Determination of lipoic acid, Trolox methyl ether and tocopherols in human plasma by liquid-chromatography and ion-trap tandem mass spectrometry

A method for the simultaneous determination of lipoic acid and/or Trolox methyl ether, along with α‐, γ‐ and δ‐tocopherol was developed using liquid chromatography–tandem mass spectrometry with negative electrospray ionization (HPLC‐ESI‐MS/MS) in an ion‐trap mass spectrometer. Detection and quantification were accomplished by a multiple reaction monitoring method, using specific transitions from precursor ion to product ion for each analyte. Chromatographic separation was achieved in a 12 min run using a C₁₈‐bonded phase and methanol–aqueous ammonium acetate elution gradient. Linear correlations of the chromatographic peak area (r.u. × s^?1) to the injected amount (ng) gave the slope values (r.u. × s^?1 × ng^?1) 2.34 × 10⁴ for α‐tocopherol, 5.05 × 10⁴ for γ‐tocopherol, 1.27 × 10⁵ for δ‐tocopherol, 8.86 × 10⁵ for lipoic acid and 1.23 × 10⁵ for Trolox methyl ether. The lower limit of quantification ranged between 0.02 and 1.22 ng for Trolox methyl ether and lipoic acid. MS³ experiments of γ‐ and δ‐tocopherol suggest ion‐radical reactions and dependence of the tocopherol fragmentation pattern on the phenolic ring methylation degree. The method is shown to be applicable to measurement of these metabolites in human serum after extraction. Copyright © 2012 John Wiley & Sons, Ltd.