Ultrasound-assisted extraction and derivatization of sterols and fatty alcohols from olive leaves and drupes prior to determination by gas chromatography–tandem mass spectrometry

A method for simultaneous determination of sterols and fatty alcohols in olive leaves and drupes based on ultrasound-assisted extraction and derivatization prior to individual identification–quantitation by chromatographic separation and mass spectrometry detection (single ion monitoring mode) is reported here. The sample preparation procedure involves the following steps: (i) leaching of the raw material accelerated by ultrasound; (ii) saponification of the leachate, also accelerated by ultrasound, and separation of the unsaponifiable matter; (iii) cleaning of the extract by solid-phase extraction; (iv) silylation of the target analytes—also assisted by ultrasound; (v) injection into the gas chromatograph for identification–simultaneous quantitation of the two families of compounds. Individual separation–determination of the fatty alcohols and sterols provide limits of detection (LOD) in the range 9.8 × 10−2 to 2 μg/l and 5.0–6.0 μg/l, respectively. The LOQs range from 0.3 to 0.9 μg/l and 17.0 to 21.0 μg/l, and the linear dynamic ranges are between LOQ and 25.0 μg/ml. The between-day precision, expressed as relative standard deviation (RSD), ranges between 3.6 and 6.1% and the within-laboratory reproducibility, also expressed as RSD, between 6.4 and 9.2%. Within the study of the metabolomic profile of the unsaponifiable fraction in olive tree, the method has been applied to the determination of the target analytes in different varieties of olive trees cultivated in the same zone, so that differences in this unsaponifiable fraction can be attributed to characteristics of the target varieties. As compared with its European Union counterpart, the method is endowed with similar analytical characteristics and drastic shortening of the operational time.