## 4 Characterization of VOC Emissions
### Sampling Methods  
#### 4.1.1 Sampling Setups  
The challenge of sampling VOCs emitted from heritage objects is that volatiles are emitted in a dynamic process, i.e., from the surface of objects. Therefore, the emission rate and the surface area from which volatiles are emitted are difficult to determine as it is often not possible to sample heritage objects and often surfaces have complex shapes. In addition, in contrast to environmental analysis, where a large volume of well-mixed air is sampled, these often involve smaller environments, and volatiles are present in lower concentrations. To better define the analytical parameters (sampled area or volume), it is therefore customary to enclose the object, or a part of it, before sampling to isolate the object from the general environment and enable the volatiles to accumulate. It is also essential to ensure that the temperature is constant or at least monitored and, if possible, increased, to increase the emission rates of volatile compounds. The more volatile compounds accumulate in the headspace (gas phase) surrounding the object, as Henry's constant increases with increasing temperature [11, 70]. However, in heritage science, heating an object is only allowed in rare cases. In most cases, the entire object is enclosed in a sampling enclosure, following which the headspace is sampled at room temperature. The migration of the compounds into the headspace depends on their volatility, and after a certain time, a dynamic equilibrium is reached [70, 71]. The most important factors to consider are the object size and whether or not the sampling involves preconcentration of the analytes. When sampling with preconcentration, the absorption of VOCs on a substrate is involved either in active or passive mode. In bulk air sampling (i.e., without preconcentration), no sorbent material is involved, and multiple analyses can be performed on the same air sample as a large volume of air is often collected. When the object under investigation is placed in a **sampling enclosure**, the latter must be non-adsorptive and non-reactive. Glass enclosures/vials can accommodate small- or medium-sized objects, while very large objects can be sampled in their entirely by enclosing them in customized emission chambers (Figure 2a) or enclosure bags, or a part of them can be isolated and sampled [25, 62]. A sealed display case also accumulates volatiles and can be used as a sampling enclosure. Attention has to be paid when the display case materials are emissive themselves, such as wood, which would influence the outcome of the analysis. Typically, emission chambers are made of stainless steel and/or glass to reduce the \"sink effect\", i.e., the adsorption and desorption (re-emission) of volatiles from the inner surfaces of the chamber [7, 73, 72]. They are available in different sizes, from ca. 50 cm\({}^{3}\) microchamber devices to 1 m\({}^{3}\) chambers (Figure 2), and they offer the possibility to adjust parameters such as temperature, relative humidity, air velocity and sampling flow. The released VOCs are then collected onto a sorbent material (sorbent tubes or fibres) selected according to the compounds of interest. The **Flat and Laboratory Emission Cell** (FLEC cell) enables standardized quantification of VOC emission rates from flat surfaces [74; 75; 76]. The geometry of the emitting surface plays a crucial role for the emission characteristics, as rough surfaces often exhibit higher emission rates due to the larger effective surface area. While FLEC cells allow for standardized emission rate determinations, they require the objects to have large flat surfaces, which is often a limitation in heritage contexts. Figure 3 describes different setups for sampling heritage objects with sample preconcentration in active or passive mode with partial or complete enclosure.