## 4 Characterization of VOC Emissions
### Sampling Methods
#### 4.1.2 Sampling Devices  
The most commonly used devices used for bulk sampling are **stainless steel** **canisters and plastic bags**. Canisters can be used passively under pressure or actively with pumps. The main parameters to consider are the moisture content of the air, polarity of the analytes and reactivity of the analytes to water [16]. An adequate level of relative humidity within the canister is fundamental to obtain good recoveries of volatiles preventing absorption and chemical interaction with the inner surface of the canister. Requirement of water vapour for volatiles stability depends on the type of analytes investigated. Polar volatiles require great relative humidity to overcome their affinity with the canister surface and vice versa when non-polar analytes are studied. The study by [PERSON] et al. [77] showed good recovery on day 0 when polar compounds were stored with RH > 27% (with the exception of alcohols with RH > 99%) and RH > 8% for non-polar VOCs. The disadvantages of sampling with canisters are large dimensions, difficult cleaning between sampling events, the risk of water condensation in the container and the tendency for reduced flow rate at the end of sampling due to a decreased difference between container and ambient pressure [16]. Sampling bags are made of polyvinyl fluoride (Tedlar), aluminized Tedlar, polytetrafluoroethylene (Teflon), Kynar(r) polyvinylidene Fluoride (Kynar(r) PVDF) or polyethylene terephthalhalate (Nalophan(r)) [78; 79; 80]. Their advantages are low cost, ease of use and availability in a variety of sizes. Similarly to canisters, they can be cleaned and reused, and unlike canisters, the sampled air needs to be pumped into the bag, which is why active sampling is only possible [16]. Extraction sampling devices are differentiated depending on whether sampling is active or passive. SPME fibres are widely used passive samplers in heritage applications and consist of a syringe with a fused silica fibre coated with a retentive coating. The coated fibre absorbs VOCs during the set sampling time, and afterwards the fibre is retracted into the needle, followed by analysis [70; 81; 82; 83; 84]. Two thermodynamic aspects are relevant: the vapour phase that interacts with the sample and the vapour phase that interacts with the fibre [2]. The advantages of SPME are that it is solvent-free and provides reproducibility, sensitivity and selectivity, with no sample preparation (exception is when an SPME fibre is derivatized, as in the case of aldehyde sampling). SPME sampling is widely used in heritage science [5; 14; 25; 56; 85; 86; 87]. SPME fibres are available with different types of retentive coatings (including mixed coatings) to improve the selectivity, such as polarity. The different criteria for selecting an SPME fibre are analyte size, polarity, analyte concentration and sample matrix complexity. Carboxen fibres are used for low-molecular-weight samples, while they are often used for general-purpose extractions due to their broad analyte compatibility. Fibres such as polyacrylate (PA)-coated fibres and polydimethylsiloxane/divinylbenzene (PDMS/DVB) are suitable for applications such as sampling of highly polar volatile analytes, e.g., amines and alcohols. Multibed fibres such as divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) are used for a wide range of analytes. After sampling, the fibre is desorbed and analysed by gas chromatography or by liquid chromatography after extraction with a solvent. DVB/CAR/PDMS fibres are most commonly used in heritage science due to their wide range of applications [11]. The duration of sampling depends on the application. For laboratory sampling in closed vials, 1 h sampling is commonly used [88; 89], while for bulk air sampling of rooms, studies have shown examples of 1 day sampling up to 15 days [86; 88; 90], with 1 day recommended to obtain the best repeatability [10]. SPMEArrow is a fibre with a larger surface area that allows for higher capacity and analyte extraction compared to conventional SPME fibres [91; 92; 93].