A method for determining regioisomer abundances of polyunsaturated triacylglycerols in omega-3 enriched fish oils using reversed-phase liquid chromatography and triple-stage mass spectrometry

Applicability of non‐linear versus linear fractional abundance calibration plots for the quantitative determination of triacylglycerol regioisomers by tandem mass spectrometry

“…For simplicity, the cations that impart charges to the molecules have been omitted from the formulae that follow. The reaction sequences for the case where ESIMS 3 is used with lithium as the cationization reagent and where non‐linearity has been observed in the calibration plots are shown below. In this case it is assumed that the second loss, that of an α,β‐unsaturated acid, occurs most favorably from the sn ‐2 position.…”

“…Fractional abundance calibration solutions were prepared containing binary mixtures of each of five pairs of positional isomers (MME/MEM, PPS/PSP, PPE/PEP, PPD/PDP and EEP/EPE), 1.0 × 10 –6 M LnLnLn and 1.0 × 10 –4 M lithium trifluoroacetate. These TAG pairs, with the exception of PPS/PSP, were selected because they are the same pairs used in our work where non‐linear fractional abundance calibration plots were observed . PPS/PSP was selected because of the expected similarity in fragmentation behavior of P and S. Regioisomers AAB and ABA were mixed to form calibration solutions with molar ratios (ABA:AAB) of 0:1.0, 0.20:0.80, 0.40:0.60, 0.60:0.40, 0.80:0.20 and 1.0:0.…”

“…In one of the MS 3 studies, the authors suggest that they were puzzled by the non‐linearity. It has been suggested to us in a review that some of our data might be of questionable quality, since the associated calibration plots were non‐linear. Lísa et al .…”

“…ESIMS 3 in the positive ion mode with lithium as cationization reagent has been employed in the quantitative analysis of regioisomers for more challenging samples . In this case the [M + Li – RCO 2 H] + ion from the first fragmentation is selected as the precursor for the second fragmentation.…”

“…In all of the quantitative studies listed above the authors have fitted the data for the fractional abundance calibration plot to a linear model with the exception of the two studies involving lithium cationization and ESIMS 3 and one study involving ammonia NICI . In the two former studies the curvature was too obvious to ignore . In the latter study, one calibration plot was said to be linear and one non‐linear.…”