Atomic Absorption
Spectroscopy (AAS)
Alex Miller
ABC’s of Electrochemistry
3/8/2012
2 Center for Electrochemical Engineering Research, Ohio University
Contents
• What is Atomic Absorption Spectroscopy?
• Basic Anatomy of an AAS system
• Theory of Operation
• Practical Operation
• Interferences
• Further Information
3 Ohio University - Avionics Engineering Center
What is Atomic Absorption
Spectroscopy?
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A Brief History
1666 – Newton’s discovery of solar spectrum
1802 – Wallaston Repeats Newton’s experiments observes “spectral
lines” in solar spectrum
1823 – Fraunhofer determines wavelengths of these “spectral lines”
1855 – Bunsen perfects the Bunsen Burner
1859 – Kirchhoff shows emission spectra to be due to elements NOT
compounds
1953 – Walsh first to use hollow cathode light source and begins to
commercialize the instrumentation using acetylene burner
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What is AAS?
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Optical Spectroscopy
Mass Spectroscopy
Atomic Spectroscopy
Atomic Emission (AES) Atomic Fluorescence (AFS)
Atomic Absorption
(AAS)
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What is AAS?
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Optical Spectroscopy
Mass Spectroscopy
Atomic Spectroscopy
Atomic Emission (AES) Atomic Fluorescence (AFS)
Taken from:
Ebdon, L. L. (1998). Introduction to
Analytical Atomic Spectrometry.
John Wiley.
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What is AAS?
Atomic Absorption Spectroscopy (AAS)
– Analytical procedure used in the qualitative and
quantitative determination of elements
• Usually metallic elements in solution
• Some techniques allow solid samples (unfortunately not at CEER)
Operates by:
1. Converting molecules or ions into free atoms
2. Measuring the absorption of radiant energy of particular
frequencies by free atoms
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What is AAS?
• If light is passed through a gas of an
element spectral lines can be observed
• Each element has a unique set of
frequencies that can be absorbed
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PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”
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Basic Anatomy of an AAS System
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Basic Anatomy of an AAS System
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Light Source
• The Hollow Cathode Lamp
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Ebdon, L. L. (1998). Introduction to Analytical Atomic Spectrometry. John Wiley.
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Light Source
• The Hollow Cathode Lamp
– Operates by exciting metal cathode which emits
radiation at the desired wavelength
– Have finite lifetime
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PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”
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Optics
• Monochromator – separates sample beam into
distinct wavelengths and sends desired wavelength
to detector
• Detector – changes light energy into electronic
signal to be interpreted by data system
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Optics
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If operating a Single beam AAS always allow ample
warm-up time for radiation sources because the
intensity drifts with time.
PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”
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Optics
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If operating a Double beam AAS, drift is minimized by
the use of a reference beam, and little to no warm-up
time is required.
PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”
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Nebulizer and Burner
• Nebulizer - Key component for operator to control
– Determines the flow rate of the sample being
introduced to the system as well as air mixture
• Flame Burner – must be positioned correctely
– Uses acetylene for fuel
– Can use air (T=2300°C) or nitrous oxide
(T=2900°C) as oxidant
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Nebulizer and Burner
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PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”
18 Ohio University - Avionics Engineering Center
Theory of Operation
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The Beer-Lambert Law
Absorbance
Intensity of light before absorption
Intensity of light after absorption
Absorption coefficient
Path length of the light through the sample
Concentration of solution
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0
1
logI
A abcI
A
0I
1I
a
b c
NOTE: According the Beer-Lambert law
absorbance is linear with concentration , But…
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The Beer-Lambert Law
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End of linear range
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Practical Operation
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Example of Standards Card
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Example of Standards Card
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Important information on Standards Card
• Characteristic concentration = Sensitivity
• Linear absorbance concentration range(s)
• Wavelengths associated with element
• Flame information (recommended conditions)
• Typical interferences
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Interferences
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Turn up heat
•Using higher or lower flame
•Using Nitrous oxide –
Acetylene flame
•Use “releasing agent” such as
Lanthanum Oxide
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Turn Down heat
•Using cooler flame
•Using doping with alkali metals
•Potassium more easily ionized
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface
tension, etc.) of calibration standards differs from
analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Method of Standard Additions
•Matching standards with samples
as closely as possible
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Diluting sample
•Turn Down heat
•Using cooler flame
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Use alternative wavelength
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Types of Interference
1. Chemical – Heat of flame does not fully atomize the
element of interest
2. Ionization – Heat of flame strips electron from
element of interest
3. Matrix – Physical characteristics (viscosity, surface tension,
etc.) of calibration standards differs from analyte
4. Emission – Only in certain “emissive” elements
(ex: Ba)
5. Spectral – Overlapping wavelengths of competing
elements
6. Background Absorption – Light scattering inefficiencies
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•Background can usually be
distinguished and subtracted
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Method of Standard Additions
• Standards are mixed with several identical sample
dilutions
– Only works in the linear range
– Does not work with certain interferences such
as background absorption and spectral
interferences
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Conclusion
• AAS is a powerful analytical technique for the
quantification of metals in solution
– Operates on the principles of Emission and Absorption
• Can be used to accurately determine concentrations
of metallic ions in solution
– Plating solutions
– Waste water
– Dissolved metals
– Blood and urine
– Food, wine, and beer
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Further Information
1. Robinson, James W. Atomic Spectroscopy. New
York: Marcel Dekker, 1996. Print.
2. Ebdon, L. L. (1998). Introduction to Analytical
Atomic Spectrometry. John Wiley.
3. Sneddon, Joseph. Sample Introduction in Atomic
Spectroscopy. Amsterdam, Netherlands:
Elsevier, 1990. Print.
4. PerkinElmer Inc. (1996). “Analytical Methods For
Atomic Absorption Spectroscopy”
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In depth overviews
of Atomic
Spectroscopy
Concise overview
and Standard
information