Multichannel FT-Raman Spectroscopy: Noise Analysis and Performance Assessment

Abstract

A Raman spectrometer based on forming an interferogram on a charge-coupled device (CCD) detector is evaluated further with respect to signal-to-noise ratio (SNR), stability, response correction, and resolution. The multichannel Fourier transform technique differs fundamentally from dispersive spectrometers and FT-Raman systems based on Michelson interferometers. Changes in entrance optics permitted multitrack operation and an improvement in collection efficiency. Both hardware and a more facile software procedure were examined for correction of noise caused by nonuniformity of the CCD response. The instrumental linewidth (ILW) for the multichannel Fourier transform (MCFT) system examined here was 14 cm⁻¹ (full width at half-maximum), close to the 13.5 cm⁻¹ predicted theoretically. An optical heterodyne method was used to downshift the observed Raman features and further reduce the ILW to 8 cm⁻¹. The unheterodyned MCFT spectrometer has a lower SNR than do dispersive systems for most samples, but has the advantages of frequency precision and large throughput. Several applications of MCFT are discussed, including examination of photolabile samples, multiple sample monitoring with fiber optics, and identification of MCFT spectra with a dispersive library.