In recent years, consumers have developed an ever-increasing interest in natural products as alternatives for artificial additives or pharmacologically relevant agents. Among them, essential oils have gained great popularity in the food, cosmetic, as well as the pharmaceutical industries. Constituting an array of many lipophilic and highly volatile components derived from a great range of different chemical classes, essential oils are known to be susceptible to conversion and degradation reactions. Oxidative and polymerization processes may result in a loss of quality and pharmacological properties. Despite their relevance for consumers, there is a paucity of information available addressing this issue. Therefore, the present review provides a comprehensive summary on possible changes in essential oils and factors affecting their stability. Focusing on individual essential oils, the various paths of degradation upon exposure to extrinsic parameters are outlined. Especially temperature, light, and oxygen availability are recognized to have a crucial impact on essential oil integrity. Finally, analytical methods to assess both genuine as well as altered essential oil profiles are evaluated with respect to their suitability to track chemical alterations. It is believed that only a careful inspection of essential oils by a set of convenient methods allows profound quality assessment that is relevant to producers and consumers alike.

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A high-performance liquid chromatography (HPLC) method was established using an analytical reversed-phase column and gradient elution to achieve chromatographic separation of typical compounds in essential oils. For detection, a diode array detector monitoring different wavelengths simultaneously as well as a mass spectrometer (MS) were used. Atmospheric pressure chemical ionization operating in the positive mode turned out to be a suitable tool to detect volatiles of different chemical classes and to identify them in essential oil matrices. Characteristic fingerprints of eucalyptus, lavender, may chang, pine, rosemary, thyme, and turpentine essential oils monitored at a representative wavelength (220 nm) demonstrated the suitability of HPLC in essential oil analysis. Additional monitoring wavelengths (210, 250, and 280 nm) provided useful information about the identity of the specific component and opened the possibility to differentiate presumably coeluting compounds by means of their distinct absorption behavior. Finally, peak assignment in seven essential oils was performed on the basis of characteristic retention times and UV and MS data of a broad set of reference volatiles.

This study evaluates a set of analytical parameters to track changes during essential oil storage. On this basis, quality alterations can be monitored.

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