Transcript Document
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