## 3 Instrumental Analytical Techniques
### Gas Chromatography (GC)  
GC is an instrumental technique of separation and analysis focused on volatile compound-containing samples. The partitioning of components is possible when the sampled VOC mixture is introduced at the beginning of a chromatographic column, and the mixture is passed through, aided by an inert gas flow. Then, the components interact differentially with the inner column material at the molecular scale according to the compound-column affinity, which is specific for each one at fixed conditions, promoting their separation. Thus, the components are singly eluted from the column for immediate detection and analysis of compound-specific signals. The process is optimized by temperature control, either with constant values or programmed increases. GC is the most commonly used technique for VOC analysis because it allows simultaneous separation and analysis of volatile components with great sensitivity, robustness, reproducibility, and versatility. This technique is suitable for thermally stable volatile compounds since high temperatures are used for sample vaporization and in the chromatographic separation itself [86]. For analysis, GC requires detectors for the recognition, identification and quantification of separated components and allows for a wide variety of them. The most convenient detector choice depends on the compound's physical and chemical properties, origin and concentration as well as the scope of the analysis, etc. In the investigation of VOCs with biological activity on insects, the detectors used with more frequency are the mass spectrometry detector (MSD) for molecular identification of compounds [6,86], flame ionization detector (FID) for quantification, and the electroantennography detector (EAD) for in-line analysis of biologically active molecules. In addition to the separation role of GC, the chemical identity of volatile semiochemicals can be achieved from retention data. Accordingly, retention indices are well-recognized as a significant parameter to the VOC identification within a standardized system [87]. The common process for this identification comprises the calculation of retention indices from retention times (t\({}_{\text{R}}\)) of each resolved compound in the GC profile. This calculation is based on the equations proposed by [PERSON] [for isothermal GC analysis, called retention index (I) or Kovats index (KI)] and [PERSON] [for programmed GC analysis, called the retention index (I), linear retention index (LRI) or programmed-temperature retention index (PTRI or IT)] [88]. KI and LRI do not have the same meaning, and usually in the literature they are misused or exchanged, so such a difference should be considered depending on the GC analysis. In the equations, the retention trend of VOCs is related to a uniform scale determined by a homologous series, having a linear relationship between the Log(t\({}_{\text{R}}\)) and the carbon number, such as n-alkanes, 2-alkanones, and linear fatty acid methyl esters, among many others [89]. These calculated RI values are related to the published information or databases; however, to ensure reliable identification, the RI calculations must be performed on retention data obtained from different runs using at least two GC capillary columns, involving a non-polar stationary phase [e.g., HP-5(r) (5%-phenyl)-methylpolysiloxane)] and a polar stationary phase [e.g., carbowax(r) (polyethylene glycols)]. A detailed compilation of the scope and characteristics of the use of LRI was published by [PERSON] et al., (2008) [88]. A larger number of peaks can be detected if the chromatogram is recorded by more than one GC column. In this sense, a vast advancement has been reached with the technique so-called comprehensive two-dimensional (2D) gas chromatography (GCxGC) [90]. The mixture is firstly separated on a high-resolution capillary GC column and resulting fractions are refocused and injected into a second GC column.