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HL Chemistry - Option A : Modern Analytical Chemistry. Principles of Spectroscopy. Definition. Spectroscopy - The study of the interaction of electromagnetic radiation with matter. Introduction. Spectroscopy is an analytical technique which helps determine structure.

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Presentation Transcript
  • Spectroscopy - The study of the interaction of electromagnetic radiation with matter
  • Spectroscopy is an analytical technique which helps determine structure.
  • It destroys little or no sample.
  • The amount of radiation absorbed by the sample is measured as wavelength is varied.
major types of spectroscopy
Major Types of Spectroscopy
  • Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.
  • Mass spectrometry (MS) fragments the molecule and measures the masses.
  • Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers.
  • Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns.
properties of electromagnetic radiation
Properties ofElectromagnetic Radiation

Electromagnetic Radiation

  • energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field
  • result of the acceleration of a charged particle
  • does not require a material medium and can travel through a vacuum
transmission of radiation
Transmission of Radiation


  • rate at which radiation passes through a transparent material is less than through a vacuum
  • depends upon the kinds and concentrations of atoms, ions, and molecules in the medium
  • radiation must interact with material
  • interaction must not undergo permanent energy transfer
reflection of radiation
Reflection of Radiation
  • Reflection always occurs when radiation passes from one medium to another
  • Reflection is greatest when two materials have large differences in their refractive indecies
scattering of radiation
Scattering of Radiation

Types of Scattering

  • Scattering by Large Molecules
    • can be measured
    • a function of the size and shape of molecule
  • Raman Scattering
    • part of the radiation undergoes quantized frequency changes
    • scattering involving molecules which are considerably smaller than the wavelength of radiation
    • blue sky results from greater scattering of shorter wavelength visible light
summary of the type of em interactions
Summary of the Type of EM Interactions
  • Absorption - EM energy transferred to absorbing molecule (transition from low energy to high energy state)
  • Emission - EM energy transferred from emitting molecule to space (transition from high energy to low energy state)
  • Scattering - redirection of light with no energy transfer
atomic absorption
Atomic Absorption
  • Absorption occurs with only a few well-defined frequencies
  • Electronic excitation
  • Several branches of spectroscopy are based on this concept, a few of which are discussed here
absorption vs emission
Absorption vs. Emission










absorption of radiation
Absorption of Radiation

The energy of an exciting photon must equal the energy difference between the ground state and one excited state for absorption to occur… “Quantum Leap”

emission of radiation
Emission of Radiation
  • Radiation results from the relaxation of electrons from higher (excited) states to lower energy states
  • “Emission Spectroscopy” and “Fluorescence Spectroscopy” are based on this concept
equation definitions
Equation Definitions
  • E = energy (Joules, ergs)
  • c = speed of light (constant)
  • l = wavelength
  • h = Planck’s constant
  • n = “nu” = frequency (Hz)
  • nm = 10-9 m
  • Å = angstrom = 10-10 m
key formulas
Key Formulas
  • E = hn
  • h = 6.626 x 10-34 J-s
  • n = frequency in Hz, E = energy
  • l = c/n
  • c = 3.0 x 108 m/s
  • l = wavelength, n = frequency in Hz
visible light22
Visible Light















700 nm

650 nm

600 nm

550 nm

500 nm

450 nm

400 nm

complementary colors
Complementary Colors

Absorbed Observed

major types of light spectroscopy
Major Types of Light Spectroscopy
  • Absorption spectroscopy
    • Measures amount of light absorbed
    • Most common, non-destructive
    • Concentration, pH measures, purity, ID
  • Atomic emission spectroscopy
    • Measures light emitted from burned sample
    • Elemental analysis
types of light spectroscopy continued
Types of Light Spectroscopy(continued)
  • Fluorescence spectroscopy
    • Samples fluoresce when they emit at higher l than what they absorb
    • Measures solvent interactions, distances, molecular shape, and motion
  • Circular Dichroism spectroscopy
    • Absorption of circular polarized light
    • Chiral compound identification
  • Transmission spect. (colorimetry)
uv vs ir vs nmr
UV vs. IR vs. NMR
  • UV has broad peaks relative to IR & NMR
  • UV has less information than IR & NMR
  • UV spectra are easier to collect
  • UV spectra are faster to collect
  • UV spectrometers are cheaper
  • UV spectra require only nanograms of material or chemicals
cuvettes sample holder
Cuvettes (sample holder)
  • Polystyrene
    • 340-800 nm
  • Methacrylate
    • 280-800 nm
  • Glass
    • 350-1000 nm
  • Suprasil Quartz
    • 160-2500 nm
  • Io = intensity of light through blank
  • IT = intensity of light through sample
  • Absorption = Io - IT
  • Transmittance = IT/Io
  • Absorbance = log(Io/IT)



absorbance beer s law
Absorbance & Beer’s Law

Increasing absorbance

beer s law
Beer’s Law





pathlength b

pathlength b

beer s law33
Beer’s Law

Absorbance = ebC

absorption methods
Absorption Methods

“Readout for an inexpensive photometer.”

transmittance scale is linear……

absorbance scale is exponential……..

thus, one usually reads transmittance,

then calculates absorbance

uv activity
UV Activity
  • Needs chromophores
  • C=C, C=O, N=N, NO2,
  • p -> p* and n -> p* transitions
  • napthoquinones, anthocyanins
  • Non-absorbers…
    • long chain aliphatics
    • alcohols, ethers, non-conjugated mols.
pk a measurement with uv
pKa Measurement with UV



Titration of Phenylephrine

Ai - A

pKa = pH + log

A - An