Optimizing Laser Ablation–CRDS Coupling for Spatially-Resolved Millimetric Isotopic Measurements on Ice Cores

Ice cores are extremely valuable archives of the past atmospheric composition, extending back more than 1 million years. In such ancient ice, ice layers become extremely thinned, and the spatial resolution of analytical techniques becomes the primary factor limiting the ability to resolve past climate signals, such as the temperature-related variability inferred from the stable isotopic composition of the ice. Laser ablation (LA) is a micro-destructive technique that has recently shown strong potential for coupling to cavity ring-down spectroscopy (CRDS) to retrieve the isotopic composition of ice-core samples with minimal sample loss. In principle, LA–CRDS not only enables substantially higher spatial resolution than conventional methods but has the potential to gain new insights into signal formation processes in shallow ice by mapping the two-dimensional distribution of stable water isotopes within the ice matrix. However, LA–CRDS hyphenation remains challenging due to several factors, including wavelength-dependent suboptimal laser–ice interaction that can induce isotopic fractionation during ablation, and limitations related to the fast detection of transient signals by commercially available CRDS analyzers. To address these challenges, it is necessary to identify and constrain the factors affecting the ablation efficiency and the aerosol transport between the two systems, while remaining within the operational specifications of both instruments. In the context of the Isotope iMAGing for Ice Core Science (IMAGICS) project, we investigate how laser energy density, artificial ice generation (slow and flash freezing), and measurement configuration affect the LA–CRDS efficiency using an ArF excimer laser (Analyte Excite+, Teledyne Photon Machines) coupled to a CRDS water vapor isotope analyzer (L2130-i, Picarro). Artificial ice samples with known isotopic composition were analyzed under varying laser fluence, dosage, and firing duration. The water-vapor-calibrated CRDS analyzer collected aerosol and vapor generated in the LA cell via an Aerosol Rapid Introduction System (ARIS), operated under its default sampling configuration (~40 ml min^-1, 1 Hz). Preliminary results from this study indicate that, although isotopic fractionation effects are observed in the retrieved aerosol and vapor composition, as previously reported in Malegiannaki et al (2024), the high repeatability of water vapor peaks and isotopic plateaus suggests that the LA–CRDS system introduces a systematic, non-random bias. This finding implies that a correction based on tailored calibration experiments to characterize ice–laser sensitivity is feasible. Such an approach would enable reproducible and accurate isotopic analyses of ice samples with reasonable analysis times (e.g., <10 s per mm² of ablated ice surface).

Malegiannaki, E., Bohleber, P., Zannoni, D., Stremtan, C., Petteni, A., Stenni, B., Barbante, C., Vinther, B. M., & Gkinis, V. (2024). Towards high-resolution water isotope analysis in ice cores using laser ablation - cavity ring-down spectroscopy. Analyst , 149 (24), 5843–5855. https://doi.org/10.1039/d4an01054j