Citrus peel oils are of great importance for the flavor and fragrance industry because they are widely used in perfumes, beverages, food and cosmetic products. Many publications cover the analysis of the volatile substances of peel oils. Nevertheless, the non-volatile ingredients seem to play an important role in citrus oils, too. These substances act as odor fixatives and therefore influence the olfactory properties. Non-volatiles found in lemon oils are mainly coumarins, psoralens, carotenoids, fatty acids and sterols. The two first-mentioned classes of substances form a large part of the non-volatile residue. The presence of psoralens can lead to the formation of sediments in citrus oils, which can cause problems during their application and therefore have to be removed. Coumarins and psoralens can be used as marker substances for the characterization or for detecting the adulteration of oils. Besides their biological properties, they also seem to be important for the stability of citrus oils.
In the past, analyses of the non-volatile substances have mainly been carried out by HPLC with UV or mass spectrometric detection. This publication describes the analysis of a residue of a cold pressed (CP) lemon oil by HPLC-NMR. Sixteen coumarins and psoralens were identified. The structure of one heretofore unknown psoralen was elucidated.
The Coupling of HPLC with NMR NMR spectroscopy is the most powerful spectroscopic method with which to determine the structures of unknown molecules. The drawback of this technique is the low sensitivity compared with other spectroscopic methods, like mass spectrometry, UV or infrared spectroscopy. With the availability of higher magnetic fields, development of special flow probes with enhanced sensitivity and improved methods for solvent suppression it was possible to couple NMR spectroscopy successfully with high performance liquid chromatography. First studies date back to the end of the 1970s and beginning of the 1980s. Meanwhile, the technique is available at a routine level.