2. 2.1 Terms employed in Absorption Spectroscopy
Term & Symbol Definition Alternative Name &
Symbol
Incident radiant
power, Po
Radiant power in
watts in incident on
sample
Incident intensity, Io
Transmitted radiant
Power, P
Radiant power
transmitted by sample
Transmitted intensity, I
Absorbance, A Log (Po/P) Optical density, D;
extinction, E
Transmittance, T P/Po Transmission, T
Path length of sample,
b
Length over which
attenuation occurs
l,d
Absorptivity, a A/(bc) Extinction coefficient,
k
Molar absorptivity, ε A/bc Molar extinction
coefficient
5. 2.1.2 Absorbance A
• The logarithm of the ratio between the initial power of a
beam of radiation Po and its power after it has traversed
an absorbing medium:
• When Absorbance of a solution increases, transmittance
decreases
7. 2.1.3 Measuring Transmittance & Absorbance
• Losses in measuring: Reflection losses and scattering
losses in solution
• To compensate these effects, the power of the beam
transmitted through a cell containing the analyte solution
is compared with one that traverses either in identical
cell containing only the solvent/reagent blank
10. 2.2 Beer’s Law
Exercise:
A 7.25 x 10-5
M solution of potassium permanganate has a
transmittance of 44.1% when measured in a 2.10 cm cell at
a wavelength of 525 nm. Calculate (a) the absorbance of
this solution; (b) the molar absorptivity of KMnO4
11. 2.2.1 Application of Beer’s Law to Mixtures
• Beer’s Law also applies to solutions containing more
than one kind of absorbance substance
• Provided that there is no interaction among the various
species, the total absorbance of multicomponent system
at a single wavelength is the sum of the individual
absorbances:
12. 2.2.2 Limitation to the applicability of Beer’s Law
• There are few exceptions to the linear relationship
between absorbance and path length at a fixed
concentration due to deviation:
– Real deviation (fundamental)
– Method:
• Instrumental
• Chemical
18. 2.2.3 Absorption Spectra
• Line spectra
– Occur when the radiating species are individual atomic particles
that are well separated in gas
– The individual particles in a gases medium behave
independently of one another, and spectrum consists of a series
of sharp lines
• Band spectra
– Are often produced in spectral source because of the presence
of gases radicals or small molecules
– This spectra is not fully resolved by the instruments
27. 2.3.2.IR Spectrophotometers – Fourier Transform
• Characteristics:
– Great speed
– High resolution
– High sensitivity
– Excellent wavelength precision and accuracy
• FTIR have been reduced to benchtop size which is
reliable and easy to maintain, less price
• Contain no dispersive elements, all wavelengths are
detected and measured simultaneously
• Interferometer is used to produce interference patterns
that contain the IR spectral information
• Types of sources are the same as dispersive
instruments
• Transducers: pyro-electric transducer, photoconductive
transducer
• Most FTIR are of the single beam type
28. 2.3.2.IR Spectrophotometers – Fourier Transform
• Advantages:
– Better speed and sensivity
– Simpler mechanical design
– Better light-gathering power
– More accurate wavelength calibration
– Elimination of stray light and IR emission
