The theory of electromagnetic spectrum, absorption,
Brief review of electromagnetic spectrum and absorption of
radiation. The Chromophore concept, absorption law and
Electromagnetic spectrum: The arrangement of all
types of electromagnetic radiations in order of their
increasing wavelengths or decreasing frequencies is
known as complete electromagnetic spectrum.
If we arrange all types of electromagnetic
radiations in order of their increasing wavelengths,
then the portion above the visible region is called
Infra-red while that below it is the ultra-violet region.
In a electromagnetic spectrum, we may note that :
i. Visible and ultra-violet radiation cover the
wavelength range from 200-800 mµ. The
absorption of radiation in this region causes the
excitation of π electron in a conjugated or an
ii. The Infra-red radiations which cover the
wavelength range from 0.8 to 2.5 µ constitute near
Infra-red region and that from 15 to 25 µ is called
far Infra-red region. The most useful region for
Infra-red spectroscopy is 2.5 to 15 µ.
iii. NMR spectroscopy provides a complete insight into
the environment and arrangement of atoms within a
molecule. For this technique, radiation of longest
wavelength range, i.e., Radio waves are useful.
Due to the different environmental effects, different
magnetic nuclei ( say protons, N15, C13, F19, P31 atoms
etc.) will feel the applied magnetic field differently.
Hence, absorptions at different field strengths will
correspond to different sets of protons or magnetic nuclei.
Chromophore: It is define as isolated
covalently bonded group that shows a characteristic
absorption in the ultraviolet or the visible region.
There are two types of chromophores:
a. Chromophore in which the group contains electrons
and they undergo n–›π* transition. Such
chromophores are ethylene's, acetylenes etc.
b. Chromophore which contain both π electrons and n
(non-bonding) electrons. Such chromophores undergo
two types of transition i.e., π–›π* and n–›π*.
There are some points:
a. Spectrum consisting of a band near 300 mµ
may contain two or three conjugated unites.
b. Absorption bands near 300 mµ with very low
intensity, €max 10-100 are due to n–›π*
transitions of the carbonyl group.
c. Simple conjugated chromophores such as
dines have high €max values, i.e., from 10,000
d. The absorption with €max value between 1000
to 10,000 shows an aerometric system.
Lambert’s Law: The intensity of the transmitted
light passing through a homogeneous medium
decreases geometrically as the thickness of the layer
Beer’s Law: Each molecule of solute absorbs the
same fraction of light incident upon it, regardless of
concentration in a non-absorbing medium.
Beer’s law does not hold over the
entire concentration range, but in very dilute
solutions, as in uv-spectroscopy, the deviations are
BEER’S LAW: LIMITATIONS
The linearity of the Beer-Lambert law is limited by
chemical and instrumental factors. Causes of
1. Deviations in absorptivity coefficients at high
concentrations (>0.01M) due to electrostatic
interactions between molecules in close proximity .
2. Scattering of light due to particulates in the sample.
3. Changes in refractive index at high analyte
4. Shifts in chemical equilibrium as a function of
5. Non-monochromatic radiation, deviations can
be minimized by using a relatively flat part of the
absorption spectrum such as the maximum of an