Laser-Induced Isotopic Discrimination Correction for Tin(Sn) in Resonance Ionization Mass Spectrometry

Abstract

Resonance ionization mass spectrometry (RIMS) is a highly sensitive technique for isobar-free analysis of long-lived isotopes, leveraging its exceptional elemental selectivity. However, the inherent laser-induced isotopic discrimination (LIID) in RIMS has posed challenges for its application in high-precision isotope ratio analysis. To address this limitation, based on the experimental phenomena observed in the analysis of Sn isotope ratios using RIMS, we investigated how isotope mass and isotope shift affect ionization efficiency, and proposed a semi-empirical internal standard correction method for LIID. Additionally, the combination of the total evaporation method, which is commonly used in thermal surface ionization mass spectrometry (TIMS), with RIMS effectively corrects the influence of mass fractionation on ratio measurements, thereby decoupling the LIID from the mass fractionation. This novel internal correction model for LIID enables RIMS, for the first time, to perform isotope ratio measurements with internal calibration capabilities comparable to those of TIMS and inductively coupled plasma mass spectrometry (ICP-MS). The application of this correction method to Sn isotopes has led to a tenfold improvement in both precision and accuracy. Post-correction analyses demonstrated isotope ratio determinations with precision better than 0.05% and accuracy exceeding 0.1%. This advancement significantly expands the potential of RIMS in fields that demand strict isotopic fidelity, such as nuclear forensics, the nuclear industry, and environmental tracer studies.