citronella oil like odour LRI 1346
40 50 60 70 80 90 100 110 120 130 140 150 m/z FIGURE 6.4 Representation of the mass spectra similarity of b-citronellol enantiomers.
In some specific cases, the information attained by means of GC is not sufficient to characterize a citrus essential oil, and the analysis of the nonvolatile fraction can be required. Oxygen hetero-cyclic compounds, which are a distinct class of flavonoids, can have an important role in the identification of a cold-pressed oil and in the control of both quality and authenticity [92-95]. The analysis of these compounds is usually performed by means of LC, also referred to as high-performance liquid chromatography (HPLC), in normal (NP-HPLC) or reversed-phase (RP-HPLC) applications. The former method, commonly used when the analytes of interest are slightly polar, separates ana-lytes based on polarity by using a polar stationary phase and a nonpolar mobile phase. The degree of adsorption on the polar stationary phase increases on the basis of analyte polarity, and the extension of this interaction has a great influence on the elution time. In general, the interaction strength is related to the nature of the analyte functional groups and to steric factors. On the other hand, RP-HPLC is based on the use of a nonpolar stationary phase and an aqueous, moderately polar mobile phase. Retention times are therefore shorter for polar molecules, which elute more readily. Moreover, retention times are increased by the addition of a polar solvent to the mobile phase, and decreased by the addition of a more hydrophobic solvent.
The on-line coupling of two columns, namely, a m-Porasil (30 cm x 3.9 mm I.D., with 10 mm particle size; Waters Corporation; Milford, USA) and a Zorbax silica (25 cm x 4.6 mm I.D., with 7 mm particle size; Phenomenex, Bologna, Italy), in the NP-HPLC analysis of bitter orange essential oils with UV detection has been reported. A large number of cold-pressed Italian and Spanish, commercial and laboratory-made oils, as also mixtures of bitter orange with sweet orange, lemon, lime, and grapefruit oils were analyzed . A total of four coumarins [osthol (1), meranzin (5), isomer-anzin (6), and meranzin hydrate (14)], three psoralens [bergapten (2), epoxybergamottin (3), and epoxybergamottin hydrate (13)], and four PMFs [tangeretin (8), heptamethoxyflavone (9), nobiletin (10), and tetra-O-methylscutellarein (11)] were identified. In addition, further three unidentified cou-marins (peaks 4, 7, and 12) were detected. The bracketed numbers refer to those in Figure 6.5. In general, Italian essential oils exhibited a higher content of oxygen heterocyclic compounds than the Spanish oils. The use of NP and RP-HPLC with microbore columns and UV detection has also been reported for lemon and bergamot , bitter orange and grapefruit  essential oils. Orange and
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