Evaluation of the long-term fluctuation in isotope ratios measured by TIMS with the static, dynamic, and multistatic methods: A case study for Nd isotope measurements

We investigated the long-term fluctuation in high-precision Nd isotope measurements by using a modern thermal ionization mass spectrometer (Triton plus) to evaluate the effect of Faraday cup deterioration caused by the accumulation of strong ion beams into the cups. A number of measurements (n = 76) were conducted by applying three distinct methods (static, dynamic, and multistatic) during an eight-month analytical period, which was further divided into seven short analytical campaigns. The reproducibilities of Nd-142/Nd-144 ratios for campaigns 1-4 were 23, 6.4, and 17 ppm (2 SD) for the static (Jump 1), dynamic, and multistatic methods, respectively. After the replacement of six out of nine Faraday cups (campaigns 5-7), the reproducibilities of Nd-142/Nd-144 ratios were improved to 18, 4.1, and 5.4 ppm (2 SD) for the static, dynamic, and multistatic methods, respectively. This implies that high-precision Nd isotope analysis with not only the static but also the multistatic method is susceptible to the deterioration of Faraday cups. Consequently, the dynamic method is the most effective to minimize the influence of Faraday cup degradation, although it is recommended to ensure that Faraday cups are as fresh as possible to avoid small, but resolvable, shifts in isotope ratios obtained by this method. In addition to Nd-142/Nd-144, we demonstrate that it is possible to determine Nd-148/Nd-144 and Nd-150/Nd-144 ratios with the dynamic method. The reproducibilities of Nd-148/Nd-144 and Nd-150/Nd-144 for campaigns 5-7 were 6.3 and 8.8 ppm, respectively, which are 1.5-4.1 times better than those obtained in previous studies. (C) 2016 Elsevier B.V. All rights reserved.