Signatures of rare-earth elements in mineralogical form using laser-ablation dual-comb spectroscopy

TL;DR

This paper explores dual-comb absorption spectroscopy for detecting rare-earth elements in mineral samples, offering high spectral resolution and potential advantages over traditional LIBS methods for mineralogical sensing.

Contribution

It demonstrates the application of dual-comb spectroscopy to identify and analyze REE signatures in mineral reference materials and alloys, highlighting improved spectral resolution and detection capabilities.

Findings

Identified REE lines with limits of detection from 54-583 ppm in reference materials.

Observed faster appearance and disappearance of REE lines in CRM samples due to matrix effects.

Compared spectral resolution of dual-comb absorption spectroscopy favorably to LIBS.

Abstract

Spectroscopy of laser-produced plasmas offers an avenue for real-time, standoff and non-preparatory sensing of rare-earth elements (REEs) within a mineralogical context with applications spanning exploration geology to ore body mapping to ore sorting. Demonstrations of laser-induced breakdown spectroscopy (LIBS) in rock samples have employed both atomic and molecular detection for REE sensors. In this work we evaluate a complementary technique of absorption spectroscopy, realized with dual-frequency combs. This approach provides multi-THz (nm) spectral coverage with simultaneous sub-GHz (pm) resolution. It can improve accuracy and line identification confidence in congested multi-species spectra, which makes it ideal for multi-species evaluations present within mineralogical samples. We analyze REE signatures in calibrated reference materials (CRMs) and a synthesized, REE-containing alloy for atomic, ionic and molecular (oxide) absorptions across three spectral windows. We identify lines from rare-earth and matrix elements, compare absorption line strengths and investigate their temporal evolution. For La I, Sm I and Ce I, preliminary limits of detection from 54-583 ppm are estimated for CRMs, using univariate analysis of selected transitions. Comparing the CRM signatures to those of REEs synthesized in a copper alloy, we observe that most all REE lines appear earlier and disappear faster in the CRM samples. We attribute these dynamics to matrix effects: Among other elements, the increased oxygen content in the CRM could favor molecular formation. For rock samples, observations will once again differ due to grain sizes and bonding mechanisms. Compared to LIBS, we can resolve individual REE and matrix lines with minimal spectral overlap. These proof-of-principle results form a foundation for further development of this laser-based method as a mining sensor.