Evaluation of laser ablation double-focusing SC-ICPMS for “common” lead isotopic measurements in silicate glasses and minerals

Abstract

An analytical method for the in situ measurement of “common” Pb isotope ratios in silicate glasses and minerals using a 193 nm excimer laser ablation (LA) system with a double-focusing single-collector (SC)-ICPMS is presented and evaluated as a possible alternative to multiple-collector (MC)-ICPMS. This LA-SC-ICPMS technique employs fast-scanning ion deflectors to sequentially place a series of flat-topped isotope peaks into a single ion-counting detector at a fixed accelerating voltage and magnetic field strength. Reference materials (including NIST, MPI-DING, and USGS glasses) are used to identify two analytical artifacts on the Pb isotope ratios (expressed here as heavier/lighter isotopes) when corrected for mass bias relative to NIST SRM610. The first artifact is characterized by anomalously low Pb isotope ratios (∼0.1% per AMU) when SRM610 is analyzed in raster mode as an unknown at small spot sizes (<25 μm), which may indicate that (1) SRM610 is isotopically heterogeneous on a small length scale and/or (2) there is a non-spectral matrix effect on the Pb isotope ratios related to differences in spot size. The second artifact is characterized by anomalously high Pb isotope ratios (<0.1% per AMU) for NIST SRM612 (in raster mode) and some Fe-rich glass reference materials (BCR-2G, GOR132-G, and T1-G). These offsets are thought to be caused by one or more non-spectral matrix effects related to differences in the ablation behavior, composition, or physical properties of these reference materials compared to the bracketing SRM610 standard. The precision (±2SD) of our LA-SC-ICPMS Pb isotopic measurements is similar to (²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, or ^20XPb/²⁰⁶Pb) or better than (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb, or ^20XPb/²⁰⁴Pb) a series of published studies that used a different type of SC-ICPMS and obtained a factor of ∼3–4 higher sensitivity for Pb. An increase in the sensitivity of our LA-SC-ICPMS would likely improve the precision of the ^20XPb/²⁰⁶Pb and ^20XPb/²⁰⁴Pb ratios for low-Pb materials (<5 ppm), possibly making the technique broadly similar to LA-MC-ICPMS (particularly compared to methods that rely upon at least one ion-counting detector). Further improvement in the precision of the ^20XPb/²⁰⁶Pb and ^20XPb/²⁰⁴Pb ratios for high-Pb materials (>5 ppm) by LA-SC-ICPMS is unlikely, and in this case, LA-MC-ICPMS remains the preferable analytical technique.