Determination of volatile thiols in lipid matrix by simultaneous derivatization/extraction and liquid chromatography–high resolution mass spectrometric analysis. Application to virgin olive oil

Abstract

A simple, reliable, selective and sensitive method for the determination of volatile thiols at trace levels in lipid matrices was developed. The method consisted of a single-step derivatization/extraction procedure followed by electrospray ionization–high performance liquid chromatography–high resolution mass spectrometry (ESI–HPLC–HRMS) analysis using ebselen as a derivatization agent. The analytical conditions were optimized in a lipid model system (triolein) and virgin olive oil (VOO), both spiked with seven representative volatile thiols. The method was validated and the sensitivity, precision, accuracy and selectivity were evaluated. The experimental limit of quantification (LOQ) of the tested thiols in the VOO matrix ranged from 0.05 to 0.5ng/kg. Moreover, the intra-day relative standard deviation (RSD) was in general lower than 10%, except for esters, which reached RSD values of around 13%. The inter-day RDS ranged between 9.6% and 36.6%. The recovery in VOO ranged from 79% to 20% for derivatives with the highest and lowest polarity, respectively. The methodology was applied to the analysis of some VOO samples. Within this first probe sample, some of the VOO analyzed presented concentrations of 4-methoxy-2-methyl-2-butanethiol above the LOQ, ranging between 0.2 and 1.9ng/kg. The advantage of using full-scan acquisition mode when working with HRMS is that it allows the use of a non-target approach based on the fragmentation of thiol derivatives yielding a main product ion at m/z 275.9922 [C13H10ONSe]+ and that corresponded to the ebselen moiety of the derivatives. Any positive finding satisfied the identification and confirmation criteria established during the validation: retention time drift <3-fold the standard deviation of the method, mass accuracy <2ppm with real resolution ≥20,000 (full width at half maximum) at the mass range of interest, and the presence of isotope ions containing 80Se and 78Se, respectively.