A grating is an optical element in a monochromator that disperses the broadband light beam into distinct wavelengths. The desired wavelength is selected by adjusting the exit slit width of the monochromator.
The angle of dispersion of the grating depends on the number of lines on the grating. Larger the number of lines (usually 1200 lines/mm to 1800 lines/mm) the larger is the dispersion power of the grating. In turn the overall efficiency of the monochromator depends on the dispersing power of a grating.
Mechanical rulings of gratings are in the form of V– shaped grooves. Interference phenomena results in divergence of different wavelengths from the grating at different angles. At the blaze angle the particular wavelength diverges from the surface at an angle corresponding to specular reflectance which means angle of incidence is equal to angle of reflectance and there is least loss in intensity as a result of diffraction. A grating can be made by controlling the angle of cut during the ruling process. The further removed a given wavelength is from the wavelength for which a grating is blazed, the greater will be the extent of light loss at that wavelength. In other words the sensitivity of detection of a particular atomic line is highest at the blaze angle and decreases gradually on either side
Atomic Absorption lines are spread from about 190nm to about 850 nm so a grating blazed somewhere in the middle region is sufficient as energy gradually falls on either side but there is a significant drop at the extremities.
Earlier instruments were equipped with two gratings, one blazed in the UV region and other blazed in the visible region. One had the option to select the grating close to the selected wavelength. Nowadays a single dual blazed grating serves the same purpose without the need for change of gratings during analysis. It has two blaze angles in the two spectral regions.
It can be observed that though maxima appear at the two selected wavelengths the overall dispersed energy sensitivity is greater over the entire wavelength range in comparison to single blazed grating which contributes to greater signal intensity over the complete wavelength range. Thus blazed gratings provide higher detection sensitivities in AAS analysis.
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