Chapter 8 Study Notes - Optical Atomic Spectroscopy

Instrumental Analysis

Chapter 8: An Introduction of Optical Atomic Spectroscopy

Optical Atomic Spectra

  • Energy Level Diagrams: Illustrate the electron configurations and energy levels for different elements, specifically Sodium (Na) and Magnesium (Mg).

    • For Sodium (Na):

    • Ground State: 1S01S_0

    • Excited States: 2S1/2,2P1/2,2P3/2,3S1/22S_{1/2}, 2P_{1/2}, 2P_{3/2}, 3S_{1/2}

    • Ionization Potential: 10.0 eV

    • For Magnesium (Mg):

    • Ground State: 1S01S_0

    • Excited State: Various states from triplet (3S,3P,3D3S, 3P, 3D) to singlet ground state.

  • Ionization Potentials: Key values for elements are summarized:

    • Na: 10.0 eV

    • Mg: 7.0 eV

Energy Level Diagrams

  • Energy levels represented in electron volts (eV), examples include:

    • Sodium (Na):

    • Ground State: 2.02.0 eV, Excited States: 4.0,6.04.0, 6.0 eV levels identified.

    • Magnesium (Mg):

    • Significant energy level states and ionization configurations presented.

Singlet and Triplet States

  • Singlet States: Characterized by paired electron spins leading to lower energy states.

    • Singlet Ground State: 1S01S_0

    • Singlet Excited State and configurations for Triplet States

  • These states impact the atomic spectra and types thereof.

Types of Atomic Spectra

  • Atomic Emission Spectra: Emission of light from atoms as electrons transition from high to lower energy levels.

  • Atomic Absorption Spectra: The absorption of light as electrons transition from lower to higher energy states.

  • Atomic Fluorescence Spectra: Involves re-emission of absorbed light, usually within a shorter time frame than in emission spectra.

Atomic Line Widths

  • Effective Line Width: riangle<br>u1/2riangle <br>u_{1/2}

  • Sources of Broadening:

    • Uncertainty Effect: Related to energy state uncertainties, where riangle
      u riangle t > 1 contributes to natural line widths of approximately 10−5ext.nm10^{-5} ext{. nm} to 10−4extA˚10^{-4} ext{Å}.

    • Doppler Broadening: Caused by the movement of atoms (thermal motion) affecting the observed wavelength of emitted/absorbed light. This broadening is typically 10-100 times greater than natural line widths.

    • Pressure Broadening: Results from collisions between atoms in the analyte and combustion products, leading to shifts in energy levels and a broader emission spectrum (possibly ranging from 100 to 1000 times the natural line width).

  • Implication: Spectral lines have finite width due to inherent energetic uncertainties.

Temperature Effects on Atomic Spectra

  • Boltzmann Equation: racNjN0=racgjg0e−Ej/kTrac{N_j}{N_0} = rac{g_j}{g_0} e^{-E_j/kT}

    • Where:

    • NjN_j = number of atoms in the excited state

    • N0N_0 = number of atoms in the ground state

    • kk = Boltzmann constant, 1.38imes10−23extJ/K1.38 imes 10^{-23} ext{ J/K}

    • TT = temperature in Kelvin

    • EjE_j = energy difference between states in joules

    • gj,g0g_j, g_0 = statistical factors dependent on quantum levels.

  • At 2500 K:

    • racNjN0=1.74imes10−4rac{N_j}{N_0} = 1.74 imes 10^{-4}

    • Excited State Percentage: 0.0174% with 99.983% in the ground state.

  • At 2510 K:

    • racNjN0=1.79imes10−4rac{N_j}{N_0} = 1.79 imes 10^{-4}

    • Excited State Percentage: 0.0179% with 99.982% in the ground state.

Band and Continuum Spectra in Atomic Spectra

  • Concepts of Relative Absorption and Emission: Displayed as continuum spectra along with manifestation of atomic spectra.

  • Specific examples include absorption and emission of CaOH, with identified wavelength ranges from 5520 Å to 5600 Å.

Atomization Methods

  • Types of Atomizers:

    • Flame Atomization: 1700-3150°C

    • Electrothermal Vaporization (ETV): 1200-3000°C

    • Inductively Coupled Argon Plasma (ICP): 4000-6000°C

    • Direct Current Argon Plasma (DCP): 4000-6000°C

    • Microwave-Induced Argon Plasma (MIP): 2000-3000°C

    • Glow-Discharge Plasma (GD): Typically non-thermal with electric arcs and sparks (up to 40,000 °C).

Atomization Process

  • Steps of Atomization:

    1. Nebulization: Spraying the sample solution.

    2. Desolvation: Drying the aerosol generated from nebulization.

    3. Volatilization: Converting the material into free atoms ready for analysis.

  • Limitations: The introduction of solution samples in atomization can limit the accuracy and precision of spectrometric methods utilized in analysis.

Methods of Sample Introduction

  • Types of Sample Introduction Techniques:

    • Pneumatic Nebulization: Utilizes a gas flow to aerosolize a solution.

    • Ultrasonic Nebulization: Employs ultrasound for fine droplet generation.

    • Electrothermal Vaporization: Suitable for solid and liquid samples.

    • Hydride Generation: For specific elements.

    • Direct Insertion: For solid samples.

    • Laser Ablation: For direct material ablation.

    • Spark or Arc Ablation: Common for conducting solids.

    • Glow-discharge Sputtering: Utilized for conducting solids and non-thermal samples.