Atomic Absorption Spectroscopy (AAS) - Ohio · PDF fileWhat is AAS? Atomic Absorption...

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?

4 Center for Electrochemical Engineering Research, Ohio University

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

5

What is AAS?

Center for Electrochemical Engineering Research, Ohio University

Optical Spectroscopy

Mass Spectroscopy

Atomic Spectroscopy

Atomic Emission (AES) Atomic Fluorescence (AFS)

Atomic Absorption

(AAS)

6

What is AAS?


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.

7

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


8

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


PerkinElmer Inc. (1996). “Analytical Methods For Atomic Absorption Spectroscopy”


Basic Anatomy of an AAS System

10

Basic Anatomy of an AAS System


11

Light Source

• The Hollow Cathode Lamp


Ebdon, L. L. (1998). Introduction to Analytical Atomic Spectrometry. John Wiley.

12

Light Source

• The Hollow Cathode Lamp

– Operates by exciting metal cathode which emits

radiation at the desired wavelength

– Have finite lifetime



13

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


14

Optics


If operating a Single beam AAS always allow ample

warm-up time for radiation sources because the

intensity drifts with time.


15

Optics


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.


16

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


17

Nebulizer and Burner




Theory of Operation

19

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


0

1

logI

A abcI

A

0I

1I

a

b c

NOTE: According the Beer-Lambert law

absorbance is linear with concentration , But…

20

The Beer-Lambert Law


End of linear range


Practical Operation

22

Example of Standards Card


23

Example of Standards Card


24

Important information on Standards Card

• Characteristic concentration = Sensitivity

• Linear absorbance concentration range(s)

• Wavelengths associated with element

• Flame information (recommended conditions)

• Typical interferences



Interferences

26

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


27



element of interest


element of interest




(ex: Ba)


elements



•Turn up heat

•Using higher or lower flame

•Using Nitrous oxide –

Acetylene flame

•Use “releasing agent” such as

Lanthanum Oxide

28



element of interest


element of interest




(ex: Ba)


elements



•Turn Down heat

•Using cooler flame

•Using doping with alkali metals

•Potassium more easily ionized

29



element of interest


element of interest

3. Matrix – Physical characteristics (viscosity, surface

tension, etc.) of calibration standards differs from

analyte


(ex: Ba)


elements



•Method of Standard Additions

•Matching standards with samples

as closely as possible

30



element of interest


element of interest




(ex: Ba)


elements



•Diluting sample

•Turn Down heat

•Using cooler flame

31



element of interest


element of interest




(ex: Ba)


elements



•Use alternative wavelength

32



element of interest


element of interest




(ex: Ba)


elements



•Background can usually be

distinguished and subtracted

33

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


34

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


35

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”


In depth overviews

of Atomic

Spectroscopy

Concise overview

and Standard

information

www.ohio.edu/ceer

For more information

contact:

[email protected]

Atomic Absorption Spectroscopy (AAS) - Ohio · PDF fileWhat is AAS? Atomic Absorption...

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