Fundamentals of Spectroscopy (LEC)

• James Maxwell (1873) - proposed that visible light consist of electromagnetic waves
• Wavelength - distance between either 2 crests of 2 troughs
• Frequency - deals with the number of cycles per second of a wave.
• Wavelength and frequency are inversely proportional to each other.
• In the electromagnetic spectrum (EMR), the types of radiation include:

  A) Gamma rays

  B) Ultra-violet

  C) Infrared

  D) Microwave

  E) Radio waves

  
• In terms of gamma rays to radio waves, the wavelength increases.
• From radio waves to gamma rays, the frequency decreases.
• Components of visible light:

  • Red
  • Yellow
  • Blue
  • Orange
  • Violet
• Crest - the highest point in a wave.
• Trough - the lowest point in a wave.
• Amplitude - distance between the crests and the equilibrium position.
• Components of electromagnetic waves:

  • Electric field
  • Magnetic field
• Isaac Newton - showed that visible light can be separated into different colors using a prism.
• Quantum Theory (1900) - states that energy can be gained or lost by matter only in multiples of the quantity hv.
• Planck's constant (h) - 6.63 x 10^-34
• Photoelectric effect - states that light exists as stream of particles called photons. It also deals with dual nature of light.
• Dual nature of light:

  • As a wave
  • As a particle: a stream of photons
• Spectroscopy - the study of the interaction between electromagnetic radiation and matter.
• Spectrometry - the measurement of electromagnetic radiation to obtain information about a system.
• Emission spectroscopy - methods in which the stimulus for the analyte is the application of heat or electrical energy.
• Chemiluminescence - refers to excitation of the analyte by chemical reaction.
• Bioluminescence - involves chemical reactions in biological or biochemical systems.
• Line Spectra - occur when the radiating species are individual atoms or ions in the gas phase.
• Band Spectra - produces due to the presence of gaseous radicals or small molecules.
• Continuous Spectra - produced when solids, such as tungsten, are heated to incandescence.
• Absorption Spectroscopy - the amount of light absorbed as a function of wavelength is measured, which can give qualitative and quantitative information about the sample.
• Atomic Absorption - when UV or VIS radiation is passed through a medium containing gaseous atoms, the outermost electron of the atom is promoted from the ground state to higher energy level orbitals.
• Molecular absorption - aside from electronic transitions, molecules also undergo vibrational and rotational transitions.
• Infrared radiation - is not energetic enough to cause electronic transitions, but can induce vibrational and rotational states.
• UV and VIS radiations - in terms of molecular absorption, they are more energetic.
• Photoluminescence Spectroscopy - the emission of photons is measured following absorption.
• Photoluminescence Spectroscopy:

  • Fluorescence Spectroscopy
  • Phosphorescence Spectroscopy
• Atomic Fluorescence - gaseous atoms fluoresce when exposed to radiation that has a wavelength that exactly matches that of one of the absorption (or emission) lines of the element (resonance fluorescence).
• Non-radiation relaxation:

  • Vibrational deactivation
  • Internal conversion
• Vibrational deactivation - occurs during collision between excited molecules and molecules of the solvent in a series of steps, with an average lifetime of 10^-15 seconds.
• Internal conversion - occurs between two vibrational levels, but less efficient than vibrational relaxation with an average lifetime of 10^-9 to 10^-6 seconds.
• Fluorescence - requires structural features that slow the rate of the non-radiative relaxation process and enhance the rate of fluorescence emission.
• Spectrophotometry is most commonly used in:

  • Biomedical and life science research.
• Typical spectrophotometry applications:

  • Measurements of nucleic acids
  • Measurements of proteins
  • Measurements of bacterial density
• Other applications of spectrophotometry:

  • Diagnostic and clinical testing
  • Drug discovery
  • Pharmaceutical research
  • Chemical engineering
  • Material science
  • Agricultural research