Rapid-scanning grating spectrometers

Figure 8 Comparison of the resolution of a FT-IR (Michelson) spectrometer to that of a grating spectrometer. The solid lines represent the resolution of a grating spectrometer with a bandpass of 10 nm and the resolution (in nm) for a FT-IR spectrometer operating at a nominal resolution of either 4 cm- or 16 cm- (because noise levels rapidly increase for narrower slitwidths, a 10 nm bandpass is typical of analytical rapid-scanning grating spectrometers). For example, a FT-IR spectrometer operating at 4 cm- resolution has an effective bandpass of 2.5 nm at 2500 nm, falling to less than 0.5 nm at 800 nm.

Several systems for automation of spectrometers have been discussed. A computer-controlled Echelle monochromator allowed wavelength increments of 0.01 nm. A wavelength-scan and lamp-intensity control scheme for the popular Bausch and Lomb high-intensity monochromator has been described. The accurate synchronization of monochromator wavelength-scan and chart-recorder speed, and the possibility of rapid scanning allowing spectra to be displayed in real time on an oscilloscope, has also been discussed. Details have been provided for the modification of a commercially available mirror mount (Oriel model 1450) for use as a stepper-motor controlled grating mount. 

Sequential and simultaneous ICP systems of many types are available from numerous instrument manufacturers. Most are available in radial or axial view, or both. Most spectrometers can be purged to reach 160 nm. Examples of some of the companies who make ICPs and DCPs are given. Jobin Yvon, Inc. makes a line of PMT-based high-resolution sequential systems, simultaneous systems, and combination sequential-simultaneous systems. PerkinElmer Inc. makes a scanning ICP with CCD detection, and a line of simultaneous CCD-based Echelle systems. Leeman Laboratories, Inc. makes an Echelle ICP spectrometer with an aperture grid available as a sequential system with rapid scanning PMT detection, a simultaneous system with a PMT array, and a combination system. Varian, Inc. makes a sequential ICP with PMT detection and a simultaneous system with CCD detection. ThermoElemental offers ICP Echelle spectrometers with CID detection and a sequential diffraction grating instmment with PMT detection. 

FIGURE 18. Rapid-scan spectrometer for in situ reflectance studies. SI, S2 entrance and exit slits SM spherical mirrors G vibrating galvanometer mirror GR grating P polarizing prism BS beam splitter PMT A, B photomultiplier tubes for sample and reference beams. After Ref. 57. 

In contrast, diode array based instruments, popular as relatively low cost spectrometers, have a reverse arrangement by having the dispersion of wavelengths post-sample by a dispersive optic (e.g. a diffraction grating) which irradiates the diode array detector with the spectrum across its elements. Whilst commercially available diode array instruments are invariably of lower optical quality than the best scanning instruments, and are limited by their spectral resolution through the number of elements in their array, they do offer the advantage of rapid spectral acquisition as the complete... 

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