An Introduction to Flame Atomic Absorption Spectrometry FAAS

1
An Introduction toFlame Atomic Absorption
Spectrometry (FAAS)

• Steve Badger
• and
• Charity Wessel

2
Introduction

• FAAS, developed in the 1950s, is a common method
  of quantitative analysis of many elements
• FAAS is extensively used to determine trace
  quantities of elements in biological,
  environmental, clinical, geological, and edible
  samples.
• Elemental health hazards can be detected with
  0.0331 error.

3
Theory of Operation

• Structure of an atom
• Atoms consist of a nucleus and electrons.
• The nucleus contains protons and neutrons.
• The negatively charged electrons are located in
  orbitals around the nucleus.
• When electrons occupy the innermost available
  orbital, their energy is at a minimum (ground
  state).

4
Theory of Operation, continued

• When atoms are subjected to heat or some form of
  EMR, one or more electrons jump to a higher
  energy level, leaving a vacancy in the inner
  shell
• We say the electron is excited
• As this happens, energy is absorbed

5
Theory of Operation, continued

• When the excited electron in the outer orbital
  returns to the lower energy level of the inner,
  vacant orbital, energy is released in the form of
  a photon

6
Theory of Operation, continued

• Because the atoms of each element have a
  different electronic structure, each one emits
  light of different wavelengths
• In FAAS, the concentration of an element present
  in a sample can be measured by noting the
  absorbance caused by the excited sample

7
Instrument Design

• FAAS components
• Fuel (acetylene)
• Oxidizer (air, N2O)
• Hollow Cathode Lamp (HCL)
• Nebulizer
• Burner Head
• Flame
• Exhaust hood

8
Instrument Design Acetylene

• Acetylene is a flammable, compressed gas that is
  used as a fuel for FAAS.
• During FAAS operation, the valve connected to the
  acetylene cylinder is opened completely.
• Storing the tank in a lab can be extremely
  dangerous.
• That is why the tanks are supported upright with
  a metal chain.

9
Instrument Design HCL

• The HCL is a glass tube filled with an inert gas
  (neon, argon, or helium) and the pure element to
  be subjected to FAAS.
• The atoms of the element become ionized and
  excited when the FAAS is on.

10
Instrument Design HCL

• The spectrum emitted by the HCL corresponds to
  the element in the cathode
• For example, if a Cd HCL is used, the
  characteristic wavelengths of Cd are emitted

11
Do you recall emission spectra from general
chemistry?
12
7.3
13
Instrument Design HCL

• Single-element HCLs generally achieve greater
  sensitivity
• On the other hand, multi-element HCLs save money

14
Instrument Design Burner Head

• The burner head is made of titanium
• This is where the sample atoms are excited

15
Instrument Design Nebulizer

• The nebulizer changes liquid sample to a mist
• This arrow is pointing to the nebulizer

16
Instrument Design Flame

• The flame is ignited by pushing the red burner
  button on the instrument
• The maximum temperature of the air-acetylene
  flame is 3,095C

17
Typical Units of Concentration

• Parts Per Million (ppm)
• A unit of concentration often used when measuring
  levels of pollutants in air, water, body fluids,
  etc.
• 1.00 ppm is equal to 1.00 mg/liter
• And 1.00 ?g/mL

18
Typical Units of Concentration

• Parts Per Billion (ppb)
• Another typical concentration unit used when
  measuring trace levels of pollutants in air,
  water, body fluids, etc.
• 1.00 ppb is equal to 1.00 ?g/liter

19
Preparation of Standards

• Typically 4-5 standards and a blank are prepared
  to construct a calibration curve (a/k/a a
  standard curve)
• When preparing your solutions, think about the
  need for precision, low contamination, and
  minimal waste
• Standard solutions should be prepared using
  appropriate glassware and distilled water
• Consult a reference to determine appropriate
  concentrations of standards for a given element

20
Constructing a Calibration Curve

• As with other analytical techniques, FAAS
  requires careful calibration
• The absorbance of the standards are plotted
  versus concentration
• The plot often deviates from a straight line

21
Preparation of Samples

• A standard reference should be consulted to
  determine appropriate methods of preparing
  various samples for FAAS

22
Operation

• Open the valve on top of the acetylene cylinder
  completely.
• Turn on the exhaust vent using the switch on the
  hood.
• Turn on the workstation connected to the FAAS.
• Load the FAAS software.

23
Operation Optimization

• Only a few steps are required for optimization
• The software steps you through the optimization,
  because a method has already been created for
  doing so
• After optimization, you are now ready to start
  the analysis

24
Analysis

• Again, the software leads you through the
  procedure
• Sequentially aspirate the blank (distilled
  water), the calibration standards, and the
  samples

25
Evaluation of Results

• After the analyses are complete, look at the
  calibration curve printed out by the FAAS
  systems printer

26
Limitations of FAAS

• The chemical form of the analyzed element is not
  detected
• For example, if copper was being analyzed, all
  that would be known is how much copper is
  present.
• It would not be known if it is copper(II)
  cyanide, copper(III) sulfate, etc.
• The preparation of the standards and samples can
  be time consuming
• The whole FAAS procedure requires detailed work
• Some elements cannot be examined by FAAS
• The FAAS procedure destroys the sample

27
Conclusion

• FAAS is an accurate method of quantitative
  analysis for many elements