Plasma AppNote
Determination of Cerium Isotope Ratios Using a High-Gain Detection System
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Radiogenic 138Ce/136Ce ratios help constrain the ancient history of light REE enrichment and depletion in Earth and planetary materials, and may also be used for geochronology as 138Ce is the daughter product of 138La. However, measurements of 138Ce/136Ce ratios are challenging due to a number of reasons: Firstly, the small variations in nature. Second, the relative low abundances of both 136Ce and 138Ce (<0.25%), and finally the significant isobaric interferences on both isotopes. Measurement of Ce as an oxide species by TIMS has yielded precise and accurate 138Ce/136Ce ratios(1). MC-ICP-MS holds considerable promise, due to the higher ionization efficiency for Ce and the potential for higher sample throughput. To date, however, the difficulty of correcting for both the larger interferences on Ce isotopes using the ICP source (due to its high ionization efficiency for all elements and the additional presence of interfering 136Xe in the Ar plasma gas) and (2) tailing of the large 140Ce beam under 138Ce has prevented this technique from achieving a level of accuracy and precision that is similar to TIMS.
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