Coloured
compounds absorbs visible light while colourless substances don’t. UV radiation
is required for the detection of colourless substances in UV visible
spectrophotometer.
Must read the Basics of UV spectroscopy first.
Principle
The sample to be
analysed contains either sigma, pi or non-bonding (n) electrons. On irradiation,
these electrons absorbs characteristic amount of radiation and undergoes
transition from ground state to the excited state.
The
greater is the number of molecules capable of absorbing light of particular
wavelength, the more will be light absorption.The intensity of light absorption
depends upon the path length and concentration as according to the BeerLambert’s law (covered earlier).
By
knowing the characteristic absorption peaks, the various types of electrons
present in the compound can be analyzed and hence would help in structure
ellucidation.
The
UV-Visible spectrum is often plotted as Absorbance versus Wavelength. However,
most of the spectra are described by characterizing the wavelength maxima and
absorbtivities of principle absorption peaks of the compound.
Instrumentation
The
typical Uv-Visible spectrophotometers composes of the following:
1. Light Source: For UV radiation – Deuterium lamp is used while
Tungsten lamp is used as a source of visible light. The light should be stable
and of proper intensity.
2. Monochromators: Diffraction gratings are used in UV-Visible
spectrophotometers. These are used to convert the polychromatic light to the
monochromatic light. As the name indicates, polychromatic light composes light
of different wavelengths while monochromatic light has single wavelength
light.
3. Sample cells: Glass or plastic cells can be used to the visible
light but in case of UV light; sample cells made up of quartz are used as glass
can absorb uv radiations while quartz couldn’t.
4. Solvents: Hexane, water and 95% Ethanol are commonly use
solvents. Following are the factors to be considered while choosing the
solvent:
a.
Solvent should
not absorb the radiation in the region in which the sample to be analyzed
absorbs.
b.
As the polar
solvents could interact with the sample species by forming H-bonds with them,
this will lead to the loss of fine spectrum.
Non polar solvents don’t interact.
c.
The solvent has
the ability to influence the property of sample to absorb the light of
particular wavelength by providing the stability to sample species in either
ground state or excited. One should consider this while choosing the solvent.
5.
Detectors: Among
various detectors available, Photomultiplier tube detectors are used. In modern
instruments, photodiodes are used.
6.
Recorders: It can
be done on plotters or digital displays.
The
UV – visible spectrophotometers can be single beam or double beam. As the name
indicates, single beam spectrophotometer has single beam of light to get passed
through the sample only for analysis. While the double beam spectrophotometer
has a rotating disc to split the single beam into two. From the resulting two beams, one is passed through sample compound and other through the reference compound. The results are
then plotted. A typical diagram of double beam spectrophotometer is given
below.
Must read : Red and blue shifts and Auxochrome
1 Comments
I find it helpful.. thnkx
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