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What is Spectroscopy?
Spectroscopy is the study of the interaction between matter and radiated
energy. Historically, spectroscopy originated through the study of visible light
dispersed according to its wavelength, e.g., by a prism. Later the concept was
expanded greatly to comprise any interaction with radiative energy as a
function of its wavelength or frequency.
Spectrometry is the spectroscopic technique used to assess the
concentration or amount of a given chemical (atomic, molecular, or ionic)
species. In this case, the instrument that performs such measurements is a
spectrometer, spectrophotometer, or spectrograph.
Spectroscopy/spectrometry is often used in physical and analytical chemistry
for the identification of substances through the spectrum emitted from or
absorbed by them.
What are electromagnetic waves?
Electricity can be static, like what holds a balloon to the wall or
makes your hair stand on end.
Magnetism can also be static like a refrigerator magnet. But
when they change or move together, they make waves electromagnetic waves.
Electromagnetic waves are formed when an electric field
(shown as blue arrows) couples with a magnetic field (shown
as red arrows). The magnetic and electric fields of an
electromagnetic wave are perpendicular to each other and to
the direction of the wave. James Clerk Maxwell and Heinrich
Hertz are two scientists who studied how electromagnetic
waves are formed and how fast they travel.
Electromagnetic waves are typically described by any of the following
three physical properties: the frequency f, wavelength λ, or photon energy
E.
Frequency
 (a) and (b) represent two waves that are traveling at the
same speed.
 In (a) the wave has long wavelength and low frequency
 In (b) the wave has shorter wavelength and higher frequency
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of
electromagnetic radiation. The "electromagnetic spectrum" of an object is the
characteristic distribution of electromagnetic radiation emitted or absorbed
by that particular object.
Electromagnetic radiation
Electromagnetic radiation (EM radiation or EMR) is a form of energy emitted
and absorbed by charged particles, which exhibits wave-like behavior as it
travels through space. EMR has both electric and magnetic field components,
which oscillate in phase perpendicular to each other and perpendicular to
the direction of energy and wave propagation. In vacuum, electromagnetic
radiation propagates at a characteristic speed, the speed of light.
What happens to absorbed radiation
• It is possible for the packet of photonic energy to be absorbed, resulting in the
promotion of one or more electrons to higher energy levels. That is, electromagnetic
radiation is absorbed by the atom, which is converted from its ground to one of many
possible excited states.
• Since the energy of electrons in orbital is fixed, it should be clear that when an electron is
promoted, a very specific amount of energy is required—corresponding to
the energy difference between the initial orbital and the final orbital.
•Note that, if the photonic energy is very high, such as might be the case with x-rays, the
electron may be totally removed from the atom, leaving an ion in its place.
•Atoms generally do not stay in an excited state and they tend to relax to their ground
states as quickly as possible. In doing so they must emit their excess energy as the electron
falls to a lower orbital.
• The end result is that the atoms emit their excess energy once again as light. And the
light they radiate will correspond to a very specific set of photon energies.
Electrons in molecule
• p, s, and n (non-bonding) electrons
Sigma and Pi orbitals
Electron transitions
Electron transitions
Roles of Chromophore and Auxochrome on absorption
A chromophore is the part of a molecule responsible for its color. The color arises
when a molecule absorbs certain wavelengths of visible light and transmits or
reflects others. Example- nitro, azo group etc.
A molecule containing a chromophore is called chomogen.
An auxochrome is a group of atoms attached to a chromophore which modifies
the ability of that chromophore to absorb light or intensify the color.
There are mainly two types of auxochromes:
Acidic -COOH, -OH, -SO3H
Basic -NHR, -NR2, -NH2
Chromophoric Structure
Group
Structure
nm
Carbonyl
>C=O
280
Azo
-N = N-
262
Nitro
-N=O
270
Thioketone
-C =S
330
Nitrite
-NO2
230
Conjugated Diene
-C=C-C=C-
233
Conjugated Triene
-C=C-C=C-C=C-
268
Conjugated Tetraene
-C=C-C=C-C=C-C=C-
315
Benzene
261
Beer – Lambert Law
When
c
b
Exponential functions
Exponential functions look somewhat similar to functions you have seen before, in that
they involve exponents, but there is a big difference, in that the variable is now the
power, rather than the base. Previously, you have dealt with such functions as f(x) = x2,
where the variable x was the base and the number 2 was the power.
In the case of exponentials, however, you will be dealing with functions such as g(x) =
2x, where the base is the fixed number, and the power is the variable.
Let's look more closely at the function g(x) = 2x. To evaluate this
function, we operate as usual, picking values of x, plugging them
in, and simplifying for the answers. But to evaluate 2x, we need to
remember how exponents work. In particular, we need to
remember that negative exponents mean "put the base on the
other side of the fraction line".
So, while positive x-values give us values
like these:
Beer – Lambert Law
Beer – Lambert Law
a=absorptivity,
ᵋ= molar absorptivity
A=
ᵋbc
Transmittance, T = P / P0
% Transmittance, %T = 100 T
Relationship between absorbance and transmittance
The relationship between absorbance and transmittance is illustrated in the
following diagram:
So, if all the light passes through a solution without any absorption, then
absorbance is zero, and percent transmittance is 100%. If all the light is
absorbed, then percent transmittance is zero, and absorption is infinite.
Why do we prefer to express the Beer-Lambert law using absorbance as a
measure of the absorption rather than %T ?
Answer : To begin, let's think about the equations...
A=abc
%T = 100 P/P0 = e -abc
Now, suppose we have a solution of copper sulphate (which appears blue because
it has an absorption maximum at 600 nm). We look at the way in which the
intensity of the light (radiant power) changes as it passes through the solution in
a 1 cm cuvette. We will look at the reduction every 0.2 cm as shown in the diagram
below. The Law says that the fraction of the light absorbed by each layer of
solution is the same. For our illustration, we will suppose that this fraction is 0.5 for
each 0.2 cm "layer" and calculate the following data:
Path length /
cm
0
0.2
0.4
0.6
0.8
1.0
%T
100
50
25
12.5
6.25
3.125
Absorbance
0
0.3
0.6
0.9
1.2
1.5
1.6
1.4
100
1.2
Absorbance
120
%T
80
60
40
1
0.8
0.6
0.4
20
0.2
0
0
0
0.2
0.4
0.6
0.8
Path length, cm
1
1.2
0
0.2
0.4
0.6
0.8
1
Path length, cm
A = abc tells us that absorbance depends on the total quantity of the
absorbing compound in the light path through the cuvette. If we plot
absorbance against concentration, we get a straight line passing through
the origin (0,0).
The linear relationship between concentration and
absorbance is both simple and straightforward, which is
why we prefer to express the Beer-Lambert law using
absorbance as a measure of the absorption rather than
%T.
1.2
Problems
Problems
Limitations of the Beer-Lambert law
The linearity of the Beer-Lambert law is limited by chemical and
instrumental factors. Causes of nonlinearity include:
-deviations in absorptivity coefficients at high concentrations
(>0.01M) due to electrostatic interactions between molecules in
close proximity
-scattering of light due to particulates in the sample
-fluoresecence or phosphorescence of the sample
-changes in refractive index at high analyte concentration
-shifts in chemical equilibria as a function of concentration
-non-monochromatic radiation, deviations can be minimized by
using a relatively flat part of the absorption spectrum such as the
maximum of an absorption band
-stray light
(1) Chemical effects - analyte associates, dissociates or reacts to give
molecule with different ᵋ
(2) Physical effects - stray light, polychromatic radiation or noise
non-linear calibration curve