Instruments

A Varian Saturn 3800 Gas chromatography/mass spectrometry (GC/MS, Varian 3800) devise with a capillary column (RTX-624, 30 m × 0.25 mm × 1.4 μm) was applied. The temperature program was initially set at 50 °C and kept constant for 5 min. Then, it was ramped to 100 °C at a rate of 4 °C/min and finally ramped to 200 °C at 10 °C/min. Consequently, the total run time was 27.80 min. To keep the pressure of carrier gas constant in the injection port, a hand-made steel bar (with the same size of the NTD needle) was fabricated and used to cover the septum hole (Online Resource 1). Ultra-high purity helium (99.999%) was used as the carrier gas at 0.5 mL/min. A hand-made narrow neck glass liner (I.D.: 1.5 mm and neck diameter of 0.5 mm) was used instead of the standard liner in the GC/MS device. The performance accuracy of this liner in the delivery of volatile compounds to column of GC/MS has been confirmed by our previous studies [14, 15]. The mass transfer line was set at 220 °C. The injection port was set at 250 °C and operated in the split mode with a split ratio of 1:10. A calibrated low-volume sampling pump (SKC 222-3 portable personal air sampler) was used for gas sampling. After sampling, the packed NTD was connected to a 2-mL medical syringe containing 1 mL of pure helium gas. Next, the NTD connected to the syringe was injected into GC/MS without drawing the carrier gas for a specific time for complete desorption of the analytes. Finally, the plunger of the medical syringe was manually pressed to deliver the carrier gas along with the desorbed analytes to the GC-MS column (Online Resource 1).