Assessment of the precision and accuracy of thorium (²³²Th) and uranium (²³⁸U) measured by quadrupole based-inductively coupled plasma-mass spectrometry: comparison of liquid nebulization, nanosecond and femtosecond laser ablation

The precision and accuracy of the ²³⁸U/²³²Th ratio were evaluated from liquid nebulization and direct solid sampling repetitive pulsed laser ablation. Nanosecond and femtosecond pulsed lasers at 266 nm wavelength were utilized for the ablation studies. The ICP-MS and sampling parameters were optimized for each procedure; flow rates, gases, laser energy and other parameters were optimized for the particular sampling approach and therefore will not be the same. The work is not a comparison per se but represents performance metrics for three optimized sampling modalities. As expected, nanosecond pulsed ablation provided the greatest inaccuracy (>30%) from the nominal ²³⁸U/²³²Th bulk ratio. This deviation from bulk ratio is attributed to incomplete vaporization of large particle agglomerates produced by nanosecond laser ablation. Femtosecond pulsed ablation provided inaccuracy (∼1–3%) approaching that of liquid nebulization (∼1%). In terms of temporal relative standard deviation (TRSD) and relative standard deviation (RSD), liquid nebulization provided the best precision for the ²³⁸U/²³²Th ratio (TRSD ∼3–5%, RSD ∼0.2–0.6%), femtosecond laser ablation (TRSD ∼5–12%, RSD ∼1%) and nanosecond laser ablation (TRSD ∼25–48%, RSD ∼9–12%). Laser ablation requires less sample to achieve these performance metrics, in some cases less than a factor of 100-times depending on the entrainment and transport efficiency.