Experimental evidence for the formation of doubly charged oxide and hydroxide ions in inductively coupled plasma mass spectrometry

The formation of doubly charged polyatomic ions in inductively coupled plasma mass spectrometers was investigated using commercially available instruments. The species observed were ThO2+ and ThOH2+, which were found in similar amounts with the different instruments used in this study, when operated under routine analytical conditions. The signal ratios for ThO2+ were between 1.8 x 10-4 and 4.2 x 10-4 relative to the singly charged elemental ion and between 1.4 x 10-2 and 2.2 x 10-2 relative to the doubly charged elemental ion. The formation of ThOH2+ was between 1.1 x 10-4 and 2.8 x 10-4 relative to the singly charged elemental ion and between 0.72 x 10-2 and 1.3 x 10-2 relative to the doubly charged elemental ion. A mechanism is proposed for the formation of the doubly charged oxide and hydroxide ions that is based on the condensation of the doubly charged elemental ion with water or oxygen molecules in the interface region of the mass spectrometer. (orig.)