EVALUATING THE IMPACT OF LASER OPERATING CONDITIONS ON PRECISION AND ACCURACY OF U/PB ANALYSIS BY LA-ICP-MS

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a valuable way to conduct zircon uranium-lead (U/Pb) dating due to rapid data collection and the small amount of sample that is destroyed, but recent technological improvements require method optimization to increase accuracy and precision. I tested a range of beam energy density, repetition rate, and signal smoothing techniques on NIST reference glasses to observe how these conditions and configurations impact signal stability, sensitivity and ²⁰⁶Pb/²³⁸U ratios (0.2236 ± 0.44 % for NIST610). The duration of the ablation was varied as a function of ablation rate such that the product of the beam energy density and the number of laser pulses per analysis remained constant to ensure similar laser pit volumes so that data from different run conditions could be compared.

Changing beam energy density from 0.8 J/cm² to 3.5 J/cm² shows that lower beam energy densities have lower precision (²⁰⁶Pb/²³⁸U ratio of 0.2354 ± 12 %) and that medium range beam energy densities have higher precision with the highest precision at 2 J/cm² (0.2232 ± 0.7%). Beam energy densities higher than 3.0 J/cm² start to decrease in precision (0.2257 ± 2 % for 3 J/cm²). Low ablation rates (2 Hz and 1 J/cm²) are less precise (0.2358 ± 3 %) for calculated ²⁰⁶Pb/²³⁸U ratios compared to medium (6 Hz and 2.8 J/cm²) and high (10 Hz and 6 J/cm²) ablation rates (0.2246 ± 0.7 % and 0.2204 ± 0.8%, respectively). Increasing repetition rates from 2 Hz to 10 Hz (at 2 J/cm²) increases sensitivity (from ~50,000 CPS to ~450,000 CPS on ²³⁸U) and signal stability (from 0.2273 ± 3 % to 0.2229 ± 1 %). I also investigated the impacts of a signal smoother on signal stability and measurement precision. I found that without a signal smoother, the highest precision was at 10 Hz (0.2020 ± 0.6 %) and effects of laser pulsing on stability increase at repetition rates lower than 7 Hz (0.2082 ± 3 % at 7 Hz to 0.2247 ± 3 % at 2 Hz) whereas with a signal smoother, laser pulsing start to impact signal stability at 3 Hz (0.2271 ± 6 %). Uranium-lead ratios collected with a signal smoother also showed less variability to the accepted value for NIST 610 (0.2273 ± 2 % at 2 Hz to 0.2229 ± 1 % at 10 Hz) over the range of repetition rates than ratios collected without a signal smoother (0.2247 ± 3 % at 2 Hz to 0.20201 ± 0.6 % at 10 Hz).