Page 10 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 549

FIGURE 4 Analytical potential for absorption spectroscopy across the electromagnetic spectrum.

investigation. Chemical evaluation is derived from obser- concentration of the analyte, and A represents a value for
vation of the extent of interaction by measurement of the absorption.
energy and intensity of transmitted, scattered, absorbed,
or latently released radiation.
1. Atomic Absorption
Absorption is said to occur when radiation passes
through matter and interacts with the ions, atoms, or The absorption of radiation by a sample of atomic parti-
molecules constituting the sample in such a way that they cles, created by vaporizing the sample, represents a rel-
gain energy and move from a quantized low-energy state atively simple spectral situation that has great practical
to a higher energy state. Analytically, this phenomenon is value for elemental identification and concentration de-
observed as a reduction of the intensity of radiation after termination. The absorption spectrum observed with poly-
passage through a sample of matter. Two distinct param- chromatic light contains only a few areas of reduced in-
eters can be investigated. The reduction of intensity is a tensity occurring at very well-defined frequencies due to
function of the probability of interaction of the radiation the small number of energy states available to the atoms.
with appropriate analyte species and indicates concentra- The natural width of such absorption lines can be less than
tion. Also, the energy of the absorbed radiation indicates 10 −4 nm, but broadening often occurs due to collisional
the quantized energy levels in which energy is deposited and Doppler effects in the sample matrix. The quantized
(Fig. 4) and therefore assists in species identification. The energy transitions responsible for atomic absorption are
Beer–Lambert law generally applies to all absorption pro- of electronic origin, indicating that electromagnetic spec-
cesses at low concentrations when monochromatic radia- trum energies from X-rays to ultraviolet–visible rediation
tion is used. It is conventionally written are sufficient to observe this process.
The technique known as atomic absorption spec-
P 0
log = εbc = A, troscopy is of particular analytical importance for the de-
P
termination of metals due to its sensitivity and potential
where P 0 is the initial power of an incident beam of radi- for selectivity by virtue of the narrow atomic absorption
ation, P is the final power (decreased due to absorption), lines. A schematic representation of the spectrophotome-
ε represents a value characteristic of the extent of absorp- ter is shown in Fig. 5. To take advantage of characteristic
tion expected for a certain species at a defined wavelength selectivity, a special radiation source must be provided
inadefined matrix, b is the sample path length, c is the to produce extremely monochromatic radiation with a

FIGURE 5 Representation of the design of a conventional atomic absorption spectrophotometer.