Spectroscopic Techniques in Optical Methods: A Comprehensive Guide

Classification of Optical Methods

Non-Spectroscopic Techniques

• Refractometry
• Polarimetry

Spectroscopic Techniques

• UV-Vis Spectrophotometry
• Atomic Absorption
• Flame Photometry

Classification of Spectroscopic Methods

Spectroscopic methods are categorized by absorption or emission.

Absorptiometry

This electromagnetic method uses light, which has both corpuscular and wave-like characteristics. Light is broken down into different wavelengths, arranged in what is called the electromagnetic spectrum.

Wave Constitution

A wave consists of two fields—electric and magnetic—perpendicularly intersecting each other and propagating in the direction of the wave.

Speed of Wave Propagation

In a vacuum, the speed of light (c) is 3x10¹⁰ cm/sec. This speed can change when light propagates through a medium, causing dispersion.

Examples of Dispersion

• Decomposition of visible light into different colors when passing through a prism, creating a spectrum.

Quanta

Quanta are fixed units of radiant energy.

Emission Spectrum

Atoms or molecules can reach a higher energy level by absorbing energy. This creates an unstable state, which they leave by emitting radiation (excess energy). This emitted radiation constitutes the emission spectrum.

Absorption Spectrum

If the added radiant energy stabilizes the atom or molecule, a portion of the incident energy is absorbed. This absorbed portion consists of specific frequencies.

Energy and Spectra

Different amounts of energy cause changes in electronic, vibrational, and rotational energy levels, resulting in different absorption and emission spectra.

Chromophore Group

A chromophore is responsible for light absorption in a molecule and confers its characteristic color.

Hyperchromic and Bathochromic Effects

When chromophores are conjugated, the maximum absorption zone shifts towards red (bathochromic effect) and absorption intensity increases (hyperchromic effect).

Hypsochromic and Hypochromic Effects

A hypsochromic effect is a blue shift in the maximum absorption area, while a hypochromic effect is a decrease in absorption intensity.

Auxochrome Groups

Auxochromes increase the dye's power by enhancing electron mobility, facilitating light absorption, though they don't absorb light themselves.

Beer-Lambert Law

When radiation strikes a transparent body, it can be absorbed, reflected, or transmitted. Beer's Law establishes a direct proportionality between absorbance and concentration, assuming constant path length.

Solvent

The solvent should not absorb in the same zone as the solute. Solvents can be polar or nonpolar, and solute-solvent interactions can occur.

Deviations from Beer's Law

Increasing concentration can lead to deviations from Beer's Law by altering the absorption spectrum.

Visible Spectrophotometry Requirements

A suitable radiation source must:

• Provide sufficient intensity for precise measurement.
• Produce a continuous spectrum within the study region.
• Emit with constant intensity over time.