Laser ablation ICP-MS analysis

Laser ablation (LA-) Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was performed at the Centre for Microscopy, Characterisation and Analysis at the University of Western Australia. A Teledyne Analyte G2 UV-excimer laser (193 nm) was coupled to a Thermo-Fisher Element XR High Resolution (HR-)ICP-MS, using a squid signal smoothing device. An effectively continuous laser ablation raster was performed along the growth axis of speleothem comprised of 21 shorter rasters, allowing interspersion of primary and secondary reference materials. A spot size of 180 by 30 μm was used at a laser repetition rate of 10 Hz and a raster rate of 4 μm/s. A full range of analytes were included with a cycle time ca. 1.3 seconds, however only Mg/Ca is reported in this study using ²⁵Mg and ⁴³Ca as the isotopes of interest. Data reduction was performed in Iolite, including correction of detector analogue-counting factor, instrumental baselines, and standard sample bracketing. NIST 614 was used as the primary reference material, with NIST 612 and a range of in-house carbonates (low-Mg calcite, high-Mg calcite and aragonite) used as secondary reference materials. These secondary reference materials indicate Mg/Ca reproducibility better than 2.2% RSE on all standards (n = 11, n = 5, n = 10, n = 10, n = 10). Four 2 mm long Mg/Ca rasters performed along individual laminations show comparable reproducibility.