Screening of hydroxytyrosol and tyrosine related metabolites in commercial wines by an UHPLC/MS validated method.

Abstract

Hydroxytyrosol (HT) is a bioactive phenolic compound with antioxidant activity. Yeast synthetise tyrosol from tyrosine by the Ehrlich pathway which is subsequently hydroxylated to HT. The aim of the present work is to develop and validate an UHPLC–HRMS method to assess the metabolites involved in this pathway as well as to screen Spanish commercial wines for HT bioactive compound.

A total of 100 samples of commercial wines were analysed including 57 red wines and 43 white. The analysis was carried out in a Waters Acquity UHPLC (Milford, Massachusetts, USA) coupled to a Waters Xevo TQ (Milford, Massachusetts, USA) triple quadrupole mass spectrometer. The MassLynx MS software was used. The column used was an Acquity UPLC BEH C18. The chromatographic conditions consisted of two mobile phases, water with 0.2% acetic acid (A) and acetonitrile (B), with a gradient elution programmed.

This analytical method was validated following AOAC instructions (AOAC 2012). Linearity, LOD, LOQ, intermediate accuracy, repeatability and matrix effects were the parameters assessed.  Calibration standards were prepared for each analytical batch and three replicates were determined at different concentrations for each compound with 7 degrees of linearity.

Linearity values were calculated through the correlation coefficient (R2) of the curves obtained for each compound. The detection limits were calculated based on the standard deviation of the response and the slope (Ich, 2005).

The intermediate precision was calculated measuring standard deviation (RSD) in a set of three concentrations (LOQ, 10x LOQ and 100x LOQ ng mL−1) for 5 days with 6 replicates per concentration. Repeatability was assessed in a single day-long work session, with six replicates of each concentration.

The matrix effect was tested in a wine synthetic matrix by spiking with the same standard
solution as described above. The slopes resulting from the spiked matrix and calibration solutions (acetonitrile 10% v/v) in the linear range were used to evaluate the matrix effect.

In order to elucidate the effect that filtration caused on the compounds, most usual filters such as nylon (NY), polytetrafluoroethylene (PTFE) and cellulose acetate (CA) were tested. In the case of hydroxytyrosol the LOD was 0.052 ng mL−1 and LOQ 0.157 ng mL-1. For tyrosol, LOD 13,020 and LOQ 39,455 ng mL -1. Tyrosine, LOD 1,567 and LOQ 4,748 ng mL−1 and hydroxyphenylpyruvic acid, LOD 6,795 and LOQ 20,591 ng mL-1. All the values had an R2 between 0.9991 and 0.9999, showing quite good linearity. As we know, this is the first study available in which all the compound of the formation route for hydroxytyrosol has been identified and quantified. This could be accomplished thanks to a validated HRM method developed specifically to diminish LOD and LOQ. Furthermore, we ascertained the differences in the content of hydroxytyrosol in a great range of Spanish wines.

References

AOAC (2012) Appendix F: guidelines for Standard Method Performance
Requirements (SMPR). AOAC Official methods of analysis.
Ich (2005). ICH Topic Q2 (R1) Validation of analytical procedures: Text and methodology. International Conference on Harmonization, 1994 (November 1996), 17.