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July 1999 Volume 1 Number
2
8.0.
Procedure
8.1.
Discussions of forensic paint analysis are provided in dated
but detailed form by Crown (3), and more recently by Nielsen
(4), Thornton (5), Maehly and Stromberg (6), and Stoecklein (7).
8.2.
A reasonable scheme for forensic paint examinations is outlined
in Figure 1. A variety of potentially
useful techniques for the discrimination of paint binders, pigments,
and additives are listed. The major steps in Figure 1 correspond
to the discussions presented in this section (e.g., 8.8.
Solvent/Microchemical Tests). For any given comparison, not
all the techniques listed in Figures 1A, 1B, and 1C are necessarily
required. Sample size, condition, and layer structure complexity
should be considered when determining which techniques to use.
The forensic coatings examiner should always use the more specific
and least destructive tests prior to those that require more
sample preparation or consumption. A review of the general technique
descriptions, listed in 8.8.
Solvent/Microchemical Tests and the subsections that follow,
provides guidance for the selection of appropriate methods.
8.3. Figure 1 does not imply
that other examinations should be excluded or that the order
of the procedures in the chart is irrevocable. Samples that are
neither constrained by amount nor condition should be subjected
to analyses that will determine the color and texture of the
paint as well as the number, order, colors, and textures of the
layers in a multilayered sample. In most cases, instrumental
techniques should be employed to analyze and compare both the
pigment and binder portions of the sample. A combination of techniques,
which provides discrimination between as many types of paints
and coatings as possible, should be used. These techniques should also be selected
to provide either or both classification or component identification
information to be used in significance assessments. For samples
that are limited in layer structure complexity, techniques for
the comparison of both the binder and pigment portion of the
coating must be used. The choice of techniques may change depending
upon sample characteristics. For instance, pyrolysis gas chromatography
(PGC) may be used for identifying and comparing the binder portion
of samples that exhibit a low binder concentration. Likewise,
scanning electron microscopy-energy dispersive X-ray analysis
(SEM-EDS), X-ray fluorescence (XRF), and X-ray diffraction (XRD)
may be used for identifying and comparing the pigment portion
of samples that exhibit a low pigment concentration.
8.4.
The flowchart in Figure 2 is a guide
to the determination of the possible origins of a motor vehicle
paint in an investigative case. It is usually possible to differentiate
motor vehicle repaint from the original equipment manufacturer
(OEM) paint by microscopical examination. If no OEM paint is
present, then only the vehicle color can be reported. For OEM
paint, the color of the topcoat layers and of the undercoat layers
will each be useful in identifying manufacturer, model, and year.
Often the two systems provide complementary information. In most
cases a range of possible makes, models, and years will be generated
by the search. Further specific information can often be developed
through chemical analysis of the individual layers. Any of the
techniques shown in Figure 1 can be used, depending on the databases
available. Reference collections and databases may fall into
one of the following categories: books of color chips produced
by automotive refinish paint manufacturers for use by body shops
and automotive repair facilities, manufacturer topcoat and undercoat
colors, chemical standards, street samples collected from damaged
motor vehicles, OEM information on paint formulations, and collections
of infrared spectra or pyrograms of known paints. Examples of
databases include the Royal Canadian Mounted Police/Scientific
Working Group on Materials Analysis (RCMP/SWGMAT) database, the
National Automotive Paint File maintained by the Federal Bureau
of Investigation, the Collaborative Testing Services reference
collection of automotive paints, and the Georgia Bureau of Investigation
Paint Library of infrared spectra.
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8.5. Sample Description
8.5.1. The first step in forensic paint analysis is the visual
and macroscopical evaluation, description, and documentation
of the original condition of the sample or samples. Occasionally,
this can be the final step in an analysis if exclusionary features
or conditions in the sample or samples are identified during
the initial evaluation.
8.5.2. The initial evaluation
should begin with a critical review of the chain of custody,
package sealing, and identification markings of the sample or
samples and any potential cross-contamination between samples.
If the items are found to be suitable for further evaluation,
a detailed accounting and description of the paint fragments
and any commingled material should be documented. This involves
describing the general condition, weathering characteristics,
size, shape, exterior colors, and major layers present in each
sample. This description can be accomplished by examining each
item using a stereo microscope.
8.5.3. Written descriptions,
sketches, photography, or other imaging methods must be used
to document each sample's characteristics. The goal is to produce
documentation that will be meaningful to a reviewer in the absence
of the recording examiner. The resulting notes must be sufficient
to document the conclusions reached in the examiner's report.
Although documentation is discussed at this point in the guidelines,
it is an essential part of all steps in an analysis.
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8.6. Physical Match
8.6.1. The most conclusive type of examination that can be performed
on paint samples is the physical matching of samples. This may
involve the comparison of edges and surface striae between samples
or the comparison of surface striae on the underside of a questioned
paint fragment to those of a parent surface. Either or both the
edges or striae in question must possess unique characteristics.
8.6.2. Physical matches must
be documented with descriptive notes. Photography, photo transparency
overlays, or other appropriate imaging techniques may be a useful
adjunct. When imaging methods are used to document a physical
match, the examiner must ensure that the method employed is dimensionally
accurate and has associated measuring scales. Images must be
well-preserved and retained as part of the documentation.
8.7. Sample Preparation
and Layer Analysis
8.7.1. The layers in a paint film are identified by viewing the
sample edges at magnifications ranging between 5× and 100×.
The obvious layers are generally visible without sample preparation.
Definitive paint layer identification usually requires sample
preparation techniques such as manual or microtome sectioning,
edge mounting and polishing, or both. A combination of techniques
may be required to fully characterize the layer structure. The
extent of sample manipulation and preparation will depend on
the amount of paint available and its characteristics.
8.7.2. Paint layer structure
can be observed by using a scalpel blade to make an oblique cut
through a sample. The larger surface area created by this angled
cut may enhance layer visualization and assist in the detection
of layer inhomogeneities. The preparation of thin-sections and
the separation of paint layers can be accomplished with a scalpel
blade. Preliminary solvent tests can be conducted on the manually
prepared sections and layer fractions.
8.7.3. Subtle differences
in color, pigment appearance, surface details, inclusions, metallic
and pearlescent flake size and distribution, and layer defects
may require microscopical comparisons of the edge, oblique cut,
and surface views of known and questioned paint samples. These
comparisons must be carried out with both samples positioned
side by side and in the same field of view.
8.7.4. Cross sections (embedded
or thin-section preparations) may provide additional information
as to the layer sequence, layer thickness, color, pigment distribution,
pigment size, and composition of the individual layers that may
not be possible to obtain with gross examination. Embedded preparations
can be prepared by polishing, microtomy, or both. Thin-sections
can be prepared using a variety of microtomy techniques. Examination
and analysis of the cross sections can be conducted using a variety
of analytical techniques that may include light microscopy, UV-visible
microspectrophotometry, infrared microspectrophotometry, and
electron microscopy. Laing et al. (8), Allen (9), and Stoecklein
and Tuente (10) offer a concise discussion of thin-section paint
analysis.
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8.8. Solvent/Microchemical Tests
8.8.1. Solvent/microchemical tests have long been used for attempting
to discriminate between paint films of differing pigment and
binder composition that are otherwise similar in visual and macroscopical
appearance. They have been described previously in the general
references for this section (Subsection
8.1.) . The tests are based not only on dissolution of paint
binders but also on pigment and binder color reactions with oxidizing,
dehydrating, or reducing agents.
8.8.2. Solvent/microchemical
tests are destructive and should be applied first to known samples
in order to evaluate their efficacy to a specific case, and they
should be used only in situations in which an adequate questioned
sample is available.
8.8.3. Solvent/microchemical
examinations should be applied to both questioned and known materials
concurrently. The effects of various tests are recorded immediately
and then at reasonable intervals for the duration of each test.
It is desirable to apply such tests not only to intact paint
films but also to peels of each individual layer to avoid interaction
with neighboring layers and to observe the dissolution process
more critically.
8.8.4. Reactions such as
softening, swelling, curling or wrinkling, layer dissolution,
pigment filler effervescence, flocculation, and color changes
are some of the features that may be noted. The results of these
tests are inherently difficult to quantify. Therefore, they are
primarily used for preliminary classification and comparison.
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8.9. Polarized Light Microscopy (PLM)
8.9.1. PLM is appropriate for the examination of layer structure
as well as either or both the comparison or identification of
particles present in a paint film including pigments, extenders,
additives, and contaminants. Extenders and other components of
a paint film are generally of sufficient size to be identified
by their morphology and optical properties using this technique.
Although some pigment particles are too small for definitive
identification by this method, exclusionary features may still
be evident between samples.
8.9.2. Suitable samples for
examination by PLM include but are not limited to thin peels,
thin-sections, pyrolysis and low-temperature ashing residues,
sublimation condensates, and dispersed particles in a solvent
oil or other mounting medium.
8.9.3. The use of PLM for
the identification of paint components requires advanced training
and experience. Preparation and identification of paint components
by PLM are discussed by McCrone (11) and Kilbourn and Marx (12).
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8.10. Vibrational Spectroscopy
8.10.1. Infrared spectroscopy (IR) may be used to obtain information
about binders, pigments, and additives used in various types
of coating materials. Because the paint fragments to be analyzed
are often quite small, a beam condensing or focusing device is
normally required, and a Fourier transform infrared (FTIR) spectrometer
is recommended. Both transmittance and reflectance techniques
may be used for the analysis of coatings, but in most cases,
transmittance methods are preferred because all the sampling
wavelengths are subjected to the same pathlengths and most of
the reference data of coatings, binders, pigments, and additives
consist of transmittance spectra. In addition, transmittance
data are not significantly affected by collection parameters
such as type of refractive element used, angle of incidence chosen
for analysis, or the degree to which the sample makes contact
with the refractive element. These factors affect spectra obtained
using internal reflectance methods.
8.10.2. If a multiple-layer
coating system is to be subjected to an infrared examination,
optimal results can be obtained if each layer is isolated and
analyzed separately. Methods that use solvents to assist in the
sample preparation should be used with caution because they might
alter the sample or result in the production of residual solvent
spectral absorptions.
8.10.3. An infrared microscope
accessory permits the analysis of a small sample or a small area
of a sample. Samples of individual layers may be prepared manually
using scalpels, blades, needles, forceps, or other similar tools.
Peels or sections can be placed on a salt plate or appropriate
mount for analysis. The infrared microscope accessory may also
be used to sequentially sample individual layers of a multiple-layer
coating system that has been cross-sectioned either manually
or by microtome. Generally, it is desirable to press such a sample
after sectioning to produce a wider width for each layer and
to produce a more uniform thickness. The aperture for an individual
layer should be chosen so that its edges are as far from the
adjacent layers as practicable. This minimizes the amount of
stray light produced by diffraction that may be detected. All
spectra of individual layers should be examined to determine
if absorptions of adjacent layers are contributing to the spectrum.
8.10.4. Certain types of
coatings, including automotive undercoats and many types of architectural
coatings (particularly those with low luster finishes), usually
contain significant amounts of inorganic pigments. These pigments
tend to have most of their significant infrared absorptions in
the lower frequency spectral regions, and several have all of
their absorptions in the region below 700 cm-1. An
FTIR spectrometer equipped with cesium iodide (CsI) optics and
a deuterated triglycine sulfate (DTGS) detector can collect spectral
data to 220 cm-1. The DTGS detector is less sensitive
than the mercury cadmium telluride (MCT) detector used with IR
microscopes, and the DTGS detector also requires a longer time
to acquire each spectrum. CsI optics also have the disadvantage
of lower energy throughput compared to potassium bromide (KBr)
optics. Thus, a far-IR instrument requires longer analysis times.
8.10.5. Transfers of coatings
resulting in smears on various substrates may be sampled in situ
using an appropriate attenuated total reflectance (ATR) accessory
or an ATR objective for an infrared microscope. As a control,
the substrate itself (not a metal) should also be analyzed to
verify that its absorptions are not contributing to the spectrum
of the smear. Any contributions from the substrate should be
considered. If the substrate is a metal or highly reflecting,
it may be possible to obtain a reflection-absorption spectrum
of the smear using the reflectance mode of an infrared microscope
accessory. This produces a double-pass transmittance spectrum
of the material, and a background spectrum of the substrate itself
(or uncoated mirror) should be used as a reference.
8.10.6. General information
about the forensic analysis of coatings using infrared spectroscopy
is discussed by O'Neill (13), Suzuki (14), and Ryland (15). Forensic
infrared microsampling of coatings using a beam condenser is
described by Tweed et al. (16), Rodgers et al. (17), and Schiering
(18). Analyses using infrared microspectroscopy are described
by Wilkinson et al. (19), Allen (20), Bartick et al. (21), and
Ryland (15). The identification of specific binders, pigments,
and additives using infrared spectroscopy is described by Rodgers
et al. (17, 22, 23), Norman et al. (24), Ryland (15), and Suzuki
et al. (25-27). Infrared spectral data for a number of binders,
pigments, additives, and solvents are presented in a compilation
produced by the Federation of Societies for Coatings Technology
(28).
8.10.7. Raman spectroscopy can also
be used to obtain information about binders, pigments, and additives
used in coatings. Because this technique is based on light scattering
rather than absorption, Raman spectra provide information that
is complementary to that produced by infrared spectroscopy. Some
paint components, for example, may give rise to both infrared
absorption bands and Raman bands, but the relative absorption
or scattering intensities of these bands will differ significantly
between the two techniques. Other paint components may have vibrational
modes that produce no infrared absorption bands but may produce
Raman bands. In addition, Raman spectroscopy can be useful for
the analysis of inorganic pigments and additives because it,
like far-infrared spectroscopy, can provide information about
low-frequency vibrational transitions.
8.10.8. In most cases, Raman
instrumentation using near-infrared lasers will be needed to
avoid strong fluorescence produced by various paint components.
Because near-infrared excitation produces considerably weaker
Raman scattering than visible excitation, dispersive instruments
equipped with diode array detection systems or Fourier transform
Raman spectrometers are recommended. Some applications of Raman
spectroscopy for the analyses of coatings are discussed by Kuptsov
(29) and Claybourn et al. (30).
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8.11. Pyrolysis Gas Chromatography
(PGC)
8.11.1. Pyrolysis gas chromatography (PGC) is a destructive technique
that uses pyrolytic breakdown products to compare paints and
to identify the binder type. As noted by Burke et al. (31), Fukuda
(32), Ryland (15), and Cassista and Sandercock (33), this method
of analysis may offer improved discrimination of chemically similar
paints. Several pyrolysis systems and techniques are available
to the forensic scientist and are discussed in overviews by Blackledge
(34), Challinor (35), Saferstein and Manura (36), Irwin (37),
Wampler (38), Freed and Liebman (39), and Liebman and Wampler
(40).
8.11.2. Pyrograms, the chromatograms
of the pyrolytic products, are influenced by numerous sample
characteristics and instrumental parameters. These may include
sample size, shape and condition, ramping rates, final pyrolytic
temperature, type of capillary column or columns, gas-flow rates,
temperature programs, and detector type or types. The resulting
patterns of peaks in the known and questioned sample pyrograms
are used for comparison purposes. If pyrolysis and chromatographic
conditions are kept constant over time, then PGC can be used
as an aid in the characterization of binder types by comparison
with pyrograms of paints or resins from a reference collection.
8.11.3. The applicability
of this technique depends on the paint type, layer complexity,
and the amount of sample consumption that can be tolerated. PGC
analysis may be conducted with as little as 5-10 micrograms of
sample. Run times are typically 20-40 minutes in duration. PGC
is best applied to individual paint layers for purposes of both
binder classification and comparison. With multilayered paint
samples, layer thickness variations, sample orientation in the
pyrolysis accessory, and incomplete pyrolysis make reproducible
pyrograms more difficult to obtain.
8.11.4. The user must ensure
that reproducibility is maintained and that there is no sample
carryover between runs. The necessity and frequency of replicate
and blank runs must be established for each system and sample
type.
8.11.5. The identification
of pyrolysis products may be accomplished by pyrolysis gas chromatography-mass
spectrometry (PGC-MS). Besides the detection of binder components,
the reconstructed total ion chromatogram may contain information
about additives, organic pigments, and impurities. McMinn et
al. (41) and Challinor (42) provide discussions of mass spectrometric
detection for PGC.
8.11.6. Information about
the binder composition of some samples can be increased if the
paints have been derivatized during pyrolysis. The use of derivatizing
reagents such as tetramethyl-ammonium-hydroxide (TMAH) is discussed
by Challinor (43-45).
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8.12. Microspectrophotometry
8.12.1. Color analysis has a long history in the pigment, paint,
dyestuff, and fabric industries and has led to numerous approaches
to color measurement and description. Absorption spectroscopy,
to discriminate the color of visually similar or metameric paint
samples is discussed by Cousins (46). Colors can be described
by systems such as those of Munsell and the Commission International
de l'Eclairage (CIE), as described by ASTM Standard Method D
1535 and Test Method E 308. These systems can be used to classify
colors for database systems, but usually absorption spectra of
known and questioned samples are directly compared in forensic
color comparisons.
8.12.2. Microspectrophotometry
may be required to provide objective color data for paint comparison
because of the typically small size of samples. The technique
can be applied to the outer surfaces of paint films by diffuse
reflectance (DR) measurements with visible spectrum illumination.
8.12.3. Diffuse reflectance
measurements of paint surfaces are affected profoundly by surface
conditions such as weathering, abrasion, contamination, and texture.
This fact can provide useful discriminating information when
an examiner is faced with distinguishing different surfaces that
were originally painted with the same paint formulation. Careful
reference sampling is essential to the success of color comparisons
of such surfaces.
8.12.4. Diffuse reflectance
techniques can also be used on the edges of thin paint layers
much as it is on outer paint surfaces. Before analysis, questioned
and known samples can be mounted side by side on edge and polished
to a smooth surface using a polish of 3-micron grit size or less.
Microtomed samples without surface defects may be used without
polishing. The requirement for consistent surface finish characteristics
for all samples is achieved easily if the known and questioned
samples are mounted and prepared in a single mount.
8.12.5. When required for
the discrimination of similarly colored paint layers, the surface
finish of a polished sample must approach the size of the smallest
pigment particles present.
8.12.6. Comparison of paint
layers by transmission microspectroscopy of thin cross sections
offers a more definite form of color analysis for these samples,
compared to reflectance techniques. Transmission microspectroscopy
demands the most care in preparation. Consistent sample thickness
and choice of measurement size and location are essential for
meaningful comparisons. Although thin cross sections can be manually
prepared, improved reproducibility can be achieved using a microtome.
Even when using a microtome, the slice thickness, blade angle,
cutting speed, lubrication, and mounting block stiffness or resilience
must be selected and controlled carefully. A discussion of these
parameters is presented by Derrick (47).
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8.13. Scanning Electron Microscopy
8.13.1. Scanning electron microscopy-energy dispersive X-ray
analysis (SEM-EDS) can be used to characterize the morphology
and elemental composition of paint samples. The SEM rasters an
electron beam over a selected area of a sample, producing emission
of signals including X-rays, backscattered electrons, and secondary
electrons. Emitted X-rays produce information regarding the presence
of specific elements, and the electron signals produce compositional
and topographical visualization of a sample.
8.13.2. X-rays are produced
as a result of high-energy electrons creating inner-shell ionizations
in sample atoms, with subsequent emission of X-rays unique to
those atoms. The minimum detection limit under many conditions
is 0.1 percent. Elements with atomic numbers greater than or
equal to 11 are customarily detectable. Detection of elements
with atomic numbers greater than or equal to 4 is possible using
a detector with an organic film window or a windowless detector.
Analysis can be performed in a rastered beam mode for bulk-layer
analysis or static beam (spot) mode for individual particle analysis.
Goldstein et al. (48) present a general treatment of all aspects
of SEM and X-ray microanalysis.
8.13.3. Comparison of the
composition of layers is generally performed by a nonquantitative
method, such as direct spectral comparison or peak ratioing.
Because accurate quantitative EDS requires sample homogeneity
to a level of several microns, quantitative methods are not generally
used for paint analysis. In order to produce a representative
spectrum of a paint layer, the summing of spectra from multiple
areas or the use of a beam raster area larger than 30 square
microns is often required. The homogeneity of modern household
paints and their examination by SEM-EDS is discussed by Gardiner
(49).
8.13.4. The analysis of individual
pigment particles in paint layers by static beam (spot) analysis
can be useful. However, variations in the absorption and fluorescence
factors caused by particle size and shape and fluorescence contributions
from adjacent particles preclude the application of small, single-particle,
X-ray correction routines.
8.13.5. The depth from which
X-rays are produced (the analytical volume) is dependent upon
beam energy, composition and density of the sample, and energy
of the X-rays. Generally, the primary X-ray spatial resolution
obtained in the analysis of paint systems is less than 10 mm.
Secondary X-ray fluorescence further enlarges the analytical
volume beyond the scanned image area visible in the SEM image.
Optimal results are obtained from samples prepared in cross section,
either by microtomy or polishing. The low bulk density and low
average atomic number of organic polymer-based paint layers make
them susceptible to electron and X-ray penetration that can yield
analytical X-ray contributions from adjacent layers. Care must
be taken to ensure that the EDS data generated are representative
only of the paint layer of interest or that any adjacent layer
contributions are reproducible.
8.13.6. Additional sample
preparation methods such as thin peels or stair stepping may
be used. Stair-step sample preparation and analysis allows larger
areas to be analyzed and possibly avoids inhomogeneity concerns
but faces the potential for penetration into the underlying layer
and difficulties in obtaining flat analytical surfaces and reproducible
layer thicknesses. If samples are prepared as single-layer peels,
the concerns of penetrating or sampling adjacent layers or both
are avoided. Elements in low concentrations may not be as readily
detected in these thin peels without longer count times.
8.13.7. Mapping of elements
across the cross section of a multilayer paint can be useful
for explaining or demonstrating elemental distributions and elemental
associations. However, elemental maps are generally not quantitative
and may lack the sensitivity to demonstrate minor sample differences.
8.13.8. Because a wavelength
dispersive spectrometer (WDS) generally has better spectral resolution,
lower detection limits, and superior light element detection
capability than EDS, its use can supplement EDS to more completely
characterize the elemental composition of paints. For example,
WDS may resolve overlapping Ti K and Ba L lines, which is not
possible by EDS. Because WDS has critical X-ray focusing requirements,
the sample analyzed must generally be flat, and the analysis
area must generally be smaller than that allowed for EDS. Goldstein
et al. (48) present a complete discussion of wavelength and energy
dispersive spectrometers.
8.13.9. The elemental composition of paint smears that cannot
be lifted from a substrate can often be estimated by subtraction
of the substrate's X-ray spectrum from the combined smear-substrate
spectrum. However, commingling of the smeared paint with substrate
surface contaminants, the low mass of the smear, and typical
inhomogeneity of paint can produce significant deviations of
the smear spectrum from that of the original paint.
8.13.10. Atomic number contrast images are produced in the SEM
by the collection of backscattered electrons. These images are
used to characterize and compare the structure of paints, including
layer number, layer thickness, distribution and size of pigment
particles, and the presence of contaminants.
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8.14. X-Ray Fluorescence Spectrometry
(XRF)
8.14.1. XRF is an elemental analysis technique based upon the
emission of characteristic X-rays following excitation of the
sample by an X-ray source. XRF analysis is less spatially discriminating
than SEM-EDS because of its larger analytical beam size and the
greater penetration depth of X-rays compared to electrons. However,
the limits of detection for most elements are generally better
than for SEM-EDS, and the higher energy X-ray lines produced
by higher energy excitation typical of XRF can be useful during
qualitative analysis.
8.14.2. Because of the significant
penetration depth of the primary X-rays, XRF analysis will generally
yield elemental data from several, if not all, layers of a typical
multilayer paint fragment simultaneously. Because variations
in layer thickness may cause variations in the X-ray ratios of
elements present, this technique can be used only comparatively
or qualitatively. Fischer and Hellmiss (50) present a general
discussion of the forensic applications of X-ray fluorescence.
Howden et al. (51) discuss XRF analysis as applied to single-layer
household paints.
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8.15. X-Ray Diffraction (XRD)
8.15.1. XRD is a nondestructive technique for the identification
of the crystal form of pigments and extenders or fillers. This
method is usually not suitable for the identification of organic
pigments. X-ray diffraction techniques for the analysis of paint
compounds are discussed by Snider (52).
8.15.2. XRD instruments usually
employ a copper target X-ray tube to generate the X-ray beam
and a diffractometer to measure both the diffraction angles and
peak intensities characteristic of the crystal structure. Beam/sample
geometry is critical in producing the correct diffraction pattern.
8.15.3. Commercially available
databases of diffraction patterns of crystalline materials can
be used to facilitate qualitative analysis. Because the diffraction
pattern of a mixture may be difficult to interpret, the identification
of each component may require information provided by other analytical
techniques such as elemental analysis.
8.15.4. Most paints need
no sample preparation. Surface contaminants (e.g., sand particles),
however, should be removed. Individual layer analysis is preferred
over multilayer or bulk analysis in order to associate components
to their respective layers.
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