Page 11 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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550 Analytical Chemistry

linewidth less than the absorption bandwidth of the atomic TABLE III Atomic Absorption Detection Limits for Common
sample. In most cases the same element being analyzed is Analytes
also used in the source lamp to provide appropriate emis- Limit of detection (ppm)
Analytical
sion line spectra by stimulation with a plasma discharge.
wavelength Flame Nonflame
Common sources are hollow cathode lamps, electrode- a
Element (nm) absorption absorption
less discharge lamps, and gaseous discharge lamps. The
source is often modulated and coupled to a lock-in am- Aluminum 396.2 0.03 0.00001
plifier in order to determine the relative absorbance of Calcium 422.7 0.001 0.00005
the atomic sample as compared with sample emission at Cadmium 326.1 0.0005 0.000003
the same wavelength. Other methods of background cor- Iron 372.0 0.003 0.00002
rection involve monitoring of a nonabsorbed radiation by Lithium 670.8 0.0005 0.0003
the use of distinct spectral lines, continuous sources, or Magnesium 285.2 0.0001 0.000004
the Zeeman effect (separation of degenerate energy states Potassium 766.5 0.005 0.0009
of an atom by application of a powerful magnetic field; a
Data for acetylene–air flame.
provides closely spaced energy levels which are sensi-
tive to polarized light). The sample itself is usually pro-
2. Molecular Absorption
duced by volatilization of solutions in a high-temperature
flame or solids and in a graphite furnance. Flame methods The absorption spectrum commonly produced by
generally provide relative errors of approximately 1–2%, molecules is significantly more complex than that
whereas the equivalent furnace methods provide only 5– produced by atoms due to the large number of energy
10%, but sensitivities up to 1000 times greater than those states available for energy deposition. The total energy of
observed for flames. Chemical interferences commonly a molecule that has no translational velocity is given by
originate from the atmosphere supporting the atomic pop-
E total = E electronic + E vibrational + E rotational ,
ulation and from other sample matrix components. The
formation of oxides in flames causes the reduction of the where the electronic levels due to electrons in bonding and
atomic population and can be controlled by the employ- nonbonding orbitals are related to numerous interatomic
ment of appropriate fuel–oxidant ratios and by sampling vibrations and molecular rotations about a center of grav-
appropriate portions of the flame. This problem is avoided ity. The large energy difference of the three energy types
in the furnace by the use of a continuous flowing inert impliesthateachcanbeindividuallystudiedwithradiation
gas atmosphere. Poor volatilization and atomization often from distinctly different portions of the electromagnetic
occur when an analyte binds with anions in the sample spectrum.
matrix. This is overcome by the use of high concentra-
tions of cationic releasing agents, which preferentially Optical polarization. Since the early 1800s it has
couple with the interferent, or by the use of protective been recognized that certain molecules have the capacity
agents such as chelating ligands, which form stable but to rotate plane-polarized light, which has since led to the
volatile species with the analyte of interest. Spectral in- development of a number of techniques suitable for quali-
terferences can also occur, though direct overlap of inter- tative structural determination and quantitative concentra-
ferent and analyte atomic absorption bands is rare. These tionanalysis.Allthesetechniquesarebasedonthefactthat
interferences include molecular band absorption such as electromagnetic radiation has wavelike properties and can
that experienced from the formation of combustion prod- be represented as a combination of electric vectors. The
ucts, which can often be eliminated by the use of higher electric vector can interact with the electrons of matter
temperatures, and ionization of the atomic population due in an absorption–reemission process taking place over a
to excess temperatures. Since many of the chemical pro- time period of 10 −14 to 10 −15 sec. Even though this pro-
cesses occurring in certain localized areas of the atomic cess does not change the energy of the radiation, a slowing
atmosphere are approximately in equilibrium, such ion- does occur. If a monochromatic beam of radiation passes
ization can be suppressed by the addition of a radiation through an anisotropic solid sample, the electric vectors
buffer, which selectively ionizes in contrast to the ana- that encounter greater particle density will be slowed more
lyte of interest and therefore increases the probability of than the vectors passing through less dense areas. Plane-
ion–electron recombination to form the required atomic polarized light is represented as the resultant of two inter-
population. Table III presents some of the analytes com- fering electric vectors such that the resultant always lies in
monly investigated by the techniques of atomic absorption a single plane. In the case of plane-polarized monochro-
spectroscopy. matic light with two coherent perpendicular electric vector