Measurement of Isotopes of Light Rare Earth Elements in the Form of Oxide Ions: A New Development in Thermal Ionization Mass Spectrometry

An isobaric interference free thermal ionization mass spectrometry (TIMS) method is presented here for the precise isotope measurements of light rare earth elements (LREEs). LREE isotopes can be measured in the form of monoxide ion (MO+), but the previous determinations of LREE isotopes in the form of oxide were hindered, notably by the 17O and 18O related isobaric interferences. In the present study, 16O-enriched oxygen was introduced into the ion source chamber via a specially designed gas inlet system. It was observed that the isobaric interferences caused by 17O and 18O were remarkably reduced by up to 6 and 70 times, respectively, compared to those caused by natural oxygen bleeding. Isotopic ratios of La, Ce, and Nd obtained without isobaric interference corrections show the excellent precision and accuracy in the present study. The results also shed some light on the formation mechanism of the measured oxide species in TIMS. Furthermore it promises more precise determination of LREE abundances and La-Ce and Sm-Nd ages in geochemistry and cosmochemistry.