Samples were analysed by means of an Automatic Thermal Desorber UNITY2 (Markes International) coupled with a GC/MS-system (Agilent, GC6890N – MS 5973i). Samples were desorbed at 320 °C for 15 min and the chromatographic separation was achieved on an Agilent DB624 capillary column (60 m × 250 μm × 1.40 μm). The column was subjected to the following temperature program: 40 °C for 2 min, followed by heating to 90 °C at 5 °C min− 1 (hold 2 min), then increased to 120 °C at a rate of 8 °C min− 1 (hold 4 min), then to 220 °C at 10 °C min− 1 (hold 14 min). A Helium flow of 1.5 ml min− 1was used as a carrier gas.