Baseline Assessment for the Consistency of Raman Shifts Acquired with 26 Different Raman Systems and Necessity of a Standardized Calibration Protocol

Although Raman shifts originate from molecular vibrations and in theory must be independent of analytical systems, acquired Raman shifts, in practice, are not so. Since the consistency of Raman shifts acquired with different systems has not been investigated previously, we have compared the Raman spectra of polystyrene, benzonitrile, and cyclohexane obtained with 26 different systems. The medians of 26 measurements for the characteristic peaks were found to be 1001.3, 1001.1, and 802.0 cm−1 for polystyrene, benzonitrile, and cyclohexane, respectively, and were consistent with their corresponding ASTM E1840 values, which have been widely used for the calibration of Raman systems. However, only 16 measurements of the Raman shift of the polystyrene peak (1001.4 cm−1) met the tolerance standard of the European Pharmacopoeia (±1.5 cm−1). Thus, consistency of Raman shifts obtained using different systems is low, and this mainly originates from differences in the Raman systems rather than materials. Although correction of the offset using the Raman shift of the peaks of cyclohexane (802.0 or 2852.4 cm−1) could improve the consistency of the Raman shifts acquired with different systems, the magnitude of improvement was not uniform over the range of shift values. Thus, there is a need for a standardized calibration protocol that can be used for multiple Raman shifts of common materials to improve the consistency of Raman shifts for different systems.