Improvement of in situ Pb isotope analysis by LA-ICP-MS using a 193 nm Nd:YAG laser

Abstract

We have tested the capabilities of a new 193 nm Nd:YAG laser ablation system for in situ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb measurements by LA-ICP-MS. Experiments were performed with MPI-DING (KL2-G, ML3B-G, ATHO-G, T1-G) and USGS (GSE-1G) reference glasses using the fast electrical scan mode (0.075 s per pass) of a single collector sector-field mass spectrometer. Mass discrimination was determined by sample-reference material (e.g., NIST SRM 612) bracketing with linear interpolation. We typically obtained an external precision for ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb of 0.05–0.2% RSD for 50–100 μm three-spot analyses of the low-Pb (1–10 μg g⁻¹) MPI-DING glasses. The precision of Pb isotope ratio data of the high-Pb GSE-1G sample (366 μg g⁻¹) was <0.2% for spot sizes ≥10 μm. Even 2 and 5 μm spot size measurements could be performed with GSE-1G. The LA-ICP-MS isotope ratio data of the MPI-DING reference glasses agree within uncertainties (about 0.05% and 0.1% for ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb, respectively, and 100 μm spot size analyses) with the reference values obtained from TIMS and solution MC-ICP-MS. A comparison of the 193 nm Nd:YAG laser results with those obtained from a 213 nm Nd:YAG laser system shows that there is almost a factor of two improvement in precision using the new 193 nm Nd:YAG laser. Because of lower sample consumption of the 193 nm Nd:YAG laser, the Pb detection efficiency (2.5 × 10⁻³) of the mass spectrometer is a factor of 2.5 higher than that using the 213 nm laser. Reasons for these improvements are the shorter wavelength (193 nm compared with 213 nm) and the shorter pulse width (2.8 ns compared to 5 ns) of the new laser ablation system. Thus, the mean particle size of the ablated material and the localized heating of the sample is reduced, leading to a more uniform mass discrimination.