Atomic Absorption Spectroscopy (AAS) What is AAS? Center for Electrochemical Engineering Research, Ohio University Optical Spectroscopy Mass Spectroscopy Atomic Spectroscopy Atomic

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

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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

22

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

www.ohio.edu/ceer

For more information

contact:

ceer@ohio.edu