Citrus Oils : Liquid Chromatography

Citrus essential oils are very complex matrices which contain numerous compounds of different chemical classes. These compounds are generally divided into two fractions: the volatile fraction, which is the most representative, and ranges between 85 and 99% in the different cold-pressed citrus oils, and the non-volatile residue, which ranges between 1 and 15%. The development of new instrumental analytical techniques, mainly chromatographic, has allowed the characterization of citrus essential oils to become more precise. Gas chromatography is an essential tool for the study of the volatile fraction, while liquid chromatography (thin-layer chromatography (TLC) and highperformance liquid chromatography (HPLC) combined with spectral absorption and Suorescence measurements) is widely used for the study of the composition of the non-volatile residue. This fraction consists largely of oxygen heterocyclic compounds (coumarins, psoralens (furanocoumarins) and polymethoxylated Savones) that exhibit strong absorption in the ultraviolet region ( max about 315 nm). The presence of coumarin compounds is widespread in plants of the Rutaceae family to which the citrus species belong. Their presence is qualitatively and quantitatively different in the different citrus oils, so knowledge of the oxygen heterocyclic fractionmay be useful to assess authenticity, the geographical origin and the possible adulteration of the oils. Figure 1 shows the basic structures of the oxygen heterocyclic compounds present in citrus oils. In the numbered position, coumarins and psoralens may contain hydroxyl, methoxyl, isopentenyl, isopentenyloxy, geranyloxy groups; polymethoxylated Savones contain methoxyl groups. Table 1 lists the oxygen heterocyclic compounds identiRed in citrus oils.