ASTM E2911-23

4.1 Generally, Raman spectra measured using grating-based dispersive or Fourier transform Raman spectrometers have not been corrected for the instrumental response (spectral responsivity of the detection system). Raman spectra obtained with different instruments may show significant variations in the measured relative peak intensities of a sample compound. This is mainly as a result of differences in their wavelength-dependent optical transmission and detector efficiencies. These variations can be particularly large when widely different laser excitation wavelengths are used, but can occur when the same laser excitation is used and spectra of the same compound are compared between instruments. This is illustrated in Fig. 1, which shows the uncorrected luminescence spectrum of SRM 2241, acquired upon four different commercially available Raman spectrometers operating with 785 nm laser excitation. Instrumental response variations can also occur on the same instrument after a component change or service work has been performed. Each spectrometer, due to its unique combination of filters, grating, collection optics and detector response, has a very unique spectral response. The spectrometer dependent spectral response will of course also affect the shape of Raman spectra acquired upon these systems. The shape of this response is not to be construed as either “good or bad” but is the result of design considerations by the spectrometer manufacturer. For instance, as shown in Fig. 1, spectral coverage can vary considerably between spectrometer systems. This is typically a deliberate tradeoff in spectrometer design, where spectral coverage is sacrificed for enhanced spectral resolution.

FIG. 1 SRM 2241 Measured on Four Commercial Raman Spectrometers Utilizing 785 nm Excitation

4.2 Variations in spectral peak intensities can be mostly corrected through calibration of the Raman intensity (y) axis. The conventional method of calibration of the spectral response of a Raman spectrometer is through the use of a National Metrology Institute (NMI), for example, NIST, traceable calibrated irradiance source. Such lamps have a defined spectral output of intensity versus wavelength and procedures for their use have been published (1)5. However, intensity calibration using a white-light source can present experimental difficulties, especially for routine analytical work. Calibrated tungsten halogen lamps have a limited lifetime and require periodic recalibration. These lamps are often mounted in an integrating sphere to eliminate polarization effects and provide uniform source irradiance. In practice, these sources can be difficult to align with the variety of sampling arrangements that are now typical with Raman spectrometers, especially microscope based systems and process Raman analyzers where electrical safety concerns persist in hazardous areas. The advantage of a standard lamp is that it can be used for multiple excitation wavelengths.

4.3 The spectra of materials that luminesce with irradiation can be corrected for relative luminescence intensity as a function of emission wavelength using a calibrated Raman spectrometer. An irradiance source, traceable to the SI, can be used to calibrate the spectrometer. Several groups have proposed these transfer standards to calibrate both Raman and fluorescence spectrometers (1-6). The use of a luminescent glass material has the advantage that the Raman excitation laser is used to excite the luminescence emission and this emission is measured in the same position as the sample. These glasses can be used in a variety of sampling configurations and they require no additional instrumentation. The glasses are photostable and unlike primary calibration sources, may not require periodic recalibration. NIST provides a series of fluorescent glasses that may be used to calibrate the intensity axis of Raman spectrometers. A mathematical equation, which is a description of the corrected emission, is provided with each glass. The operator uses this mathematical relation with a measurement of the glass on their spectrometer to produce a system correction curve.

4.4 This guide describes the steps required to produce a relative intensity correction curve for a Raman spectrometer using a calibrated standard source or a NIST SRM and a means to validate the correction.