Oxygen heterocyclic compound screening in Citrus essential oils by linear retention index approach applied to liquid chromatography coupled to photodiode array detector

To date, liquid chromatography coupled to a photodiode array (PDA) detector is the main technique employed to analyse furocoumarins (FCs) in essential oils. This type of detection may lead to poor performance because of the low sensitivity and selectivity of PDA detection. In this work, the linear retention index (LRI) was used in combination with a UV-Vis library for the identification of 35 oxygen heterocyclic compounds in Citrus essential oils, making it possible to distinguish compounds with very similar spectra. As opposed to retention time, which can significantly change with the instrument or laboratory used, LRI represents a kind of normalized relative retention time, as independent as possible from operating conditions. From a qualitative point of view, reproducibility results are provided to definitively assess the suitability of such an approach for inter-laboratory use and for a fully automated process, even when pure and very expensive standard materials are not available, as just the LRI database and the UV-Vis library are needed. From a quantitative point of view, calibration curves were created in pure solvent and by adding standard compounds to several blank matrices, with the aim of evaluating differences, overcoming matrix effects, achieving the correct limits of quantification, and providing the quantitative profile of real samples. The LRI system is not listed in the Commission Decision 2002/657/EC as criteria for the Full Scan UV-Vis detection; however, its validity and applicability are here investigated and proposed for future applications, in combination with the UV-Vis library.