Page 8 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

Interaction of a Substance with Electromagnetic
Radiation (EMR)

The absorption and emission of EMR

• If a substance is irradiated with EMR, the energy of the incident
photons may be transferred to the atoms or molecules raising them
from the ground state to an excited state. This process is called
absorption.

• The energy of absorption is given by the equation:

∆ E = Es – Eg = hν
where ν is the frequency of EMR at which absorption takes place

which is characteristic for each molecule.
• Then, the absorbed energy is rapidly lost in the form of heat, light or

molecular collision, and the molecule returns to the ground state.

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• In absorption spectroscopy, we measure the amount of light absorbed by
the substance. Absorption measurements can give both qualitative and
quantitative information about the sample.

• In emission (luminescence) spectroscopy, the emission of photons is
measured following absorption. Important forms of luminescence for
analytical purposes are fluorescence and phosphorescence spectroscopy.

Simplified energy
diagram showing
the absorption and

emission of a
photon by an atom

or a molecule.

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