1 atomic absorption spectroscopy prof mark a. buntine school of chemistry dr vicky barnett...
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Atomic Absorption Spectroscopy
Prof Mark A. BuntineSchool of Chemistry
Dr Vicky BarnettUniversity Senior College
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“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.”
Atomic Absorption Spectroscopy
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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 Spectroscopy:An 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 theelectric 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.
1020 1018 1016 1014 1012 108
-rays X-rays UV IRMicro-wave
Frequency (Hz)
Wavelength (m)10-11 10-8 10-6 10-3
Visible
400 500 600 700 800 nm
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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:
E h
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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
Groundstate
Excitedstate
photon
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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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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 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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• 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
Atomic Absorption Spectroscopy
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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:
Abs logIIo
Abscb
= extinction coefficient
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Atomic Absorption Spectroscopy
• But what if 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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Use of Calibration curve to determine sodium concentration {sample
absorbance = 0.65}
Concentration (ppm)
Absorbance
2 4 6 8
0.2
0.4
0.6
0.8
1.0
Concentration
Na+ = 7.3ppm
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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
• 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
MM
MM**
M + e MM + e M**
M + ArM + Ar** M M**
M
M*
MM** M + M + hh
collision-inducedexcitation
spontaneousemission
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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
• Degree of atomisation is temperature dependent.
• Vary flame temperature by fuel/oxidant mixture.
Fuel Oxidant Temperature (K)Acetylene Air 2,400 - 2,700Acetylene Nitrous Oxide 2,900 - 3,100Acetylene Oxygen 3,300 - 3,400Hydrogen Air 2,300 - 2,400Hydrogen Oxygen 2,800 - 3,000Cyanogen Oxygen 4,800
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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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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.