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July 1999 Volume 1 Number
2
9.0.
Other Techniques
9.1. Fluorescence Microscopy
9.1.1. Fluorescence microscopy of thin or bulk cross sections,
as an aid in differentiating samples or various layers within
intact paint fragments, is discussed by Stoecklein and Tuente
(10). When using an excitation wavelength of 365 nanometers,
the technique may be sensitive to differences in organic pigments,
additives, and film-forming components. Allen (53) reports it
to be most useful with light-colored architectural coatings.
9.2. Low-Temperature Ashing
9.2.1. The low-temperature asher is a device in which an oxygen
plasma is used to remove organic materials from a complex matrix.
Materials that produce volatile oxides (principally organic components)
are removed from the matrix with minimal elevation of the sample
temperature in contrast to pyrolysis systems. Ashing usually
continues until all such volatile oxides are removed.
9.2.2. Inorganic pigments,
extenders, and some additives in the different layers of the
ashed paint film will remain after the organic material is volatilized.
The relative size and morphology of the different particles,
noted during previous tests, serve to help identify and separate
these residue components for additional analysis. A brief description
of the technique is provided by McCrone and Delly (54) and Brown
(55).
9.2.3. Ashing residues can
be analyzed by a variety of methods, including PLM, SEM-EDS,
analytical electron microscopy (AEM), or XRD techniques.
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9.3. Solvent Extraction
9.3.1. Solvent extraction can be used to separate some of the
organic components from paint films, depending on the paint system
or systems in question. The objective of the procedure is to
recover a solute that can be examined by IR, GC, or GC-MS techniques.
It is especially useful for coatings where the volume of pigments
or extenders is very high. The identification of the binders
by FTIR is often only possible after separation of the pigments
and extenders.
9.3.2. When using solvent
extraction, separation of paint layers is very important. If
this is impossible, it is important that identical conditions
(e.g., time, and temperature) be applied to both the known and
questioned samples.
9.4. Analytical Electron Microscopy
9.4.1. Analytical electron microscopy (AEM) is the term applied
to the use of transmission electron microscopy (TEM) in conjunction
with both selected area electron diffraction and EDS. The combination
of techniques can provide more definitive identification of some
pigment grains that cannot be identified conclusively by PLM
because of their extremely small size or opacity.
9.4.2. AEM requires that
the sample be sufficiently thin to permit transmission of the
analytical electron beam. It is thus applied only to dispersions
of extracted inorganic particulates, such as those recovered
from low-temperature ashing, dissolved paint layers, or ultramicrotomed
sections of a paint film (56).
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9.5. Cathodoluminescence
9.5.1. Cathodoluminescence (CL) is the emission of radiation
from the sample in the visible light region and neighboring wavelengths
following excitation by electrons where these, originating from
a cathode and accelerated in an electric field, strike upon an
insulator or semiconductor (e.g., inorganic pigments or fillers).
This phenomenon can be observed through the use of specially
equipped optical or scanning electron microscopes.
9.5.2. Cathodoluminescence
can be performed on embedded sections as an aid in differentiating
samples or various layers within intact paint fragments. The
technique has been found to be most useful for characterizing
and comparing multilayered white or beige architectural and marine
paint fragments (57).
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10.0.
References
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Federal Bureau of Investigation, Washington, DC, 1999.
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(55) Brown, R. Light and
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11.0.
Endnotes
1 Annual Book
of Standards, Vol. 06.01.
2 Annual Book
of ASTM Standards, Vol. 14.02.
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FORENSIC SCIENCE COMMUNICATIONS JULY 1999 VOLUME 1 NUMBER 2 |
