SPECTROSCOPIC METHODS OF ANALYSIS

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CHAPTER 4

• SPECTROSCOPIC METHODS OF ANALYSIS

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OUTLINE

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

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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.

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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.

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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.

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Overview of spectroscopy

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

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Overview of spectroscopy

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

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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.

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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)

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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

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Basic components of spectroscopic instrumentation

• Wavelength selection
• Filters
• Monochromators
• Interferometers
• Detectors
• Photon transducers
• Thermal transducers
• Signal Processors

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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.

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Absorption spectra
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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.

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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)

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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)