Multichannel wavelength-dispersive instruments

It should also be noted at this point that a multichannel detector can have multiple detector elements along two axes, one parallel to the direction of wavelength dispersion, and one perpendicular. The latter is parallel to the entrance slit in most dispersive instruments. For example, a CCD may have 1024 pixels along the wavelength axis and 256 along the vertical axis, for a total of 262,144 independent elements. This second dimension of the detector may be used in a variety of applications involving Raman imaging, multiple detection tracks, or echelle spectrometers. 

Spectroscopic instruments in the UV and visible regions are usually equipped with one or more devices to restrict the radiation being measured to a narrow band that is absorbed or emitted by the analyte. Such devices greatly enhance both the selectivity and the sensitivity of an instrument. In addition, for absorption measurements—as we saw in Section 24C-2—narrow bands of radiation greatly diminish the chance of Beer s law deviations due to polychromatic radiation. Many instruments use a monochromator or filter to isolate the desired wavelength band so that only the band of interest is detected and measured. Others use a spectrograph to spread out, or disperse, the wavelengths so that they can be detected with a multichannel detector. 

Polychromators. Polychromators are multichannel spectrometers with PMTs as detectors. Instmments with up to 64 PMTs are commercially available. These instruments generally use a concave diffraction grating as the dispersion device, as shown schematically in Fig. 7.13. As can be seen, a concave grating focuses light of different wavelengths, Ai, A2, A3, and so on, at different points on the circumference of a circle. 

Unlike IR spectroscopy where nowadays FT instrumentation is solely used, in Raman spectroscopy both conventional dispersive and FT techniques have their applications, the choice being governed by several factors. The two techniques differ significantly in several performance criteria, and neither one is best for all applications. Contemporary dispersive Raman spectrometers are often equipped with silicon-based charge coupled device (CCD) multichannel detector systems, and laser sources with operating wavelength in the ultraviolet, visible or near-infrared region are employed. In FT Raman spectroscopy, the excitation is provided exclusively by near-infrared lasers (1064 nm or 780 nm). 

A spectrometer is an instrument that provides information about the intensity of radiation as a function of wavelength or frequency. The dispersing modules in some spectrometers are multichannel so that two or more frequencies can be viewed simultaneously. Such instruments are sometimes called polychromators. A spectrophotometer is a spectrometer equipped with one or more exit slits and photoelectric transducers that permit the determination of the ratio of the radiant power of two beams as a function of wavelength as in absorption spectroscopy. A spectrophotometer for fluorescence analysis is sometimes called a speciroftttorometer. 

It is difficult to say with certainty how Raman instrumentation will evolve over the next 10 years. However, there are several instrumentation developments that are beginning to appear as commercial products. These include more systems engineered for dedicated process and/or QA/QC applications, NIR multichannel detectors for use on a dispersive Raman instrument with long-wavelength lasers, UV microprobes, and near-field Raman microprobes that can complement atomic force microscopes (AFMs) or scanning tunneling microscopes (STMs), or their variants. It is an exciting time to be a Raman researcher. 

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