MEVE 018: Unit 05 - Atomic Absorption and Emission Spectrometry

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UNIT 5: ATOMIC ABSORPTION AND EMISSION SPECTROMETRY

5.0 Introduction

Atomic absorption and emission spectrometry are powerful analytical techniques used to determine the concentration of metals and metalloids in samples. These methods are widely used in environmental analysis, including monitoring of water, soil, and air pollutants. The techniques are based on measuring the light absorbed or emitted by free atoms in the gaseous state.

5.1 Objectives

After completing this unit, you should be able to:

  • Understand the origin and classification of atomic spectra.
  • Describe the principles and instrumentation of Flame AAS, Graphite Furnace AAS, Flame AES, and ICP-AES.
  • Understand interferences associated with each technique.
  • Identify the environmental applications of atomic absorption and emission spectrometry.

5.2 Origin and Classification of Atomic Spectra

Atomic spectra arise from electronic transitions between discrete energy levels in atoms. When atoms absorb energy, electrons are excited to higher levels. The return of electrons to lower energy levels results in the emission of radiation at characteristic wavelengths.

Classification:

  • Absorption spectra: Dark lines on a bright background; occurs when atoms absorb specific wavelengths.
  • Emission spectra: Bright lines on a dark background; occurs when atoms emit light upon relaxation.

Each element produces a unique spectral signature, enabling its identification and quantification.

5.3 Flame Atomic Absorption Spectrometry (FAAS)

5.3.1 Flame and its Structure

The flame in FAAS serves two purposes:

  • Atomization: Converts analyte ions into free atoms.
  • Excitation: Provides thermal energy for atomic transitions.

Structure of the flame:

  • Primary combustion zone
  • Interconal region (used for analysis)
  • Outer cone

Fuel-oxidant mixtures (e.g., acetylene-air or acetylene-nitrous oxide) are commonly used.

5.3.2 Principle of FAAS

In FAAS, a light beam from a hollow cathode lamp (HCL) specific to the element of interest passes through the flame. The amount of light absorbed by the ground-state atoms is proportional to the concentration of the element.

Based on Beer-Lambert Law:

A = εbc

5.3.3 Instrumentation for FAAS

Key components:

  • Light source: Hollow cathode lamp
  • Atomizer: Burner and nebulizer
  • Monochromator: Selects specific wavelength
  • Detector: Photomultiplier tube
  • Readout: Digital display or computer interface

5.4 Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

Also known as Electrothermal AAS, it uses a graphite tube to atomize the sample.

5.4.1 Electrothermal Atomizers

  • A small sample (few microliters) is introduced into a graphite tube.
  • The tube is electrically heated in stages:
    • Drying
    • Ashing
    • Atomization
  • This method allows high sensitivity and low detection limits.

5.4.2 Advantages and Disadvantages of GFAAS

Advantages:

  • Requires small sample volume
  • Excellent for trace analysis
  • Higher sensitivity than FAAS

Disadvantages:

  • Slower analysis
  • More complex instrumentation
  • Prone to matrix interferences

5.5 Flame Atomic Emission Spectrometry (FAES)

5.5.1 Principle of FAES

In FAES, atoms are excited in a flame, and as they return to the ground state, they emit light at characteristic wavelengths. The intensity of emitted light is directly proportional to the concentration.

Example: Sodium emits intense yellow light at 589 nm.

5.5.2 Instrumentation for FAES

Components include:

  • Flame: Source of atomization and excitation
  • Monochromator
  • Detector: Typically a photomultiplier
  • Readout system

FAES is simpler than FAAS but limited in sensitivity for some elements.

5.6 Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)

ICP-AES is a highly sensitive, multi-element technique.

5.6.1 Plasma and Its Characteristics

Plasma is a high-temperature ionized gas composed of ions, electrons, and neutral atoms. It provides energy to atomize and excite the sample atoms.

  • Temperature: 6000–10,000 K
  • Argon is used as a carrier and plasma gas.

5.6.2 Inductively Coupled Plasma (ICP)

An RF (radio frequency) generator creates a magnetic field that induces current in the argon gas, forming a plasma torch. The sample (in aerosol form) is introduced into this torch.

5.6.3 Instrumentation for ICP-AES

  • Sample introduction system: Nebulizer and spray chamber
  • Plasma torch
  • Optical system: Monochromator or polychromator
  • Detector: CCD or photomultiplier tube

5.7 Interferences in Atomic Absorption and Emission Spectrometry

5.7.1 Interferences in AAS and GFAAS

  • Spectral interference: Overlapping absorption lines
  • Chemical interference: Formation of non-volatile compounds
  • Ionization interference: Ionization of analyte atoms

Solutions:

  • Use of releasing agents
  • Addition of ionization suppressors

5.7.2 Interferences in AES

  • Background emission
  • Self-absorption (high analyte concentrations)
  • Flame temperature variations

5.7.3 Interferences in ICP-AES

  • Spectral overlap: Due to multi-element detection
  • Matrix effects: High salt content can affect plasma stability
  • Physical interferences: Sample viscosity, flow rate

5.8 Environmental Applications of Atomic Absorption and Emission Spectrometry

These techniques are extensively used in environmental chemistry, including:

  • Water quality analysis: Detection of lead, cadmium, arsenic, and mercury
  • Soil and sediment testing: For heavy metal contamination
  • Air monitoring: Analysis of metal particulates
  • Waste analysis: Characterization of industrial effluents

ICP-AES and GFAAS are preferred for trace metal analysis due to their sensitivity and multi-element capabilities.

5.9 Let Us Sum Up

In this unit, we learned:

  • The principles, instrumentation, and applications of AAS and AES techniques.
  • FAAS and GFAAS are best for single-element analysis, while ICP-AES allows rapid, simultaneous multi-element detection.
  • These spectrometric methods are indispensable in monitoring environmental pollution, especially for detecting trace and toxic metals in various matrices.

Keywords

  • Atomic Absorption Spectrometry (AAS)-A technique that measures the absorption of light by free atoms in a flame or furnace.
  • Atomic Emission Spectrometry (AES)-A method that quantifies elements based on light emitted by excited atoms.
  • Graphite Furnace AAS (GFAAS)-A type of AAS using a graphite tube for atomization, allowing trace analysis.
  • Inductively Coupled Plasma (ICP)-A high-temperature plasma source used to excite atoms in emission spectrometry.
  • Spectral Interference-Occurs when two spectral lines overlap, affecting accuracy.
  • Ionization Suppressor-A substance added to prevent ionization of analyte atoms.
  • Flame Atomization-Use of a flame to convert a sample into free atoms for analysis.
  • Nebulizer-A device that converts liquid sample into fine aerosol droplets for introduction into a flame or plasma.

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