A study has been made of the use of polynomial curve fitting for removal of nonlinear background and high-spatial-frequency noise components from Raman spectra. Two variations on polynomial curve fitting through a least-squares calculation are used. One, involving fitting data x values to corresponding y values, was used to approximate background functions, which are subtracted from the original data. For smoothing, a reference matrix of six vectors that contains a unity d.c. level, a ramp made up of x values, a quadratic made up of x2 values, etc., is fitted to a section of data. The reference vectors are scaled by the fit values and added to give the smoothed estimate of a spectral peak. It is demonstrated, with factor analysis as a test procedure, that the background removal procedure does remove nonlinearities that were present in the original data. The smoothing procedure rejects high-spatial-frequency noise without introducing detectable nonlinearities.

A readily automated procedure for wavelength calibration of multichannel spectrometers is described. Once applied, line positions can be read from the multichannel display with good accuracy and precision for any spectrometer setting. The procedure uses Ne atomic lines as wavelength standards. A novel apodization procedure is used for accurate measurement of the pixel positions of the neon lines. Raman line positions can be determined with an average error of less than 0.2 cm−1.

The effect of particle size on Raman intensity has been measured for a number of crystalline solids with a fiber-optic-based Raman spectrometer. Particle sizes ranged from 76 to 605 μm. Materials examined were sodium nitrate, sodium chlorate, sodium bromate, potassium ferrocyanide, potassium ferricyanide, and copper(II) sulfate. Raman intensity was found to decrease with increasing particle size. The factors responsible for this trend are discussed. We conclude that the major factor is diffuse reflectance that enhances the overlap between the excitation and collection beams. The depth of sample contributing to the Raman signal has been examined for both powders and tablets as a function of powder particle size. Materials examined in this study were sodium nitrate, sodium sulfate, sodium chlorate, sodium bromate, potassium ferrocyanide, potassium ferricyanide, and copper(II) sulfate. For nonabsorbing powders, the depth of sample contributing to the signal exceeded 15 mm. The effect of the pressure used in forming tablets on the Raman signal strength and reproducibility has been examined for sodium nitrate. The Raman intensity was found to decrease with increasing pressure until a tablet of constant density was formed. The effect of particle size and particle size mismatch on the sodium nitrate Raman signal in binary mixtures with potassium chloride, potassium bromide, and potassium iodide has been examined. Good reproducibility was found to require matching of component particle sizes.

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