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

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