## 2 VOC Sampling and Detection
### VOC Detection by Gas Chromatography  
The most common technique used in VOC detection and quantification is gas chromatography. However, in most of the cases due to the low concentration of these compounds in the samples to analyze, a pre-concentration step is needed to ensure the complete detection of all VOCs present and the accuracy of the analysis. Different materials and settings are used with this purpose. [PERSON] _et al_. [20] pre-concentrated gaseous samples from a biofilter surface by sorption onto Tenax TA prior to their analysis by GC-MS preceded by thermal desorption (220 \(\,\)C) and cold trapping using liquid nitrogen. A 60 m dimethylpolysiloxane column (film thickness 1.5 \(\,\)m, internal diameter 0.53 mm) was used. The GC was provided with a splitter that allows analyzing part of the eluate in a FID detector for VOC quantification and the other part in a MS for VOC identification. [PERSON] _et al_. [13] collected air samples from a composting hall in a full scale plant into Tenax-GR adsorption tubes using an air pump. VOC analyses were performed in the laboratory with a thermal desorption/gas chromatograph/mass spectrometer system in combination with simultaneous sniffing. Desorption temperature was maintained during 10 min at 250 \(\,\)C and during this time desorbed compounds were concentrated in a cold trap (-120 \(\,\)C). The GC was equipped with a double column system (HP 5, 30 m, 1 \(\,\)m film) with compounds eluted from one column diverted to MS and those from the other column sniffed. Concentration of single VOCs (25 different compounds) were calculated according to an external standard. Adsorption onto Tenax cartridges was also suggested and used by [PERSON] _et al_. [28], who state that the low water affinity of this material and the high number of substances that is able to adsorb make them an ideal material for the sampling of heterogeneous organic compounds such as those produced in waste treatment plants. These authors use diffusive samplers made up of a cylindrical adsorbing cartridge coaxially put inside a cylindrical diffusive body (polycarbonate and microporous polyethylene) holding a known amount of Tenax. The analysis of the compounds was undertaken by GC-MS with previous desorption.  
[PERSON] _et al_. [14] used activated coconut charcoal traps to obtain gaseous samples for VOC determination in composting experiments at laboratory level. VOCs were then extracted with high purity carbon disulfide under agitation and then analyzed using GC-FID (60 m \(\times\) 0.32 mm SPB-5 capillary column). GC-MS determinations were performed using a 30 m \(\times\)0.25 mm HP-5 MS capillary column. [PERSON] _et al_. [12] collected VOCs from the headspace of MSW samples (kept in vials and heated to 85 \({}^{\circ}\)C) and also from air bag samples using solid phase microextraction (SPME). Different fibers were compared presenting different coating materials and a three-phase fiber polydimethylsiloxane-carboxen-divinylbenzene (PDMS-Carboxen-DVB) was finally selected. GC-MS was then used for analysis and VOC identification. This type of fibers was also used by [PERSON] _et al_. [19] when determining odors and VOCs in an anaerobic digestion treatment plant. In this case, solid samples were carried to the laboratory and gaseous samples collected using a flux chamber. The waste sample was put in a tray container and covered with a Plexiglas chamber continuously flushed with a known air flow. Gas samples were collected from the output port of the chamber in Naplohane bags. The SPME fiber was exposed in the bags for VOC adsorption. VOCs were desorbed for GC-MS analysis exposing the fiber in the GC injection port for 3 min at 250 \({}^{\circ}\)C. Some authors have also used gas chromatography without samples pre-concentration to determine VOC emissions from the composting process of several wastes, although they are mainly focused on the composting of olive oil wastes and their effect on carbon sequestration in soil [37, 38].