Dispersive Raman Spectroscopy, Current Instrumental Designs

Raman spectroscopy (RS) is based on an incoherent inelastic scattering process of light by matter, in either its solid, liquid, or gas state. This interaction between ultraviolet (UV), visible (VIS) or near-infrared (NIR) photons with matter gives rise to a Raman spectrum characterizing its different vibrational transitions. These vibrational transitions may be active for the infrared (IR) absorption or the Raman scattering processes, or for both of them, depending on their selection rules and so IR frequencies may be present or absent in the Raman spectrum of a compound and vice versa. IR and RS are therefore closely related. It is noteworthy that the first Raman spectrometer used by Sir C.V. Raman in 1928 was a dispersive Raman spectrometer (DRS), composed of a mercury lamp when the sun itself was not the exciting source, with a prism monochromator (MC) as the light dispersor and a photographic film as detector. However, it must be emphasized that the Raman effect is extremely weak, and a good Raman spectrum was very difficult to obtain in a reasonable recording time. Progress in the fields of excitation sources, MCs, and of detectors has been constant. Lasers (mainly gas and solid state) are now currently used as sources. Thanks to the considerable improvements in the other components of a Raman spectrometer, the output power of the laser can now be quite low: a few milliwatts for the excitation light is all that is needed to obtain good Raman signals. As for MCs, the already highly efficient ruled diffraction gratings (RDGs) were followed by the excellent holographic diffraction gratings (HDGs) used in DRSs. Fourier transform (FT) principles have been also developed, giving birth to FT-Raman spectrometers. Hadamard transform principles have also been used to develop new instruments and Hadamard spectrometers are now being designed. The ever-present photomultiplier tube (PMT) for the VIS range and semiconductors for the NIR region are largely used as detectors, especially for monochannel and FT-Raman detection. Multichannel detectors with very high quantum efficiency (QE) have appeared on the market opening up new avenues for use of dispersive Raman spectroscopy: High speed and/or compact Raman spectrometers, Raman imaging spectrometers, Raman microscopes, and time-resolved Raman spectrometers are now available using dispersive Raman spectroscopy principles. The Raman technique is one of the most elegant analytical tools: A Raman spectrum gives rich structural information, useful for both fundamental and applied research. All the states of matter can be studied, whether solid, liquid, gas, glass, or solutions in water and in organic solvents. There are no sampling problems since a raw material can be studied without any preparation of the sample. The quantity needed for an examination may be very small: a few microliters in size, a few milligrams to micrograms in weight, or some tens of parts per million in a mixture; Several derived techniques such as resonance Raman spectroscopy (RRS), time-resolved Raman spectroscopy (TRRS), surface-enhanced Raman spectroscopy (SERS), coherent anti-Stokes Raman Spectroscopy (CARS), hyper-Raman effect, constitute a large panel of methods, where the same instrumentation can sometimes be used giving complementary results. Raman microscopes and Raman imaging spectrometers are also now available, extending the field of exploration to new domains. Optical fibers used to transport the excitation and Raman light make possible both remote and in situ analyses. These instruments are now used in industry. The present article deals with Dispersive RS and current instrumental designs, but does not tackle other techniques such as the FT-Raman and Raman imaging which are treated in other articles in this Encyclopedia.