The high purity helium flow rate during the first stage of thermal desorption was set to 50 ml/min, while during the second stage the high purity helium flow rate was 1.5 ml/min. The injection split ratio was set to 1:7. The transfer line temperature TD–GC was set to 150 °C. Detailed information about the two-stage thermal desorption technique was described in previous literature ( Król et al., 2012, Król et al., 2014 and Mar? et al., 2014).