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A monochromator is a device that isolates and transmits a band of wavelength from a wider range of wavelengths available at the inlet slit. The dispersion of light can be obtained by means of a prism or diffraction grating. The Czerny- Turner monochromator using a pair of concave mirrors and a plane grating is most widely used in atomic absorption spectroscopy.
Broadband beam reaches the entrance slit positioned at the focal length of the collimating mirror and the parallel beam is diffracted by the plane grating and after reflection from the second mirror is focused on the exit slit. As each color (wavelength) arrives at a separate point in the exit plane, a series of images are focused on the exit slit. As the exit slit has a finite width, parts of nearby images overlap. The light leaving the exit slit contains the image of entrance slit along with images of nearby colors Rotation of dispersion grating causes the band of colors to move relative to fixed exit slit so that the desired entrance slit image can be centered on the exit slit. Thus the range of colors leaving the exit slit is a function of slit width. Slit size is variable, though usually not continuously.
Mirrors used in the monochromator must be highly reflecting in the wavelength range of interest. This can be achieved by polishing the front surface with aluminium, silver or gold. The metal layer is covered with a protective coating that prevents the metal from tarnishing.
The dispersion of light takes place on the grating. Parallel beam striking the grating leaves the grating at slightly different wavelengths. The angle of dispersion at the grating is controlled by the density of lines on the grating, i.e. number of lines/mm. High dispersion is achieved by increasing the line density. In order to isolate desired line from nearby lines narrower exit slit is used. The use of a wider slit width allows more light thereby enhancing sensitivity but at the cost of resolution.
Blaze angle governs the efficiency of the grating. The slope of the triangular groove in a ruled grating is adjusted to enhance the brightness of a particular diffraction order. The further removed a given wavelength of light is from the wavelength for which the grating is blazed the greater will be the extent of light loss at that wavelength.
The wavelength range normally used in atomic absorption spectroscopy is from 185nm to about 900 nm. With a grating blazed somewhere in the middle of this range significant energy fall–of occurs at the wavelength extremities due to the energy inefficiencies of the diffraction process. Dual blazed gratings with a blaze in both UV and visible regions offer better energy efficiency over the entire wavelength range.
Slit width is the width in millimetres of the entrance and exit slist of the monochromator. Narrow slit width gives better resolution. In standard monochromator design both entrance and exit slits have equal width. Wider the slit widths more wavelengths passes through the monochromator. Research grade instruments have user controlled slit widths.
Monochromator Focal Length
Greater the focal length of collimating mirrors the larger their resolution. The resolving power of a monochromator is governed by both focal length and slit width.
The dispersion of a monochromator is characterized as the width of band of wavelengths per unit of slit width, i.e, nm of wavelengths per mm of slit width
Spectral bandwidth is the width of the triangle at the points where the light has reached half the maximum value defined as Full Width at Half Maximum (FWHM)
Stray light is light other than selected wavelength reaching the detector.
We shall now introduce you to different types of interferences in Atomic Absorption Spectroscopy in the next module.