CHEM220-Section06

Section 06 Spectroscopy

  • Definition: Interaction between electromagnetic radiation and matter; widely used analytical method.

  • Spectrophotometry: Specific subset of spectroscopy, quantifies electromagnetic spectra in visible, UV, and IR regions.

  • Typical Method: Sample absorbs electromagnetic radiation, amount related to analyte concentration.

Electromagnetic Spectrum

  • Properties: Electromagnetic radiation possesses wave and particle properties.

    • Wave Properties: Described by wavelength (位 - distance of one complete cycle) and frequency (谓 - cycles per unit time).

    • Wavenumber (饾泿虆): Reciprocal of wavelength, number of waves per unit length.

Components of Electromagnetic Radiation

  • Electric Field Component (E): Oscillating field, perpendicular to magnetic field component (M).

  • Magnetic Field Component (M): Also oscillates perpendicular to E and the direction of propagation.

Wave-Particle Duality

  • Photons: Discrete packets of energy corresponding to electromagnetic radiation.

  • Energy Relationships:

    • Wavelength and frequency: 位 = c/谓.

    • Photon energy: E = hv = hc/位.

  • Quantum Mechanics: Energy is quantized; interaction with atoms causes electronic excitation.

Absorption and Emission

  • Absorption: Process where light increases energy of an atom or molecule to higher energy levels.

  • Emission: Ejection of light from an excited state back to a lower energy level.

  • Absorption Spectroscopy: Measurement of light absorption; Emission Spectroscopy: Measurement of light emission.

    • Transitions: Rotational, vibrational, and electronic transitions occur as a molecule absorbs energy.

Spectrophotometer

  • Components:

    1. Light Source

    2. Wavelength Selector

    3. Sample Cell

    4. Detector

    5. Readout Device

Light Sources and Selection

  • Common light sources include H2/D2 lamps (UV), tungsten lamps (visible), and globar (IR).

  • Monochromator: Selects specific wavelength; consists of lenses, slits, and dispersing elements (prisms and diffraction gratings).

Sample Cells and Detectors

  • Sample cells must be transparent at the measured wavelengths; typically made of glass or quartz.

  • Photodetectors: Detect light intensity changes; include phototubes, PMTs, photodiodes.

  • Photomultiplier Tubes (PMT): Amplifies the signal through a cascading electron effect.

Beer鈥檚 Law

  • Statement: Absorbance (A) is proportional to concentration (c) and path length (b).

  • Formula: A = 蔚bc, where 蔚 = molar absorptivity.

  • Limitations: Deviations occur at higher concentrations; interactions may affect measurements.

Titration and Calculation Examples

  • Spectrophotometric titration uses absorbance data to identify endpoints; requires absorbance corrections.

  • Fluorescence: Emission of light upon returning to the ground state; related terms include luminescence types (bioluminescence, chemiluminescence, etc.).

  • Pathways: Nonradiative (e.g., vibrational relaxation) and radiative pathways (e.g., fluorescence vs phosphorescence).

Atomic Spectroscopy

  • Analyzes free atoms, requires high temperatures for atomization.

  • Types include atomic emission and absorption spectroscopy, using very sharp lines for analysis.

  • Interferences: Can be spectral, chemical, or ionization interferences, which can be managed through specific techniques.