High-precision in situ analysis of Pb isotopes in glasses using 1013 Ω resistor high gain amplifiers with ultraviolet femtosecond laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry

Abstract

We report high-precision in situ determination of Pb isotope ratios in glass samples, using 200 nm ultraviolet femtosecond laser ablation coupled with a multiple Faraday collector-inductively coupled plasma mass spectrometer (MFC-ICPMS), equipped with 10¹³ Ω resistor high gain Faraday amplifiers. The use of the highly sensitive ion interface of MFC-ICPMS together with the state-of-the-art amplifiers enabled determination of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb isotope ratios at the highest precision ever achieved from a laser crater with 30 μm diameter and 30 μm depth dug on glass samples containing 1.7–39 ppm Pb. The signal responses of the 10¹³ Ω amplifiers were slower than those of the 10¹¹ and 10¹² Ω amplifiers. We confirmed a strong linear correlation between the rates of signal intensity change for D²⁰⁸Pbi/dt and the measured isotope ratios [^20XPb/²⁰⁶Pb]/dt for the same time intervals. The D²⁰⁸Pbi/dt values deviated around zero, and the [^20XPb/²⁰⁴Pb]/dt value at the zero intercept of the linear regression line represents the Pb isotope ratio of the sample. The slope of the linear regression line was either positive or negative because of different combinations of the amplifiers, indicating that the response of the amplifiers differed individually. The slope also changed with the signal intensity for ²⁰⁸Pbi or D²⁰⁸Pbi/dt, i.e., it was flatter at low levels and steeper at high levels. By using these relationships, corrections were made on the time resolved data measured from a single crater. Furthermore, using the proposed analytical method, ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb isotope ratios in BHVO-2G (1.7 ppm Pb) and BCR-2G (11 ppm Pb) basalt glass samples were analysed by using the National Institute of Standards and Technology (NIST) standard reference material (SRM) 612 (38.57 ppm Pb) synthetic glass as an external standard. The laboratory bias of the basalt glass samples was ±0.05–0.15‰ RD (per mille relative difference) and intermediate precisions were ±3–7‰ 2 SD (per mille 2 standard deviation) for BHVO-2G and ±0.6–3.7‰ 2 SD for BCR-2G. These intermediate precisions, along with repeatability, were approximately 2–3 times better than those obtained by using either multiple ion counter ICPMS or MFC-ICPMS with 10¹² Ω resistor amplifiers.