INTRODUCTION TO

SPECTROCHEMICAL METHODS
 Lecture Content
• Properties of Electromagnetic Radiation
• Interaction of Radiation and Matter
• Radiation Absorption
• Beer’s Law and Quantitative Analysis
 Properties of Electromagnetic Radiation
Spectrochemical analysis is one of the major tools of analytical
chemistry.

"Spectrochemical" is a compound word that comes from spectrum and

chemical.

A spectrum is a display of the intensity of radiation emitted, absorbed,

or scattered by a sample versus a quantity related to photon energy,
such as wavelength or frequency.

The term spectrochemical implies that a spectrum or some aspect of a

spectrum is used to determine chemical species and to investigate the
interaction of chemical species with electromagnetic radiation.
 Properties of Electromagnetic Radiation
Spectroscopy:
A term used to describe techniques based on the measurement of
absorption, emission, or luminescence of electromagnetic radiation.

Spectrometry:
Spectrometry is based on the absorption of photons by the analyte.
The more concentrated the analyte solution, the more light is
absorbed, and the deeper the resulting color of the solution.
The color of an object we see is due to the wavelength transmitted or
reflected. The other wavelengths are absorbed.

Spectroscopic analytical methods:

Based on measuring the amount of radiation produced or absorbed
by molecular or atomic species of interest. The amount absorbed is
related to the concentration of the analyte in the solution.
 Properties of Electromagnetic Radiation
Colorimetric measurement:
The use of instrumentation to perform for more accurate
estimates of the intensity of the colour of a sample.

Electromagnetic radiation:
- is a form of energy that is transmitted trough space at
enormous velocities.
- Can be described as a wave with properties of wavelength,
frequency, velocity, and amplitude.
- Requires no transmitting medium; thus it can travel readily
through a vacuum.
- The electromagnetic spectrum covers an enormous range of
energies (frequencies) and thus wavelengths.
 Properties of Electromagnetic Radiation
Wave nature of a beam of single frequency electromagnetic radiation.

Fig. 1

(a) A plane-polarized waves is shown propagating along the x axis. The electric field oscillates
in a plane perpendicular to the magnetic field. If the radiation were unpolarized, a
component of the electric field would be seen in all planes.
(b) Only the electric field oscillations are shown. The amplitude of the wave is the length of
the electric field vector at the wave maximum, while the wavelength is the distance
between successive maxima.
 Properties of Electromagnetic Radiation
Amplitude (a) of a wave is the distance from the centre line
(or the still position) to the top of a crest or to the bottom of
a trough.
Wavelength (λ) of a wave is the distance from any point on
one wave to the same point on the next wave along. – metres
(m) unit.
Frequency (ƒ) of a wave is the number of waves passing a
point in a certain time. - hertz(Hz) unit
Velocity (v)of a wave is the speed of wave travels in a certain
time. v=fλ

wavenumber (ṽ) is defined as the number of wavelengths per

unit distance, typically centimeters (cm−1)
ṽ = 1/ λ
 Properties of Electromagnetic Radiation

The units used to express wavelength in various regions of the

spectrum (Table 1).

Table 1
Calculate the wavenumber of a beam of infrared radiation with
a wavelength of 5.00 μm.

ṽ = 1/ λ
= 1/ 5.00 μm x 10-4 cm/μm
= 2000 cm-1
Interaction
of Radiation
and Matter
 Interaction of Radiation and Matter
• The most interesting and useful interactions in spectroscopy
are those in which transitions occur between different energy
levels of chemical species.
• The electromagnetic spectrum covers an enormous range of
energies (frequencies) and thus wavelengths (see Table 2).
Table 2
Interaction of Radiation and Matter
Interaction of Radiation and Matter
 Interaction of Radiation and Matter
• Spectroscopists use the interactions of radiation with matter to
obtain information about a sample. Several of the chemical
elements were discovered by spectroscopy.
• The sample is usually stimulated in some way by applying
energy in the form of heat, electrical energy, light, particles, or
a chemical reaction.
• Prior to applying the stimulus, the analyte is predominantly in
its lowest-energy or ground state. The stimulus then cause
some of the analyte species to undergo a transition to a higher-
energy or excited state.
• We acquire information about the analyte by measuring the
electromagnetic radiation emitted as it returns to the ground
state or by measuring the amount of electromagnetic radiation
absorbed as a result of excitation.
Interaction of Radiation and Matter
Emission or chemiluminescence processes.

Fig. 2
 Interaction of Radiation and Matter
Emission or chemiluminescence processes (Fig. 2):

(a) The sample is excited by applying thermal, electrical, or

chemical energy. No radiant energy is used to produce excited
states, and so, these are called non-radiative processes.
(b) In the energy level diagram, the dashed lines with upward
pointing arrows symbolize these non-radiative excitation
processes, while the solid lines with downward pointing arrows
indicate that the analyte loses its energy by emissions of a
photon.
(c) The resulting spectrum is shown as measurement of the
radiant power emitted, as a function of wavelength, λ.
 Interaction of Radiation and Matter

Fig. 3

When the sample is stimulated by applying an external

electromagnetic radiation source, several processes are possible.
For example, the radiation can be scattered or reflected. What is
important to us is that some of the incident radiation can be
absorbed and promote some of the analyte species to an excited
state (Fig. 3).
 Interaction of Radiation and Matter
- In absorption spectroscopy, we measure the amount of light
absorbed as a function of wavelength. Absorption measurements can
give both qualitative and quantitative information about the sample.
- In photoluminescence spectroscopy (Fig. 4), the emission of photons
is measured following absorption. The most important forms of
photoluminescence for analytical purpose are fluorescence and
photophorescence spectroscopy.

Fig. 4
 Radiation Absorption
Wavelength, frequency and wavenumber are interrelated

C
λ =ν
λ = Wavelength (cm)
ν = Frequency (s-1 or Hertz, Hz)
C = Velocity of light (3 x 1010 cm/s)

Ṽ = λ= C
1 ν
Ṽ = wavenumber (cm-1)

Wavelength in the ultraviolet and visible regions are on the order of

nanometers. In the infrared region, they are micrometers, but the reciprocal of
wavelength is often used (wavenumbers in cm-1)
 Radiation Absorption

ℎ𝐶
E = hv =
𝜆
E = energy of the photon in ergs (unit of energy)
h = is Planck’s constant (6.63 x 10-34 joule-second (J-s) or 4.14 x 10-15 eV.s)
V = frequency of photon/ electromagnetic radiation
λ = wavelength of photon/ electromagnetic radiation

The shorter the wavelength or the greater the frequency, the greater the energy

Most of the energy from the absorbed radiation is lost as heat, via collisional
processes, that is, by increasing the kinetic energy of the collided molecules.
 Radiation Absorption
- Every molecular species is capable of absorbing its own
characteristic frequencies of electromagnetic radiation (Fig. 5).
- This process transfers energy to the molecule and results in a
decrease in the intensity of the incident electromagnetic radiation.
- Absorption of the radiation thus attenuates the beam in accordance
with the absorption law (Beer-Lambert Law).

In spectroscopy attenuates means to decrease the energy per unit

area of a beam of radiation.
In terms of the photon model, attenuate means to decrease the
number of photons per second in the beam.
 Radiation Absorption

𝑃𝑜 𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡
A = log ≈ log
𝑃 𝑃𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛

T = P/Po
Fig. 5

As light traverses a medium containing an absorbing analyte, the intensity decreases

as the analyte becomes excited. For an analyte solution of a given concentration,
the longer the length of the medium through which the light passes (path length of
light), the more absorbers are in the path, and the greater the attenuation. Similarly,
for a given path length of light, the higher the concentration of absorbers, the
stronger the attenuation.
The transmittance T of the solution is the fraction of incident radiation transmitted
by the solution. Transmittance is often expressed as a percentage and called the
percent transmittance.
The amount of radiation absorbed may be measured in a number of
ways:
Transmittance, T = P / P0

% Transmittance, %T = 100 T

Absorbance,

A = log10 P0 / P
A = log10 1 / T
A = log10 100 / %T
A = 2 - log10 %T
Calculate the absorbance of a solution
having a %T of 89 at 400 nm.

A = 2 - log%T
= 2 - log89
= 0.051
 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.
 Beer-Lambert Law @ Beer’s Law
According to Beer’s Law, absorbance is directly proportional to the
concentration of the absorbing species, c, and to the path length, b, of
the absorbing medium as expressed by the following equation:

A = log (Po/P) = abc

T = P/Po
𝑃𝑜 𝑃𝑜
A = - log T = - log = log
𝑃 𝑃
A = Absorbance
a = Absorptivity (Lg-1cm-1)
b = path length (cm)
c = concentration (gL-1)
T = transmittance
Po= power of a beam (W)
P = transmitted power of a beam (W)
 Beer-Lambert Law @ Beer’s Law

a is a proportionality constant called the absorptivity. Because

absorbance is a unitless quantity, the absorptivity must have units that
cancel the units of b and c. For example, c has the units of gL-1 and b
has the units of cm, absorptivity has the units of Lg-1cm-1.

When we express the concentration in moles per liter and b in cm, the
proportionality constant is called the molar absorptivity and is given
the symbol ε. Where ε has the units of Lmol-1cm-1.

A = εbc

A = Absorbance
ε = molar absorptivity (Lmol-1cm-1)
b = path length (cm)
c = concentration (molL-1)
 Beer-Lambert Law @ Beer’s Law
- Beer-Lambert Law also known as absorption law (Beer’s Law).
- Beer-Lambert Law relates the absorption of most molecular species to the
concentration (c), the path length (b) and the molar absorptivity (ε).

A = εbc
A = Absorbance
ε = Molar absorptivity (Lmol-1cm-1)
b = path length (cm)
Fig. 6 c = concentration (mol/L)

Beer’s Law- relating the amount of radiation absorbed to concentration.

The absorptivity varies with wavelength and represents the absorption spectrum.
The absorbance is directly proportional to the concentration.
A solution of Co(H2O)2+ has an absorbance of 0.20 at 530
nm in a 1.00 cm cell. ε is known to be 10 L mol-1 cm-1
what is its concentration?

A = 0.20
b = 1.00 cm
ε = 10 L mol-1 cm-1

A = εbc
0.20 = 10 x 1 x c
c = 0.02 M
The absorbance of an unknown MnO4- solution is 0.50 at
525 nm. When measured under identical conditions, a
1.0×10-4 M MnO4- is found to have an absorbance of
0.20. Determine the conc. of the unknown.

Solution:
A = εbc
ε and b are constant
A is directly proportional to c

𝐴1 𝑐1
=
𝐴2 𝑐2
0.5 𝑐1
=
0.2 1.0 𝑥 10−4

c1 = 2.5 x 10-4 M
A CaCO3 solution shows a transmittance of 90%, when taken in
a cell of 1.9 cm thickness. Calculate its concentration, if the
molar absorption coefficient is 9000 dm3/mol/cm.
Solution:
A = 2 - log10 %T
= 2 - log10 90
= 2 – 1.954
= 0.045

A = εbc, b= 1.9 cm, ε = 9000 dm3/mol/cm, A=0.045

c=?
So, c = A/εb
= 0.045/ 9000 × 1.9
= 2.631 × 10-6 mol/dm3
 Radiation Absorption

Fig. 7

Reflection and scattering losses with a solution contained in a

typical glass cell. Losses by reflection can occur at all the
boundaries that separate the different materials. In this example,
the light passes through the following boundaries, called
interfaces: air-glass, glass-solution, solution-glass, and glass-air.
Radiation Absorption
Table 3
 Radiation Absorption
Absorption Spectra

An absorption spectrum is a plot of absorbance versus

wavelength (Fig. 8). Absorbance could also be plotted against
wavenumber or frequency. Modern scanning spectrophotometers
produce such an absorption spectrum directly. The colour of a
solution is related to its absorption spectrum (Table 4).

Fig. 8
 Radiation Absorption
Absorption Spectra

Colors of different wavelength regions:

Table 4
Radiation Absorption
 Beer’s Law and Quantitative Analysis

Absorbance spectrum
Calibration plot

Absorption spectra at Linear equation:

different concentrations y = mx + c
The Beer-Lambert law using absorbance as a
measure of the absorption rather than %T ?

absorbance is directly proportional to the other parameters

 Beer’s Law and Quantitative Analysis

Calibration curve Deviation from Beer’s Law result in

nonlinear calibration curves, especially
at higher concentrations.
Beer’s Law and Quantitative Analysis
Example:
If given A = 0.25
Must use the y = mx + c
y = 1.32x + 0.004
0.25 = 1.32x + 0.004
x = 0.186 M
Instrumental analysis

• Instruments are more selective and

sensitive than volumetric and gravimetric
methods.
• More expensive
• More rapid, more automated
• May be capable of measuring more than
one analyte at a time