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

diffraction lines and comparing them to standards, it is possible to quantitatively analyze
crystalline mixtures; however, if the species is a hydrated form or has a preferred orientation,
this method is only semiquantitative at best. Since this technique represents a bulk analysis,
no particle size or lability information can be extracted from the patterns.
Particle Induced X-Ray Emission (PIXE and µPIXE). Particle induced X-ray emission
(PIXE) uses a beam, ~4 µm in diameter, of heavy charged particles (generally He) to irradiate
the sample. The resulting characteristic X-rays are emitted and detected in a similar manner as
XRF, using Si-Li detectors. Particles generated from a small accelerator or cyclotron, with a
potential of 2 to 4 MeV, are commonly used. Detection limits on the order of 1 mg/kg are
achieved on thin-film samples. Disadvantages to its use for speciation include (1) only a small
volume of material can be analyzed (1 to 2 mg/cm 2 ); (2) no particle size information is provided;
(3) peak overlaps associated with Si-Li detectors limit identification of species; (4) limited
availability; and (5) high cost. For a further review of PIXE analysis and applications, see
Maenhaut (1987).
Electron Probe Microanalysis (EPMA). Electron probe microanalysis uses a finely
focused (1 µm) electron beam (generated by an electron gun operating at a 2 to 30 kV
accelerating voltage and pico/nanoamp currents) to produce a combination of characteristic
X-rays for elemental quantification along with secondary electrons and/or backscatter electrons
for visual inspection of a sample. Elements from beryllium to uranium can be nondestructively
analyzed at the 50-ppm level with limited sample preparation. X-ray spectra can be rapidly
acquired using either wavelength dispersive spectrometers (WDS) or energy dispersive
spectrometers (EDS).
With WDS, a set of diffracting crystals, of known d-spacing, revolve in tandem with a
gas-filled proportional counter inside the spectrometer housing so that Bragg’s law is satisfied
and a particular wavelength can be focused. Photon energy pulses reflecting off the crystal are
collected for an individual elemental line by the counter as a first approximation to
concentration. For quantitative analysis, these intensities are compared to those of known
standards and must be corrected for background, dead time, and elemental interactions (ZAF)
(Goldstein et al., 1992). ZAF correction is in reference to the three components of matrix
effects: atomic number (Z), absorption (A), and fluorescence (F).
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