Near-Infrared Raman Spectroscopy of Liquids and Solids with a Fiber-Optic Sampler, Diode Laser, and CCD Detector

Abstract

Fluorescence interference often present in conventional Raman techniques may be considerably reduced by moving from a visible laser source to one in the near-infrared (NIR) region. Fourier transform Raman spectroscopy (FT-Raman) performed at 1064 nm dramatically reduces fluorescence background and extends the utility of Raman spectroscopy to a wider range of samples. However, FT-Raman exhibits a lower signal-to-noise ratio (S/N) than visible Raman because of reduced scattering (due to the v4 factor) and higher noise associated with detectors useful in the range above 1064 nm. An alternative procedure for NIR Raman involves a 783-or 830-nm GaAlAs diode laser, a charge-coupled device (CCD) detector, and a single-grating spectrograph. Excellent reduction of fluorescence background has been achieved, yet Raman spectra are obtained at wavelengths within the quantum efficiency range of the CCD detector. Because the CCD detector is shot-noise limited, a much higher S/N is attained in comparison with FT-Raman, permitting the use of low laser power and weakly scattering samples. The approach is similar to other CCD Raman spectrometers based on He-Ne11 or NIR dye lasers, and exhibits a trade-off between quantum efficiency and fluorescence reduction since the laser wavelength is extended into the NIR.