## 2 Tools for Sampling Semiochemicals to Study Insect Interactions
### Most Commonly Used Methods for Collecting (Trapping) Semichemicals
#### 2.2.1 Enclosure Techniques  
These techniques consist of the confinement of a portion of VOC-containing atmosphere inside a recipient, e.g., canister, cuvette, flask or a special bag. The VOCs collection can, therefore, be performed in passive mode (i.e., no gas flow is employed) or by means of a pump or ventilator, trapping them in a sorbent material, or the container itself. The collected VOCs are subsequently analyzed through analytical platforms. The concept of this method is to keep the VOCs in their original concentrations within the source atmosphere as much as possible until analysis. Diverse factors must be considered for avoiding the loss of some compounds, such as temperature, relative humidity, light exposure, containing recipient surface and sorbent materials, or the control of reactive species (e.g., oxidizers) and therefore ensure reliable measurements [34]. Static (SHS) and Dynamic Headspace (DHS) Sampling  
The vapor above a condensed phase or surrounding a sample/organism, so-called headspace (HS), contains the volatiles released by the target itself, so it can be sampled and analyzed to provide insights into the composition of this HS. There are two main classifications of HS sampling [35]: static (SHS), in which the analytes on the vapor phase are accumulated passively on capture material by sorption; and dynamic (DHS), which employs a gas flow to assist with the extraction and concentration of volatile compounds on the adsorbent phase [36]. There are a huge number of published cases using DHS sampling, so the collection of VOCs has been achieved, for instance, from whole intact plants [37] or those with controlled herbivory [38; 39], flowers [40; 41], fresh fruits [42], insect dress [37], roots [43], and living insects [44]. For DHS, there are many materials explicitly designed for this purpose, and their use is common. An interesting application has been reported using the same principle with an alternative device consisting of a portion of a gas chromatography column for the collection of pheromones from _Grapholita molesta_, with results comparable to those from a commercial product SuperQ [45]. A variant of dynamic HS is the purge-and-trap method, which consists of the confinement of a VOC source on a recipient attached via conduits to a pump that passes inert gas through the sample (purging), facilitating the release of VOCs that are then carried to an adsorbent trap where the compounds concentrate. The abovementioned trap is then extracted with the solvent for VOC recovery [46]. Sorbent-Based Trapping [Solid-Phase Extraction (SPE)]  
Semiochemicals can be passively collected (even at ppm to ppb levels) over a defined period from the HS using small loads (20-200 mg) of a polymeric sorbent packed into a single tube (solid-phase extraction (SPE) from the HS). This tube is made of durable, chemically-inert materials, e.g., borosilicate glass or stainless steel. To improve the trapping, the HS is either pulled or pushed through the packed sorbent at a constant flow-rate (0.1-2 L/min) using a vacuum pump or compressor, respectively [47]. The trapped volatiles are subsequently eluted using low-polar organic solvents (although thermal desorption can be possible) for further analysis typically using gas chromatography (GC) or GC coupled to mass spectrometry (GC/MS). [48]. A common setup of this kind of sampling is known as volatile collection traps (VCTs), comprising the sorbent material packed into a borosilicate glass tube, which is retained with a borosilicate glass wool plug and a PTFE compression seal. The type of sorbent determines the semiochemical selectivity during trapping. Therefore, a careful choice of the appropriate sorbent material should be made. The most common commercial options are Tenax (2,6-diphenyl-\(p\)-phenylene oxide) GC(r) (60 to 80 mesh) and Porapak (ethylvinylbenzene) Q(r) (80 to 100 mesh size), their refined versions Tenax TA(r) and Super-Q(r), and activated charcoal.