Atomic absorption spectroscopy (AAS)
electrothermal atomization
-a few ul of sample is evaporated at low temp then ashed at a higher temp in electrically heated graphite tube -after ashing, current is rapidly increased and the sample is atomized in a few milliseconds -absorption of atomic vapor is measured in region immediately above heated surface -enhanced sensitivity because entire sample is atomized in short period, and average residence time of atoms in optical path is a second or more -not as precise as flame atomizer, heating and cooling takes longer, analytical range is narrow
advantages
-analyze ~70 elements -highly selective because atomic lines are very narrow (2~5 pm) -sensitive (ppm, ppb) -light emitted by atom (lamp) is absorbed by atom of same element
collisional broadening (pressure effect)
-collisions of the emitting or absorbing species with other atoms/ions in the heated medium -produce broadening that is two or three orders of magnitude greater than natural line widths
hollow cathode lamp (HCL)
-consists of tungsten anode in glass tube filled with neon or argon at pressure of 1~5 Torr -300V is applied across electrode, ionization of filler gas: Ne + e- → Ne⁺ + 2 e- -gaseous cations acquire enough KE to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud in process called splintering: M(s) + Ne⁺ → M(g) + Ne -excitation of cathode element: M(g) + Ne⁺ → M*(g) + Ne -emit photon of characteristic frequency M*(g) → M(g) + hν
electrothermal atomizer
-graphite tube open on both ends, hole for sample introduction -external gas stream prevents outside air from entering the tube -internal stream flows into both ends of tube and out sample port - excludes air and carries away vapor generated from sample matrix during first two heating stages -reducing porosity of graphite tube by coating the tube with pyrolytic carbon increases the reproducibility and gives stronger analyte signals
typical aas instrument
-monochromator: used to isolate specific wavelength of emission spectrum -detector: photomultiplier tube
line width importance
-narrow lines are important to reduce the possibility of interference due to overlapping lines -line width is important in design of instrument
disadvantages
-only 1 element can be determined at a time (element in cathode of HCL must be same as analyte element) -no information is obtained on chemical form of analyte
Beer's law
-only applies to monochromatic radiation: linewidth of radiation being measured is less than the bandwidth of the absorbing species -typical monochromator has bandwidth of 1 nm, but atomic absorption line is very narrow/sharp -need source with line width <0.01 nm
flame atomizer
-solution of sample is nebulized by a flow of gaseous oxidant, mixed with gaseous fuel, and carried into a flame where atomization occurs -desolvation occurs, solvent evaporates to produce a finely divided solid moleculer aerosol -aerosol is volatilized to form gaseous molecules, atoms, and atomic ions in equilibrium (reduce the cations) -air-acetylene flame: 2300°C -N₂O-acetylene flame: 2700°C -most reproducible of all liquid-sample-introduction methods, although the sampling efficiency and sensitivity of electrothermal atomization is better (large portion of sample flows down the drain, and residence time of individual atoms in the optical path in the flame is brief, 10⁻⁴ s)
What is aas?
-spectroanalytical technique for the quantitative determination of chemical elements employing absorption of light by free atoms in the gas phase (UV light range)
natural broadening effect (uncertainty effect)
-the energy level of an atom or ion can never be determined precisely because its lifetime ∆t is finite |∆λ½|=λ²∆ν/c -10⁻⁸ s leads to width of 10⁻⁵ nm
doppler broadening
-the wavelength of radiation admitted or absorbed by a rapidly moving atom decreases if motion is toward a transduce, and increases if the atom is receding from the transducer -frequency is offset by net speed of radiation that hits the detector
