Insights into Analysis of Phenolic Secoiridoids in Extra Virgin Olive Oil

Extra virgin olive oils (EVOOs) containing more than 5 mg/20 g of tyrosol, hydroxytyrosol and their secoiridoids can be recognized with the health claim related to the protection of blood lipids from oxidative stress. Therefore, a reliable, accurate and standardized analytical procedure is needed to determine these markers of EVOO quality. In order to overcome the limitations of current methods, a detailed investigation of sample preparation and chromatographic conditions was performed by UHPLC-UV-HRMS. The use of C18 fused-core column and a non-acidified gradient elution provided single sharp peaks for oleocanthal and oleacein, allowing their reliable quantitation in UV profile. Positive and negative UHPLC-HRMS/MS characterization of methanolic extracts revealed the presence of dimethyl acetal, methyl hemiacetal and monohydrate derivatives of all secoiridoids. These artefacts were formed in aqueous methanol, usually employed to extract and analyse the EVOO phenols, making the HPLC profiles more complex and the measurements less accurate and reproducible. Acetonitrile proved to be a suitable solvent to avoid the formation of secoiridoid dimethyl acetals and methyl hemiacetals, and to efficiently extract EVOO bioactive phenols. Finally, the phenolic contents of Italian EVOO samples were determined by UHPLC-UV analysis of acetonitrile extracts before (direct method) and after acid hydrolysis (indirect method). The results indicated that the use of tyrosol and hydroxytyrosol as reference standards allowed more accurate quantitative data to be obtained. Direct and indirect methods provided comparable levels of EVOO phenols, highlighting the usefulness of acid hydrolysis in routine analyses. The improved procedure defines the most reliable conditions to provide an analytical method with suitable accuracy and repeatability in the analysis of healthy and functional EVOO phenols.