Atomic Absorption Spectroscopy

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Atomic Absorption Spectroscopy
Prof Mark A. Buntine School of Chemistry
Dr Vicky Barnett University Senior College
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Atomic Absorption Spectroscopy

• This material has been developed as a part of
  the Australian School Innovation in Science,
  Technology and Mathematics Project funded by the
  Australian Government Department of Education,
  Science and Training as a part of the Boosting
  Innovation in Science, Technology and Mathematics
  Teaching (BISTMT) Programme.

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Professor Mark A. Buntine Badger Room
232 mark.buntine_at_adelaide.edu.au
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Atomic Absorption Spectroscopy

• AAS is commonly used for metal analysis
• A solution of a metal compound is sprayed into a
  flame and vaporises
• The metal atoms absorb light of a specific
  frequency, and the amount of light absorbed is a
  direct measure of the number of atoms of the
  metal in the solution

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Atomic Absorption SpectroscopyAn Aussie
Invention

• Developed by Alan Walsh (below) of the CSIRO in
  early 1950s.

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Electromagnetic Radiation
Sinusoidally oscillating electric (E) and
magnetic (M) fields.
Electric magnetic fields are orthogonal to each
other.
Electronic spectroscopy concerns interaction of
the electric field (E) with matter.
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The Electromagnetic Spectrum

• Names of the regions are historical.
• There is no abrupt or fundamental change in going
  from one region to the next.
• Visible light represents only a very small
  fraction of the electromagnetic spectrum.

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The Visible Spectrum

• l lt 400 nm, UV
• 400 nm lt l lt 700 nm, VIS
• l gt 700 nm, IR

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The Electromagnetic Spectrum

• Remember that we are dealing with light.
• It is convenient to think of light as particles
  (photons).
• Relationship between energy and frequency is

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Energy Frequency

• Note that energy and frequency are directly
  proportional.
• Consequence higher frequency radiation is more
  energetic.

E.g. X-ray radiation (? 1018 Hz) 4.0 x 106
kJ/mol IR radiation (? 1013 Hz) 39.9
kJ/mol (h 6.626 x 10-34 J.s)
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Energy Wavelength

• Given that frequency and wavelength are related
  ?c/?
• Energy and wavelength are inversely proportional
• Consequence longer wavelength radiation is less
  energetic

eg. ?-ray radiation (? 10-11 m) 1.2 x 107
kJ/mol Orange light (? 600 nm) 199.4 kJ/mol
(h 6.626 x 10-34 J.s c 2.998 x
108 m/s)
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Absorption of Light

• When a molecule absorbs a photon, the energy of
  the molecule increases.
• Microwave radiation stimulates rotations
• Infrared radiation stimulates vibrations
• UV/VIS radiation stimulates electronic
  transitions
• X-rays break chemical bonds and ionize molecules

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Absorption of Light

• When light is absorbed by a sample, the radiant
  power P (energy per unit time per unit area) of
  the beam of light decreases.
• The energy absorbed may stimulate rotation,
  vibration or electronic transition depending on
  the wavelength of the incident light.

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Atomic Absorption Spectroscopy

• Uses absorption of light to measure the
  concentration of gas-phase atoms.
• Since samples are usually liquids or solids, the
  analyte atoms must be vapourised in a flame (or
  graphite furnace).

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Absorption and Emission
Excited States
Ground State
Multiple Transitions
Absorption
Emission
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Absorption and Emission
Excited States
Ground State
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Atomic Absorption

• When atoms absorb light, the incoming energy
  excites an electron to a higher energy level.
• Electronic transitions are usually observed in
  the visible or ultraviolet regions of the
  electromagnetic spectrum.

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Atomic Absorption Spectrum

• An absorption spectrum is the absorption of
  light as a function of wavelength.
• The spectrum of an atom depends on its energy
  level structure.
• Absorption spectra are useful for identifying
  species.

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Atomic Absorption/Emission/Fluorescence
Spectroscopy
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Atomic Absorption Spectroscopy

• The analyte concentration is determined from the
  amount of absorption.

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Atomic Absorption Spectroscopy

• The analyte concentration is determined from the
  amount of absorption.

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Atomic Absorption Spectroscopy

• Emission lamp produces light frequencies unique
  to the element under investigation
• When focussed through the flame these frequencies
  are readily absorbed by the test element
• The excited atoms are unstable- energy is
  emitted in all directions hence the intensity
  of the focussed beam that hits the detector plate
  is diminished
• The degree of absorbance indicates the amount of
  element present

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Atomic Absorption Spectroscopy

• It is possible to measure the concentration of an
  absorbing species in a sample by applying the
  Beer-Lambert Law

e extinction coefficient
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Atomic Absorption Spectroscopy

• But what if e is unknown?
• Concentration measurements can be made from a
  working curve after calibrating the instrument
  with standards of known concentration.

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AAS - Calibration Curve

• The instrument is calibrated before use by
  testing the absorbance with solutions of known
  concentration.
• Consider that you wanted to test the sodium
  content of bottled water.
• The following data was collected using solutions
  of sodium chloride of known concentration

Concentration (ppm) 2 4 6 8
Absorbance 0.18 0.38 0.52 0.76
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Calibration Curve for Sodium
A b s o r b a n c e
1.0
0.8
0.6
0.4
0.2
2
4
6
8
Concentration (ppm)
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Use of Calibration curve to determine sodium
concentration sample absorbance 0.65
A b s o r b a n c e
1.0
0.8
0.6
0.4
?Concentration Na 7.3ppm
0.2
2
4
6
8
Concentration (ppm)
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Atomic Absorption Spectroscopy

• Instrumentation Light Sources
  Atomisation Detection Methods

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Light Sources

• Hollow-Cathode Lamps (most common).
• Lasers (more specialised).
• Hollow-cathode lamps can be used to detect one or
  several atomic species simultaneously. Lasers,
  while more sensitive, have the disadvantage that
  they can detect only one element at a time.

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Hollow-Cathode Lamps

• Hollow-cathode lamps are a type of discharge lamp
  that produce narrow emission from atomic
  species.
• They get their name from the cup-shaped cathode,
  which is made from the element(s) of interest.

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Hollow-Cathode Lamps

• The electric discharge ionises rare gas(Ne or Ar
  usually) atoms, which in turn, are accelerated
  into the cathode and sputter metal atoms into the
  gas phase.

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Hollow-Cathode Lamps
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Hollow-Cathode Lamps

• The gas-phase metal atoms collide with other
  atoms (or electrons) and are excited to higher
  energy levels. The excited atoms decay by
  emitting light.
• The emitted wavelengths are characteristic for
  each atom.

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Hollow-Cathode Lamps
collision-induced excitation
M
spontaneous emission
M
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Hollow-Cathode Spectrum
Harris Fig. 21-3 Steel hollow-cathode
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Atomisation

• Atomic Absorption Spectroscopy (AAS) requires
  that the analyte atoms be in the gas phase.
• Vapourisation is usually performed by
• Flames
• Furnaces
• Plasmas

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Flame Atomisation

• Flame AAS can only analyse solutions.
• A slot-type burner is used to increase the
  absorption path length (recall Beer-Lambert Law).
• Solutions are aspirated with the gas flow into a
  nebulising/mixing chamber to form small droplets
  prior to entering the flame.

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Flame Atomisation
Harris Fig 21-4(a)
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Flame Atomisation

• Degree of atomisation is temperature dependent.
• Vary flame temperature by fuel/oxidant mixture.

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Furnaces

• Improved sensitivity over flame sources.
• (Hence) less sample is required.
• Generally, the same temp range as flames.
• More difficult to use, but with operator skill at
  the atomisation step, more precise measurements
  can be obtained.

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Furnaces
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Furnaces
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Inductively Coupled Plasmas

• Enables much higher temperatures to be achieved.
  Uses Argon gas to generate the plasma.
• Temps 6,000-10,000 K.
• Used for emission expts rather than absorption
  expts due to the higher sensitivity and elevated
  temperatures.
• Atoms are generated in excited states and
  spontaneously emit light.

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Inductively Coupled Plasmas

• Steps Involved
• RF induction coil wrapped around a gas jacket.
• Spark ionises the Ar gas.
• RF field traps accelerates the free electrons,
  which collide with other atoms and initiate a
  chain reaction of ionisation.

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Detection

• Photomultiplier Tube (PMT).pp 472-473 (Ch. 20)
  Harris

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Photomultiplier Tubes

• Useful in low intensity applications.
• Few photons strike the photocathode.
• Electrons emitted and amplified by dynode chain.
• Many electrons strike the anode.