UV-Visible Spectrometry

1. UV-Visible Spectrometry
Theory
Instrumentation
Application

2. T = I0 / It
Transmittance T is a ratio of intensity of transmitted light
to that of the incident light.
A = log I0 / It
The more general equation can be written as follows:
A = log I0 / It = log 1/ T = – log T = abc = εbc
 The molar absorption coefficient is a measurement of
how strongly a substance absorbs light.
 The larger its value, the greater the absorption
Beer's and Lambert's Law

4. INSTRUMENTATION
Instruments for measuring the absorption of U.V. or visible
radiation are made up of the following components;
1. Sources (UV and visible)
2. filter or monochromator
3. Sample containers or sample cells
4. Detector

5. Radiation source
 It is important that the power of the radiation source does not change
abruptly over its wavelength range.
 The electrical excitation of deuterium or hydrogen at low pressure
produces a continuous UV spectrum
 The mechanism for this involves formation of an excited molecular species,
which breaks up to give two atomic species and an ultraviolet photon
 Both Deuterium and Hydrogen lamps emit radiation in the range 160 - 375
nm
 Quartz windows must be used in these lamps, and quartz cuvettes must be
used, because glass absorbs radiation of wavelengths less than 350 nm.

6. Various UV radiation sources
a. Deuterium lamp
b. Hydrogen lamp
c. Tungsten lamp
d. Xenon discharge lamp
e. Mercury arc lamp
Various Visible radiation sources
a. Tungsten lamp
b. Mercury vapour lamp
c. Carbonone lamp

7. Filters or monochromators
All monochromators contain the following component parts;
• An entrance slit
• A collimating lens
• A dispersing device (a prism or a grating)
• A focusing lens
• An exit slit
Steps Involved
a. Polychromatic radiation enters the monochromator through the entrance slit.
b. The beam is collimated, and then strikes the dispersing element at an angle.
c. The beam is split into its component wavelengths by the grating or prism.
d. By moving the dispersing element or the exit slit, radiation of only a
particular wavelength leaves the monochromator through the exit slit.

8. Sample containers or sample cells
 A variety of sample cells available for UV region. The
choice of sample cell is based on
a) the path length, shape, size
b) the transmission characteristics at the desired
wavelength
c) the relative expense
 The cell holding the sample should be transparent to the
wavelength region to be recorded.

9.  Quartz or fused silica cuvettes are required for
spectroscopy in the UV region.
 Silicate glasses can be used for the manufacture of
cuvettes for use between 350 and 2000 nm.
 The thickness of the cell is generally 1 cm. cells may
be rectangular in shape or cylindrical with flat ends.

10. Detectors
In order to detect radiation, three types of photosensitive devices
are
a. photovoltaic cells or barrier- layer cell
b. phototubes or photo emissive tubes
c. photomultiplier tubes

11. Photovoltaic cell
 Also known as barrier layer or photronic cell.
 It consists of a metallic base plate like iron or aluminium which acts as one
electrode.
 On its surface, a thin layer of a semiconductor metal like selenium is
deposited. Then the surface of selenium is covered by a very thin layer of
silver or gold which acts as a second collector tube.
 When the radiation is incident upon the surface of selenium, electrons are
generated at the selenium- silver surface and the electrons are collected by
the silver. This accumulation at the silver surface creates an electric voltage
difference between the silver surface and the basis of the cell.

12. Phototubes
 Also known as photo emissive cells.
 A phototube consists of an evacuated glass bulb.
 There is light sensitive cathode inside it. The inner
surface of cathode is coated with light sensitive layer
such as potassium oxide and silver oxide.
 When radiation is incident upon a cathode,
photoelectrons are emitted.
 These are collected by an anode. Then these are
returned via external circuit.
 And by this process current is amplified and recorded.

13. The photomultiplier tube
 It consists of a photoemissive cathode , several dynode and an anode.
 A photon of radiation entering the tube strikes the cathode, causing the
emission of several electrons.
 These electrons are accelerated towards the first dynode then onwards
 Each original photon has produced 106 - 107 electrons. The resulting current is
amplified and measured.
 Photomultipliers are very sensitive to UV and visible radiation. They have fast
response times. Intense light damages photomultipliers; they are limited to
measuring low power radiation.

15. Single Beam Spectrometer
Benefits of Single Beam Design
 Less number of optical components results in improved light transmission
and reduced noise
 Affords greater sensitivity due to higher light throughput

16. Double Beam Spectrometer
Benefits of Double Beam Design
 Correction of absorbance for solvent blank
 In single beam design there can be fluctuations in lamp energy between
the ratio recording measurements.
 Correction due to fluctuations in stray light, beam intensity variations
and electronic noise are applied in real-time

18. Absorption of EMR by Organic molecules
 Absorption of light result in the promotion of an electron from HOMO
to LUMO
 i.e movement of valence shell electron ,typically from a filled bonding
MO to unfilled antibonding MO
 As conjugation increases HOMO-LUMO gap decreases and the position
of absorption shift to longer wavelength
 In saturated alkyl halide energy required decrease with decrease in
electronegativity
 The conjugated system needs lower energy for excitation because
energy gap separation is low

19. The Influence of Functional Group in absorption peak
 Absorption peaks are also influenced by functional groups.
 The functional groups influence the conjugated systems, causing the
absorption peaks to appear at longer wavelengths than the peak wavelength
of benzene, although they do not go beyond 400 nm and enter the visible
region.
 The colour of organic compounds, then, is influenced more strongly by the
size of the conjugated system.

20. Chromophore
Functional group not in conjugation with any other group is said to be
chromophore
Generally groups having unsaturated centers or heteroatoms or both
NO2,N=O ,C=O ,C=N ,C=S
Auxochrome
Don’t have selective absorption above 200nm on its own
Along Chromophore shift to a longer wavelength and increase in intensity of
absorption peak
Acidic: −COOH, −OH, −SO3H
Basic: −NH2, −NHR, −NR2

21. The Relationship Between Conjugated Double Bond
Systems and Absorption Peaks
 This absorption spectra obtained by dissolving
compounds in ethanol and analysing the resulting
solutions.
 The concentrations were adjusted so that the
absorption intensities of the components were
roughly the same.
 Peak wavelengths tend to be shifted toward the
long wavelength region as the conjugated system
gets larger
 With larger conjugated systems, the absorption
peak wavelengths tend to be shifted toward the
long wavelength region and the absorption peaks
tend to be larger.

22. Substance Absorption Peak Molar Absorption Coefficient
Ethylene(CH2=CH2)
1.3-butadiene
Vitamin A
β-carotene
180nm
217nm
328nm
450nm
10000
21000
51000
140000
Benzene
Naphthalene
Anthracene
Naphthacene
255nm
286nm
375nm
477nm
180
360
7100
110000

23. Solvent Effect
 Compound peak could be obscured by the
solvent peak
 So a most suitable solvent is one that does
not itself get absorbed in the region under
investigation.
 A solvent should be transparent in a
particular region.
 A dilute solution of sample is always
prepared for analysis.
 Most commonly used solvents are as
follows.
Solvent λ of absorption
Water 191 nm
Ether 215 nm
Methanol 203 nm
Ethanol 204 nm
Chloroform 237 nm
Carbon
tetrachloride
265 nm
Benzene 280 nm
Tetrahydrofuran 220 nm

25. Detection of Impurities
 One of the best methods for determination of
impurities in organic molecules.
 Additional peaks can be observed due to
impurities in the sample and it can be
compared with that of standard raw material.
 By also measuring the absorbance at specific
wavelength, the impurities can be detected
 Benzene appears as a common impurity in
cyclohexane. Its presence can be easily
detected by its absorption at 255 nm.
Applications of Absorption Spectroscopy (UV, Visible)

26. Structure elucidation of organic
compounds.
 To check the presence or absence of
unsaturation, the presence of hetero
atoms
 From the location of peaks and
combination of peaks, it can be
concluded that whether the compound
is saturated or unsaturated, hetero
atoms are present or not etc.

27. Quantitative analysis
 Quantitative determination is based on Beer’s law
which is as follows.
A = log I0 / It = log 1/ T = – log T = abc = εbc
Where,
ε is extinction co-efficient,
c is concentration
b is the length of the cell that is used in
UV spectrophotometer

28. Qualitative analysis
 Identification is done by comparing the
absorption spectrum with the spectra of
known compounds
 UV absorption spectroscopy is generally
used for characterizing aromatic
compounds and aromatic olefins.

29. Chemical kinetics
 Kinetics of reaction can also be studied using UV spectroscopy.
 The UV radiation is passed through the reaction cell and the
absorbance changes can be observed.
Quantitative analysis of pharmaceutical substances
 Many drugs are either in the form of raw material or in the form of
formulation
 They can be assayed by making a suitable solution of the drug in a
solvent and measuring the absorbance at specific wavelength.
 Diazepam tablet can be analyzed by 0.5% H2SO4 in methanol at
the wavelength 284 nm.

30. As HPLC detector
 The presence of an analyte gives a response which can be
assumed to be proportional to the concentration.
 For more accurate results, the instrument's response to the
analyte in the unknown should be compared with the response
to a standard; as in the case of calibration curve.
 Three types
- Fixed Wavelength
- Variable Wavelength
- Diode array

31. THANK YOU