Page 17 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

556 Analytical Chemistry

Properties of interest for chemical analysis are the so- This technique allows a multitude of species to be de-
called isomer shifts, which represent the displacement of termined concurrently since characteristic lines originate
resonance from zero Doppler velocity, and two types of from all suitable species simultaneously. A schematic rep-
line splitting. Splitting can be induced by interaction of resentation of a simple instrument would be equivalent
the electric field gradient about the nucleus with the elec- to that shown in Fig. 5 if the hollow cathode lamp and
tric moment of the excited nucleus and also by interaction modulation system were removed. A more energetic atom
of the nuclide magnetic dipole moment with internal or source such as an inductively coupled plasma (a zone of
external magnetic fields to produce magnetic hyperfine highly energetic ionized inert gas such as argon, which
structure. Instrumentation is based on the γ -ray source, a provides a high constant operating temperature, a long
Doppler motion device, and an energy-proportional γ -ray sample residence time, and a nonoxidating amosphere) is
detector coupled to a multichannel analyzer. Chemical usually employed. Detection limits are generally in the
information extracted from M¨ossbauer spectra indicate range observed for atomic absorption methods, though
oxidation states, sample identity with respect to mineral the two techniques are complementary, each surpassing
structure, and quantitative sample mineral compositions. the other in the analysis of certain species.
The technique is employed most often for iron and cobalt,
though over 50 nuclides have demonstrated the M¨ossbauer Atomic fluorescence spectroscopy. Fluorescence
effect. refers to a process whereby absorption and reemission
of radiation are separated temporally. A pulsed source
of high intensity such as a laser, electrodeless discharge
B. Emission of Electromagnetic Radiation
lamp, gaseous discharge lamp, or specially adapted hol-
Matter can exist in a physically excited energy level and low cathode lamp at the required resonant frequency is
can relax to a lower ground state energy by releasing the used to irradiate an atomic population created usually by
difference in energy. One mechanism of such relaxation a nonflame method. Emission can occur in any direction
can occur by emission of electromagnetic radiation. Con- and is commonly observed by standard atomic absorption
tinuous broadband radiation is observed from a thermally instrumentation set at an angle of 90 to the high-intensity
◦
excited continuous solid and results from the complicated source beam. This arrangement has been shown to im-
oscillations originating from the molecules and atoms of prove detection limits over those for atomic absorption or
the sample. If these molecules or atoms emit indepen- emission for up to 10 elements.
dently, then a discontinuous spectrum composed of dis-
tinct bands or lines can be observed. The structure of this X-ray fluorescence spectroscopy. This technique
discontinuous emission is related directly to the quantized is one of the most widely used for qualitative and quan-
electronic energy levels of molecules and atoms as dis- titative elemental determination for elements of atomic
cussed previously, and energy is released in a reversal of number greater than 8. A beam of X-rays is directed from
the absorption process. Though this can be strictly true a source such as a Coolidge tube or radioactive substance
for atomic samples, often radiationless decay either elim- to a sample. Figure 4 indicates that such radiation is suf-
inates or precedes radiative emission from molecular sam- ficient for ionization of the core electronic level in atoms
ples and provides interesting chemical information. of the sample. The process of absorption of X-rays can be
used to provide structural information about atomic po-
sitions in three-dimensional space by a technique known
1. Atomic Emission
as Extended X-ray Absorption Fine Structure (EXAFS).
Emission spectroscopy. Atomic or ionic popula- Relaxation of such excited elements occurs by the shifting
tions can be prepared by the use of high-energy sources of an electron from a higher energy level to fill the elec-
such as flames, electrical discharges, and plasmas. If the tronic vacancy. The difference in energy between the two
energy in these sources is sufficient, electronic excitation electronic levels is released as quantized fluorescence in
occurs by collision processes, and relaxation can take the the X-ray region of the electromagnetic spectrum. A large
form of discrete narrow line emission. Since the wave- number of different X-ray energies can be emitted from a
lengths of such emissions reflect the energy differences single sample simultaneously, necessitating the employ-
in the quantized electronic energy level distributions, the ment of wavelength- and/or energy-dispersive devices in
emission wavelengths are characteristic of the excited el- many cases. Wavelength dispersion is accomplished in a
ement and can be used for identification purposes. The manner analogous to that for grating monochromators,
intensity of emitted lines can be used for quantitative but in this case a collimated beam of fluorescent X-rays
analysis by comparison with calibration standard signals is allowed to impinge on a solid crystal of well-defined
recorded on film or collected by photoelectric detectors. lattice spacing d (e.g., lithium fluoride, sodium chloride,