The absorbance for a given quantity of a compound in spectroscopic analysis is absolute and depends only on the fraction of that quantity which interacts with the electromagnetic radiation. Also, to maximize the signal-to-noise (S/N) ratio the total optical power of the spectrophotometer should be allowed to interact with the sample. Since the design of the monochromator is normally based on line-shaped entrance and exit slits, the optimum geometry for a sample pellet would be a rectangle of dimensions comparable to the entrance slit. Circular pellets which cover the entire slit keep part of the sample outside the beam; when the pellet diameter is made comparable to the slitwidth, excessive vignetting will occur which will adversely affect the S/N ratio.
KeywordsEntrance Slit Thrust Bearing Exit Slit Cylindrical Pellet Percent Transmittance
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