Selecting a laser excitation source and sampling strategy for Raman spectroscopy

Selection of an excitation source precedes the collection of Raman spectra. While principles exist to guide this process based on Raman scattering efficiency or fluorescence, these general trends do not converge to a single wavelength. Background signals competing with Raman scattering are also highly compound dependent. Adding to these factors are additional considerations including the wavelength-dependence of detector sensitivity, laser source power availability, and spectrometer optical efficiency. Given this multidimensional problem, an empirical approach was taken to guide laser source selection based on five classes of materials commonly subjected to Raman spectroscopy: silica-based (SiO₂) materials, pharmaceuticals, organic polymers, carbons, and inorganic salts. Largely, peak position was shown to be invariant based on laser excitation wavelength unless thermal effects or known enhancements of dispersive modes were induced, as in graphitic carbons. Other factors that influence Raman data quality include substrate choice, sample orientation, fluorescence, and signal-to-noise.