The off-line combination of high performance liquid chromatography and comprehensive two-dimensional gas chromatography–mass spectrometry: A powerful approach for highly detailed essential oil analysis

The present contribution is focused on the off-line combination of high performance liquid chromatography (HPLC) and comprehensive two-dimensional gas chromatography–quadrupole mass spectrometry (GC × GC–quadMS), and its application to the detailed qualitative analysis of essential oils. Specifically, a silica column was exploited for the separation of the essential oil constituents in two groups, namely hydrocarbon and oxygenated compounds. After, each HPLC-fraction was reduced in volume, and then subjected to cryogenically modulated GC × GC–quadMS analysis. The volatiles were separated on a normal-phase GC × GC column set, and identified through database matching and linear retention index information. The concentrated HPLC fractions gave origin to unexpectably crowded chromatograms, due to two fundamental GC × GC characteristics, namely the enhanced separation power and sensitivity. The results attained were particularly stimulating with regards to the oxygenated compounds, namely those constituents which contribute most to the essential oil aroma, and are of more use for the evaluation of quality and genuineness. Two genuine Citrus essential oils, bergamot and sweet orange, were subjected to analysis, and compared to applications carried out with a GC–quadMS instrument.