Improving of HS-SPME-GC-MS quantitation of volatile compounds in virgin olive oils by multiple internal standard normalization

This work was focused on the improvement of the HS-SPME-GC-MS analytical method to evaluate the volatile organic compounds (VOCs) of virgin olive oils (VOOs). A reliable analytical method, able to work in an wide working range, is needed to support the standard panel test method usually used to classify VOOs. VOO is a challenging matrix due to the simultaneous presence of: i) compounds at ppm and ppb concentrations; ii) molecules belonging to different chemical classes and iii) analytes with a wide range of molecular mass. Therefore, HS-SPME-GC-MS quantitation based upon the use of external standard method or of only a single internal standard (ISTD) for data normalization in an ISTD method, may be troublesome. As a consequence, we propose an analytical method based on multiple ISTD normalization, in which 11 ISTDs are used for quantitation of 71 VOO-VOCs: a suitable ISTD was selected for each identified VOCs, so expanding the linear working range of each of them. The linear working range obtained with the suitable ISTD was wider than without ISTD or with an unsuitable ISTD, with the only exception for E-2-hexenal. In particular, linear ranges as wide as 10-100 fold of the LOQ were obtained. The method requests the initial preparation of the standard solutions and the optimization for the quantitation; after these steps the standard solutions can be easily stored over time at -20°C until used and the method can be used as a routinely analytical tool for a reliable quantitation of VOCs profile in VOOs of different characteristic. The method was validated and applied to a series of LVOO and EVOO and showed a wide variability of VOCs profile, confirming the usefulness of wide linear working ranges of quantitation. The method was able to quantify the main VOCs responsible for the defectiveness in LVOO and for green attributes of EVOO. In this way, it can be used to support sensorial analysis for the evaluation of positive and negative attributes of VOOs, aimed at the creation of a objective tool for the classification of the VOOs. Future developments are possible for this methodology, ie by the use of larger number on isotopologues and with the use of SPME fibers of larger capacity.