Identification and quantification of electrochemically generated metabolites of thyroxine by means of liquid chromatography/electrospray-mass spectrometry and countergradient liquid chromatography/inductively coupled plasma-mass spectrometry.

Thyroxine (T4) is one of the major thyroid hormones, which regulates cellular metabolism, central nervous system development, body temperature, reproduction and growth. The simulation of oxidation reactions of T4 may provide further information about the fate of T4 in cells without using laborious in vitro and susceptible in vivo tests. In this study, oxidation products of T4, generated inside an electrochemical (EC) cell, were separated and identified by on-line EC/liquid chromatography/electrospray ionization-mass spectrometry (EC/LC/ESI-MS). In another experimental setup, the electrogenerated metabolites were separated by LC, subsequently mixed with a compensating countergradient (cg), and finally introduced into an inductively coupled plasma-mass spectrometer (ICP-MS). The gradient compensation was achieved by an additional pump module which generated a reversed gradient to the analytical gradient used for the separation. This setup enabled a constant composition of the LC eluent flowing into the plasma so that stable plasma conditions and a uniform response over the complete elution time could be achieved. Combined with identification information from online-coupled EC/LC/ESI-MS, robust and reliable quantification of T4 and its oxidation products was accomplished by on-line coupled EC/cgLC/ICP-MS, with LOD of 33nM of iodine.