UV-Vis Spectroscopy Notes

UV-Vis Spectroscopy

Chapter Overview

  • This chapter covers UV-Vis spectrophotometry, an instrumentation method used in arts and sciences.

Learning Objectives

  • Describe the basic components of a UV-Vis spectrophotometer, including:
    • Light source
    • Monochromator
    • Sample holder
    • Detector
  • Differentiate between single-beam and double-beam spectrophotometer designs, discussing their:
    • Advantages
    • Limitations
    • Applications
  • Illustrate the laboratory workflow for UV-Vis spectrophotometric analysis, from:
    • Sample preparation
    • Result reporting
  • Explain the function and importance of each step in the UV-Vis lab workflow, including:
    • Blanking
    • Wavelength selection
    • Calibration
  • Apply Beer-Lambert’s Law to determine the concentration of an unknown sample using absorbance data obtained from a UV-Vis instrument.
  • Interpret UV-Vis laboratory data, including absorbance readings and calibration curves, to solve for unknown concentrations with proper unit conversions and significant figures.

Review: What is Spectroscopy?

  • Spectroscopy is the study of the interaction of electromagnetic radiation with matter.
  • Process:
    • An emitter produces electromagnetic radiation.
    • The radiation interacts with a sample (matter).
    • Absorption: Ground state to excited state.
    • Emission: Excited state to ground state.
    • A spectrometer analyzes the resulting spectrum.
    • Readout provides the data.

UV-Visible Spectrophotometer

  • Spectroscopy studies the interaction of electromagnetic radiation (EMR) with matter.
  • A UV-Visible Spectrophotometer measures absorbance in the UV (200-400 nm) or Visible (400-800 nm) region.
  • The spectrophotometer records the degree of absorption by a sample at different wavelengths.
  • A spectrum is the resulting plot of absorbance (A) versus wavelength (λ\lambda).

Instrumentation: The Spectrophotometer

  • Basic Components:
    • Light source
    • Wavelength isolation (monochromator)
    • Fiber optics
    • Cuvettes (sample holders)
    • Photodetector
    • Readout device
    • Recorder
    • Computer

Instrument Components

  1. Source of energy (lamp)
  2. Wavelength selector (monochromator)
  3. Sample container (cuvette)
  4. Detector
  5. Signal processor and readout

Optical Diagrams

  • Conventional Spectrophotometer:
    • Light Source (White Light) -> Entrance Slit -> Monochromator (Reflectance Grating) -> Exit Slit (Monochromatic Light) -> Sample Compartment -> Detector -> Readout
  • Array Detector Spectrophotometer:
    • Light Source (White Light) -> Grating -> Exit Slit -> Sample Compartment -> Detector

Light Source

  • A continuous source of radiant energy covers the spectrum the instrument is designed to work in.
  • Requirements:
    • Stable
    • Provides continuous radiation
    • Sufficient intensity for detection at the end of the optical path

Light Source: Hydrogen and Deuterium Lamps

  • Produce a continuous spectrum in the UV region via electrical excitation of deuterium or hydrogen at low pressure.
  • Range: 3500-1200 Å (160-800 nm)
  • Stable, robust, and widely used.
  • Deuterium lamps increase emission intensity compared to hydrogen lamps.
  • Deuterium lamps are more expensive but used when higher intensity is needed.

Light Source: Tungsten Filament Lamp

  • Most common light source in spectrophotometers.
  • Also called