2.4. Head Space Solid Phase Micro-Extraction Coupled with Multidimensional Gas Chromatography Mass Spectrometery (HS-SPME-MDGC-MS)

The three-phase divinylbenzene/carboxen/polydimethylsiloxan (DVB/CAR/PDMS) Stableflex SPME fiber (50/30 µm thick, 2cm long, 24 Ga, SUPELCO, Bellefonte, PA, USA) with Shimadzu heart cut-MDGC-MS fitted with a Shimadzu AOC-5000 plus auto-sampler was used to perform sample analysis. Prior to SPME fiber extraction, the samples were equilibrated with agitation for 20 min (5 s on, 2 s off) at 60 °C. They were then extracted for 50 min with no further agitation. The fiber was injected into the MDGC-MS for 5 min at 250 °C, followed by further conditioning in an NDL heater for 3 min at 250 °C.

An Rtx-wax column (Restek Corporation, PA, USA) was applied in the first GC (gas chromatography). An Rt^®-βDEXsm connected using a zero dead volume internal union (Valco Instruments Co. Inc., Houston Texas, USA) to a Rt^®-βDEXse column (Restek Corporation, Bellefonte, PA, USA) in series in the second GC. Method parameters were altered from Song et al. [16], due to the inclusion of nerol oxide isomers, as follows. Injector temperature was at 250 °C. The column oven for the first GC was held at 65 °C for 2 min, and then increased to 80 °C at 8 °C/min, following ramp to 125 °C at 2 °C/min for 20 min, then further increased to 230 °C at 8 °C/min and held at this temperature for 10 min. The column oven for the second GC was held at 40 °C for 2 min, then increased to 95 °C at 5.0 °C/min, held for 45 min, followed by an increase of 2.0 °C/min to 100 °C for 10 min, then further increased to 130 °C at 4.0 °C/min for 4 min, and finally increased to 220 °C at 20.0 °C/min and held at this temperature for 3 min. Quantifier, qualifier ions and heart-cut timings in the first GC selected for monoterpene isomers can be found in Table S1.