Comparison of Red (785 nm) and Blue (457 nm) Excitation Lasers for Raman Analysis of Synthetic and Natural Calcium Carbonate Samples

ABSTRACT Raman spectroscopy has become a prominent analytical technique in calcium carbonate geochemistry, crystallography, and mineralogy. Many Raman‐based calcium carbonate studies have commonly utilized the 785‐nm (near‐infrared or “red”) excitation laser for its relatively high scattering intensities (peak intensities) and low background fluorescence. Although the 457‐nm (blue) laser has a theoretically higher spatial resolution due to its lower wavelength and experiences relatively less background fluorescence compared to the 532‐nm (green) laser, it remains largely unused and untested for Raman analyses of calcium carbonate crystals. Here, we performed comparative Raman analyses (area mapping and vertical profiling) on various abiogenic and biogenic aragonite and calcite samples to assess spatial resolution differences in acquired Raman spectral data (ν 1 full width at half maximum or FWHM, Raman shift, and peak intensity) between the red and blue excitation laser settings. Area maps and vertical profiles demonstrate that analyses with the blue laser provide higher lateral and vertical spatial resolution than those with the red laser, likely due to the smaller lateral spot size and depth penetration of the blue laser. We also performed spot measurements on precipitated aragonite samples from previous studies, which generated distinct trend lines between ν 1 FWHM and aragonite saturation state (Ω Arag ) for specific excitation laser wavelengths and instrument models. Blue excitation lasers provide a mode of producing higher quality Raman area maps, which will be useful for high‐resolution spatial analyses on natural and synthetic calcium carbonate samples known to contain crystallographic and structural heterogeneity.