## 4 Characterization of VOC Emissions
### Analysis of Volatiles  
The technique of analysis is selected depending on the compounds of interest and the method by which they are sampled. **Direct-reading devices** allow for real-time readings without the need for sampling and subsequent laboratory analysis. Examples include devices based on photoionization detectors (PIDs), flame ionization detectors (FIDs), electrochemical sensors (ECs, amperometric or potentiometric) and metal oxide semiconductor sensors (MOSs) [105; 106]. **PIDs** are based on ionization of the target molecule by a UV lamp, which generates a current flow that is then converted into a voltage signal. The signal is proportional to the ionized compound, and only compounds with an ionization potential lower than the energy of the UV lamp photons are detected [105]. The greater the difference between these two, the better the response of the detector is. In addition, these detectors are only sensitive to volatiles within a certain range of photoionization energy or measuring a potential or current difference and therefore may exclude some compounds that are present in high concentrations in the environment. PIDs are typically used to measure the tVOCs present, rather than being compound-specific, and their sensitivity is in the range of 1 ppb-1000 ppm [105]. Electrochemical sensors detect organic volatiles based on an electric current generated through the interaction of the analytes with an electrode. The compounds detected are organic volatiles such as formaldehyde and ethanol as well as inorganic ones such as NH\({}_{3}\), NO\({}_{2}\), SO\({}_{2}\) and CO, with a detection range of 100 ppb-50 ppm [105]. MOS sensors are based on a change in resistance when the target volatile interacts with a metal oxide receptor surface. Sensors are available in two types: n-type, in which the change in resistance is given by a reducing gas; p-type, in which the change in resistance is given by an oxidizing gas [106]. They can be used to measure the concentration of a wide range of volatiles, such as alcohols, ethers, ketones, esters, carboxylic acids, nitroalkanes, amines or aromatic compounds. Nevertheless, in contrast to other sensors, MOS sensors have a lower sensitivity and can only measure volatiles in the ppm range. Sensor-based devices are portable, easy to use and require minimal training to operate, making them suitable for applications in heritage environments. However, there are limitations in terms of lower accuracy and often higher limits of detection compared to laboratory-based analytical methods. In particular, if the sensor only measures the sum of all tVOCs such as a PID sensor, it needs to be coupled with some analytical techniques to obtain more information about which species are present in the environment. An established method for the separation and analysis of VOCs is **chromatography**, usually gas chromatography (GC) in conjunction with mass spectrometric detection [78; 105; 107]. The sample, e.g., collected on SPME, is introduced directly, or else extracted first, e.g., from sorbent tubes, where extraction proceeds thermally with solvents. Liquid extracts can also be analysed using high-performance liquid chromatography (HPLC), commonly used for the analysis of aldehydes [108; 109], or with ion chromatography (IC) for the analysis of low-molecular-weight acids, such as formic and acetic acid [110; 111]. To complement the analytical information with sensory information, it is necessary to use **olfactometric detection**, in which the human nose is used to detect and describe odour signals [78]. In GC-MS-O, the two detection techniques are used in parallel and are complementary, as not all MS-detectable compounds are odour-active and vice versa [62; 72; 81; 82]. The output is the chromatogram, which is overlaid with the aromagram (or olfactogram) representing the eluted odorous VOCs [72; 78].