Instrumental Lecture 9

  • Class Schedule

    • Next class: Tuesday due to Mardi Gras festivities.
    • Finish UV-Vis spectroscopy today; start Raman spectroscopy related to ancient dyes and pigments.

  • Detectors Overview

    • Focus on:
    • Photomultiplier tubes (PMTs)
    • Charge-coupled devices (CCDs)
    • Photomultiplier Tube (PMT):
    • Operates from 200 to 900 nm
    • Incident photons eject electrons, causing amplification (electron multiplication).
    • Involves photocathode and dynodes for sensitivity.
    • Charge-Coupled Devices (CCD):
    • Involves p-type and n-channel semiconductors.
    • Accumulates charge based on light interaction, requiring cooling to reduce thermal noise.

  • Noise Sources in Spectroscopy

    • Stray light causes erroneous absorbance readings.
    • Types of Noise:
    • Detector noise (dark current & shot noise)
    • Source noise (thermal noise from conductor movement)
    • Trade-offs: Cooling electronics may introduce experimental trade-offs regarding response times.

  • Raman vs. IR Spectroscopy

    • Both are vibrational spectroscopy techniques.
    • IR Requirements:
    • Must involve a change in dipole moment for absorption.
    • Raman Requirements:
    • Change in polarizability needed for activity.
    • They are complementary techniques:
    • Can yield insight on functional groups and molecular structures.

  • Dyes vs. Pigments

    • Dyes: Chemically bound to materials (e.g., indigo).
    • Pigments: Require a binder; often inorganic materials.
    • Mordants: Metal ions aiding dye binding to substrates.

  • Historical Use and Composition of Dyes/Pigments

    • Early dyes were often organic (e.g., indigo); pigments were inorganic.
    • Examples:
    • Maya Blue: Organic dye (indigo) with clay binding.
    • Egyptian Blue: Synthetic pigment with calcium and copper.

  • Application of Raman Spectroscopy

    • Non-invasive method for analyzing artwork and materials without destruction.
    • Allows for the investigation of molecular interactions and structural determinations.

  • Raman Scattering

    • Involves inelastic scattering of photons.
    • Ranges include:
    • Rayleigh scattering (elastic, no energy change)
    • Raman scattering (inelastic, energy change)
    • Stokes and Anti-Stokes lines: Stokes are typically more intense than Anti-Stokes.

  • Conclusion

    • Focus on the importance of understanding historical pigments/dyes through spectroscopy for cultural heritage preservation and chemistry development.
    • Reminder for upcoming research papers or reviews due at the next class.