Isomeric Discrimination and Quantification of Thyroid Hormones, T3 and rT3, by the Single Ratio Kinetic Method Using Electrospray Ionization Mass Spectrometry

The single ratio kinetic method is applied to the discrimination and quantification of the thyroid hormone isomers, 3,5,3′-triiodothyronine and 3,3′,5′-triiodothyronine, in the gas phase, based on the kinetics of the competitive unimolecular dissociations of singly charged transition-metal ion-bound trimeric complexes [MII(A)(ref*)2–H]+ (MII = divalent transition-metal ion; A = T3 or rT3; ref* = reference ligand). The trimeric complex ions are generated using electrospray ionization mass spectrometry and the ions undergo collisional activation to realize isomeric discrimination from the branching ratio of the two fragment pathways that form the dimeric complexes [MII(A)(ref*)–H]+ and [MII(ref*)2–H]+. The ratio of the individual branching ratios for the two isomers Riso is found strongly dependent on the references and the metal ions. Various sets are tried by choosing the reference from amino acids, substituted amino acids, and dipeptides in combination with the central metal ion chosen from five transition-metal ions (CoII, CuII, MnII, NiII, and ZnII) for the complexes in this experiment. The results are compared in terms of the isomeric discrimination for the T3/rT3 pair. Calibration curves are constructed by relating the ratio of the branching ratios against the isomeric composition of their mixture to allow rapid quantitative isomer analysis of the sample pair. Furthermore, the instrument-dependence of this method is investigated by comparing the two sets of results, one obtained from a quadrupole ion trap mass spectrometer and the other from a quadrupole time-of-flight mass spectrometer.

Graphical AbstractThe single ratio kinetic method is applied to the discrimination and quantification of thyroid hormone isomers using the ion trap and the QTOF mass spectrometer.Figure optionsDownload full-size imageDownload high-quality image (227 K)Download as PowerPoint slide