Determination of levothyroxine and its degradation products in pharmaceutical tablets by HPLC-UV-ICP-MS

Abstract

A new analytical methodology using high-performance liquid chromatography (HPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) was applied to study the presence of possible degradation products in levothyroxine tablets. The analytical methodology takes advantage of the element-specific and highly-sensitive I detection provided by ICP-MS and is applied to the speciation of: 3,3′,5,5′-tetraiodothyronine (T₄); 3,3′,5-triiodothyronine (T₃); 3,5-diiodothyronine (T₂); 3,3′,5,5′-tetraiodothyroacetic acid (TTAA₄); 3,3′,5-triiodothyroacetic acid (TTAA₃); and 3,5-diiodothyroacetic acid (TTAA₂). Chromatographic conditions were optimized by using UV absorption at 225 nm and ICP-MS detection of ¹²⁷I. Complete chromatographic resolution of the levothyroxine degradation products within 25 min was obtained using 22% (v/v) acetonitrile at pH 2.3, adjusted with 0.08% (v/v) trifluoroacetic acid. The introduction of the mobile phase containing acetonitrile was performed by post column on-line dilution (1∶3.3) of the chromatographic eluent with a 2% (v/v) nitric acid solution using a PTFE tee. The separations with either detector were good with little detector effect on the resolution. The peak broadening caused by on-line dilution was insignificant. The detection limits obtained with UV detection ranged from 28.9 to 34.5 µg l⁻¹, whereas those obtained with the plasma detector were about 175–375 times better (lower). Finally, the analytical methodology was applied to the determination of possible iodine species originated by degradation of T₄ in synthetic and commercial levothyroxine sodium tablets.