Electrospray Ion Chromatography−Tandem Mass Spectrometry of Oxyhalides at Sub-ppb Levels

An electrospray ion chromatography−tandem mass spectrometry (IC-MS/MS) method has been developed for simultaneous analysis of oxyhalide ions (chlorite, chlorate, bromate, iodate) in water. Oxyhalide ions are extracted, from pretreated sample, in solid phase extraction (SPE), using an ion exchange column. Elution with water/methanol ammonium nitrate eluent is performed on-line with negative ion electrospray mass spectrometry detection. Sample pretreatment is required in order to remove any major ions that displace oxyhalides during SPE, consisting of eliminating SO 4 2 -, Cl -, and HCO 3 - with ion exchange resins. Oxyhalide ion fragmentation yields mass spectra with successive oxygen losses. Isotope contribution allows the selection of two precursor ions per analyte (except IO 3 - ), highly enhancing the selectivity of the method. Eluent ion choice is discussed, particularly because most of the tested ammonium salts were shown to be chlorate contaminated. In water samples, the limits of quantitation are 0.05 (BrO 3 - and ClO 3 - ), 0.5 (IO 3 - ), and 1.0 μg/L (ClO 2 - ). In some ammonium salts, the microgram per kilogram level is reached for chlorate impurity quantitation.