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

material to describe the chemical or kinetic factors controlling a metal’s release, transport,
and/or exposure.
X-Ray absorption Spectroscopy (XAS). X-ray absorption spectroscopy (XAS) is a
powerful technique using the tunable, monochromatic (white light) X-rays produced by a
synchrotron (2-4 GeV) to record oscillations in atomic absorption within a few 100 GeV of an
element’s absorption edge. Spectra provide information on both chemical state and atomic
structure. Measurements are theoretically available for all elements and are not surface-sensitive
nor sample-sensitive (i.e., gases, liquids, solids, and amorphous materials are testable).
High-energy spectra within 30 eV of the edge, termed XANES (X-ray absorption near
edge structure spectroscopy (Fendorf et al., 1994; Maginn, 1998), are particularly suited for
determination and quantification (10 to 100 ppm) of metal in a particular oxidation state
(Szulczewski et al., 1997; Shaffer et al., 2001; Dupont et al., 2002). The lower-energy spectra
persist some 100 eV above the edge. These oscillations are termed EXAFS (extended X-ray
absorption fine structure) and are more commonly used for speciation analyses (Welter et al.,
1999; Manceau et al., 1996, 2000a; Isaure et al., 2002).
The main limitations to XAS techniques are (1) the lack of spatial resolution; (2) XAS
techniques provide only a weighted average signal of structural configurations, providing
information on the predominant form of the metal, while minor species, which may be more
bioavailable, can be overlooked; (3) access to synchrotrons is limited and the beam time required
to conduct a site investigation would be prohibitive; (4) a large spectral library must be
developed; (5) generally, poor fits to solution models are achieved when the compound list is
large; and (6) high atomic number elements have masking problems based on compound density.
X-Ray Diffraction (XRD). In X-ray diffraction, a monochromatic Fe, Mo, Cr, Co, W, or
Cu X-ray beam rotates about a finely powdered sample and is reflected off the interplanar
spacings of all crystalline compounds in the sample, fulfilling Bragg’s law (nλ = 2dsinθ). The
identification of a species from this pattern is based upon the position of the lines (in terms of
θ or 2θ) and their intensities as recorded by an X-ray detector. The diffraction angle (2θ) is
determined by the spacing between a particular set of atomic planes. Identification of the species
is empirical, and current available databases contain more than 53,000 compounds.
If a sample contains multiple compounds, interpretation becomes more difficult and
computer-matching programs are essential. In some instances, by measuring the intensity of the
AX7-13