First results from a dual-wavelength (157 & 193 nm) LA-ICP-MS/MS System for spatially-resolved chemical analysis of ice cores

In recent years, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) applied to ice-core samples has become the go-to method to investigate climate signals in highly thinned sections of ice cores and the interaction of impurities and the ice’s microstructure. Ablation is typically performed using DUV (deep UV, 193 nm or 213 nm) excimer laser sources. However, at these wavelengths ice is virtually transparent leading to high penetration of the laser energy into the ice. That means that ablation is sometimes non-controlled and likely depends on the impurity load of the ice, and may require very high on-sample fluence. This makes it challenging to generate calibrated ice-core impurity records using cryo-LA-ICPMS. One approach to overcome this is to utilize a laser wavelength that is absorbed by the ice, resulting in shallower penetration. To implement this, we have built a unique custom-designed dual-wavelength LA system that can use both 193 nm and 157 nm excimer lasers. At 157 nm, ice is strongly absorbent, which implies good energy transfer into the sample. Our setup has already been successfully used to ablate other DUV-transparent materials such as fused silica and quartz.

Here we present the design of the system and the accompanying purpose-built cryo sample holder that allows us to use both 193 nm and 157 nm laser light for the analysis of ice-core samples. The holder is designed to enable high sample throughput by keeping three 14 cm long ice core samples alongside reference materials and frozen standards inside the proven Laurin Technic S155 ablation chamber. In addition to showcasing the design of our system we will show initial results of laser ablation analyses from Greenland ice core samples over Stadial/Interstadial transitions using an Agilent 8900 ICP-MS/MS. In the presented setup the system can be used both to generate high-depth-resolution down-core time series as well as high-resolution impurity maps, both of which are essential to further our understanding of the signal preservation in the ice and to ultimately reconstruct climate variability from highly thinned ice-core records such as the >1Ma old Beyond EPICA Oldest Ice core.