Spectrochemical

Analysis
Spectroscopy

 Spectroscopy is the study of the

properties of matter through its
interaction with various types of
radiation (mainly electromagnetic
radiation) of the electromagnetic
spectrum.
 Spectrometric Techniques are a
large group of analytical methods
that are based on atomic and
molecular spectroscopy.
 Spectrometry and spectrometric
methods refer to the
measurement of the intensity of
radiation with a photoelectric
transducer or other types of
electronic device.
Common terms
 Electromagnetic radiation: form of energy
having both electric and magnetic
characteristics wherein both the fields oscillate
perpendicularly along the direction of
propagation
 Wavelength (λ): distance between two
successive waves or distance between two
successive crests or troughs of waves (nm)
 Frequency (ν): number of waves produced per
second (Hz)
 Amplitude: maximum distance a wave extend
beyond its middle position
 Electromagnetic spectrum: it range from very
short wavelength (gamma rays) to very long
wavelength (radio waves)

 Relationship between λ and ν is c= νλ

Cont. • Nature of interaction between
radiation and matter may
include
✓Absorption
✓Emission
✓Scattering
• The effect of interaction
between radiation and matter
depend on the energy
associated with radiation
✓Very high energy radiation (UV
& X ray) may cause an electron
to be ejected from the
molecules
✓Radiation in the IR region have
much less energy which can
cause vibrations in molecule
Interaction of Radiation with Matter
 When a beam of radiant energy strikes the surface of a
substance, the radiation interacts with the atoms and molecules
of the substance or molecular ions or solids.
 The radiation may be transmitted, absorbed, scattered or
reflected, or it can excite fluorescence depending upon the
properties of the substance. The interaction, however, does not
involve permanent transfer of energy
 The velocity at which radiation is propagated through a medium
is less than its velocity in vacuum. It depends upon the kind and
concentration of atoms, ions or molecules present in the
medium.
 Figure shows various possibilities which might result when a
beam of radiation strikes a substance. These are:
 (a) The radiation may be transmitted with little absorption taking
place, and therefore, without much energy loss.
 (b) The direction of propagation of the beam may be altered by
reflection, retraction, diffraction or scattering.
 (c) The radiant energy may be absorbed in part or entirely by
the substance
 In absorption spectrophotometry, we are usually concerned with
absorption and transmission. Generally, the conditions under
which the sample is examined are selected to keep reflection
and scattering to a minimum.
EM radiation and matter interaction

 When an Electromagnetic
radiation is incident on a
matter, phenomena like
reflection, transmission,
absorption ,are occurring.
 Spectroscopy is the study of
interaction of electromagnetic
radiation with matter based on
the Bohr-Einstein frequency
relationship E=hv , here h is
the proportionality constant
called Planck’s constant (6.626
x 10-34 J s) and v is frequency.
 Cont.
 Measurement of radiation intensity as a function of wavelength is
described by spectroscopy, as shown in figure.

 All forms of spectroscopy use part of the electromagnetic radiation to give

us information about the materials.
SPECTRUM
 The spectrum is formed by electromagnetic waves and the wavelength is
varies.

 When a narrow beam of light is allowed to pass through a prism/grating, it is

dispersed into seven colors from red to violet and the band
is called Spectrum..
 PRINCIPLE
Basic principle of spectroscopy is the
Beer-Lambert’s law.

BEER LAW
 Beer's law stated that absorbance
is proportional to the concentrations
of the material sample.

LAMBERT LAW
 Lambert's law stated that
absorbance of a material is directly
proportional to its thickness (path
length).
  The modern
derivation of the
Cont. Beer–Lambert law
combines the two
laws and
correlates the
absorbance to
both the
concentrations
and the thickness
of the material.
 Cont.
 It may be noted that e is a function of wavelength. So, the Beer-Lambert
Law is true only for light of a single wavelength or monochromatic light.
 Absorptivity is a constant, depending upon the wavelength of the radiation
and nature of the absorbing material. Absorptivity is also sometimes
referred to as specific extinction and absorbance as ‘Optical Density’.
 Absorbance is the property of a sample, whereas absorptivity is the
property of a substance and is a constant
 The relationship between energy absorption and concentration is of great
importance to analysis.
 The amount of monochromatic radiant energy absorbed or transmitted by a
solution is an exponential function of concentration of the absorbing
substance present in the path of radiant energy.
 This means that successive equal thickness of a homogenous absorbing
medium will reduce the intensity by successive equal fraction and,
therefore, radiant energy will diminish in geometric or exponential
progression.
Cont.
 Consider a condition when three samples (standard solutions) having
identical absorption are introduced in a beam of monochromatic light.
Each of the samples is chosen so that precisely onehalf of the intensity of
the incident radiation is transmitted (T = 50%). If the intensity of the
incident radiation is 100% T, then their intensity after each sample will be:

 However, the expression relating to absorbance ‘A’ to transmittance ‘T’

(A = log 100/T) shows that the absorbance after each sample will be

 An alternative to plotting calibration curves is to make use of the

relationship:
 ABSORPTION INSTRUMENTS
 These essential components are:
 • A source of radiant energy, which may be a tungsten lamp, xenon-mercury arc,
hydrogen or deuterium-discharge lamp, etc.
 • Filtering arrangement for selection of a narrow band of radiant energy. It could be a
single wavelength absorption filter, interference filter, a prism or a diffraction grating.
 • An optical system for producing a parallel beam of filtered light for passage through
an absorption cell (cuvette). The system may include lenses, mirrors, slits, diaphragm, etc.
 • A detecting system for measurement of unabsorbed radiant energy, which could be
human eye, barrier-layer cell, phototube or photomultiplier tube.
 • A readout system or display, which may be an indicating metre or numerical display
Radiation Sources
 Blackbody sources
 A hot material, such as an electrically heated filament in a
light bulb, emits a continuum spectrum of light. The spectrum is
approximated by Planck’s radiation law for blackbody
radiators:

 Where h is Planck’s constant, u is frequency, c is the speed of

light, k is the Boltzmann constant, and T is temperature in K. The
most common incandescent lamps and their wavelength
ranges are:

 the emitted energy increases with temperature and the

wavelength of maximum energy shifts to shorter wavelengths.
– Blackbody radiation
Discharge lamp
 Common discharge lamps and their wavelength
ranges are given below:

 Deuterium lamps are the UV source in UV-Vis

absorption spectrophotometers.
 Mercury lamps are usually run direct from the AC
power line via a series ballast choke. This method
gives some inherent lamp power stabilisation and
automatically provides the necessary ionising
voltage. The ballast choke is physically small and a
fast warm-up to the lamp operating temperature is
obtained
Lasers
 The term laser has been coined by taking the first letters of the expression
‘Light amplification by simulated emission of radiation’.
 The laser beam has spatial and temporal coherence, and is
monochromatic (pure wavelength). The beam is highly directional and
exhibits high density energy which can be finely focused.
 A high peak power will allow a measurable amount of light to get through
an optically dense sample so high absorbance can be measured.
 Another advantage of the highly collimated beam of a laser is to measure
absorption over a very long path length.
Light Emitting Diodes (LEDs)
 The development of LEDs has resulted in the appearance of a new optical
light source in analytical instrumentation. The first LED was developed in
1962 based on GaAsP layers, which emitted red light.
 Significant advances in III-V nitride manufacturing processes have resulted
in high power commercially available LEDs in the region of 247–1,550 nm,
covering UV, visible and near infrared regions.
 LEDs offer a number of advantages compared to existing light sources in
optoelectronic applications. These include increased lifetime, low cost,
reduced power consumption, higher brightness, rugged construction,
flexible configuration, enhanced spectral purity, small size, and breadth of
spectral range
 The most popular type of tri-colour LED has a red and a green LED
combined in one package with three leads. When both the red and green
LEDs are turned on, the LED appears to be yellow. Figure 2.15 shows the
construction of a tri-colour LED.
 The centre lead (K) is the common cathode for both LEDs, the outer leads
(A1 and A2 ) are the anodes to the LEDs allowing each one to be lit
separately, or both together to give the third colour.
 The use of bi-/tri- colour LEDs can provide a compact rugged multi-
wavelength photometer source that can facilitate multi-component
analysis.
Optical Filters
 A filter may be considered as any transparent
medium which by its structure, composition or
colour enables isolation of radiation of a
particular wavelength. For this purpose, ideal
filters should be monochromatic (i.e. they must
isolate radiation of only one wavelength). A
filter must meet the following two requirements:
 (a) high transmittance at the desired
wavelength
 (b) low transmittance at other wavelengths
 However, in practice, the filters transmit a
broad region of the spectrum.
 Referring to Figure, they are characterized by
the relative light transmission at the maximum
of the curve T𝝀 the width of the spectral region
transmitted (the half-width – the range of
wavelength between the two points on the
transmission curve at which the transmission
value equal 1/2 T𝝀 ) and Tres, (the residual value
of the transmission in the remaining part of the
spectrum).
 The ideal filter would have the highest value of
Tl and the lowest values for the transmission
half-width and Tres.
Absorption filters
 The absorption type optical filter usually consists of colour media:
colour glasses, colour films (gelatine, etc.), and solutions of the
colour substances.
 This type of filter has wide spectral bandwidth, which may be 40–
50 m in width at one-half the maximum transmittance. Their
efficiency of transmission is very poor and is of the order of 5–25%.
 Composite filters consisting of sets of unit filters are often used. In
the combination, one set consists of long wavelength, sharp cut-
off filters and the other of short wavelength, cut-off filters;
combinations are available from about 360–700 nm.
 The glass filter consists of a solid sheet of glass that has been
coloured with a pigment, which is either dissolved or dispersed in
glass; whereas the gelatine filter consists of a layer of gelatine
impregnated with suitable organic dyes and sandwiched
between two sheets of glass.
 Gelatine filters are not suitable for use over long periods. With the
absorption of heat, they tend to deteriorate due to changes in
the gelatine and bleaching of the dye.
Interference filters
 Interference filters usually consist of two semitransparent layers of silver,
deposited on glass by evaporation in vacuum and separated by a layer of
dielectric (ZnS or MgF2 ).
 Figure 2.17 shows the path of light rays through an interference filter. Some
part of light that is transmitted by the first film is reflected by the second film
and again reflected on the inner face of the first film, as the thickness of the
intermediate layer is one-half a wavelength of a desired peak wavelength.
 Only light which is reflected twice will be in phase and come out of the
filter, other wavelengths with phase differences would cause destructive
interference.
 Constructive interference between different pairs in superposed light rays
occurs only when the path difference is exactly one wavelength or some
multiple thereof. The relationship expressing a maximum for the transmission
of a spectral band is given by

 when light is incident normally, sin𝜽 = 1

where d is the thickness of the dielectric spacer, whose refractive index is n.

The multiple of frequencies harmonically related to the wavelength of the
first order rays is the order (m) of the interference.
Monochromators

 Monochromators are the optical systems,

which provide better isolation of spectral
energy than the optical filters, and are
therefore preferred where it is required to
isolate narrow bands of radiant energy.
 Monochromators usually incorporate a
small glass of quartz prism or a diffraction
grating system as dispersing media.
 The radiation from a light source is passed
either directly or by means of a lens or
mirror into the narrow slit of the
monochromator and allowed to fall on the
dispersing medium, where it gets isolated.
 The efficiency of such monochromators is
much better than that of filters and
spectral half-bandwidths of 1 mm or less
are obtainable in the UV and visible
regions of the spectrum.
Prism monochromators
 Isolation of different wavelengths in a
prism monochromator depends upon the
fact that the refractive index of materials
is different for radiation of different
wavelengths.
 If a parallel beam of radiation falls on a
prism, the radiation of two different
wavelengths will be bent through different
angles.
 The greater the difference between these
angles, the easier it is to isolate the two
wavelengths.
 This becomes an important consideration
for selection of material for the prisms,
because only those materials are
selected whose refractive index changes
sharply with wavelength.
 Figure shows the use of a prism as a
monochromator.
 UV-VISIBLE SPECTROSCOPY
 Ultraviolet–visible spectrum can be generated when ultraviolet light and
visible light(100-900nm) are absorbed by materials. The spectrum can be
used to analyze the composition and the structure of the materials. For a
particular wavelength in the ultraviolet–visible ranges, the absorption
degree is proportional to the components of the materials. Therefore, the
characteristics of the materials are quantitatively reflected by the
spectrum, which changes with the wave-length.

 Ultraviolet–visible spectrum consists of an absorption spectrum. An

absorption spectrum gives information about the molar absorptivity,
concentration of the sample, optical bath length.
 The UV-VIS spectrometry
 The UV-VIS spectrometry is one of the oldest instrumental techniques of
analysis and is the basis for a number of ideal methods for the
determination of micro and semimicro quantities of analytes in a sample.
 It concerns with the measurement of the consequences of interaction of
Electromagnetic radiations in the UV and/or visible region with the
absorbing species like, atoms, molecules or ions.

 Spectrophotometer device is used in UV-VIS Spectroscopy.

 Components of spectrophotometer
❑ Sources of light(200nm to 900nm).
❑ Monochromator.
❑ Sample solution in cuvette.
❑ Detectors.
❑ Readout devices.
 INSTRUMENTATION
 SOURCE of LIGHT.
 MONOCHROMATOR.
 SAMPLE SOLIOTION in CUVETTE.
 PHOTO DETECTOR.
 READOUT DEVICE.
SOURCE of LIGHT

 Part of the UV and Visible radiation source is Tungsten

lamp.

 UV radiation source is Deuterium or Hydrogen lamp .

Range of wavelength 200-400 nm.
 Cont.
 MONOCHROMATOR
 It is a device that breaks the polychromatic radiation into component
wavelengths.

Entrance slit narrow beam of radiation from source.

Monochromator
Collimating collimates the lights.
mirror
Diffraction
disperses the light into specific wavelength
grating or Prism
captures the dispersed light & sharpens the same to
Focusing mirror the sample via exit slit

allows the corrected wavelength of light to the

Exit slit sample .
 SAMPLE SOLIOTION in CUVETTE
 liquid sample is usually contained in a cell called a cuvette.
 Fingerprints and droplets of water disrupt light rays during measurement.
 Cuvette from Quartz can be used in UV as well as in visible
spectroscopy.
 Cuvette from Glass is suitable for visible but not for UV spectroscopy
because it absorbs UV radiation.
PHOTO DETECTOR
 A photo detector is a semiconductor device which converts light energy to
electrical energy. It consists of a simple P-N junction diode and is designed
to work in reverse biased condition. The photons approaching the diode
are absorbed by the photodiode and current is generated.
READOUT DEVICE.
 Digital screen to record an uv spectrograph with absorbance against the
wavelength

TYPES of SPECTROPHOTOMETER
Advantages and disadvantages
 Major advantages of uv-vis spectroscopy are:
1. High sensitivity.
2. Require only small volume of sample.
3. Linearity over wide range of concentration.
4. Can be used with gradient elution.

 Major disadvantages of uv-vis spectroscopy are:

1. Not linear for high concentration.
2. Does not work with compounds that do not absorb light
at this wavelength region.
3. Generates significant heat and requires external
cooling.