Organic Chemistry.

1. MOLECULAR ABSORPTION
SPECTROSCOPY; THEORY,
INSTRUMENTATION AND
APPLICATION

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

3. 2.1.1 Transmittance T

4. 2.1.1 Transmittance T

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

6. 2.1.2 Absorbance A

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

8. 2.1.4 Sample Containers

9. 2.2 Beer’s Law

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

13. 2.2.2 Limitation to the applicability of Beer’s Law

14. 2.2.2 Limitation to the applicability of Beer’s Law

15. 2.2.2 Limitation to the applicability of Beer’s Law

16. 2.2.2 Limitation to the applicability of Beer’s Law

17. 2.2.2 Limitation to the applicability of Beer’s Law

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

19. 2.2.3 Absorption Spectra
• Continuum Spectra

20. 2.2.3 Absorption Spectra

21. 2.3 Instruments for Optical Absorption Instruments
• Ultraviolet/Visible Spectrophotometers
– Single beam instruments
– Double beam instruments
– Multichannel instruments
• Infrared Spectrophotometer
– Dispersive Instruments
– Fourier Transform Instruments

22. 2.3.1.1 Single Beam Instruments

23. 2.3.1.2 Double Beam Instruments

24. 2.3.1.2 Double Beam Instruments

25. 2.3.1.3 Multichannel Instruments

26. 2.3.2.IR Spectrophotometers – Dispersive 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