High-precision MC-ICP-MS static measurements of uranium isotopes using Faraday cups

Abstract

<p indent="0mm">High-precision measurements of ²³⁸U/²³⁵U and ²³⁴U/²³⁸U atomic ratios are essential for U-Pb and ²³⁰Th/U dating, as well as for a wide range of Earth and environmental studies. The ²³⁸U/²³⁵U ratio of a natural material has been considered to be a constant of 137.88 since the 1970s. However, recent advances in analytical techniques, such as TIMS (Thermal Ionization Mass Spectrometry) and MC-ICP-MS (Multiple Collector Inductively Coupled Plasma Mass Spectrometry), allowed the measurement of ²³⁸U/²³⁵U ratios at a precision of 10⁻⁵, revealing up to 5‰ variations in terrestrial ²³⁸U/²³⁵U ratios. <sc>The ²³⁴U/²³⁸U</sc> ratios are commonly observed at the 10⁻⁴–10⁻⁵ levels in the natural environment. These large differences in abundance make it impossible to simultaneously measure U isotopes in Faraday cups connected to the standard 10¹¹ Ω current amplifiers using the MC-ICP-MS. To obtain epsilon precision levels in the ²³⁴U/²³⁸U measurements, previous studies employed a 10⁹ or 10¹⁰ Ω amplifier for the ²³⁸U Faraday cup and a 10¹¹ Ω amplifier for the ²³⁴U Faraday cup. However, these methods require a large quantity of sample material of <sc>400–1000 ng</sc> ²³⁸U, as the 10¹¹ Ω amplifier has a relatively low signal/noise ratio. In 2014, Thermo Fisher Scientific launched a new Faraday cup current amplifier with a 10¹³ Ω feedback resistor, which offers a signal/noise ratio improved by a factor of five compared with standard 10¹¹ Ω amplifiers. Therefore, we are attempting to use the new 10¹³ Ω amplifier in combination with the 10¹¹ and 10¹² Ω amplifiers to perform high-precision static measurements of U isotopes with Faraday cups using a small amount of sample. In the present study, we simultaneously used a 10¹³ Ω amplifier, two 10¹² Ω amplifiers, and two 10¹¹ Ω amplifiers for the ²³⁴U, the ²³³U and ²³⁶U, and the ²³⁵U and ²³⁸U Faraday cups, respectively. To evaluate the relative gain of the 10¹³ Ω current amplifier prior to sample analyses, we repeated the measurements of the ²³⁸U/²³⁵U ratios in a uranium solution with Faraday cups connected to 10¹³ and 10¹¹ Ω amplifiers. The relative gain of the 10¹³ Ω amplifier was calculated as (²³⁸U₁₁/²³⁵U₁₃)/(²³⁸U₁₁/²³⁵U₁₁), where the subscript indicates the resistors of the Faraday cup amplifiers. We also developed approaches to correct for the effects of baseline, memory, tailing, hydride, and mass biases. Using the new static measurement method, we repeatedly measured ²³⁴U/²³⁸U and ²³⁸U/²³⁵U ratios for the international uranium standards HU-1 and SRM950a, as well as for the national uranium standards GBW04412 and GBW04428. Our results (presented with ±2<italic>σ</italic>) show that the international standards HU-1 (<italic>n</italic>=10) and SRM950a (<italic>n</italic>=10) have ²³⁴U/²³⁸U atomic ratios of (54.90±0.03)×10⁻⁶ and (53.88±0.03)×10⁻⁶, and ²³⁸U/²³⁵U ratios of 137.765±0.010 and 137.844±0.005, respectively. On the other hand, the national standards GBW04412 (<italic>n</italic>=8) and GBW04428 (<italic>n</italic>=10) have ²³⁴U/²³⁸U ratios of (101.66±0.04)×10⁻⁶ and (53.86±0.03)×10⁻⁶, and <sc>²³⁸U/²³⁵U</sc> ratios of 137.786±0.018 and 137.995±0.008, respectively. Our results are consistent with previously published data and demonstrated high reliability. Our study shows that using the new method we can rapidly measure ²³⁴U/²³⁸U and ²³⁸U/²³⁵U ratios with precision of 0.3‰–1‰ and 0.3<italic>ε</italic>–0.7<italic>ε</italic>, respectively, consuming <sc>20–30 ng</sc> uranium within ~20 min. In addition, our method is flexible: The combination of different amplifiers can be easily modified according to the characteristics of the analyzed samples and the required measurement precision.