Scanning-Grating Spectrometers

In this section the mainstream NIRS technologies in current widespread practical use for PAC applications will be examined. These are scanning grating monochromators, grating polychromator PDA spectrometer, AOTF analysers and FTNIR analysers. 

Figure 2 A scanning grating-based spectrometer (a, source b, entrance slit c, grating d, exit slit e, sample e, detector).

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.

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Three different types of grating spectrometer detection sterns are used (Figure 3) sequential (slew-scan) monochromators, simultaneous direct-reading polychroma-... 

Figura 3 Grating spectrometers commonly used for ICP-OES (a) monochromator, in which wavelength is scanned by rotating the grating while using a singie photomultiplier tube (PMT) detector (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40 or more exit slits and PMTs can be arranged along the focal plane) and (c) spectrally segmented diode-array spectrometer.

Fig. 23 P6 of 28SiH4 recorded on a grating spectrometer. Trace (a) is from a continuous scan of natural-abundance silane at a resolution of 0.020 cm-1. Trace (b) is the same region recorded separately at a resolution of 0.012 cm-1 and is the average of 12 scans.

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