The underlying principle of Flame Emission Spectroscopy (FES) may be explained when a liquid sample containing a metallic salt solution under investigation is introduced into a flame, the following steps normally take place in quick succession, namely :

**THEORY**

The underlying principle of **Flame Emission Spectroscopy (FES)** may be explained when a liquid
sample containing a metallic salt solution under investigation is introduced
into a flame, the following steps normally take place in quick succession,
namely :

(*i*) the solvent
gets evaporated leaving behind the corresponding solid salt,

(*ii*) the solid
salt undergoes vaporization and gets converted into its respective gaseous
state, and

(*iii*) the
progressive dissociation of either a portion or all of the gaseous molecules
gives rise to free neutral atoms or radicals.

The resulting neutral atoms are excited by the thermal
energy of the flame which are fairly unstable, and hence instantly emit photons
and eventually return to the ground state (*i.e.,*
the lower energy state). The resulting emission spectrum caused by the emitted
photons and its subsequent measurement forms the funda-mental basis of FES.

If we consider two quantized
energy levels** ***e.g.,*** **higher as E_{2}** **and lower as E_{1}, the** **radiation given out during the
transition from E_{2} to E_{1} may be expressed by the
following equation :

E_{2} – E_{1}
= *h* *ν* ...(*a*)

where, *h* = Planck’s constant, and

*v*= Frequency of emitted light,

now, the frequency *v*
may be defined as follows :

*ν *=* **c*/λ ...(*b*)

where, *c *= Velocity of light, and

= Wavelength of the absorbed radiation. Combining
equations (*a*) and (*b*) we have :

E_{2} – E_{1} = *hc*/λ

λ = *hc*/E_{2} – E_{1} ...(*c*)

The expression (*c*)
is the **Bohr’s equation** which enables
us to calculate :

·
Wavelength of the emitted radiation which is
characteristic of the atoms of the particular element from which it was
initially emitted,

·
Wavelength of radiation given out from a flame is
indicative of the element(s) that might be present in that flame, and

·
Intensity of radiation may quantify the exact amount of
the elements present.

The fraction of free atoms
which are excited thermally, or in other words, the** **relationship between the ground-state and the excited-state
quantum is exclusively represented by the Boltzmann equation given below :

.......................................(d)

where,

N_{1} = Number of
atoms in the excited state (high energy level),

N_{0} = Number of
ground state atoms,

*g*_{1}/*g*_{0}* *= Ratio of
statistical weights for ground and excited states,* *E = Energy of excitation (= *h**υ*),

*k *= The Boltzmann’s constant,
and

T = Temperature (in Kelvin).

Form equation (*d*)
it may be observed that :

·
Fraction of atoms excited (N_{1}) solely depends
upon the temperature of the flame (T), and

·
Ratio N_{1}/N_{0} is dependent upon the
excitation energy (∆E).

Therefore, the fraction of atoms excited critically
depends on the temperature of the flame thereby emphasizing the vital
importance of controlling the temperature in **Flame Emission Spectroscopy (FES).**

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Pharmaceutical Drug Analysis: Flame Spectroscopy : Flame Spectroscopy: Theory |

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