Accurate and precise measurement of Ce isotope ratios by thermal ionization mass spectrometry (TIMS)

Abstract

The La-Ce isotope system has been rarely applied, mostly due to analytical difficulties. Recent technical refinements of mass spectrometers have overcome some of these technical limitations, but Ce isotope analyses still face considerable analytical challenges. These are mainly related to the low abundance of the minor isotopes 136Ce and 138Ce relative to the main isotopes 140Ce and 142Ce (136Ce=0.19%, 138Ce=0.25%, 140Ce=88.45%, 142Ce=11.11%). Hence simultaneous measurement of ion beams over a large dynamic range is required, resulting in large differences in count statistical uncertainty on the individual ion beams. In addition, the large abundance of 140Ce introduces a tailing effect of the large 140CeO ion beam onto the 136CeO and 138CeO ion beams, which requires adequate correction. Here, we present a chemical purification scheme and high-precision thermal ionization mass spectrometric (TIMS) method for analyzing CeO isotope ratios in silicate samples. The advantages and disadvantages of different mass spectrometric strategies for data acquisition by TIMS were evaluated, including the use of 1010, 1011, and 1012Ω amplifiers and different strategies for the 140CeO tail correction. An optimization scheme was developed for different on-peak and off-peak collection schemes, in combination with different tail and baseline correction methods. It is shown that, as long as the integration times for the on-peak, off-peak (half-mass), and baseline signals are adequately optimized for the employed collection scheme and tail correction method, different strategies yield Ce isotope ratios with similar precision of 20–40ppm (2 S.D.). In contrast to previous studies, we have acquired 140CeO by using a 1010Ω amplifier, and have determined a long-term average 140Ce/142Ce of=7.94319±2. Using a common 136Ce/142Ce=0.01688 for mass fractionation correction, the 136Ce/138Ce of the international rock reference materials BCR-1, BCR-2, and BHVO-2 of this study agree well with those recently reported, when all Ce isotope ratios are reported relative to a common 138Ce/136Ce=1.337366 for the average Ames Ce metal. In addition, Ce isotope ratios for several other widely available international rock reference materials (AGV-2, BE-N, BIR-1, DNC-1, W2A) are presented, and facilitate easy inter-laboratory comparison.