Development and certification of NIST standard reference materials for relative Raman intensity calibration

In analytical Raman spectroscopy it becomes increasingly important to employ a procedure for the correction of the relative intensity of Raman spectra. The determination of the intensity response function of a Raman instrument traditionally has been carried out through a white light source that has been calibrated for its relative spectral irradiance. While this method will furnish a correction curve to yield spectra corrected to relative Raman intensity, it is often cumbersome and fraught with experimental difficulties that can profoundly affect the reliability of the correction procedure. An alternate methodology that permits a simplified calibration of the Raman instrument response function is based on the use of luminescent glass standards that transfer a white light calibration onto the Raman measurement system. In this procedure, a measurement of the luminescence of an intensity standard, whose relative irradiance has been determined, provides a means to establish the instrument response function. Correction of measured spectra by this function furnishes spectra that are free of instrumental intensity artifacts. Based on this approach, NIST is developing a series of Standard Reference Materials (SRMs) for the calibration of Raman intensity. This process, and the results obtained thereby, is described for Raman spectroscopy measurements employing 785 nm excitation. The procedure is valid for both macro-sampling and micro-sampling Raman work.