Sequentially shifted excitation: A tool for suppression of laser-induced fluorescence in mineralogical applications using portable Raman spectrometers
Abstract
Laser-induced fluorescence is a phenomenon that can be encountered in the Raman spectra of minerals, and it complicates the acquisition of high quality Raman spectra when using portable instruments. A novel portable Raman spectrometer utilizing sequentially shifted excitation was used for analyses of the minerals anhydrite, apatite, and zircon that frequently exhibit laser-induced fluorescence, especially when 785 nm excitation is used.
Fluorescence centers, responsible for the laser-induced fluorescence in these minerals, are generated primarily by the presence of rare earth elements. The narrow and intense fluorescence bands that arise can be confused with or mask the Raman bands of the minerals when analyzing with the conventional 785 nm excitation.
It has been found that the fluorescence is completely removed in those spectral regions containing no Raman bands of the minerals. In the areas of those Raman spectra of minerals where the fluorescence and Raman bands coincide, the sequentially shifted excitation was able to suppress effectively the fluorescence.
In some cases, the fluorescence removal process in the final spectra of the most fluorescence samples introduced new instrumental artifact bands. Nevertheless, the sequentially shifted excitation was able to suppress this specific laser-induced fluorescence quite effectively; and Raman features became well pronounced, especially for the zircon specimens.
This novel approach of obtaining high quality Raman spectra of minerals containing trace amounts of fluorescence-inducing components (such as rare earth elements) with portable instrumentation opens up new possibilities for in situ spectroscopic analyses in the fields of mineralogy and geology.