FLAME EMISSION SPECTROSCOPY

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RAJARAMBAPU COLLEGE OF PHARMACY,
SANGLI
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CONTENTS :
 1. INTRODUCTION
 2. PRINCIPLE
 3. INSTRUMENTATION
 4. INTERFERENCES
 5. APPLICATIONS
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INTRODUCTION :
 “The determination of elemental
composition by its
electromagnetic or mass
spectrum.”
 Atomic Spectroscopy is assumed
to be the oldest instrumental
method for the determination of
elements.
 These techniques are introduced
in the mid of 19th Century, when
Bunsen and Kirchhoff showed that
the radiation emitted from the
flames depends on the
characteristic element present in
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TYPES OF ATOMIC
SPECTROSCOPY :
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WHAT IS FLAME PHOTOMETRY ?
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• Flame photometry (more accurately called Flame Atomic Emission
Spectrometry)is a branch of spectroscopy in which the species
examined in the spectrometer are in the form of atoms.
• A photoelectric flame photometer is an instrument used in
inorganic chemical analysis to determine the concentration
of certain metal ions among them sodium, potassium,
calcium and lithium.
• Flame Photometry is based on measurement of intensity of
the light emitted when a metal is introduced into flame.
– The wavelength of colour tells what the element is
(qualitative)
– The colour's intensity tells us how much of the element
present
(quantitative).

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• The basic principle upon which Atomic Spectroscopy
works is based on the fact that "Matter absorbs light at
the same wavelength at which it emits light".
• Atoms of elements - subjected to hot flame specific
quantum of thermal energy absorbed by orbital
electrons - become unstable at high energy level -
release energy as photons of particular wavelength -
change back to ground state.
• When a metal salt solution is burned, the metal
provides a colored flame and each metal ion gives a
different colored flame.
• Flame tests, therefore, can be used to test for the
absence or presence of a metal ion.

PRINCIPLE :
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• Liquid sample contaning metal salt solution
is introduced into a flame.
• Solvent is first vaporized, leaving particles
of solid salt which is then vaporised into
gaseous state
• Gaseous molecule dissociate to give
neutral atoms which can be excited (made
unstable) by thermal energy of flame
• The unstable excited atoms emit photons
while returning to lower energy state
• The measurement of emitted photons
forms the basis of flame photometry.

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• Under constant and controlled conditions, the light intensity of
the characteristic wavelength produced by each of the atoms
is directly proportional to the number of atoms that are
emitting energy, which in turn is directly proportional to the
concentration of the substance of interest in the sample.
• Various metals emit a characteristic colour of light
when heated.

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Structure of Flame:
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 As seen in the figure, the flame
may be divided into the following
regions or zones.
– Preheating zones
– Primary reaction zone or inner
zone
– Internal zone
– Secondary reaction zone

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• preheating zone- In this, combustion mixture is heated
to the ignition temperature by thermal conduction from the
primary reaction zone.
• primary reaction zone- This zone is about 0.1 mm thick
at atmospheric pressure
– There is no thermodynamic equilibrium in this zone and the
concentration of ions and free radicals is very high.
– This region is not used for flame photometry.
• interconal zone – It can extend up to considerable
height. The maximum temperature is achieved just
above the tip of the inner zone.
– This zone is used for flame photometry.
• secondary reaction zone - In this zone, the products of
the combustion processes are burnt to stable
molecular species by the surrounding air.

INSTRUMENTATION:
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Major Components:
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1. Sample Delivery
System
2. Source
3. Monochromator
4. Detector
5. Read out device

1. Sample Delivery System:
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 There are three components for introducing liquid sample:
• Nebulizer – it breaks up the liquid into small droplets.
– Nebulization the is conversion of a sample to a mist of finely
divided droplets using a jet of compressed gas.
– The flow carries the sample into the atomization region.
– Pneumatic Nebulizers: (most common)
• Aerosol modifier – it removes large droplets from the
stream and allow only smaller droplets than a certain
size to pass
• Flame or Atomizer – it converts the analyte into free atoms.

2. Source:
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• A Burner used to spray the sample solution into fine
droplets.
• Several burners and fuel+oxidant combinations have been
used to produce analytical flame including: Premixed,
Mecker, Total consumption, Lundergarh, Shielded burner,
and Nitrous oxideacetylene flames
• Pre-mixed Burner:
– widely used because uniformity in flame intensity
– In this energy type of burner , aspirated sample , fuel and
oxidant are thoroughly mixed before reaching the burner
opening.

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• Total Consumption Burner:
– In this fuel and oxidant are hydrogen and oxygen gases
– Sample solution is aspirated through a capillary by high
pressure of fuel and Oxidant and burnt at the tip of burner
– Entire sample is consumed.
Pre-mixed burner Total Consumption Burner

MECKER BURNER:
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 This burner was used earlier and employed natural gas and
oxygen. Produces relatively low temp. and low excitation
energies. This are best used for ALKALI metals only. Now-a-
days it is not used.

LUNDERGRAPH BURNER :
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In this sample and air is mixed in a chamber, this mixed
composition is send to fuel nozzle where it is atomized.
Here the sample reaches
the flame is only about 5%.

SHIELDED BURNERS :
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In this flame was shielded from the ambient atmosphere
by a stream of inert gas. Shielding is done to get better
analytical sensitivity and quieter flame.

NITROUS OXIDE ACETYLENE FLAME:
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 These flames were superior to other flames for
effectively producing free atoms. The drawback of it is
the high temperature reduces its usefulness for the
determination of alkali metals as they are easily ionized
and Intense background emission, which makes the
measurement of metal emission very difficult

3. MONOCHROMATOR :
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1. MIRRORS :
The radiation from the flame is emitted in all the directions in
space. Much of the radiation is lost and loss of signal results.
A mirror is located behind the burner to reflect the radiation
back to the entrance slit of the monochromator. The
reflecting surface of the mirror is front-faced.

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2. SLITS :
The entrance and exit slits are used before and after the
dispersion elements.
The entrance slit cuts off most if radiation from the
surroundings and allows only the radiation from the
flame and the mirror reflection of flame to enter the
optical system.
The exit slit is placed after the monochromator and
allows only the selected wavelength range to pass
through the detector.

4. DETECTORS :
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1. PHOTOMULTIPLIER TUBE :
The third major type of light detector is the photomultiplier (PM)
tube, which detects and amplifies radiant energy.
Incident light strikes the coated cathode, emitting electrons. The
electrons are attracted to a series of anodes, known as
dynodes,each having a successively higher positive voltage These
dynodes are of a material that gives off many secondary electrons
when hit by single electrons. Initial electron emission at the
cathode triggers a multiple cascade of electrons within the PM tube
itself. Because of this amplification, the PM tube is 200 times more
sensitive than the phototube.

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PM tubes are used in instruments designed to be extremely
sensitive to very low light levels and light flashes of very short
duration.
The accumulation of electrons striking the anode produces a
current signal, measured in amperes, that is proportional to the
initial intensity of the light. The analog signal is converted first to a
voltage and then to a digital signal through the use of an analog to-
digital (A/D) converter. Digital signals are processed electronically
to produce absorbance readings.

5. READ OUT DEVICES :
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 In the past nearly all spectrophotometer used ammeters or
galvanometers. Newer digital devices and printers have now
replaced these, and many instruments relay their electrical
output directly to computer circuits where calculations are
performed, allowing direct
reporting of sample concentration.
 Microprocessor and recorders.

INTERFERENCES :
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• In determining the amount of a particular element present, other elements
can also affect the result.
 Such interference may be of 3 kinds:
1. Spectral interferences: Occurs when the emission lines
of two elements cannot be resolved or arises from the
background of flame itself.
– They are either too close, or overlap, or occur due to high
concentration of salts in the sample
2. Ionic interferences: High temperature flame may cause
ionisation of some of the metal atoms, e.g. sodium.
– The Na+ ion possesses an emission spectrum of its own
with frequencies, which are different from those of atomic
spectrum of the Na atom.

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3. Chemical interferences: The chemical interferences
arise out of the reaction between different interferents and the
analyte.
 Includes:
• Cation-anion interference:
– The presence of certain anions, such as oxalate,
phosphate, sulfate, in a solution may affect the
intensity of radiation emitted by an element. E.g.,
– calcium + phosphate ion forms a stable substance, as
Ca3(PO4)2 which does not decompose easily, resulting
in the production of lesser atoms.
• Cation-cation interference:
– These interferences are neither spectral nor ionic in
nature
– Eg. aluminum interferes with calcium and magnesium.

APPLICATIONS:
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 To estimate sodium, potassium, calcium, lithium etc. level in
sample of serum, urine, CSF and other body fluids.
 Flame photometry is useful for the determination of alkali and
alkaline earth metals.
 Used in determination of lead in petrol.
 Used in the study of equilibrium constants involving
in ion exchange resins.
 Used in determination of calcium and magnesium in
cement.

ADVANTAGES:
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 Simple quantitative analytical test based on the flame
analysis.
 Inexpensive.
 The determination of elements such as alkali and alkaline
earth metals is performed easily with most reliable and
convenient methods.
 Quite, convenient, selective and sensitive to even parts per
million (ppm) to parts per billion (ppb) range.

DISADVANTAGES:
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 The concentration of the metal ion in the solution cannot be
measured accurately.
 A standard solution with known molarities is required for
determining the concentration of the ions which will
corresponds to the emission spectra.
 It is difficult to obtain the accurate results of ions with higher
concentration.
 The information about the molecular structure of the
compound present in the sample solution cannot be
determined.
 The elements such as carbon, hydrogen and halides cannot be
detected due to its non-radiating nature.

LIMITATION :
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 Limited number of elements that can be analyzed.
 The sample requires to be introduced as solution into
fine droplets. Many metallic salts, soil, plant and other
compounds are insoluble in common solvents. Hence,
they can’t be analyzed by this method.
 Since sample is volatilized, if small amount of sample
is present, it is tough to analyze by this method. As
some of it gets wasted by vaporization.
 Further during solubilisation with solvents, other
impurities might mix up with sample and may lead to
errors in the spectra observed.

REFERENCES:
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 Tietz Textbook of Clinical Chemistry and Molecular
Diagnostics.
 Basic Clinical Biochemistry Practice, second edition, editied by O.
A. Afonja.
 College Analytical Chemistry, Himalaya Publishing House, 19th
Edition (2011), By K.B.Baliga et al. Chapter 4 - Optical Methods,
Pages : 135-148.
 http://www.hindawi.com/journals/chem/2013/4658 25/
 Practical Biochemistry, Principles & Techniques, Cambridge
lowprice editions, 5th Edition, Edited By Keith Wilson & John
Walker, Chapter: Spectroscopic Techniques, Pages : 486-490.

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FLAME EMISSION SPECTROSCOPY

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