Page 37 - Instrumental Analysis - Pharm D Clinical- 07-PA403

Spectrofluorimetry

Instrumental Analysis (07-PA 403)

Molecular emission spectroscopy

Introduction

• These are analytical methods based on emission of radiation by a molecule
after it has been excited by absorbing EMR in the UV or visible region.

• The lifetime of an excited species is generally transitory (10−9 to 10−6 sec),
and relaxation to the ground state takes place with a release of the excess
energy in the form of electromagnetic radiation (photons), heat, or perhaps
both.

• The excited states for some substances return to the ground state with
emission of light (photons) called Luminescence or Fluorescence.

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Relaxation Processes 2

• The electronic energy states are the ground
state (S0) and the excited state (S1). Both have
several vibrational levels closely spaced.

• Once the molecule is excited to S1, several
processes can occur that cause the molecule to
lose its excess energy. Two of the most
important of these processes are non-radiative
relaxation and fluorescence emission.

• Non-radiative relaxation: loss of excess energy
without emission of light through: vibrational
relaxation, internal conversion or external
conversion.

• Fluorescence or photon emission (radiation):
in this case the excited molecule emits radiation
that can be measured.

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