Abscissa calibration and transfer for the development of instrument independent Raman spectra

The advent of the laser and low noise multichannel detectors, such as the charged coupled device (CCD), have aided the recent growth of Raman spectroscopy as an analytical method. A significant limitation to the use of Raman spectroscopy for qualitative and quantitative analysis is the lack of reference spectra in the form of digital libraries.;A major difficulty in the development of reference spectra is the lack of standardized wavenumber shift and intensity calibration procedures. Calibration of the focal plane is a critical step in determining the accuracy of a Raman band. Any calibration must be capable of both high precision and accuracy in terms of either frequency or wavelength. The assignment of Raman features with band positions acceptable for use in a spectral library, requires calibration of the spectrograph with an absolute standard; such as the emission spectrum of neon.;This treatise describes a calibration routine to generate a piece-wise linear model of dispersion at the focal plane of a multi-channel detector. For short focal length spectrographs, the dispersion of light at the focal plane is only approximately linear in wavelength. A computer program was written that automatically assigns wavelengths from a master "look-up" table to the atomic line spectra of neon and generates a piece-wise linear model of the dispersion plane. Using the procedures outlined, the Raman shift positions of cyclohexane were measured on two separate instruments and over a period of four months. The wavenumber reproducibility of these measurements was less than 0.1 cm;Also described is a numerical method of transferring reference spectra acquired on an FT-Raman instrument to the abscissa space of a multichannel dispersive instrument. From the calibration of each spectrometer, a linear array of wavenumber shift positions is generated. Polynomial interpolation is used to determine the intensity of the reference spectrum at each wavenumber shift value in the abscissa space of the target spectrograph. The comparison of spectra and sample spectra acquired with different instruments, grating densities, and excitation sources are shown as proof of the methods utility.