MEVE 018: Unit 04 - Molecular Spectroscopy

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UNIT 4: MOLECULAR SPECTROSCOPY

4.0 Introduction

Molecular spectroscopy refers to the study of the interaction between electromagnetic radiation and matter, specifically molecules. It plays a critical role in qualitative and quantitative analysis of substances. In environmental science, molecular spectroscopy is widely used to monitor pollutants, contaminants, and trace elements in air, water, and soil.

4.1 Objectives

After studying this unit, you should be able to:

  • Understand the basic principles of UV-Vis, fluorescence, and vibrational spectroscopies.
  • Explain the origin and characteristics of various spectra.
  • Describe instrumentation used in different spectroscopic techniques.
  • Identify key environmental applications of molecular spectroscopy.

4.2 UV-VIS Spectrometry

4.2.1 Origin of Spectrum

UV-VIS spectroscopy is based on electronic transitions in molecules. When molecules absorb ultraviolet (200–400 nm) or visible (400–800 nm) light, electrons are excited from lower energy molecular orbitals (like π or n orbitals) to higher energy orbitals (like π* or σ*).

4.2.2 Characteristics of Spectrum

The UV-Vis spectrum of a compound shows absorbance peaks that correspond to electronic transitions. The spectrum provides:

  • λmax (wavelength of maximum absorbance)
  • Intensity (absorbance value)

These features are influenced by:

  • Molecular structure
  • Conjugation
  • Solvent effects

4.2.3 Principle of UV-VIS Spectrophotometry

Based on Beer-Lambert Law:

A = εcl
Where:

  • A = absorbance
  • ε = molar absorptivity
  • c = concentration
  • l = path length

This law forms the basis for quantitative analysis.

4.2.4 Quantitative Methodology

Used to determine:

  • Concentrations of organic/inorganic compounds
  • Trace metal ions (after complexation)
  • Pesticides, nitrates, phosphates in water samples

4.2.5 Instrumentation

Main components include:

  • Light Source: Deuterium (UV), Tungsten (visible)
  • Monochromator: Selects specific wavelength
  • Sample Holder: Usually quartz cuvette
  • Detector: Photodiode or photomultiplier
  • Display/Data system: Converts signals to absorbance values

4.3 Fluorescence Spectrometry

Fluorescence involves emission of light by a substance that has absorbed light or other electromagnetic radiation.

4.3.1 Origin of Spectrum: Jablonski Diagram

The Jablonski Diagram explains electronic transitions, including:

  • Excitation: Absorption of energy and promotion to excited singlet state.
  • Fluorescence: Return from excited singlet state to ground state with photon emission.
  • Phosphorescence: Involves intersystem crossing to triplet state, followed by emission.

4.3.2 Excitation vs Emission Fluorescence

  • Excitation Spectrum: Wavelengths that excite the molecule.
  • Emission Spectrum: Wavelengths emitted during relaxation.

Emission typically occurs at longer wavelengths than excitation due to energy loss (Stokes Shift).

4.3.3 Instrumentation

  • Excitation Source: Xenon or mercury lamp
  • Monochromators: For both excitation and emission paths
  • Sample Cell: Usually quartz cuvette
  • Detector: Photomultiplier tube for high sensitivity
  • Output: Fluorescence intensity vs wavelength

4.4 Vibrational Spectroscopy

4.4.1 Origin of Raman Spectrum

Raman spectroscopy is based on inelastic scattering of monochromatic light (usually from a laser). It provides information on:

  • Bond vibrations
  • Molecular symmetry
  • Chemical environment

Unlike IR, Raman-active vibrations depend on polarizability changes rather than dipole moment changes.

4.4.2 Instrumentation

  • Laser Source: Provides monochromatic light
  • Sample Holder
  • Monochromator/Spectrograph
  • Detector: CCD (Charge Coupled Device)
  • Computer System for spectrum analysis

4.5 Applications of Spectrometric Methods in Environmental Monitoring

Spectroscopic methods are essential in pollution detection and environmental quality control.

UV-Vis Spectrometry Applications:

  • Measurement of nitrate, phosphate, and arsenic in water
  • Detection of organic pollutants and pesticides
  • Analysis of industrial dyes in wastewater

Fluorescence Spectrometry Applications:

  • Detection of polycyclic aromatic hydrocarbons (PAHs) in air and water
  • Monitoring of humic substances in soil and water
  • Sensitive detection of biomarkers and toxins

Raman Spectroscopy Applications:

  • Characterization of microplastics
  • Identification of soil contaminants
  • Real-time in-situ analysis of water pollutants

4.6 Let Us Sum Up

In this unit, we explored:

  • The principles and instrumentation of UV-Vis, fluorescence, and Raman spectroscopies.
  • Their role in understanding electronic and vibrational transitions in molecules.
  • Spectroscopic methods as sensitive, reliable tools for environmental analysis, including water quality testing, pollutant detection, and trace-level monitoring of organic compounds.

Keywords

  • UV-Vis Spectroscopy-A technique measuring light absorption in the ultraviolet and visible ranges.
  • Fluorescence-Emission of light by a substance that has absorbed light, typically at longer wavelengths.
  • Jablonski Diagram-A visual representation of electronic states and transitions in fluorescence.
  • Beer-Lambert Law-A linear relationship between absorbance and concentration of an analyte.
  • Raman Spectroscopy-Technique that measures inelastic scattering of light to study vibrational modes.
  • Stokes Shift-The difference between the positions of the band maxima of the absorption and emission spectra.
  • Monochromator-An optical device that isolates specific wavelengths of light.
  • Polarizability-The ability of a molecule’s electron cloud to be distorted by an electric field.

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