An Introduction toFlame Atomic

Absorption Spectrometry (FAAS)

Steve Badger and

Charity Wessel

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.

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).

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

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

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

Instrument Design• FAAS components:

– Fuel (acetylene)

– Oxidizer (air, N2O)

– Hollow Cathode Lamp (HCL)

– Nebulizer – Burner Head– Flame– Exhaust hood

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.

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.

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

Do you recall emission spectra from general

chemistry?

7.3

Instrument Design: HCL

• Single-element HCLs generally achieve greater sensitivity

• On the other hand, multi-element HCLs save money

Instrument Design: Burner Head

• The burner head is made of titanium

• This is where the sample atoms are excited

Instrument Design: Nebulizer

• The nebulizer changes liquid sample to a mist

• This arrow is pointing to the nebulizer

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 3,095°C

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/literAnd 1.00 g/mL

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

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

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

Preparation of Samples

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

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.

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

Analysis

• Again, the software leads you through the procedure

• Sequentially aspirate the blank (distilled water), the calibration standards, and the samples

Evaluation of Results

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

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

Conclusion

FAAS is an accurate method of quantitative

analysis for many elements