## 2 Materials and Methods
### Identification of Volatile Organic Compounds (VOCs)  
#### 2.3.1 Identification of Targeted VOCs  
The identification of targeted (VOCs) was performed with a static head space sampler connected to a Gas Chromatograph with Flame Ionization Detector (GC-FID) (Perkin Elmer, MA, USA) and equipped with a HP-FFAP column (length: 25 m; internal diameter: 0.20 mm; layer thickness: 0.33 \(\upmu\)m; Agilent Technologies, Glostrup, Denmark). Helium was selected as the carrier gas. The chemicals that were used for dilution and acidification were 1-methyl-2-pyrrolidone, MQW, and 4N H\({}_{2}\)SO\({}_{4}\). The compounds for the preparation of the standards were the following: acetone (100 mg), acetaldehyde (100 mg), ethyl acetate (100 mg), 3-methyl-butanol (100 mg), ethanol (100 mg), diacetyl (100 mg), butan-1-ol (500 mg), and acetoin (500 mg). The chemicals were weighed and transferred to a 100 mL measuring flask containing ca, 50 mL methyl-pyrrolidone. Acetaldehyde was weighted as the last compound due to its volatility. Methyl-pyrrolidone was added until 100 mL and stirred for 10 min. This was equivalent to a 1000/5000 ppm solution. 1 mL of the stock solution was added to 99 mL MQW in a measuring flask and mixed carefully. This was equivalent to a 10/50 ppm solution. For the analysis of the standard solution, 10 headspace vials (20 mL) with 1 mL of the prepared standard plus 200 \(\upmu\)L 4N H\({}_{2}\)SO\({}_{4}\) were prepared, sealed with teflon-lined aluminium caps and analyzed on the day of preparation. The remaining volume of the calibration solution was used as aqueous control solution. It was frozen in tubes containing 3 mL, sealed and stored at \(-18\)\({}^{\circ}\)C prior to analysis. On the day of analysis, the solution was thawed and 1 mL of the solution was transferred to headspace vials with 200 \(\upmu\)L 4N H\({}_{2}\)SO\({}_{4}\) and sealed with teflon-lined aluminium caps. Kovats retention index solution (KI) was prepared with a mix of 25 \(\upmu\)L hexane, 25 \(\upmu\)L heptane, 40 \(\upmu\)L octane, 80 \(\upmu\)L nonane, 160 \(\upmu\)L decane, 360 \(\upmu\)L undecane, 640 \(\upmu\)L dodecane, 2500 \(\upmu\)L tridecane, 100 \(\upmu\)L 1-heptanol, 500 \(\upmu\)L 1-octanol, and 1000 \(\upmu\)L 1-nonanol, from which 25 \(\upmu\)L were transferred to 20 mL vials. For the preparation of the samples, 1 mL of each sample was transferred to a headspace vial (20 mL) with 200 \(\upmu\)L of 4N H\({}_{2}\)SO\({}_{4}\) and sealed with teflon-lined aluminium caps. The samples were analyzed with Masked Clean Air for volatile compounds (MCA-VOC) method with a split flow of 10 mL/min. The identification of VOCs was based on retention time in comparison with that of standards. The injector and detector were maintained at 180 \({}^{\circ}\)C and 220 \({}^{\circ}\)C, respectively. The oven was initially heated to 60 \({}^{\circ}\)C and held for 2 min, then increased to 230 \({}^{\circ}\)C and held for 0.5 min. The calculation of the concentration of each compound in each sample was based on the peak height divided by the response factor (Equation (2)). The response factor was previously established with the standard solutions by the quotient of the peak height divided by the known sample concentration.  
\[\text{Sample concentration }=\frac{\text{Peak height}}{\text{Response factor}} \tag{2}\]