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AAS
Atomic Absorption
Spectrophotometry
• AAS
– Widely in clinical laboratories to measure
elements such as aluminum, calcium, copper,
lead, lithium, magnesium, zinc, & other metals.
• Atomic
• Unexcited or ground state (neutral atom)
• at a low energy level
• Absorption
– Capable of absorbing radiation at a very narrow
bandwidth corresponding to its own line spectrum
• Photometry
– Original light , enters the flame, some of it is
absorbed, net decrease in the intensity of the beam
COMPONENTS OF AAS
• Essentials of an atomic absorption spectrophotometer
Kinds of burners
• Total consumption burner
– Disadvantage
• large droplets are produced in the flame
– Scatter light and thus cause signal noise
• Acoustical noise produced
– Advantage
• the flame is more concentrated, and it can be made
hotter
– Molecular dissociation
» Desirable for some chemical systems
• Premix burner
– The sample is aspirated, volatilized, and burned
– Advantage
• Larger droplets go to waste
– Producing a less noisy signal
• Pathlength through the flame of the burner is longer
– Increase the sensitivity of the measurement
– Disadvantage
• Flame is usually not as hot as that of the total
consumption burner
– Cannot sufficiently dissociate certain metal complexes
• When light leaves the flame, it is composed of
– Pulsed unabsorbed light from the lamp
– a small amount of unpulsed flame spectrum
– Sample emission
• Detector senses all light
• Tuned amplifier accept only pulsed signals
Flameless atomic absorption
• Sample cup
– Sample is placed in a depression on a carbon rod in an
enclosed chamber
– Dry, char, and finally atomize the sample into the
chamber
– atomized element then absorbs energy from the
corresponding hollow cathode lamp
• Advantage
– More sensitive
• permits determination of trace metals in small samples of
blood or tissue
Flameless atomic absorption
• Zeeman effect (Background correction)
– In an intense magnetic field
– the energy levels in the atom are shifted slightly
– The magnetic field can be pulsed alternately on
and off
– The difference between the two signals is
background-corrected absorbance
INTERFERENCE IN AAS
• Chemical
• Ionization
• Matrix effects
• Chemical interference
– The flame cannot dissociate the sample into free
atoms
– Example
• Phosphate interference in the determination of calcium
– Being solved, by
• Using a special high-temperature burner
• Adding a cation that competes with calcium for the
phosphate
• Ionization interference
– When atoms in the flame become excited
– Overcome by
• Adding an excess of a more easily ionized substance
– absorb most of the flame energy
• Reducing the flame temperature
• Matrix interference
– Enhancement of light absorption by organic
solvents
– Formation of solids from sample droplets
• Concentrations greater than 0.1 mol/L
– Refractory oxides of metals
Summary
• AAS
– Advantages
•
•
•
•
Sensitive
Accurate
Precise
Highly specific
– Disadvantage
• the problem of interferences
• Does not require excitation of the element
– Less affected by
• Temperature variations in the flame
• Transfer of energy from one atom to another
• High specificity
– Light used has an extremely narrow bandwidth
(0.01 nm)
• Selectively absorbed by the atoms being measured