LS_CHT_BIA_L23_Ltr04_Instrumentation of AAS(Part1)

Basic Instrumental Analysis

  • Course Code: LS5006FPR/5012FP

  • Lesson 23: Instrumentation of AAS (Part 1)

Lesson Objectives

  • At the end of this lesson, students should be able to:

    • Identify the parts in AAS instruments.

    • Understand the role of the radiation source and atomizer.

Key Components of AAS Instruments

  1. Radiation Source: Hollow Cathode Lamp (HCL)

  2. Sample Conversion: Converts sample to free atoms using:

    • Atomizer (Flame or Graphite Furnace)

Atomic Absorption Spectrometry (AAS)

  • AAS measures the concentrations of analytes in solid, liquid, or solution form by converting them to gaseous atoms.

  • Key Requirement: Conversion of the analyte to free gaseous atoms is essential for analysis.

Atomization Process

  • Atomization: The process of converting the analyte into free atoms to emit or absorb energy.

  • Atomization can occur via:

    • Heat processes (e.g. flame or electrothermal methods).

Types of Atomizers

  1. Flame Atomizer

    • Most common device for introducing samples.

  2. Flameless Atomizer

    • Graphite Furnace Atomizer (GFA)

    • Hydride Vapor Generator

Components of Hollow Cathode Lamp (HCL)

  1. Transparent Window

  2. Hollow Cylinder Cathode (same element as analyte)

  3. Tungsten Anode

  4. Buffer Gas

Working Principle of HCL

  • Argon ions (Ar+) are produced from the collision of electrons with argon gas, resulting in the formation of excited metal atoms which emit energy.

Flame Atomization

  • Flame Atomizer Key Components:

    1. Burner Head: Produces stable flames for sample introduction.

    2. Sample Introduction System: Nebulizer and spray chamber break the sample into fine mist.

    3. Drain Line: Allows excess solution to drain away, ensuring efficient atomization.

  • Process Flow:

    1. Nebulization: Converts solution to mist.

    2. Desolvation: Evaporates solvent, leaving dry aerosol.

    3. Volatilization: Heats particles to form free atoms.

Factors Affecting Volatilization Efficiency

  • Depends on bond dissociation energies, temperature, and aerosol size.

  • Incomplete vaporization can lead to inaccuracies in calibration.

Graphite Furnace Atomization (GFA)

  • Key Features:

    • Consists of a cylindrical tube for atomization under controlled conditions.

    • Uses electrical heating to achieve higher temperatures for better atomization.

  • GFA Atomization Steps:

    1. Drying: Remove solvent, leaving solid residue.

    2. Ashing: Convert organic materials into gaseous products.

    3. Atomization: Vaporize the sample to produce gaseous atoms.

    4. Cleaning: (Optional) Remove remaining materials from the furnace.

Comparison of Flame AAS vs. Graphite Furnace AAS

Feature

Flame AAS

Graphite Furnace AAS

Sensitivity

Low

High

Detection Limits

ppm level

ppb level

Precision

Good

Poor due to contamination

Summary of AAS Instrumentation

  • Six components comprise AAS:

    1. Radiation source (Hollow Cathode Lamp)

    2. Atomizer (Flame or Graphite Furnace)

    3. Optical system

    4. Monochromator

    5. Detector (Photomultiplier tube)

    6. Signal processor (Computer)

References

  • Fundamentals of Analytical Chemistry, 7th Edition by Skoog West Holler, Chapter 26

  • Instrumental Methods of Analysis, 7th Edition by Hobart H. Willard, Chapter 9 and 10

  • Analytical chemistry and quantitative analysis, international edition by David S Hage, James D Carr, Chapter 19

  • ChemWiki Analytical Chemistry - Atomic Absorption Spectroscopy