Deiodination of iodinated aromatic compounds with electrospray ionization mass spectrometry

Abstract

Dehalogenation of iodinated X-ray contrast media (ICM) has been reported using electrochemical and bioelectrochemical systems. Correspondingly, dehalogenation of aromatic halogens has also been reported in mass spectrometry (MS) using different ionization techniques like chemical ionization (CI), thermospray, fast-atom bombardment (FAB) and FAB-liquid secondary ionization mass spectrometry (LSIMS). The aim of the present work was to study deiodination of iodinated aromatic compounds in MS with electrospray ionization (ESI). The iodinated aromatic compounds were characterized by liquid chromatography/tandem mass spectrometry (LC/MS/MS) using a quadrupole time-of-flight (QTof)-micro MS instrument and ESI in both positive and negative ion mode. The effect of mobile phase additives like formic acid, acetic acid, trifluoroacetic acid, ammonium formate and ammonium acetate on the negative and positive ESI mass spectra of the iodinated aromatic compounds was studied. Formic acid and ammonium formate induced deiodination of the iodinated aromatic compounds with ESI-MS. Neither acetic acid, trifluoroacetic acid nor ammonium acetate induced the deiodination reaction. The effect was most pronounced with negative ESI where the HI product of the deiodination reaction easily adhered to the aromatic compounds giving rise to HI adducts in the mass spectra. The deiodination reaction was shown to take place in the ESI capillary, since the extent of the reaction was largely dependent on the capillary voltage. The calculated heat of reaction for deiodination of the iodinated aromatic compounds was significantly exothermic for formic acid. This was not the case for acetic acid and trifluoroacetic acid. Care should be taken when using formic acid as a mobile phase additive in LC/MS analyses of iodinated aromatic compounds, since the interpretation of the mass spectra might be influenced by potential dehalogenation reactions taking place in the ESI capillary.