Proposal of a Linear Retention Index System for Improving Identification Reliability of Triacylglycerol Profiles in Lipid Samples by Liquid Chromatography Methods

The retention index system was proposed in 1958 by Kováts. It is based on the correlation between the retention time of the analytes and the ones of a series of reference standards, making retention data dependent on the chromatographic phenomenon only, viz., on the three-term interaction analyte-stationary phase-mobile phase. Particularly in GC, because the mobile phase has a negligible influence, the retention of the analytes depends almost entirely on the stationary phase and it is as independent as possible from operating conditions. This makes retention index databases usable in samples identification at both the intra- and interlaboratory levels. The scope of this research was to create a similar identification system in LC, where the identification of unknowns is still a challenge because of the unavailability of spectral databases related to the low reproducibility of mass spectra generated by means of the atmospheric pressure ionization technique normally interfaced to LC. Lipid compounds, specifically triacylglycerols, were selected as target analytes mainly due to their regular chromatographic LC profile under reversed phase conditions. The odd carbon chain number triacylglycerol series from trinonain to trinonadecanoin was chosen as the basis of the retention index scale and a database of 209 triacylglycerols was built. For this purpose an ultra high-performance LC method, able to maximize the baseline separation of triacylglycerols in different real-world samples (vegetable oils, fish, and milk samples) was developed.