2.3. Heracles e-Nose Analysis

The analysis of the volatile molecules in the sample headspace was carried out using a Heracles II (Alpha MOS, Tuluse, France) ultra-fast chromatography electronic nose [18]. The instrument consists of a double-columns ultra-fast-chromatography system, with FID detectors, interfaced with a PAL-RSI automatic headspace autosampler, after injection a Tenax TA polymer trap is employed. The two columns were mounted in parallel in the oven; they had different polarities, namely, an MXT-5 (non-polar) and MXT-1701 (slightly polar) were employed, both 10 m in length, with internal diameters of 0.18 mm and phase thicknesses of 0.40 μm. A temperature ramp was employed, starting from 50 °C for 2 s, then going to 80 °C at 1 °C·s⁻¹ and finally reaching 250 °C at 3 °C·s⁻¹. The total fast GC analysis time was 110 s. The carrier gas was hydrogen.

The different replicates of each extracted sample were loaded in the instrument autosampler and incubated for 20 min at 40 °C before injection with 500 rpm agitation (5 s on, 2 s off). Then, 1 mL of air headspace was injected with a syringe temperature of 50 °C. Trap loading conditions were 18 s at 40 °C, then flashed to 250 °C for the release in the two columns at split ratio 1:1.

The AlphaSoft v 16.0 software was used to process the data. Volatile compounds were identified on the basis of Kovats’ relative retention indices (KI) and can be linked to specific molecules that are collected in the AroChemBase v 7.0 database (Alpha MOS., Tuluse, France). In this way, eighteen compounds were tentatively identified as further discussed in Section 3.