MEV 013: Unit 13 - Spectrometry

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UNIT 13: SPECTROMETRY

13.0 Introduction

Spectrometry involves the interaction of electromagnetic radiation with matter and is an essential tool for identifying, quantifying, and understanding the chemical composition of substances. It plays a vital role in environmental monitoring, chemical analysis, biomedical research, and forensic investigations. This unit focuses on three key types of spectrometry: UV-Vis spectrophotometry, Infrared (IR) spectrometry, and Mass spectrometry (MS).

13.1 Objectives

After studying this unit, you will be able to:

  • Understand the principles of UV-Vis, IR, and Mass Spectrometry.
  • Explain the origin and nature of spectra in different techniques.
  • Describe instrumentation used in various spectrometric techniques.
  • Apply these methods in environmental analysis.
  • Recognize the advantages and limitations of each technique.

13.2 UV-Vis Spectrophotometry

13.2.1 Origin and Nature of Spectrum

  • UV-Visible spectrophotometry involves absorption of ultraviolet (200–400 nm) and visible light (400–700 nm) by molecules.
  • Electronic transitions occur when electrons move between molecular orbitals (e.g., π → π*, n → π*).
  • The spectrum is a plot of absorbance vs. wavelength, providing information about molecular structure and concentration.

13.2.2 Beer-Lambert’s Law

  • Governs the relationship between absorbance and concentration:

A = εcl

Where:

    • A = absorbance
    • ε = molar absorptivity (L mol⁻¹ cm⁻¹)
    • c = concentration (mol/L)
    • l = path length (cm)
  • This law is the foundation of quantitative analysis in UV-Vis spectrophotometry.

13.2.3 Basic Instrumentation for UV-Vis Spectrophotometry

Key components:

  • Light Source: Deuterium (UV), Tungsten (Visible)
  • Monochromator: Separates wavelengths
  • Sample Cell (Cuvette): Typically quartz or glass
  • Detector: Measures transmitted light (e.g., photodiodes)

Types:

  • Single-beam
  • Double-beam spectrophotometers

13.2.4 Environmental Applications of UV-Vis Spectrophotometry

  • Measurement of nitrate, phosphate, and heavy metals in water
  • Detection of organic pollutants
  • Estimation of biological oxygen demand (BOD) using colorimetric methods
  • Air quality testing via color-developing reactions

13.3 IR Spectrometry

13.3.1 Origin and Nature of Spectrum

  • IR spectrometry is based on absorption of infrared light by molecular vibrations (stretching, bending).
  • Spectra appear as absorption bands corresponding to functional groups.
  • Useful in qualitative identification of organic and inorganic compounds.

13.3.2 Basic Instrumentation for IR Spectrometry

Components:

  • IR Source: Globar or Nernst filament
  • Sample Handling: Solid (KBr pellets), Liquid (salt plates), Gas (cells)
  • Monochromator: Grating or prism
  • Detector: Thermocouple, bolometer, or pyroelectric detector
  • Interferometer: Used in FTIR (Fourier Transform Infrared) systems

13.3.3 Environmental Applications of Infrared Spectrometry

  • Identification of organic pollutants in soil and water
  • Monitoring volatile organic compounds (VOCs) in air
  • Analysis of plastics, hydrocarbons, pesticides, and industrial waste
  • Greenhouse gas detection (e.g., CO₂, CH₄)

13.4 Mass Spectrometry

13.4.1 Principle

  • Mass spectrometry (MS) involves ionizing chemical compounds to generate charged fragments and measuring their mass-to-charge ratio (m/z).
  • It gives both qualitative (structure) and quantitative (concentration) data.
  • Especially powerful for detecting trace-level contaminants.

13.4.2 Instrumentation for Mass Spectrometry

Key components:

  1. Sample Inlet System
  2. Ionization Source: Electron ionization (EI), chemical ionization (CI), electrospray ionization (ESI)
  3. Mass Analyzer: Quadrupole, Time-of-Flight (TOF), Magnetic sector
  4. Detector: Electron multiplier or photomultiplier
  5. Data System: Software to process spectra

13.4.3 Hyphenation of MS with Other Techniques (e.g., ICP-MS)

  • GC-MS (Gas Chromatography-Mass Spectrometry): Used for volatile compounds
  • LC-MS (Liquid Chromatography-Mass Spectrometry): For non-volatile or thermally unstable compounds
  • ICP-MS (Inductively Coupled Plasma Mass Spectrometry): For metals and isotopic analysis

These hybrid methods improve sensitivity, selectivity, and versatility.

13.4.4 Environmental Applications of Mass Spectrometry

  • Detection of pesticides, herbicides, PAHs, and pharmaceuticals in water
  • Analysis of airborne toxins, heavy metals, and organic aerosols
  • Studying biodegradation pathways in soils
  • Isotope ratio analysis for tracing pollution sources

13.5 Summary

  • UV-Vis spectrophotometry is useful for quantitative analysis of molecules absorbing in the UV-Visible range.
  • IR spectrometry aids in identifying functional groups in organic and inorganic compounds.
  • Mass spectrometry provides highly sensitive and accurate identification of chemical species, even at trace levels.
  • These techniques are vital for environmental analysis, industrial monitoring, and scientific research.

13.6 Keywords

  • Spectrometry – Analytical technique for measuring the interaction between matter and electromagnetic radiation.
  • UV-Vis Spectrophotometry – Technique to measure light absorption in the ultraviolet and visible regions by substances.
  • Beer-Lambert’s Law – States that absorbance is directly proportional to concentration and path length.
  • Infrared Spectrum – Graph showing absorption of infrared radiation by molecules due to vibrational transitions.
  • Functional Group Identification – Determination of specific groups of atoms in molecules using characteristic spectral peaks.
  • Mass-to-Charge Ratio (m/z) – Ratio of the mass of an ion to its electrical charge, used in mass spectrometry.
  • Ionization – Process of converting atoms or molecules into ions, often by removing or adding electrons.
  • Chromatography-MS Coupling – Combination of chromatography (GC/LC) with mass spectrometry for enhanced separation and detection.
  • Environmental Monitoring – Systematic sampling and analysis of environmental components to detect pollution or changes.
  • Trace Analysis – Detection and quantification of very small amounts (traces) of substances in samples.

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