VOCs were analysed using a thermal desorber (Markes International Ltd, Unity) equipped with an autosampler (Markes mod.ULTRA TD) provided with 100 positions and coupled with a gas chromatograph (Agilent GC-6890 PLUS) and a mass selective detector (Agilent MS-5973 N). The thermal desorber provides a twostage mechanism: first the analytes are desorbed from the sample
tube and refocused into a cold trap; then they are desorbed from the trap and carried into the GC column. The parameters of the thermal desorption and GCeMS analysis are listed in Table 1.
Compounds were identified based on their retention times, target and qualifier ions and then confirmed with standard solutions. The standard solutions were prepared by the successive dilution in methanol of a VOC standard mixture at 2000 mg ml1 (Cus-5997 Ultra scientific, Bologna, Italy). To quantify the samples, a calibration curvewas prepared injecting 1 ml of a standard solution into an