Thorium and iodine memory effects in inductively-coupled plasma mass spectrometry

Thorium and iodine memory effects have been characterized experimentally fo r inductively-coupled plasma mass spectrometry by adding ammonia gas direct ly to the spray chamber and nebulizing aqueous ammonia sample solutions to assess analyte memory retention sites. Thorium memory effect originates fro m the tendency of an unidentified thorium compound to volatilize from the s pray-chamber walls, and not from Th compound adsorption to nebulizer tubing . The mass spectrometer skimmer and sampler cones, ion optics, quadrupole, and other components are not responsible for the memory effect. Unlike that of thorium the iodine memory effect originates from adsorption of iodine c ompounds on nebulizer tubing surfaces and from volatilization of HI and I-2 from the spray-chamber walls. Addition of ammonia sample solutions or ammo nia gas directly to the spray chamber eliminated the Th and I memory effect s in practical analyses, and blank levels were achieved after 2 min wash-ou ts. Quantitative recoveries were obtained for Th and I in reference materia ls.