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SPECTROSCOPIC METHODS OF ANALYSIS

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... the difference in energy, ?E, between two of the analytes and a higher energy states. ... Molecular absorption. Limitations to Beer's law. Instrumental deviation ... – PowerPoint PPT presentation

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Title: SPECTROSCOPIC METHODS OF ANALYSIS


1
CHAPTER 4
  • SPECTROSCOPIC METHODS OF ANALYSIS

2
OUTLINE
  • Overview of spectroscopy
  • Basic components of spectroscopic instrumentation
  • Spectroscopy Based on Absorption

3
Overview of spectroscopy
  • Electromagnetic Radiation
  • From of energy whose behavior is described for
    the properties of both waves and particles.
  • Optical properties of electromagnetic radiation,
    such as diffraction, are explained best by
    describing light as a wave.
  • Electromagnetic radiation consists of oscillating
    electric and magnetic fields that propagate
    through space along a linear path and with a
    constant velocity.

4
Overview of spectroscopy
  • Group Assignment
  • Discuss about the wave properties of
    electromagnetic radiation and define and explain
    frequency, wavelength and wavenumber. Submit with
    ID only.

5
Overview of spectroscopy
  • 2 additional wave properties are
  • Power, P the flux of energy per unit time
  • Intensity, I the flux of energy per unit time
    per area (I).
  • To understand the interaction between the sample
    radiation and the electromagnetic radiation, we
    assume that electromagnetic radiation consists of
    a beam of energetic particles called photons.

6
Overview of spectroscopy
  • Energy of photons (unitJoules) is related to
    frequency, wavelength, or wavenumber

7
Overview of spectroscopy
  • Elecromagnetic spectrum the division of
    electromagnetic radiation on the basis of a
    photons energy.

8
Overview of spectroscopy
  • Measuring Photons as a signal
  • Spectroscopy is possible only if the photons
    interaction with the sample leads to a change in
    one or more of these characteristic properties.
  • In one of spectroscopy classes, energy is
    transferred between a photon of electromagnetic
    radiation and the analyte.
  • The source of energy state depends on the
    photons energy. Refer figure 10.4. it shows that
    absorbing a photon of visible light causes a
    valence electron in the analyte to move to a
    higher energy level.

9
Overview of spectroscopy
  • The intensity of photons passing through a sample
    containing the analyte is attenuated because of
    absorption.
  • Absorbance the attenuation of the photons as
    they pass through a sample (A)
  • Absorbance spectrum a graph of a samples
    absorbance of electromagnetic radiation versus
    wavelenght
  • Emission the release of a photon when an
    analyte returns to a lower-energy state from a
    higher-energy state.
  • What are photoluminescence, chemiluminescence and
    emission spectrum? Explain. (Note Combine with
    previous assignment stated)

10
Basic components of spectroscopic instrumentation
  • Sources of energy
  • Electromagnetic radiation either continuum
    source or line source.
  • Continuum source a source that emits radiation
    over a wide range of wavelenghts
  • Line source a source that emits radiation at
    only select wavelengths
  • Thermal energy (flame and plasmas)
  • Flame can achieve T of 2000-3400K
  • Plasma can achieve up to 6000-10,000K of T
  • Chemical Energy
  • Exothermic reactions

11
Basic components of spectroscopic instrumentation
  • Wavelength selection
  • Filters
  • Monochromators
  • Interferometers
  • Detectors
  • Photon transducers
  • Thermal transducers
  • Signal Processors

12
Spectroscopy Based on Absorption
  • 2 general requirements must be met if an analyte
    is to absorb electromagnetic radiation
  • 1st requirement there must be a mechanism by
    which the radiations electric field or magnetic
    field interacts with the analyte.
  • 2nd requirement the energy of the
    electromagnetic radiation must exactly equal the
    difference in energy, ?E, between two of the
    analytes and a higher energy states.

13
Absorption spectra
14
Absorbance and Concentration Beers Law
  • a constant, called the analytes
    absorptivity (cm-1 conc-1)
  • b samples overall thickness (cm).
  • C concentration of the absorbing
    species.
  • e constant, called the molar
    absorptivity (using when concentration in moles
    per liter or M) (cm-1 M-1)
  • a and e must have unit that cancel the units of b
    and c.

15
Limitation to beers law
  • There are 3 exceptions in beers law
  • Fundamental limitations fundamental and
    represent real limitation to the law
  • i. too high concentration of the analyte.
  • ii. Dependence of the molar absorptivity upon
    the refractive index of the medium
  • Use low concentration of analyte
  • Chemical limitations chemical change that occur
    when the concentration changes.
  • i. Chemical reaction of analyte with solvent
  • Same wavelength or use buffer system (to
    maintain pH)

16
Limitation to beers law
  • Instrumental limitations method to make
    absorbance measurements
  • Polychromatic radiation give negative deviation
    (beers law valid for purely monochromatic
    radiation only one wavelength)
  • Stray radiation (accidental splitting of
    radiation from its original source)
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