FLAME PHOTOMETRY By- Dr. UMESH KUMAR SHARMA AND SHYMA M S

1. FLAME EMISSION
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SPECTROSCOPY
By :
Dr. UMESH KUMAR SHARMA & SHYMA M. S.
DEPARTMENT OF PHARMACEUTICS
MAR DIOSCORUS COLLEGE OF PHARMACY,
THIRUVANANTHAPURAM, KERALA, INDIA

2. FLAME EMISSION SPECTROSCOPY
 Flame photometry is also known as flame emission
spectroscopy.
 Neutral atoms are involved in emission of radiation
when introduced into flame.
 Atoms are simplest & pure form of matter.
 Atoms exhibit electronic transitions when absorb
energy.
 Such discrete transitions are quantised & line spectra is
observed.
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3.  Atoms in the form of atomic vapour are produced in
higher energy level.
 It returns to ground energy state by emitting photons.
 Generating sharp line emission spectra.
 Flame photometry is based on the measurement of
intensity of light emitted when a metal is introduced into
flame
 Wavelength of light emitted indicates the type of element
present
 Intensity of light indicates the quantity of element.
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4.  When a liquid sample containing a metallic salt is
introduced into the flame :
 Solvent vaporises leaving particles of solid salt.
 Salt vaporises into gaseous state.
 Gaseous molecules dissociated to neutral atoms.
 Neutral atoms are excited by thermal energy of flame.
 Excited unstable atoms quickly emit photons and returns
to ground state.
 Emitted radiation intensity is measured.
 Permitted energy levels of all atoms can be represented
diagrammatically in Grotrian chart.
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5.  In flame spectroscopy source of excitation energy is a
flame.
 It is a low energy source.
 Emission spectrum produced is simple.
 It has few emission lines.
 Quantitative determinations are made by aspirating
sample into flame.
 Intensity of characteristic radiation emitted by flame for
individual elements is correlated with concentration of
element in sample.
 Specific wavelength emitted by elements appear as
spectral lines in UV & visible regions. 5

6. 6

7.  Wave length of light emitted is :
λ =hc /∆E
Where: h – Planks constant, c –velocity of light, ∆E-Difference energy levels of ground state & excited state.
 Intensity of radiation emitted depends upon concentration of
element in solution.
 Higher the concentration the more is flame intensity.
 Intensity of spectral emission line :
I =
VAThυNg 𝒆−𝑬/𝑲𝑻
B(T)
Where : E – Energy of excited state, T – Absolute temperature, υ – Frequency of radiation, AT -No of transitions each excited atom
undergoes, N - No of free metal atoms in ground state per unit volume, g-Statistical weight of excited atomic state, B-Partial function
of atom
 Fraction of free atoms thermally excited:
𝑁∗/𝑁 𝑂 =A𝑒−∆𝐸/𝐾𝑡
Where : 𝑁∗
-Number of atoms in excited state, 𝑁 𝑂 -Number of atoms in ground state, A - Constant for element, ∆𝐸 -Difference in
energy levels of excited & ground state, K - Boltzman constant, T -Flame temperature. 7

8. MEANS OF EXCITATION
ELECTRIC ARC
ELECTRIC SPARK
THERMAL ENERGY OF FLAME
 Powdered sample is incorporated in carbon electrode.
 Series of sparks applied which carries current across
gaps.
 Arc source has high sensitivity of detection than spark.
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9. STRUCTURE OF FLAME :
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PREHEATING ZONE
In this combustion mixture is heated to ignition
temperature by thermal conduction from primary reaction
zone.

10. PRIMARY REACTION ZONE :
 About 0.1mm thickness.
 No thermodynamic equilibrium.
 Concentration of ions & free radicals are very high.
 This region is not used for flame photometry.
INTERCONAL ZONE:
 Can extend up to considerable height.
 Maximum temperature is attained just above tip of inner zone.
 This zone is used for flame photometry.
SECONDARY REACTION ZONE :
 Products of combustion process burnt to stable molecular species by
surrounding air.
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12. 12

13. Characteristics of suitable flame for producing
emission spectra :
 Should have proper temperature.
 Temperature should remain constant throughout
operation.
 There should not be any fluctuations during burning
 Spectrum of flame must not interfere with observations
when emissions is being measured.
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14. When sample in the form of aerosol is uniformly delivered
into flame:
 Water or other solvent is vaporised leaving minute
particles of dry salt.
 At high temperature of flame, dry salt is vaporised.
 Part or all of gaseous molecules are dissociated to give
neutral atoms which are potentially emitting species.
 Some of free metal atoms may unite with other radical or
atoms in flame gases.
 They are introduced into flame with test element.
 Vapours of neutral atoms of test elements are excited by
thermal energy of flame.
 Ionisation of neutral atoms occur to some extent.
 Excited atoms fall back to ground state by collision & by
emission of light. 14

15. PROCEDURE :
 A small volume of sample is dissolved in water or organic solvent
placed in a cup of atomiser.
 Air, oxygen & combustible gas is fed into atomiser under
controlled conditions.
 This allows a thin spray of solution to be introduced in the flame.
 Solvent evaporates rapidly to give dehydrated salt.
 It then dissociated into free gaseous atoms in ground state.
 Some of atoms absorb energy from flame & raised to excited
electronic states.
 They drop back to ground level emitting photon.
 Characteristic wavelength are detected with a monochromator -
detector assembly
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16. OPTIMISATION OF FLAME
 Number of free atoms will be an equilibrium value which depend
on rate of nebulization & atomization.
Rate of fuel flow :
 Rate of nebulization & residence time of atoms with in flame
should be properly controlled.
Viscosity of solvent :
 If sample solution is viscous, rate of nebulization is largely
diminished.
Chemical nature of solvent:
 Sometimes a stable solvated species may be formed.
 Hence a modification in flame process is necessary.
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17. Other chemical species in solution / flame
 These may form non volatile stable compounds with sample.
Flame temperature
 Rate of solvent evaporation, molecular dissociation, molecular
association, extent of ionization depends on flame temperature.
 Flame photometry is now mainly used for analysis of sodium,
potassium in biological fluids & tissues.
 It is widely applicable, specific & sensitive.
 Useful in case of elements whose resonance lines associated with
relatively low energy value.
 Sodium, potassium, lithium calcium shows higher sensitivity to
this technique.
 More than half of elements in periodic table have analysed by
flame photometry.
 Many exist in flame as molecules, especially such as oxide
produce molecular band spectra.
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18.  Only those elements which give line spectra are generally
determined by flame emission spectroscopy in a clinical
laboratory.
 Cool gaseous atoms in outer region of source cause self
absorption.
 It decrease the intensity of transition.
 Use of high energy source is not always helpful since it
ionizes gas with loss of one or more electrons.
 Spectrum of singly ionised ion (Mg+) is similar to neutral
Na atom with an atomic number less than one unit.
Na - 1𝑠2
2𝑠2
2𝑝6
3𝑠1
Mg+ -1𝑠2
2𝑠2
2𝑝6
3𝑠1
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19.  Hottest practical flame temperature (~4575) is produced by
burning cyanogen in oxygen.
C2N2+O2→ 𝟐𝑪𝑶 + 𝑵𝟐
 Optimum flame temperature depends on:
 Excitation energy of flame
 Sensitivity of measurement
 Presence of other elements
 Element combination in sample
 High temperature increase emission intensities & thereby provide
higher sensitivity.
 Sensitivity depends upon response & stability of detector &
stability of flame aspiration system.
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20.  Measurement of intensities of spectral lines depends on :
 Amount of salt impregnated in flame.
 Amount of salt dissociated.
 Degree of ionization of compound.
 Number of atoms excited.
 Chances of transition from excited to ground state.
 Self absorption.
 Fraction of excited atom depend on temperature of flame.
 Temperature of flame depends on type of fuel & oxidant used.
 Flames have limited amount of energy compared to excitation
sources.
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21.  They are useful to detect alkali & alkali earth metals.
 Cannot be used for transition metals or other metals which
requires significant energy.
 Fuel rich oxy acetylene flame generates intense radiation bands at
shorter wave length (300-200nm).
 Ion emission lines can be detected.
 If the flame is oxygen rich they operate at same temperature, but
ion emission lines are not observed.
 Nitrous oxide-Acetylene flame gives higher temperature
 Useful for oxides of Aluminium, Titanium etc.
 But high temperature ionises alkali metals.
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22. LIMITATIONS OF FLAME PHOTOMETRY:
 As natural gas & air flame is used for excitation, temperature is
not high enough to excite transition metals.
 Method is selective towards detection of alkali & alkaline earth
metals.
 Low temperature makes this method susceptible to interference,
stability of flame, aspiration conditions.
 Identical conditions are necessary for measuring emission of
standard & unknown solutions.
 Relatively low energy available from the flame leads to low
intensity of radiation.
 It helps in determining total metal concentration.
 It tells us nothing about molecular form of metal in original
sample.
 Only liquid samples can be used.
 Preparation of liquid samples involves lengthy steps & time
consuming. 22

23. REFERENCE:
Instrumental methods of analysis, by Willards, 7th
edition.
Pharmaceutical Analysis, by P.C Kamboj.Volume
3rd.
Principle of instrumental analysis, by Doglas A
Skoog, James Holler.5th edition.
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24. THANK YOU
For Feedback / Comments
Write to umeshpanditjp@gmail.com
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