Analytical Chemistry II: Spectroscopy

• Analytical Chemistry II, PMC 104, Part 1 focuses on spectroscopy.
• Spectroscopy involves the study of the interaction of electromagnetic radiation with matter.

Spectroscopy

• Spectroscopy studies the interactions of radiation and matter.

Spectroscopic analysis

• Spectroscopic analysis measures the amount of radiation absorbed or produced by molecular or atomic species.
• Examples of spectroscopic analysis include:
  • Molecular absorption spectroscopy.
  • Molecular fluorescence spectroscopy.
  • Atomic absorption spectroscopy (AAS).
  • Atomic emission spectroscopy.

Electromagnetic Radiation (EMR)

• Electromagnetic radiation (EMR) is a form of radiant energy with both particle and wave properties.
• Light technically refers only to visible radiation, but UV, Visible, and sometimes IR radiation are referred to as light.
• Electromagnetic radiation is divided into different regions based on the type of atomic or molecular transition that gives rise to the absorption or emission of photons.
• The boundaries describing the electromagnetic spectrum are not rigid, and an overlap between spectral regions is possible.

Important Concepts in the History of Light Theory

• Corpuscular Theory of Light (1672): Newton argued that light was made up of particles called corpuscles.
• Wavelets (1690): Huygens developed the wave theory of light.
• Young's Double Slit Diffraction (1803): Diffraction patterns prove light has wave behavior.
• Photoelectric Effect (1887): Hertz observes photoelectric effect, which proves that light has particle behavior.
• Quantised Energy Theory (1900 – 05): Planck identifies the Planck constant.
• Einstein: Determined the quantum of energy, which became known as the photon.
• Wave Particle Duality (1924): De Broglie theorized that particles could show wave properties. He also theorized the de Broglie matter wavelength.

Wave Properties of EMR

• Wavelength: Distance measured in cm and traversed by one complete cycle.
• Frequency: Number of wave cycles passing a fixed point per second (cycles/sec). 1 cycle/sec (s⁻¹) = 1 Hz.
• The formula v = c / λ, where C represents the velocity of light (3 × 10¹⁰ cm/sec or 3 × 10⁸ m/sec).
• Wave number (ú): Number of cycles or waves per cm and the reciprocal of wavelength. The formula for this is ú = 1/λ = v/c (cm⁻¹).

Photon Properties of EMR

• In many radiation/matter interactions, consider light as photons or quanta.
• Relationship between energy of photon (E) to its wavelength, frequency, and wave number: E = h v = h c / λ = h c ú.
• Planck Constant (h) is 6.63 × 10⁻³⁴ J s.
• Frequency (v) and wave number (ú) are directly proportional to energy (E).
• Wavelength (λ) is inversely proportional to energy (E). The shorter the wavelength, the greater the energy of the photons and the more powerful the radiation.
• The UV range, which contains shorter λ, carries more energy photons than a beam of visible region.

Interaction of Matter with EMR

• Absorption of UV or Vis radiation excites bonding electrons, making absorption spectroscopy valuable for identifying functional groups in a molecule.
• Materials contain both bonding (involved in bonds) and non-bonding electrons.
• The energy of incident photons may be transferred to atoms or molecules, raising them from the ground state to an excited state when a substance is irradiated with EMR. This process is called absorption.
• The energy of transition: ΔE = Es – Eg = h v.
• Where v is the frequency of EMR at which the transition takes place, which is characteristic for each molecule.

Additional Info

• Absorbed energy is rapidly lost as heat, light, or through molecular collision.
• Excited electrons emit light or heat when they return to the ground state.
• σ → σ* never is observed in the UV region due to the large energy requirement.
• n → σ* in saturated compounds containing atoms with unshared electrons can be brought about by EMR in the region 125 – 250 nm with absorption peaks less than 200 nm.
• Most applications of absorption spectroscopy in organic compounds are based upon the transition ∏ → ∏*, corresponding to absorption peaks into the spectral region 200 – 700 nm.
• The greater the number of conjugated multiple bonds in a compound, the longer the wavelength of the light absorbed.

Light

• A beam of light carrying only one discrete λ is monochromatic.
• A beam carrying radiation of several λ (s) is polychromatic or heterochromatic.

Theory of Light Absorption

• When a beam of light, of intensity Io, passes through a transparent cell containing a solution of an absorbing substance, the intensity of light may be reduced.
• A portion of the incident light is reflected at the cell faces (Ir), absorbed by the solution (Ia), scattered (Is), refracted (In), and the remainder is transmitted (It).
• For a clear solution, Is = 0, and using a blank consisting of a similar cell filled with the solvent used, Ir and In are cancelled; lo = la + It.
• In chemical analysis, we measure the amount of light absorbed (la) or transmitted (It) by a solution and relating this to the concentration of some particular ion or compound in the solution.