, the sampling volume, and the gas flow rate [47; 53]. Solid-Phase Micro-Extraction (SPME)  
SPME is a non-exhaustive technique that isolates and concentrates volatile analytes on a miniaturized extraction phase (a polymeric coating material as sorbent) from a matrix/medium in a liquid or gas phase in one step without the intervention of any solvent. In the process, the volatile molecules enter into contact with the sorbent material and deposit onto its surface by intermolecular interactions, with strengths that depend on the affinity of the material for the volatile molecules. This method is primarily useful for VOC collection for qualitative purposes; although quantitative analysis can be performed, a calibration curve is required for each compound using appropriate standards for absolute quantification (see more details in Section 3.1.1) [51; 54]. The efficiency of VOC capture by SPME depends on assorted variables, including the coating material itself, which is probably the most important factor because the diversity of available compositions and dimensions is broad and these parameters determine the affinity for specific groups of molecules. If possible, to enhance the choice of the best option, a preliminary evaluation of different coatings is advisable, considering their performance in the recovery of desired metabolites through thermal or solvent desorption. In the first case, the metabolites are desorbed directly from the material with a high efficiency and can be coupled directly to GC, but desorption must be performed quickly and below the decomposition temperature of the sorbent to avoid permanent damage. In solvent desorption, the main advantage is the possibility of storing the dissolved VOCs for ulterior analysis or bioassays, but some of the original compounds can be lost due to solubility in addition to the masking of most volatile VOCs by the solvent [51; 54]. Apart from the adsorbent selection, there are more variables that intervene in the extracted VOC profile and should be controlled as much as possible, including the humidity range, biological sample uniformity, temperature, and device geometry. Among these, the exposure time is a parameter that is convenient to normalize depending on the experiments, since short periods result in low concentrations of captured compounds, but long periods favor fixation of less volatile compounds while more volatile compounds are desorbed, according to the affinity between adsorbent-VOCs [54; 55]. A SPME device is comprised of a protection layer, coating material and support [56]. Currently, different device geometries have been developed to improve the sampling and extraction such as fibers (polymeric coating supported onto a fused silica rod); in-tube (a tube internally coated with extraction phases); and other coated supports such as particles, fabric/blade and stir bar [55].