Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

The greater spatial resolving power and the
crystallographic analysis abilities of TEM and
TEM-ED are used in some cases for standardized
workplace industrial hygiene characterizations.
TEM methods (e.g., NIOSH 7402)
are used to complement PCM in cases where
there is apparent ambiguity in EMP identification
[NIOSH 1994b] and, under the Asbestos
Hazardous Emergency Response Act of
1986, the EPA requires that TEM analysis be
used to ensure the effective removal of asbestos
from schools [EPA 1987]. Each of these methods
employs specific criteria for defining and
counting visualized fibers and reporting different
fiber counts for a given sample. These data
subsequently can be independently interpreted
according to different definitional criteria,
such as those developed by the International
Organization for Standardization (ISO), which
provides methods ISO 10312 and ISO 13794
[ISO 1995b, 1999].
Improved analytical methods that have become
widely available should be reevaluated for complementary
research applications or for ease of
applicability to field samples. SEM is now generally
available in research laboratories and
commercial analytical service laboratories. SEM
resolution is on the order of ten times that of
optical microscopy, and newly commercial field
emission SEM (FESEM) can improve this resolution
to about 0.01 µm or better, near that of
TEM. SEM-EDS and SEM- wavelength dispersive
spectrometers (WDS) can identify the elemental
composition of particles. It is not clear
that SEM- backscatter electron diffraction analysis
can be adapted to crystallographic analyses
equivalent to TEM-ED capability. The ease
of sample collection and preparation for SEM
analysis in comparison with TEM and some
of the advantages of SEM in visualizing fields
of EMPs and EMP morphology suggest that
SEM methods should be reevaluated for EMP
64
analyses, both of field samples and for research
[Goldstein 2003].
Research on mechanisms of EMP toxicity includes
concerns for surface-associated factors.
To support this research, elemental surface
analyses can be performed by scanning Auger
spectroscopy on individual particles with
widths near the upper end of SEM resolution. In
scanning Auger spectroscopy, the Auger electrons
stimulated by an incident electron beam
are detected; the energy of these secondary
electrons is low, which permits only secondary
electrons from near-surface atoms to escape
and be analyzed, thus analyzing the particle elemental
composition to a depth of only one or a
few atomic layers [Egerton 2005]. This method
has been used in some pertinent research studies
(e.g., assessing effects on toxicity of leaching
Mg from chrysotile fiber surfaces) [Keane et al.
1999]. Currently, this form of analysis is timeconsuming
and not ideal for the routine analysis
of samples collected in field studies.
Surface elemental composition and limited valence
state information on surface-borne elements
can be obtained by X-ray photoelectron
spectroscopy (XPS or ESCA), albeit not for individual
particles. XPS uses X-ray excitation of
the sample, rather than electron excitation as
used in SEM-EDS or TEM-EDS. The X-rays
excite sample atom electrons to higher energy
states, which then can decay by emission of
photoelectrons. XPS detects these element-specific
photoelectron energies, which are weak
and therefore emitted only near the sample surface,
similar to the case of Auger electron surface
spectroscopy. In contrast to scanning Auger
spectroscopy, XPS can in some cases provide
not only elemental but also valence state information
on atoms near the sample surface. However,
in XPS the exciting X-rays cannot be finely
focused on individual particles, so analysis
NIOSH CIB 62 • Asbestos