aas atomic absorption spectrophotometry. aas – widely in clinical laboratories to measure elements...
TRANSCRIPT
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