Chapter 10 Atomic Emission Spectroscopy Study Notes

INSTRUMENTAL ANALYSIS

Chapter 10: Atomic Emission Spectroscopy

Overview

• Definition: Emission spectroscopy is a technique that analyzes the light emitted by atoms or ions to determine the chemical composition of a sample.

• Key focus: Techniques utilizing plasma sources such as Inductively Coupled Plasma (ICP) and others.

Emission Spectroscopy with Plasma Sources

Inductively Coupled Plasma (ICP) Sources

• Flow Rate and Components:

  • Argon gas (Ar) flow rates: 5-20 L/min.

  • RF generator power: 2 kW, utilizing a Tesla coil for ionization.

• Process:

  • Ar gas flows through quartz tubes at 5-20 L/min.

  • Ionization begins due to the Tesla coil's action.

  • The ions interact with a fluctuating magnetic field produced by an induction coil, powered by an RF generator at 0.5 to 2 kW at frequencies of 27.12 MHz or 40.68 MHz.

Sample Introduction and Nebulization

• Sample introduction parameters:

  • Ar flow: 0.3-1.5 L/min.

  • Nebulization process involves using a capillary to introduce liquid samples into the gas stream.

• Instrumentation Overview:

  • Components include:

  • 25 mm capillary for liquid input.

  • Shell nozzle for gas input.

  • Plasma tube temperatures reaching 6000 K to 10,000 K.

Plasma Characteristics and Emission

• Visual Appearance:

  • Typical plasma has a very intense, brilliant white, nontransparent core with a flame-like tail.

• Spectral Observations:

  • Observations are made 15-20 mm above the inductive coil at temperatures of 6000 - 6500 K.

ICP-AES Instrumentation

• Key components include:

  • RF Plasma: Creates ionization necessary for spectroscopy.

  • Diffraction Grating: Used to resolve the emitted light into individual spectral components.

  • Entrance Window and CCD Detector: Integral for capturing and analyzing emitted light.

  • Torch and spray chamber: Mixes the sample aerosol with argon.

Three Electrode Direct Current Plasma (DCP) Source

• Electrode Configuration:

  • Anode and cathode arrangement comprising three electrodes including:

  • Cathode block.

  • Anode block.

  • Excitation region noting plasma column.

• Operating Conditions:

  • DC current flow: 10-15 A.

  • Plasma temperatures: core at 10,000 K, viewing at approximately 5,000 K.

• Comparative Advantages:

  • Requires less argon, lower cost, and simplicity compared to ICP.

Advantages of ICP Techniques

• Advantages of ICP Atomization:

  • More complete atomization process compared to other methods.

  • Reduced chemical interferences.

  • Minimal ionization effects.

  • Inert atmosphere prevents formation of oxides.

  • Provides a uniform temperature cross-section, minimizing self-absorption.

  • Ensures linear calibration across a wide range of concentrations.

Desirable Properties of Emission Spectroscopy Techniques

• Table of Desirable Properties:

  1. High resolution: 0.01 nm or A/AA > 100,000.

  2. Rapid signal acquisition and recovery pace.

  3. Low stray light effects.

  4. Wide dynamic range: > 10^6.

  5. Accurate and precise wavelength identification and selection.

  6. Degree of precision in intensity readings: < 1% relative standard deviation at 500 times the detection limit.

  7. High stability under varying environmental conditions.

  8. Effective background correction capabilities.

  9. Computerized operations for data manipulation and storage.

Categories of Emission Spectrometry Instruments

1. Sequential Instruments:

   • Operate one wavelength after another, often used for detailed elemental analysis.

2. Simultaneous Multichannel Instruments:

   • Analyze multiple wavelengths at once for rapid analysis across several elements.

   • Also called multichannel spectrometers.

3. Fourier Transform Instruments:

   • Use Fourier transformation techniques for complex spectra analysis.

Echelle Monochromators in Spectrometry

• Components of Echelle Monochromators:

  • Entrance slit, concave mirror, echelle grating, culminating lens, prism, and charge-coupled device (CCD).

• Functionality:

  • Converts light into spectral data efficiently with high resolution capabilities.

Advantages of Plasma Emission Spectrometry

• Key Benefits:

  • Detection limits can reach the ppb (parts per billion) range.

  • Ability to observe many spectral lines simultaneously.

  • High stability, resulting in consistent results.

  • Low levels of noise and background interference.

  • Minimization of analytical interferences during analysis.

Calibration in Plasma Emission Spectrometry

• Example Calibration Curve:

  • Internal standard (yttrium) used for improved accuracy.

  • Concentration and intensity ratio measurements depicted for various solutions (e.g., deionized water, sodium concentrations).

• Variables Used:

  • Concentration in μg/mL against the intensity ratio.

Emission Spectroscopy Using Arc and Spark Sources

• Sample Handling:

  • Applicable to solids, liquids, and metals.

  • Involves electrodes, counter electrodes, and high temperature applicability.

  • Micro and graphite electrodes are commonly utilized due to their low cost and adaptability.

• Arc and Spark Sources Overview:

  • Arc sources: operating temperatures between 4000-5000 K, ideal for producing intense CN bands.

  • Spark sources: operate at temperatures exceeding 40,000 K, useful for qualitative and quantitative analysis.

• Internal Standard Requirements:

  • Concentration consistency between standards and samples.

  • Consideration for similar chemical properties and emission line energies.

Comparison of Detection Limits for Various Spectroscopic Methods

• Tabulated Values for Detection Limits:

  Method	< 1 ppb	1-10 ppb	11-100 ppb
  ICP Emission	9	32
  Flame Atomic Emission	4	12
  Flame Atomic Fluorescence	4	14
  Flame Atomic Absorption	1	14	25

• Observations:

  • ICP emission is superior with detection limits under 1 ppb compared to traditional flame methods.