CHEM 2301 W10

Applications of Spectrophotometry

  • Chapter 19 highlights various applications of spectrophotometry in chemical analysis.

Photoluminescence

  • Luminescence: Emission of electromagnetic radiation by chemical compounds due to:

    • Photoinduced excitation

    • Chemical excitation

    • Electrical excitation

  • Photoluminescence is specifically triggered by the absorption of light in the UV-Vis range.

  • Compounds containing aromatic rings tend to have the most intense fluorescence emissions. Examples include:

    • Unsubstituted aromatic hydrocarbons

    • Highly conjugated double bond structures

  • Fluorescence is favored in rigid molecules since non-radiative vibrational relaxation is less favored.

  • High sensitivity is a key feature of fluorescence, known to detect concentrations as low as sub ppb (parts per billion) or even single molecules.

Electronic Transitions in Absorption and Photoluminescence

  • Various electronic states involved include:

    • S0: Ground state

    • S1: Excited singlet state

    • T1: Excited triplet state

  • Processes:

    • Internal Conversion (IC): Conversion between different electronic states without photon emission.

    • Intersystem Crossing (ISC): Transition between singlet and triplet states.

    • Fluorescence (F): Emission of photon upon returning to a lower energy state.

    • Phosphorescence (P): Emission lasting longer due to the transition from triplet back to singlet state.

  • Difference in lifetime between fluorescence (short, nanoseconds) and phosphorescence (longer, milliseconds to seconds).

Stokes Shift

  • Stokes Shift: Phenomenon when the energy of emitted photons is lower than that of absorbed photons.

    • Result of molecules adopting different geometries after excitation, affecting energy transitions.

Other Fluorescent Compounds

  • Phosphorescence: Rarely observed in solution at room temperature because non-radiative relaxation dominates in the triplet excited state.

  • Rigid compounds exhibit phosphorescence more prominently than non-rigid ones.

Photoluminescence Spectra

  • Emission Spectrum: Produced by holding the excitation wavelength fixed and scanning the emitted radiation.

  • Excitation Spectrum: Generated by fixing the emission wavelength and varying the excitation wavelength.

  • Synchronous Spectrum: Both monochromators are scanned simultaneously at a fixed constant wavelength difference.

Fluorescence Emission and Excitation Spectra

  • Two significant spectra for substances like retene and benzo(k)fluoranthene:

    • Emission occurs at specific wavelengths depending on substance concentration and solvent composition.

    • Lower temperatures can enhance emission intensity due to reduced roto-vibrational relaxation.

PAH Identification through Emission Spectra

  • Analyzing fluorescence emissions of 16 EPA PAHs using broad-band excitation.

  • Spectral overlaps may require chromatographic techniques prior to measurements.

  • Cryogenic temperatures potentially enhance specificity and reduce overlapping spectra.

Quantitative Response

  • Measuring fluorescence emissions concerning concentrations of compounds like pyrene and benzo[a]pyrene illustrates linear relationships.

  • Calibration graphs are essential for quantifying concentrations via fluorescence.

Beer-Lambert Law and Quantum Yield

  • The Beer-Lambert law relates fluorescence intensity to concentration, absorptivity, path length, and the quantum yield.

  • Quantum yield: Defined as the ratio of radiative energy decay rates of emitted light.

  • Changes in quantum yield can affect fluorescence intensity measurements, complicating quantitative analysis.

Quantitative Fluorescence Measurements

  • Quenching: Any process that reduces fluorescence intensity through mechanisms such as collisions or energy transfer.

Quenching of Fluorescence

  • Real-time quenching studies using quinine sulfate and NaCl illustrate the relationship between concentration and fluorescence intensity plotted via Stern-Volmer equation.

Analysis Based on Quenching

  • Discusses specific analytes and their sensitivity to various quenching conditions can aid in quantitative analysis of substances like TNT.

Studying Oxygen Exchange via Fluorescence Measurements

  • Measurement setup includes sophisticated gas flow systems and dyes to analyze oxygen changes in biological contexts using fluorescence techniques.

Fluorescence Resonance Energy Transfer (FRET)

  • FRET is a method for studying interactions between molecules based on energy transfer from a donor to an acceptor under specific conditions.

FRET-based Optical Fiber Biosensor

  • Innovative biosensor design uses FRET for rapid detection of pathogens, enhancing sensitivity through unique molecular binding interactions.

Chemiluminescence

  • Defines the phenomenon of light emission resulting from a chemical reaction, relevant in specific chemical analysis applications.