Use of calcite for evaluation of spectral resolution of Raman spectrometers

Raman microscopes are widely used in various fields and their spectral resolutions differ greatly depending on the system and optical components. Thus, it is important to evaluate the spectral resolution of Raman systems under measurement conditions. Although both atomic emission lines and calcites have been used for the evaluation of spectral resolution and described in some guidelines, calcite is preferable because it contains information on laser width. Calcite is a crystal with a trigonal structure and the peak width of Raman spectra is sensitive to crystallinity, so the peak width of calcite may change depending on its quality. In this study, we examined differences in the peak width of atomic emission lines and calcites as well as laser line. The peak width of calcite at ~ 1086 cm^-1 was wider than that of laser and atomic emission lines due to contribution of laser width and calcite itself. Among various calcite samples, calcite blocks for optical prisms showed the narrowest peak width (< 1.3 cm^-1) followed by high-grade reagent calcites (1.35-1.43 cm^-1) and usual-grade reagent calcites (2.23-2.53 cm^-1). Although the peak width of calcite slightly changed with temperature (~ 0.2 cm^-1 between 5 °C and 45 °C), it was comparable to the differences among high-grade reagent calcites. Thus, calcite is preferable for evaluating spectral resolution of Raman systems but its quality requires attention.