Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

The major disadvantage of SIMS to species identification is that each element or isotope
must be tuned and analyzed sequentially. This makes the identification of a metal form highly
time-consuming and, thus, the characterization of a multiphase medium impractical.
X-Ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy or ESCA
(electron spectroscopy for chemical analysis, as it was previously known) is a classical surface,
10 to 50 Å in depth, analytical technique for determining qualitative elemental concentrations of
elements greater than He in atomic number and provides limited structural and oxidation state
information. In XPS, the high-energy (15 kV) electrons are typically produced from a dualanode
(Al-Mg) X-ray tube. The excitation or photoionization of atoms within the near surface of
the specimen emit a spectrum of photoelectrons. The measured binding energy is characteristic
of the individual atom to which it was bound. Monochromatic sources are often employed to
improve energy resolution, allowing one to infer oxidation states of elements or structure of
compounds (organic and inorganic) by means of small chemical shifts in binding energies
(Hercules, 1970). The major disadvantages of XPS for environmental speciation studies is its
poor sensitivity, especially in complex matrices and its large, 100-200 µm, spatial resolution.
The direct speciation techniques discussed above are summarized in Table AX7-1.1.3.
Indirect Approaches
A more indirect approach to speciation than the methods previously described include the
functional or operational extraction techniques that have been used extensively over the years
(Tessier et al., 1979; Tessier and Campbell, 1988; Gupta and Chen, 1975). These methods use
either a single or sequential extraction procedure to release species associated with a particular
metal within the media. Single chemical extractions are generally used to determine the
bioavailable amount of metal in a functional class: water-soluble, exchangeable, organically
bound, Fe-Mn bound, or insoluble.
In a similar approach, sequential extractions treat a sample with a succession of reagents
intended to specifically dissolve different, less available phases. Many of these techniques have
been proposed, most of which are a variation on the classical method of Tessier et al. (1979),
in which metal associated with exchangeable, carbonate-bound, Fe-Mn bound, organically
bound, and residual species can be determined. Beckett (1989), Kheboian and Bauer (1987),
and Foerstner (1987) provide excellent reviews on the use and abuse of extractions. These
AX7-16