Dispersive grating spectrometer

Until the early 1980s, most IR spectrometer systems were double-beam dispersive grating spectrometers, similar in operation to the double-beam system for UV/VIS spectroscopy described in Chapter 2. These instruments have been replaced almost entirely by FTIR spectrometers because of the advantages in speed, signal-to-noise ratio, and precision in determining spectral frequency that can be obtained from a modern multiplex instrument. There are NIR instruments that are part of double-beam dispersive UV /VIS/NIR systems, but many NIR instruments are stand-alone grating instruments. 

Mazzacurati V., Montagna M., PiUa 0., Viliani G., Ruocco G., Signorelli G. Vibrational dynamics and Raman scattering in fractals A numerical study. Phys. Rev. B 1992 45 2126-2137 Mazzacurati V., Benassi P., Ruocco G. A new class of multiple dispersion grating spectrometers. J. Phys. E Sci. Instrum. 1988 21 798-804... 

Let us cite an example to help us judge the equivalence between Fourier and dispersive instruments. A grating spectrometer employing a four-passed 8 x 104-line grating in the first order has a resolving power of 4 x 8 x 104 = 3.2 x 105. At 3200 cm -1 in the near infrared, this instrument has a Rayleigh resolution of 10" 2 cm- L The same resolution can be achieved by a Fourier... 

Sometimes the spectrometer completely obliterates the information at all Fourier frequencies co beyond some finite cutoff Q. This is specifically true of dispersive optical spectrometers, where the aperture determines 1. The cutoff Q may be extended to high Fourier frequencies by multipassing the dispersive element or employing the high orders from a diffraction grating. 

As described in Section 8.2.1.1, there is a trade-off between resolution and spectral coverage for a dispersive CCD spectrometer. For a given grating... 

The procedure below was developed for a dispersive CCD spectrometer used daily to acquire response-corrected Raman spectra. The calibration procedure was conducted at the beginning of each session or after the grating was repositioned to cover a different Raman shift range. The calibration steps were automated for the most part, so the time required from the user was approximately 5 min before each session. This procedure may be adapted to a particular spectrometer and application, guided by the objective of resulting in a known level of accuracy of Raman shift and relative intensity, and providing a daily record of instrument performance. 

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

The spectrometer used is a modified Spectrametrics Spectraspan III echelle grating spectrometer with a quartz prism cross disperser. An echelle spectrometer was chosen because of its two dimensional format display. This format allows efficient simultaneous examination of a much wider spectral range than with a linear dispersion spectrometer when a two dimensional television camera type detector is used. 

---