Optimization of comprehensive two-dimensional gas-chromatography (GC × GC) mass spectrometry for the determination of essential oils

► Optimized parameters, modulation period, discharge time, 1st and 2nd column flows. ► Essential oil concentrations 0.04-6.6 μg/g for rosemary and 0.04-0.5 μg/g for oregano. ► Possible qualitative assignment of the chemical class of the unidentified compounds. ► The most frequent observed structures were: C10H16, C10H14O, C10H16O, C10H18O, C15H24. ► Up to 30 compounds identified for rosemary and oregano in the MS.