## 2 VOC Sampling and Detection
### VOC Sampling: Obtaining Representative Samples from Area Sources  
As can be deduced from the nature of the emitting surfaces, obtaining representative and comparable samples of gaseous emissions is not straightforward. In addition, no impact on the characteristics and conditions of the source should be caused when sampling and no influence of the equipment used on the sample should be guaranteed [31]. In the case of area sources, it is generally very difficult to cover the entire emission area during sampling. Representative sampling sites have to be established, but there are no regulations on how to select the sites [31]. [PERSON] _et al_. [32] located odor sampling points according to a regular array at the surface of different shape piggery anaerobic treatment ponds, being the number of points related to the pond surface area. Odor monitoring was conducted at different periods during a year to take into consideration the possible seasonal variations in the waste characteristics and climatic conditions. Average values were calculated for each pond. A wind tunnel device was used to collect air samples forcing carbon filtered ambient air into the tunnel and measuring air velocity with a hot wire anemometer. [PERSON] _et al_. [33] also used the wind tunnel to obtain air samples for odor analysis at different points of mechanical-biological treatment plants of MSW. These authors also reported the use of a flux chamber and a static hood in the study of a composting plant [34]. The flux chamber was positioned on the heaps during sufficient time to reach equilibrium conditions between the gas and the solid phase. Afterwards, the air sample was collected sucking the air by means of a depression pump inside Nalophan(tm) bags. The static hood was used to obtain air samples on a biofilter surface with the function of isolating the sampling point from the external conditions and to channel the air stream in a separated stack. To obtain representative air samples from a biofilter surface, [PERSON] _et al_. [20] covered the entire surface with a polyethylene foil fixing three of the four sides on the biofilter material. Gaseous samples were obtained by a PTFE tube from the open side of the covering foil.  
[PERSON] _et al_. [35] proposed a methodology to obtain gaseous emission samples from windows and biofilters surfaces in waste treatment plants. The methodology consists of determining the air velocity and the concentration of selected compounds in a matrix of points on the emitting surface defined on the basis of the emitting surface dimensions (although different sampling points at the top and both sides of the composting windows were considered, maximum emission was always found at the top). Air velocity was directly measured at field by means of a thermo anemometer, which limited sensitivity was overcome using a home-made Venturi, protected by a plastic box [18; 36]. Air samples were collected in Tedlar(tm) bags for VOC analysis. The product of VOC concentration (mg m\({}^{-3}\)) and air velocity (m s\({}^{-1}\)) results in VOC mass flow released per window surface area unit (mg s\({}^{-1}\) m\({}^{-2}\)). Measures of VOC emissions were repeated at different days. Data obtained from emission measurements during a single sampling day were represented in a three dimensional graph with window length and perimeter in x and y axes respectively (or alternatively the biofilter dimensions). VOC mass flow values per square meter were placed in the z-axis to obtain an emission surface. The three dimension emission surface was then projected in a two-dimension graph (window perimeter at x-axis and window length at y-axis). Multiplying the pollutant mass flow per area unit by the corresponding area in the graph resulted in the compound mass flow and the sum of the different quantities obtained corresponds to the total mass flow of VOC (g s\({}^{-1}\)). VOCs were determined as total carbon concentration by GC using a FID (flame ionization detector).