Components of Optical Instruments

1

• Components of Optical Instruments
• Some definitions - Anal. Chem. 1974, 46, 2257.
• Spectrograph an instrument with an entrance slit
  and dispersing device that uses photography to
  obtain a record of the spectral range
• The radiant power is integrated over time giving
  a function of radiant energy
• Spectrometer an instrument with an entrance
  slit, dispersing device and one or more exit
  slits
• Measurements are made at selected wave lengths or
  wave length range
• The quantity detected is a function of radiant
  power
• Spectrophotometer an instrument with an entrance
  slit, dispersing device, and exit slit that
  furnishes the ratio or a function of the ratio
  of radiant power of two beams as a function of
  wavelength
• The two beams can be separated in time, space or
  both

2

• Flame Atomic Spectroscopy
• Sources - there are numerous sources that can be
  used to produce a reservoir of free atoms
• A characteristic of these sources is they operate
  at high temperatures and are thus energetic
• Not only a reservoir but an excitation source for
  emission spectroscopy
• Flames are useful for emission, absorption and
  fluorescence
• Electrical excitation, including sparks and arcs,
  are used for emission
• Electrothermal sources are useful for absorption
• Inductively coupled plasmas are emission sources
• Atomic spectra in the vapor or plasmas are quite
  simple, consisting of discrete lines 0.01 nm
  wide
• Formation of atoms in a flame is a very complex
  process requiring careful control of a number of
  variables

3

• Flame Atomic Spectroscopy
• Desirable characteristics of flames, burners and
  nebulizers
• Stable
• Safe
• Inexpensive
• High temperature
• Slow rise velocity
• Reducing atmosphere
• Two types of burners
• Total consumption burners consume all the analyte
  solution reaching the flame

• Gas flow around the tip creates a vacuum sucking
  liquid up capillary and into combustion region
  of the flame
• Turbulance mixes the gases and they burn
• Droplets leaving capillary tube are shattered in
  the turbulent region
• Advantages safe inexpensive combustible
  mixture formed at burner outlet can use H2/O2
  which are usually dangerous high temperature,
  3,000 K good oxide reducing qualities
• Disadvantages expect high efficiency, yet not
  all sample desolvated or vaporized turbulence
  produces unsteady flame short residence time
  of sample in flame

4

• Flame Atomic Spectroscopy
• Two types of burners
• Advantages
• Premix burner mixes fuel, oxidant and nebulized
  analyte solution in combustible combination in
  a chamber prior to reaching the burner tipA
  steady flame is produced because fuel and
  oxidizer premixed
• A slotted burner head provides a long path length
  for AA
• Use of N2O/acetylene gives a high temperature
  comparable to the total consumption burner and
  excellent reducing properties and good
  residence times
• Acetylene/air flames produce less background but
  with a lower temperature
• Disadvantages
• Can be unsafe
• Expensive
• Inefficient in use of analyte sample solution
• Factors influencing the production of atoms in a
  flame
• Nebulization is the production of a fine aerosol
  of a liquid
• Its efficiency determines the fraction of sample
  solution that is converted to droplets reaching
  the flame
• Viscosity and surface tension of solution and the
  flow rate of nebulizer gas affect the efficiency
• Organic solvents have lower viscosity and enhance
  nebulization
• High solute concentration increase viscosity and
  decrease nebulization efficiency

5

• Flame Atomic Spectroscopy
• Factors influencing the production of atoms in a
  flame...
• Desolvation is the evaporation of solvent leaving
  solid particles
• Critical in determining the the number of analyte
  atoms in a flame
• Influenced by
• Flame temperature
• Nature of solvent
• Residence time of aerosol droplets in the flame
• Improve efficiency by
• Using a fuel/oxidant combination that burns at a
  high temperature
• Using a fuel/oxidant combination that has a low
  combustion velocity
• Use organic solvents that evaporate rapidly
• Use organic solvents that have a low DHvap and
  thus cools flame less
• Vaporization is the conversion of desolvated
  particles into gaseous atoms
• Factors affecting vaporization similar to those
  affecting desolvation
• High flame temperature and particle residence
  time improve vaporization
• The nature of the particle determines the
  efficiency of vaporization
• Metal oxides are refractory but metal halides are
  quite volatile
• Contaminants in analyte solution can influence
  vaporization