Monochromator is a vital optical component of the Atomic Absorption Spectrometer. It plays the function of isolating the wavelengths of interest from the broad spectrum of wavelengths emanating from the hollow cathode lamp. For a brief description of monochromator component parts and their role browse through the article Dispersion and resolution of light in atomic absorption spectroscopy.
The ideal distribution of monochromator output is triangular shaped. The selected wavelength lies in the centre with nearby wavelengths decreasing linearly on both sides till a cut-off value is reached where it stops decreasing further. The cut-off value is typically about one thousandth of the peak value, or 0.1%.
In the present article these parameters are discussed in some more detail so that you can appreciate their contribution towards performance of the monochromator.
Monochromator Performance Parameters:
Monochromator Focal Length
The focal length of the collimating mirrors has a bearing on their resolving power. Large focal lengths improve resolution of overlapping or closely spaced absorption bands. The exit slit is positioned at the focus of the collimating mirrors so that a sharp image of dispersed light reaches the exit slit.
Slits are provided for permitting entry of broad wavelength light into the monochromator and to allow exit of isolated wavelength light. Slit width is expressed in mm. Narrow slits give better resolution but limit the sensitivity as these prevent a fraction of dispersed light from reaching the detector. In standard design both entrance and exit slits have equal width but in research grade instruments variable slit widths are provided which can be controlled by the user.
The monochromator output is a narrow band of isolated wavelengths. The bandwidth is inversely related to the resolving power of the monochromator. It is defined as the width of the triangle at the points where the light reaches half the maximum value and is referred to as full width at half maximum (FWHM).
Dispersion is the ability of a monochromator to separate different wavelengths.
Angular dispersion refers to change in angle of reflection r with change of wavelength
Angular Dispersion= dr/dλ
Linear dispersion D represents change in distance at focal plane with change of wavelength
D = dy/dλ
where y represents distance along the focal plane and λ is wavelength.
When focal length of collimating mirror is F then
dy = Fdr which on substitution in the linear dispersion equation gives
D = Fdr/dλ
Commonly reciprocal linear dispersion \(D^-^1\) is used to express the dispersion of grating monochromators
\(D^-^1\) = dλ/Fdr
Stray light is the radiation that emerges from the monochromator without passing through the dispersing elements. Scattered radiation arises due to the presence of dust particles inside the monochromator as well as on various optical surfaces.
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