Isotope dilution-gas chromatography/mass spectrometry and liquid chromatography/electrospray ionization-tandem mass spectrometry for the determination of triiodo-L-thyronine in serum

Isotope dilution-gas chromatography/mass spectrometry (ID-GC/MS) and isotop e dilution-liquid chromatography/tandem mass spectrometry (ID-LC/MS/MS) met hods have been developed for an determination of triiodothyronine (T3) in s erum and their potential as candidate reference methods investigated. In bo th methods, C-13(9)-T3 was used as internal standard, Sample pretreatment c onsisted of deproteinization, extraction and high performance liquid chroma tography (HPLC) purification. Conversion of serum thyroxine (T4) to T3 was controlled by adding C-13(6)-T4. For GC/MS, T3 and C-13(9)-T3 were converte d to the N,O-di-heptafluorobutyryl (HFB) methyl ester derivatives and monit ored at m/z 844 and 853. For LC/MS with electrospray ionization, the transi tions m/z 652/661 to 606/614 were monitored. For use of the methods as cand idate reference methods, special attention was paid to the calibration and the measurement protocol (duplicate analysis of each sample on three occasi ons). Evaluation of the ID-GC/MS and ID-LC/MS/MS methods showed the absence of interference by reverse T3 and T4, a limit of detection of 100 pg (GC/M S) and 18 pg (LC/MS), a recovery of 100 +/- 1.5% (95% confidence interval) and goad precision (the total coefficient of variation, n = 6, was typicall y 1.5%). In addition to the recovery study, the accuracy of the methods was proven by method comparison (ID-GC/MS vs. LC/MS/MS) on three sera, showing a maximum deviation of 1.1%. Finally, the ID-GC/MS method was applied for measurement of 10 different human sera with a T3 concentration range from 0 .6 to 7.3 ng/mL, Copyright (C) 1999 John Wiley & Sons, Ltd.