Spectrophotometry Chapter 17, Harris. Spectrophotometry is the use of the measurement of the interaction of Electromagnetic radiation (EMR) with matter to quantize the concentration of an analyte. There are many different types of
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Spectrophotometry is the use of the measurement of the interaction of Electromagnetic radiation (EMR) with matter to quantize the concentration of an analyte. There are many different types of
spectrophotometers, based on the wavelength region of the EMR they measure. Examples are uv-vis, IR, microwave, x-ray, etc.
Electromagnetic Radiation travels at the speed of light (c), 2.997 x 108 m/s
Monochromatic light has a very small wavelength spread or narrow bandwidth; one
Polychromatic light has several wavelengths or in its beam.
Frequency (, Greek nu): Number of peaks that pass a given point per unit time.
Wavelength (, Greek lambda): Distance from one wave peak to the next.
= 1/, cm-1
Amplitude: Height measured from the center of the wave. The square of the amplitude gives intensity.
c = = 2.997 X 108m/s
Energy and frequency are related by the expression
E = h
where h (Planck’s constant = 6.626 X 10-34 J s)
symbolRegion of EMR
meter m radio
millimeter mm microwave
micrometer m infrared
nanometer nm visible/ultraviolet
Angstrom (10-10 m) Å X-ray
picometer pm -ray
Now, for a molecule,
Energy TOTAL =Eelectronic + Evibrational + Erotational
Where Eelectronic occursin the UV-Vis
Evibrational in the IR, and
Erotational in the microwave region of the EMR
Electronic transitions are accompanied by fine structure, i.e., vibrational and rotational transitions.
There are two quantities that relate the change in
the intensity of EM before and after interaction
A = log10 (P0 / P ) = - log10 (P/ P0 )
Beer’s Law (or the Beer-Lambert Law) expresses the relationship between absorbance (A), the length of cell(b), and the concentration of the absorbing species (c). The proportionality constant is and is known the molar absorptivity. This relationship is given by the equation
A = b c