A new gain calibration protocol for Faraday amplifiers equipped with a 1013 Ω resistor

Abstract

A new gain calibration method (roundabout method) that entails the use of a 10¹³ Ω amplifier-equipped multiple-collector inductively coupled plasma mass spectrometer was developed to correctly determine amplification gains of Faraday detectors. The amplification gains were successfully calibrated by implementing various configurations of Faraday cups and amplifiers in ¹⁴²Nd/¹⁴⁴Nd, ¹⁴³Nd/¹⁴⁴Nd, ¹⁴⁵Nd/¹⁴⁴Nd, ¹⁴⁸Nd/¹⁴⁴Nd, and ¹⁵⁰Nd/¹⁴⁴Nd isotopic measurements. Here, Sr, Hf, W, and Pb isotopic analysis was applied for several reference materials (NIST SRM 987, JMC475, NIST SRM 3163, NIST SRM 981, and NIST SRM 982) to validate the new calibration method. Regarding the resulting isotopic data, the roundabout method yielded isotopic results that were dissimilar to those of an already existing protocol based on constant current calibration. The ²⁰⁸Pb/²⁰⁴Pb isotopic analysis of NIST SRM 981 and 982 resulted in the observed biases reaching >10ε. Alternatively, the isotopic data obtained by using a conventional 10¹¹ Ω amplification system were largely consistent with the roundabout calibration protocol-derived results. This indicates that gains of 10¹³ Ω amplifiers can be accurately determined by using the roundabout technique. Compared to the previous gain calibration technique, the roundabout method shows approximately twice better analytical precision. A significant advantage of the new calibration protocol is that it does not require a reference material for each element. This advantage is critical to improving the analytical accuracy of gain measurements and extending the analytical applicability of MC-ICPMS isotopic measurements in the field of isotope geoscience. The resulting isotopic data clearly demonstrate that precise isotopic measurements can be performed by applying 10¹³ Ω resistors to Faraday detectors. The new calibration protocol has the potential to validate a gain calibration protocol, and its practical advantage is that it effectively downsizes the sample amount used for high-precision isotopic analysis.