A spectral library for laser-induced fluorescence analysis as a tool for rare earth element identification

With the recurring interest on rare-earth elements (REE), laser-induced fluorescence (LiF) may provide a powerful tool for their rapid and accurate identification at different stages along their value chain. Applications to natural materials such as rocks could complement the spectroscopy-based toolkit for innovative, non-invasive exploration technologies. However, the diagnostic assignment of detected emission lines to individual REE remains challenging, because of the complex composition of natural rocks in which they can be found. The resulting mixed spectra and the large amount of data generated demand for automated approaches of data evaluation, especially in mapping applications such as drill core scanning. LiF reference data provide the solution for robust REE identification, yet they usually remain in the form of tables of published emission lines. We show that a complete reference spectra library could open manifold options for innovative automated analysis.

We present a library of high-resolution LiF reference spectra using the Smithsonian rare-earth phosphate standards for electron microprobe analysis.We employ three standard laser wavelengths (325 nm, 442 nm, 532 nm) to record representative spectra in the UV-visible to near-infrared spectral range (340–1080 nm). Excitation at all three laser wavelengths yielded characteristic spectra with distinct REE-related emission lines for EuPO4, TbPO4, DyPO4 and YbPO4. In the other samples, the high-energy excitation at 325 nm caused unspecific, broadband defect emissions. Here, lower energy laser excitation showed successful for suppressing non-REE-related emission. At 442 nm excitation, REE-reference spectra depict the diagnostic emission lines of PrPO4, SmPO4 and ErPO4. For NdPO4 and HoPO4 most efficient excitation was achieved with 532 nm. Our results emphasise on the possibility of selective REE excitation by changing the excitation wavelength according to the suitable conditions for individual REEs. Our reference spectra provide a database for transparent and reproducible evaluation of REE-bearing rocks. The LiF spectral library is available at https://zenodo.org/ and the registered DOI: http://doi.org/10.5281/zenodo.4054606 (Fuchs et al., 2020). It gives access to traceable data for manifold further studies on comparison of emission line positions, emission line intensity ratios and splitting into emission line sub-levels or can be used as reference or training data for automated approaches of component assignment.