Use and abuse of retention indices in essential oils analysis.

The identification of essential oil (EO) constituents is a challenging task. Automatic searching in mass spectra libraries is not sufficient, as compounds in EO, usually, present isomeric structural relationships. Even before the coupling of gas chromatographs to mass spectrometry (MS) lead to commercial equipment, identification of EO components was possible by co-injection of authentic standards, whenever available, and/or by the calculation of their retention indices (RI). The RI concept was developed by Kóvats in 1958 to be used in isothermal analyses, as was the common approach at that time. A few years later, van den Dool and Kratz published a RI modification for programmed-temperature analyses. RI do provide a useful tentative indication of the possible molecule(s), especially if combined with MS data. After more than 60 years the system proved its efficacy. However, a worrying trend is the increasing number of articles in which the original concepts or RI are misused, leading to misidentification and, what is worse, propagating wrong data which, in turn, serve as a (wrong) basis for new misidentification, perpetuating the propagation of error. Herein we discuss some causes of this misuse and propose a roadmap for the reliable use of retention indices.