Monochromator Nondispersive

A nondispersive system of monochromation filters either the radiation or the electrical signal generated in the detector. In a dispersive monochromator, radiation is separated according to wavelength. The resolution of a monochromator is defined as where 8X is the minimum distance between the centers of two spectral lines which can just be dis-... 

The basic components of spectrophotometers are a light source, wavelength selector, absorption cell (cuvette), and photodetector. Colorimeters or absorptiometers commonly use nondispersive wavelength selection (a filter with bandwidth 4 -40 nm) and solid state or simple phototube detectors, while spectrophotometers employ a prism or grating monochromator (with bandwidth down to 0.2 nm) and a photomultiplier. Colorimeters are inexpensive and most appropriate for repetitive measurements of absorption at a fixed wavelength. The more expensive spectrophotometer can also fulfill this function, but its main purpose, by virtue of its accurate and variable wavelength control, is the measurement of absorption spectra. 

In theory, no monochromator or lilter should be necessary for alomic fluorescence measurements when an LDI. or hollow-cathode lamp serves as the excitation source becau.se the emitted radiation is, in principle, that of a single element and will thus excite onlv atoms of that element. A nondispersive system then could be... 

Spectrometers are used to produce the spectrum or to isolate narrow spectral ranges, without further deconvolution. In dispersive apparatus, the spectrum is produced with a prism or diffraction grating. In nondispersive apparatus, spectral regions are isolated from the radiation beam by reflection, interference, or absorption in interferometers or filter monochromators. The latter are of use only in flame emission spectrometry. 

A conventional monochromator (either Ebert, Czerny-Turner, Littrow or Echelle) may be used (see AAS sections for details), but some of the more basic instrumentation uses interference filters. These are optical filters that remove large bands of radiation in a nondispersive way. A dispersion element such as a prism or a grating is therefore not required. Only a relatively narrow band of radiation is allowed to pass to the detector. The disadvantage with such devices is that they are not particularly efficient and hence much of the fluoresced light is lost. An alternative development is the multi-reflectance filter. This is shown diagramatically in Figure 3, and has the... 

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