Question Topics :
A slit is a mechanical opening in a black painted metallic sheet, with a circular or triangular hole through which incident rays pass which lead to a monochromator. The slit permits passage of only a small fraction of incident radiation, which is selected for further processing such as wavelength selection and detection etc.
Usually the output of the radiation is in the image of the slit. When the slit is wide relative to the wavelength, diffraction is difficult to observe. But when the wavelength and slit opening are of the same order of magnitude, diffraction is observed easily. Here the slit behaves as if it is a source of radiation. The output in this case occurs in a series of 1800 arcs.
The plot of the frequency of the radiation as a function of peak transmittance is in the form of a Gaussian curve with maximum intensity being the midpoint of the frequency range.
In contrast, monochromator may be defined as a device to select different wavelength of the original source radia
1.In a UV- Visible spectrophotometer, the monochromator is placed between the source of light and the sample. The light source gives white light which is dispersed by the monochromator and only light of the selected wavelength falls on the sample, gets absorbed by the sample and the decreased intensity of the emerging light is measured.
2.Slit is present before the flame in AAS.
3.For AAS, the characteristic resonance absorption wavelength of the analyte is selected. This information can be obtained from the literature on atomic spectra of the elements. The instrument manufacturer usually provides these data. The wavelength selected is independent of the matrix (other elements and compounds present).
For atomic emission spectrometer, most elements have several prominent lines that can be used for identification and determination purposes. Wavelength data recorded up to three decimal places with corresponding intensity information for prominent lines of more than 70 elements