GC–MS, GC–Flame Photometric Detector (FPD) and GC–O analysis

The GC–MS analysis was performed using a Hewlett-Packard 7890A GC with a 5975C mass selective detector (MSD) (Agilent Technologies, USA) working under electron ionisation (EI) circumstance (70 eV, ion source temperature 230 °C) with the ion trap operating in a scanning mode (the scan range was 30–450 m/z at a scan rate of 1 scan/s). The separation was performed using DB-INNOWAX column (60 m × 0.25 mm i.d. × 0.25 μm film thickness, Agilent Technologies, USA) and DB-5 analytical fused silica capillary column (60 m × 0.25 mm i.d. × 0.25 μm film thickness, Agilent Technologies, USA). Helium (purity 99.999%) was used as the carrier gas with a constant flow velocity of 1 mL/min. The temperature of quadrupole mass filter was set at 150 °C. The transfer line temperature was set at 250 °C. The oven temperature was 60 °C ramped at the rate of 3 °C/min to 230 °C and held for 10 min. The volatile compounds were positively identified by comparing retention indices (RIs) and retention times with those obtained for authentic standards, or by matching the measured mass spectra with the standards in the Wiley 7n.l Database (Hewlett-Packard, Palo Alto, CA). The RIs were determined via sample injection with a homologous series of alkanes (C6–C30) (Sigma-Aldrich, St. Louis, MO, USA). GC–O was performed on a 7890A GC equipped with a flame ionization detector (FID), olfactory detection port (ODP, Gerstel, Mülheiman der Ruhr, Germany). Working conditions were same to the GC–MS. Postcolumn flow was split at a ratio of 1:1 to the FID and the ODP using two deactivated and uncoated fused silica capillaries (106 cm × 0.15 mm I.D., 139 cm × 0.1 mm I.D.) at the end of the capillary. Each sample was run in triplicate.

Sulfur compounds were detected using a flame photometric detector (FPD) (Agilent Technologies, USA) installed on the Agilent 7890A GC. Separations were accomplished using two different capillary columns as described above in the GC–MS and GC–O analysis. The oven temperature program used was the same as that employed for the GC–O studies. Identification of sulfur compounds was based on matching RIs from authentic sulfur standards and RIs reported in the literature with those observed in the samples.

The quantification was performed using the internal standard method. A set of standard mixtures, previously prepared and containing known concentrations of the chemical standards (C_analyte) and the I.S. concentration (C_I.S.), were analyzed and their peak areas (A_analyte and A_I.S.) recorded. For each chemical standard, a six-point calibration line of relative peak area (A_analyte/A_I.S.) versus C_analyte was drawn to confirm a linear detector response, from which the amount of the analyte could be determined. The unknown concentration of an analyte (C_analyte,X) was calculated according to the interpolation of the calibration line as:

where DRF_analyte was the Detector Response Factor (DRF) for an individual analyte. The internal standard (1-decanol) using a DRF of 1.0.