## 3 Instrumental Analytical Techniques
### Nuclear Magnetic Resonance (NMR)  
The current state of VOC investigation is so widely documented that the use of tools associated with compound databases is usually satisfactory for profiling compounds, in addition to complementary chemotaxonomic knowledge. However, previously uncharacterized compounds can be found, and it is important for an entire structural elucidation of the compound to be carried out, which is possible by means of NMR, which provides highly precise information on the structural properties of small molecules.  
The basis of NMR consists of the application of a high-power magnetic field to a sample that is often highly purified; then, this field interacts with atomic nuclei that have a spin distinct from zero (for example, \({}^{1}\)H, \({}^{13}\)C, \({}^{15}\)N, etc.), promoting a change in their orientation. When the nuclear spins have a parallel or antiparallel orientation with respect to the external magnetic field, a magnetic pulse is applied for a time and then removed, so the nuclear spins return to their initial orientation, which produces a radiofrequency emission that is conditioned not only by the magnetic field but also by the nuclei chemical environment, i.e., the neighboring atoms' spatial distribution and type. The NMR equipment then measures the sample emitted radiofrequencies and compares them to corresponding values of the induced magnetic field, registering a spectrum that shows a signal-pattern with values denoted as \"chemical shifts\" that tend to appear in well-defined ranges according to their corresponding functional group and experimental conditions. The information is further interpreted to correlate the nuclei to each other, and in this way, the whole chemical structure may be deduced [124].  
In the investigation of naturally occurring volatile compounds, NMR is usually separated from other instrumental techniques, such as GC, even when GC systems coupled to NMR (GC-NMR) are currently available [125]. This is because of the NMR sample requirements, which often require high purity, as well as compared to other detection techniques (e.g., GC-MS) where a higher amount of sample is needed, i.e., in the order of micrograms. In natural sources, VOCs are often found in lower quantities and there is often interference from other compounds present on the matrix, which makes their purification, recovery and analysis [126] challenging. As stated by [PERSON] et al. (2015), \"NMR is a valuable technique for structure elucidation but suffers from low sensitivity\" [127].  
Therefore, the application of NMR in topics related to VOCs that are biologically active on insects has been widely referenced for structural elucidation after the isolation of VOCs. Such cases can be exemplified (but not extensively) by the evaluation of desert locust (_Schistocera gregaria_) secretions [127] and the compounds present in the venom of the little black ant (_Monomorium minimum_) [128], and can be used as a tool for the identification of pheromones from different insects, such as the tea moth [129], _Sitophilus oryzae_[130], _Ectropis obliqua_[131], and the azuki bean beetle (_Callosobruchus chinensis_ L.) [132]. In the field of plants, the investigations are much more extended regarding structural elucidation and the identification of purified compounds from essential oils with activity on insects [133; 134; 135] and the analysis of isolated volatile compounds from _Citrus australascia_[136] and _Punica granatum_ L. [137].