1. Scope
|
1.1.
|
This guideline describes methods for determining the concentrations
of major, minor, and trace elements in glass fragments. The
methods described may be used to measure either absolute or
relative element concentrations in glass fragments ranging in
mass from gram(s) to less than one microgram. |
|
1.2.
|
The analytical considerations for the use of scanning electron
microscopy-energy dispersive X-ray spectrometry (SEM-EDS), X-ray
fluorescence spectrometry (XRF), inductively coupled plasma-optical
emission spectrophotometry (ICP-OES), and inductively coupled
plasma-mass spectrometry (ICP-MS) are described. Other analytical
techniques, such as atomic absorption spectrophotometry may
also be used but are not specifically included in this guideline
because they are not as widely used as those listed. |
|
1.3.
|
Several of the analytical methods described are destructive.
Therefore, all nondestructive examinations must be completed
and legal considerations concerning the destruction of evidence
must be satisfied prior to conducting these measurements. |
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1.4.
|
This guideline does not purport to address all of the safety
problems, if any, associated with its use. It is the responsibility
of the user of this guideline to establish appropriate safety
and health practices and to determine the applicability of regulatory
limitations prior to use. |
|
1.5.
|
This guideline provides general considerations, rather than
detailed instrumental operating instructions. Measurement of
element concentrations in glass fragments can be reliably obtained
using any of many makes and models of instruments, each with
its own specific instructions. The examiner should use this
guideline in conjunction with instructions provided by the manufacturer
of the particular instrument being used and with validated internal
laboratory procedures for the characterization of glass evidence. |
2. Reference Documents
|
2.1.
|
Scientific Working Group for Materials Analysis Documents
Trace evidence recovery guidelines
Quality assurance guidelines
|
|
2.2.
|
American Society for Testing and Materials Standards
D1193 Specifications for Deionized Water
E50 Reagent Purity
E135 Standard Terminology Relating to Emission Spectroscopy
|
|
2.3.
|
Environmental Protection Agency Test Method
Method 200.7 Inductively Coupled Plasma-Atomic Emission
Spectrometric Method for Trace Element Analysis of Water
and Wastes
|
3. Terminology
Analytical blank is a solution containing all reagents in
the proportions used to prepare glass samples, processed in the
same manner as a glass sample without the presence of the glass.
Analytical curve is the functional relationship between instrument
response and analyte concentration.
Analytical sample is that portion of a specimen analyzed.
Calibration standards are a series of known standards used
for calibration of the instrument (i.e., preparation of the analytical
curve). A calibration standard containing zero added concentrations
of analytes is referred to as a calibration blank.
Calibration verification standard is a single-element
or multielement standard of known concentrations, obtained from
a different source than those used for the calibration, used to
monitor and verify instrument performance on a daily or case-by-case
basis.
Classification is the placement of a specimen into a particular
product-use or manufacturer source category based upon the comparison
of measured attributes with a database of known attributes for each
category. Examples of classes include sheets, containers, light
bulbs, tableware; or Libby-Owens-Ford, Pilkington, Corning; or float
line A, float line B from a given manufacturing plant.
Dead time in X-ray spectrometry is the amount of time that
a detector is receiving a signal that is not being counted.
Discrimination is the ability to distinguish between two
glass objects in the same class based on comparison of their measured
attributes.
Escape peak in energy dispersive X-ray spectrometry
is a spurious peak whose energy is equal to the difference in energies
between an analyte element's characteristic X-ray and a detector
photon, such as SiK.
Internal standard is an element or isotope either inherent
in or added to samples and calibration standards at a known concentration.
It is used to correct for differences in sensitivity between samples
or among samples and standards.
Limit of detection (solution methods) is the analyte concentration
equivalent of a signal that is equal to three times the standard
deviation of a series of ten replicate measurements of an analytical
blank signal.
Limit of detection (X-ray methods) is the analyte concentration
equivalent of a signal that is equal to three times the square root
of the background in the energy region of the spectrum for the analyte
of interest.
Linear dynamic range is the concentration range over which
the analytical curve remains linear.
Relative standard deviation (RSD) is the standard deviation
divided by the mean.
Sensitivity is the slope of the analytical curve (i.e., the
increase in analytical response corresponding to an increase in
one unit of analyte amount, either in mass or concentration units).
Units of response and analyte amount must be stated.
Specimen is a portion of a glass object available for examination.
Take-off-angle in X-ray spectrometry is the angle formed
between the surface of the specimen and the line from the average
point of origin of fluorescent X-rays to the center of the detector.
4. Summary of Guideline
|
4.1.
|
This guideline describes several techniques for determining
selected major, minor, and trace elements in glass fragments.
Each technique described may either be used to determine the
concentrations of elements in a glass fragment or to determine
the concentrations of several elements relative to each other.
A brief introduction to the principles, analytical methodologies,
uses, and advantages and limitations of scanning electron microscopy-energy
dispersive X-ray spectrometry, energy dispersive X-ray fluorescence
spectrometry, inductively coupled plasma-optical emission spectrophotometry,
and inductively coupled plasma-mass spectrometry in forensic
glass examination is presented. The reader is encouraged to
see the cited references and bibliography for more specific
information. |
5. Significance and Use
|
|
The concentrations of certain elements in glass serve to chemically
characterize its source. The concentrations of several elements
are intentionally controlled by the manufacturers to impart
specific end-use properties to a particular glass product, and
in some instances can be used to identify the product type of
a recovered glass fragment. However, even individual glass objects
that have major element concentrations within the manufacturer's
acceptable ranges display variations that can be measured and
provide useful points for a forensic comparison. Glass manufacturers
generally do not control the concentrations of trace elements,
except as needed to impart color or to keep them below levels
that would impart undesirable physical or optical properties
to the glass. The differences in concentrations of manufacturer-controlled
elements or uncontrolled trace elements may be used to differentiate
sources when the variation among objects exceeds the variation
within each object. Element concentrations may be used to differentiate
among glasses made by different manufacturers, glasses from
different production lines of a single manufacturer, specific
production runs of glass from a single manufacturer, and in
some instances individual glass objects produced at the same
production facility. |
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|
The discrimination potential of element concentrations in
glass was documented as early as 1973. Several instrumental
methods have been used by forensic scientists including neutron
activation analysis (Coleman and Goode 1973), flameless atomic
absorption (Hughes et al. 1976), inductively coupled plasma-optical
emission spectrophotometry techniques (Hickman 1987; Koons et
al. 1988), energy dispersive X-ray fluorescence spectrometry
(Andrasko and Machly 1978; Reeve et al. 1976), scanning electron
microscopy-energy dispersive X-ray spectrometry (Ryland 1986),
and inductively coupled plasma-mass spectrometry (Haney 1977;
Parouchais et al. 1996). |
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|
Comprehensive reviews of the literature on the application
and advantages and limitations of several of the analytical
techniques used for the elemental analysis of forensic glass
samples have been reported (Almirall 2001; Buscaglia 1994). |
|
Several factors that can be considered
in selecting the appropriate analytical method for the analysis
of glass in the forensic laboratory are shown in the following
table. |
Table 1: Analytical Methods for Glass Analysis
Click
here to see Table 1
6. Sample Handling
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6.1. |
Sample Preparation |
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The selection of a sample preparation technique
will depend on the particular method to be used for analysis,
fragment size and shape, and the purpose of the examination. |
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6.2 |
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All samples must be cleaned prior to elemental
analysis to remove surface contamination or residual material
from previous analytical determinations. Cleaning may include
washing samples with soap and water, with or without ultrasonication,
and rinsing in water, followed by rinsing in acetone, methanol,
or ethanol, and drying. Soaking in various concentrations
of nitric acid for 30 minutes or longer and rinsing with deionized
water and ethanol prior to analysis removes most surface contamination
without affecting the measured concentrations of elements
inherent in the glass. For very small fragments sampled by
laser ablation, preablation may substitute for cleaning
(see Section 6.5 in the Scientific
Working Group for Materials Analysis Collection, Handling,
and Identification of Glass).
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|
6.3. |
In order to assess the variability within
each glass source, multiple analytical samples should be selected
wherever possible. The size and number of samples selected
for analysis will depend upon the analytical technique, interpretive
criteria, and size of the fragments available for analysis.
In general, measurement precision decreases with decreasing
sample size.
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7. Analysis
7.1. |
Scanning Electron Microscopy with X-Ray Spectrometry
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7.1.1. |
Instrument description and operating
principle |
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7.1.1.1
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In scanning electron microscopy, a focused
electron beam is scanned over the surface of a sample, causing,
among other things, the emission of X-rays. The wavelengths
or energies of the detected X-rays are used to identify the
elements, and the intensities of the X-ray peaks in the measured
spectrum correlate with the quantities of each element present
in the sample area exposed to the electron beam.
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7.1.1.2.
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Two methods of X-ray detection can be defined
by the manner in which the data are collected and displayed.
The methods described are wavelength dispersive X-ray spectrometry
(WDS) and energy dispersive X-ray spectrometry (EDS).
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7.1.1.2.1. |
Wavelength dispersive X-ray spectrometry
sorts the generated characteristic and background X-ray emissions
by their wavelengths using a crystal monochromator. Wavelength
dispersive X-ray spectrometry offers the best spectral resolution
of all of the X-ray emission methods. Due to the high resolution
and low background, the lowest levels of detection and most
reliable quantitation are attainable. Cost and complexity of
instrumentation have limited its use with scanning electron
microscopy in forensic science laboratories. For this reason,
scanning electron microscopy-wavelength dispersive X-ray spectrometry
is not discussed in further detail in this guideline. |
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7.1.1.2.2.
|
Energy dispersive X-ray fluorescence spectrometry
sorts the generated characteristic and background X-ray emissions
by their energies. Energy dispersive X-ray fluorescence spectrometry
offers fast, simultaneous data collection, and the cost of
instrumentation is significantly lower than that of wavelength
dispersive X-ray spectrometry. Energy dispersive X-ray spectrometry
is most commonly used with scanning electron microscopy in
forensic laboratories.
|
|
|
7.1.1.3. |
Because a scanning electron microscope
can scan a focused electron beam over a small area, scanning
electron microscopy-energy dispersive X-ray spectrometry has
the ability to detect the presence of elements in a small sample,
such as a questioned glass fragment. |
|
|
|
|
7.1.2. |
Uses of scanning
electron microscopy-energy dispersive X-ray spectrometry in
glass examination |
|
|
7.1.2.1. |
In most published studies, scanning
electron microscopy-energy dispersive X-ray spectrometry measurements
of element intensity ratios have been applied to classification
of glass types (Ryland 1986; Terry et al. 1982). An analytical
scheme that combines measurement of Ca/Mg intensity ratios obtained
using scanning electron microscopy-energy dispersive X-ray spectrometry
with Ca/Fe ratios obtained using X-ray fluorescence spectrometry
has been used with good success by several forensic laboratories
to classify glass fragments into sheet and container categories
(Keeley and Christofides 1979; Ryland 1986). |
|
|
7.1.2.2. |
For discrimination among glass sources,
a scanning electron microscopy-energy dispersive X-ray spectrometry
protocol was reported for determining the ratios of the intensities
of Na/Mg, Na/Al, Mg/Al, Ca/Na, and Ca/K in glass fragments (Andrasko
and Machly 1978). Measurement of these ratios by scanning electron
microscopy-energy dispersive X-ray spectrometry was incorporated
into a scheme with refractive index, density, and emission spectrography.
Thirty-eight out of 40 window glasses analyzed by this scheme
were found to be distinguishable. The variation in the measured
element intensity ratios by scanning electron microscopy-energy
dispersive X-ray spectrometry was found to be consistent across
a new sheet, an old sheet, and within a single fragment of glass. |
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7.1.3. |
Analytical considerations |
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|
7.1.3.1. |
Sample preparation
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7.1.3.1.1.
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Samples should be cleaned and dried
according to Section 6.2. prior to embedding. Small irregularly
shaped samples may be analyzed, but flat sample surfaces are
recommended whenever possible and are particularly important
when accurate, precise quantitative results are desired. This
may be achieved by embedding the sample in a resin that is subsequently
cured and polished to provide a flat surface. The embedded sample
may need to be coated with a thin conductive layer, such as
carbon, to reduce charging from the electron beam. The glass
sample is altered and partially consumed during the embedding
and polishing process. |
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|
7.1.3.2.
|
Scanning electron microscopy-energy
dispersive X-ray spectrometry measurements are made on individual
fragments of glass. The beam current and magnification used
will depend on sample size and instrument capabilities. Magnifications
on the order of 1,000X are adequate for most samples, but magnifications
up to 10,000X can be used. |
|
|
7.1.3.3 |
An accelerating voltage of 10 to 20 kilovolts
is typically used.
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7.1.3.4. |
The scan area used should be as large as possible
to obtain representative spectra from the sample. The magnification,
scan area, and operating parameters should be consistent among
known and questioned specimens being compared.
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7.1.3.5. |
The operating current
should be adjusted for optimal count rates so that analytical
time is not excessive. It should be noted that Na migration
could occur at high operating currents. Na migration is evidenced
by a significant drop in
NaKa intensity with time. |
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7.1.3.6. |
The detector-to-specimen distance
and the takeoff angle should be optimized for each instrument. |
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7.1.3.7. |
Acquisition times are selected based
on sample size and count rate. These can be either based on
a fixed time during which the detector is acquiring data (e.g.,
a specified number of live seconds) or on achieving a specified
intensity for a selected X-ray energy (e.g., acquiring spectral
data until the intensity of the Ca Ka
peak reaches 50,000 counts). The advantage of using the latter
method is that it aids in normalizing the data and yields similar
precision for samples of different sizes and shapes. |
|
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7.1.3.8. |
Spectra should be acquired from
replicate samples in order to obtain a measure of the variability
within the sample and specimen. The most frequently used methods
of data interpretation are spectral comparison by simple overlaying
of X-ray spectra of two fragments or by calculating the element
intensity ratios. |
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7.1.3.9. |
Full quantitative analysis is possible
but requires calibration with matrix-matched standards embedded
in each stub along with the analytical samples under identical
operating conditions. |
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7.1.4.
|
Advantages of scanning electron
microscopy-energy dispersive X-ray spectrometry for glass examinations |
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7.1.4.1. |
The most significant advantages
of scanning electron microscopy-energy dispersive X-ray spectrometry
for determining element concentrations in glass fragments are
that it is nondestructive, applicable to very small samples,
and readily available to many forensic laboratories. |
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|
7.1.4.2. |
Scanning electron microscopy-energy
dispersive X-ray spectrometry, when combined with energy dispersive
X-ray fluorescence spectrometry, permits product-use classification
of glass samples. |
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7.1.4.3. |
Scanning electron microscopy-energy
dispersive X-ray spectrometry is useful in forensic laboratories
as a rapid, screening method that can add some discrimination
capability to optical and physical measurements. |
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7.1.5. |
Limitations of scanning electron microscopy-energy dispersive
X-ray spectrometry for glass examinations
Several characteristics of scanning electron microscopy-energy
dispersive X-ray spectrometry have limited its widespread
application in forensic laboratories for the comparison of
glass fragments.
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7.1.5.1. |
The irregular shape of some small
glass fragments makes precise and accurate quantitative determination
of element concentrations difficult. Variations in fragment
surface orientation contribute to relatively poor precision
and accuracy of results obtained by scanning electron microscopy-energy
dispersive X-ray spectrometry compared with other methods of
analysis. |
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7.1.5.2. |
The compositional differences distinguishable
by scanning electron microscopy-energy dispersive X-ray spectrometry
will, in most instances, manifest themselves in readily distinguishable
refractive index and/or density values. |
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7.1.5.3. |
The detection limits on a concentration
basis of scanning electron microscopy-energy dispersive X-ray
spectrometry are poor, typically in the 0.1 percent range; and
therefore, the number of elements that can be determined is
limited. Many elements, such as Ba and Sr, which are useful
for both classification and source discrimination, are present
in most glasses at levels that are not detectable using scanning
electron microscopy-energy dispersive X-ray spectrometry. |
|
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7.1.5.4. |
The detection limits for scanning
electron microscopy-energy dispersive X-ray spectrometry are
approximately an order of magnitude poorer than for scanning
electron microscopy-wavelength dispersive X-ray spectrometry. |
7.2. |
X-Ray Fluorescence Spectrometry
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7.2.1. |
Instrument description and operating
principles |
|
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7.2.1.1. |
X-ray fluorescence spectrometry
is an elemental analysis technique based upon the measurement
of characteristic X-rays emitted from a sample following excitation
by an X-ray source. The energies or wavelengths of the detected
X-rays are used to identify the elements, and the intensities
of the X-ray peaks in the measured spectrum correlate with the
quantities of each element present in the sample area exposed
to the X-ray beam. |
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7.2.1.2 |
As with X-ray methods associated with scanning
electron microscopy, two X-ray fluorescence spectrometry methods,
wavelength dispersive (WDXRF) and energy dispersive (EDXRF)
X-ray fluorescence spectrometry are also defined by the manner
in which the data are collected and displayed.
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7.2.1.2.1.
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Wavelength dispersive X-ray spectrometry
(see Section 7.1.1.2.1. for a description) offers the best spectral
resolution of all of the X-ray fluorescence methods. The glass
manufacturing industry for quality control purposes uses wavelength
dispersive X-ray spectrometry extensively. However, in addition
to the high cost of instrumentation, wavelength dispersive X-ray
spectrometry requires relatively large, flat samples, both of
which severely limit its forensic use. Wavelength dispersive
X-ray fluorescence spectrometry is not discussed in further
detail in this guideline. |
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7.2.1.2.2.
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Energy dispersive X-ray fluorescence
spectrometry (see Section 7.1.1.2.2. for a description) methods
are considered nondestructive, surface or near surface techniques.
Fluorescent X-rays that reach the detector typically originate
from a glass sample region no deeper than about 100µm.
To analyze small, irregularly shaped fragments common to forensic
casework, micro- and capillary X-ray fluorescence spectrometry
techniques are appropriate. In these techniques, the beam is
collimated down to micrometer size beam diameters, typically
100 to 300µm for forensic glass analyses. |
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7.2.2.
|
Uses of energy dispersive X-ray
fluorescence spectrometry in glass examination |
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7.2.2.1. |
Energy dispersive X-ray fluorescence
spectrometry has been used to classify unknown source glass
samples as to their product-use types (Dudley et al. 1980; Howden
et al. 1977; Ryland 1986). |
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7.2.2.1.1.
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Energy dispersive X-ray fluorescence
spectrometry is capable of distinguishing glass samples of different
product-use type whose refractive indices are indistinguishable
(Dudley et al. 1980). |
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7.2.2.1.2.
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Using element intensity ratios determined
by energy dispersive X-ray fluorescence spectrometry, sheet
and container sources were correctly classified in 93 percent
of the cases tested (Ryland 1986). A combined energy dispersive
X-ray fluorescence spectrometry and scanning electron microscopy-energy
dispersive X-ray spectrometry procedure was reported to be useful
for classification of modern sheet and container glasses. These
samples could not be separated by scanning electron microscopy-energy
dispersive X-ray spectrometry determination of Ca/Mg alone.
Using a 35 kilovolts accelerating voltage in energy dispersive
X-ray fluorescence spectrometry, a low Ca/Fe ratio is indicative
of sheet glass and a high Ca/Fe ratio indicates a container
or tableware source. Some samples had intermediate values and
were unclassifiable with this technique. |
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7.2.2.1.3.
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Energy dispersive X-ray fluorescence spectrometry
was used to measure the concentrations of 10 elements as ratios
to Ca in 50 pairs of window (nonfloat) and nonwindow glasses
having refractive indices that are indistinguishable in the
fourth decimal place (Howden et al. 1977). When element intensity
ratios were used, 95 percent of the individual glass specimens
were correctly classified as to source type. The classification
rules were made using only refractive index, As, Fe, and Mg
as points of comparison.
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7.2.2.2. |
Discrimination among sources of
glass may be accomplished by energy dispersive X-ray fluorescence
spectrometry using both qualitative and semiquantitative methods. |
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7.2.2.2.1.
|
Some glasses contain elements, such as As,
Rb, and Mn, which are readily measured by energy dispersive
X-ray fluorescence spectrometry but are not present at detectable
levels in all glasses. In such instances, discrimination of
samples may be attained by a qualitative comparison of the
spectra. Exclusion cannot be made based on the absence of
element(s) in one sample that are present at or near the limit
of detection in the other comparison sample (Dudley et al.
1980).
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7.2.2.2.2.
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Semiquantitative analysis of the energy dispersive
X-ray fluorescence spectra can be used to discriminate
among glass samples of different origins. Energy dispersive
X-ray fluorescence spectrometry using element intensity ratios
successfully discriminated all but two of 81 window glass
samples in one study (Reeve et al. 1976). These two samples
were distinguishable from each other by optical properties.
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7.2.2.2.3.
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The addition of energy dispersive X-ray fluorescence
spectrometry measurements of Si/Ca, Fe/Ca, and Sr/Zr to an
analytical scheme improved the source discrimination capability
among automobile side window glasses over the use of refractive
index measures alone (Koons et al. 1991). In this study, energy
dispersive X-ray fluorescence spectrometry was slightly more
discriminating than refractive index but not as discriminating
as inductively coupled plasma-optical emission spectrophotometry.
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7.2.2.2.4.
|
One study (Dudley et al. 1980) evaluated the
ability of energy dispersive X-ray fluorescence spectrometry
measurements to discriminate glass sources with indistinguishable
refractive indices, in addition to assessing its classification
capability. Using element ratios to Ca, the glasses from 49
of the 50 pairs of samples of window and nonwindow origin
were distinguished.
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7.2.3. |
Analytical considerations |
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7.2.3.1. |
Sample preparation |
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7.2.3.1.1.
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Samples should be cleaned and dried according
to Section 6.2. prior to sample mounting or analysis.
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7.2.3.1.2.
|
The selection of a sample mounting technique
depends on the sample size, beam collimator size, X-ray fluorescence
spectrometry sample chamber design, availability of mounting
material and polishing instrumentation, and the purpose of
the examination. The following is a list of some methods that
have been used successfully for mounting glass samples in
the sample chamber: placing directly over the aperture with
no support; using a plastic support; suspending over the aperture
by thread; affixing on X-ray film followed by inverting; suspending
a particle of glass over the aperture by super-gluing the
back side to a small diameter high-purity graphite spectrographic
electrode (with pointed tip); and embedding in a resin that
is subsequently cured and polished to provide a flat surface.
Flat sample surfaces and thick samples are recommended whenever
possible but are essential when accurate, precise quantitative
results are desired. When attempting to compare irregularly
shaped samples without polishing, precision may be improved
by selecting known samples of similar shape and size to the
questioned samples.
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7.2.3.2. |
Energy dispersive X-ray fluorescence
spectrometry measurements are generally made on individual fragments
of glass using a beam collimator of 3mm or less, depending on
sample size and instrument capabilities. |
|
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7.2.3.3. |
An accelerating voltage of 35 kilovolts
can be used for end-use classification purposes, and a fixed
setting in the range of 35 to 50 kilovolts is suitable for discrimination
purposes. |
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7.2.3.4. |
The operating current should be
adjusted as needed to obtain good count rates, optimally less
than 50 percent detector dead time. |
|
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7.2.3.5. |
Acquisition times can be selected
based on sample size and count rate, as with scanning electron
microscopy-energy dispersive X-ray spectrometry. See Section
7.1.3.7. for more details. |
|
|
7.2.3.6. |
Quantitative concentrations are
routinely determined with energy dispersive X-ray fluorescence
spectrometry on comparatively large, flat samples. For the most
accurate quantitative determinations, calibration with matrix-matched
standards is required. Standardless quantitative mathematical
models for calibration do exist; however, they may result in
poorer accuracy. Because primary and fluorescent X-rays do not
reproducibly penetrate the small size and the irregular shape
of forensic glass fragments, quantitative elemental concentrations
generally cannot be determined with adequate precision and accuracy
without sample preparation to provide a flat surface. However,
ratios of the intensities of two X-ray lines of similar energies
are reasonably constant. Therefore, comparison of the elemental
compositions of glass fragments of varying masses and shapes
can be performed with good precision using intensity ratios.
Intensity data for selected elements should be ratioed after
baseline subtraction and escape peak corrections have been performed.
Like scanning electron microscopy-energy dispersive X-ray spectrometry
spectra, energy dispersive X-ray fluorescence spectra
are best compared either by overlaying spectral images or by
comparing calculated peak intensity ratios. |
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7.2.4. |
Advantages of X-ray fluorescence
spectrometry for glass examinations |
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7.2.4.1.
|
The most significant advantage of any of the
X-ray techniques is that they are nondestructive. They are
rapid, fairly sensitive, and can be performed with minimal
sample preparation.
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7.2.4.2.
|
Energy dispersive X-ray fluorescence spectrometry
analysis is less spatially discriminating than scanning electron
microscopy-energy dispersive X-ray spectrometry due to its
larger analytical beam size and the greater penetration depth
of X-rays compared to electrons. However, the limits of detection
of energy dispersive X-ray fluorescence spectrometry for most
elements are generally better than for scanning electron microscopy-energy
dispersive X-ray spectrometry. Specifically, the higher energy
excitation typical of energy dispersive X-ray fluorescence
spectrometry yields better sensitivity for higher atomic number
elements than scanning electron microscopy-energy dispersive
X-ray spectrometry. Thus, energy dispersive X-ray fluorescence
spectrometry has better analytical capabilities for several
good glass source-discriminating elements, such as Mn, Sr,
and Zr. Significantly better peak-to-background ratios can
be obtained using energy dispersive X-ray fluorescence spectrometry
compared with scanning electron microscopy-energy dispersive
X-ray spectrometry, particularly with instruments that allow
the incident X-ray beam to be collimated to a small spot size.
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7.2.5.
|
Limitations of X-ray fluorescence spectrometry
for glass examinations
|
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|
7.2.5.1. |
Most of the disadvantages pertaining
to quantitation of element concentrations in small irregularly
shaped particles are more pronounced in energy dispersive X-ray
fluorescence spectrometry than they are in scanning electron
microscopy-energy dispersive X-ray fluorescence spectrometry. |
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7.2.5.2
|
The greatest limitation of energy
dispersive X-ray fluorescence spectrometry is the necessity
to employ matrix-matched multielement standards in order to
obtain accurate quantitative results. |
|
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7.2.5.3.
|
Disadvantages of wavelength dispersive X-ray
spectrometry compared to energy dispersive X-ray fluorescence
spectrometry are that the instrumentation is more expensive
and that larger samples are required.
|
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7.2.5.4. |
The operation of X-ray
tubes at high powers may cause the discoloration of some glass
samples. |
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7.3. |
Inductively Coupled Plasma-Optical Emission
Spectrophotometry
|
|
7.3.1.
|
Instrument description and operating
principles |
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|
7.3.1.1. |
In most inductively coupled plasmas,
an electrical discharge is initiated in a flowing stream of
inert gas, usually argon, and then sustained by a surrounding
radio frequency field. The resulting stable discharge, or plasma,
has the appearance of a small continuously glowing flame, with
temperatures in the range of 7,000-10,000K. When a sample is
introduced into the plasma, extensive atomization, ionization,
and excitation of the sample atoms occur. |
|
|
7.3.1.2. |
As the ions and atoms present in
the sample enter cooler portions of the plasma and drop to lower
excited states, they emit light at characteristic wavelengths.
In an inductively coupled plasma-optical emission spectrophotometer,
this emission is dispersed with a spectrophotometer and its
intensity is measured. Comparison of the emission intensities
of a sample with those of standard solutions is used to determine
the concentration of the elements in the sample. |
|
|
7.3.1.3. |
Glass samples can be introduced
into the plasma either by dissolving them and nebulizing the
resulting solution or by direct solid sampling. Only solution
methods are discussed in the inductively coupled plasma-optical
emission spectrophotometry portion of this guideline. Direct
solid sampling is outlined in the inductively coupled plasma-mass
spectrometry portion of the guideline because solid sampling
is more commonly used with mass spectrometry than with optical
emission spectophotometry. |
|
|
|
|
7.3.2.
|
Use of inductively coupled plasma-optical
emission spectrophotometry in glass examination |
|
|
7.3.2.1. |
The initial inductively coupled
plasma-optical emission spectrophotometry methods developed
for glass analysis were primarily designed for purposes of classification.
An inductively coupled plasma-optical emission spectrophotometry
analytical method was developed to determine the concentrations
of Mn, Fe, Mg, Al, and Ba in glass fragments (Catterick and
Hickman 1981). Over the next several years, the concentrations
of additional elements in glass by inductively coupled plasma-optical
emission spectrophotometry were determined, and 6 to 10 element
classification schemes based on comparison with a glass database
divided into nine product categories were developed (Hickman
1981; Hickman et al. 1983). Currently, the protocol most widely
used for casework was developed for determining the concentrations
of 10 elements (Al, Ba, Ca, Fe, Mg, Mn, Na, Ti, Sr, and Zr)
with excellent analytical precision in milligram-sized glass
fragments (Koons et al. 1988). A combination of five of these
elements was shown to provide good classification into the two
categories of sheet and container glass. Inductively coupled
plasma-optical emission spectrophotometry has also been used
to associate food containers to the manufacturing plants in
which they were made and to identify sources of contaminant
glass in cases involving product tampering (Wolnik et al. 1989). |
|
|
7.3.2.2. |
In further studies, the distributions
of up to 22 elements, most measured by inductively coupled plasma-optical
emission spectrophotometry, in various glasses were shown to
provide excellent discrimination capability among sources within
a product class (Hickman 1983; Hickman et al. 1983). In a study
measuring the concentrations of 10 elements in automobile side-window
glasses, the probability that two glasses from different vehicles
would be indistinguishable was reported to be one in 1,080,
compared with one in five for refractive index alone and one
in ten for energy dispersive X-ray fluorescence spectrometry
analysis alone (Koons et al. 1991). Studies have shown that
using inductively coupled plasma-optical emission spectrophotometry
sheets of glass produced within minutes of each other in a single
float-glass production line can be differentiated. In a recent
study using statistical analysis of samples collected in casework,
it was reported that inductively coupled plasma-optical emission
spectrophotometry measurements provide very high discrimination
capability. The probability that two glass fragments from different
sources will have indistinguishable concentrations of ten elements
is extremely small (Koons and Buscaglia 1999). |
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|
7.3.3. |
Analytical considerations |
|
|
7.3.3.1.
|
Prior to analysis, each fragment
must be washed to remove surface contamination and residual
fluids from refractive index and density measurements. Typically,
this is done by soaking in HNO3,
rinsing repeatedly with deionized water, then ethanol, followed
by drying. |
|
|
7.3.3.2.
|
Fragments, typically weighing 0.2
to 8mg, are obtained by crushing the specimen between sheets
of clean plastic. Prior to dissolution, analytical samples must
be weighed to the nearest 0.01mg, or better, on a microbalance,
whose calibration is checked immediately preceding each use. |
|
|
7.3.3.3.
|
Dissolution procedures involve the
use of high purity HF, HNO3,
and, in some cases, HCl (Catterick and Hickman 1981; Koons et
al. 1988). A typical procedure consists of the digestion of
glass fragments in polypropylene tubes by the addition of concentrated
HF and other mineral acids, taking the solution to dryness (to
remove all residual HF) by heating the tubes, and complete redissolution
of the residue in a strong acid, such as HCl. This dissolution
procedure typically requires several days. The complete removal
of HF is required to prevent deterioration of glass or quartz
spray chambers, nebulizers, and torches present on most inductively
coupled plasma instruments. |
|
|
7.3.3.4. |
An internal standard, usually scandium,
must be added to each sample and standard solution in order
to correct for minor differences in analytical sensitivity among
samples and among samples and standards. |
|
|
7.3.3.5. |
The analytical curve for each element
is prepared using a minimum of four calibration standards, including
a calibration blank. Multielement calibration standards containing
the internal standard and the diluting acids in the same concentration
as the sample solutions are used. The concentration ranges for
each analyte element must span the concentrations in the sample
solution. |
|
|
|
|
7.3.4 |
Advantages of inductively coupled plasma-optical
emission spectrophotometry
|
|
|
7.3.4.1. |
Analytical characteristics of inductively
coupled plasma-optical emission spectrophotometry instruments
include the capability to determine a wide range of elements,
long linear response ranges, a limited number of spectral and
matrix interferences, low detection limits for many elements
of interest, and ease of automation of data handling. |
|
|
7.3.4.2. |
The detection limits of this method
vary slightly from day to day. Typical values are 0.01 to 0.1µg/g
in the glass. For samples smaller than 5mg, these detection
limits are raised somewhat because of increased dilution factors.
Precision measurements for all elements when present in the
middle of their respective concentration ranges are generally
better than two percent relative standard deviation. |
|
7.3.5. |
Limitations of inductively coupled
plasma-optical emission spectrophotometry |
|
|
7.3.5.1 |
The dissolution of glass fragments is destructive,
requires the use of corrosive acids and high purity reagents,
and sample digestion is time-consuming compared to sample
preparation for X-ray methods.
|
|
|
7.3.5.2. |
In comparison to most forensic laboratory
equipment, inductively coupled plasma-optical emission spectrophotometry
instrumentation is costly, requires more extensive operator
training, and, to date, a limited number of forensic applications
for it have been found.
|
|
|
|
|
|
7.4.
|
Inductively Coupled Plasma-Mass
Spectrometry |
|
7.4.1. |
Instrument description and operating principles
|
|
|
7.4.1.1.
|
The inductively coupled plasma torch is an
excellent ionization device. Instruments made by coupling
inductively coupled plasma with mass spectrometry as an ion
isolator and detector have shown improved analytical capabilities
suitable for glass fragment analysis. Mass spectrometry instruments
may be of quadrupole, time-of-flight, or magnetic-sector design
with single or multiple electron multiplier detectors.
|
|
|
7.4.1.2.
|
The useful features of inductively
coupled plasma-mass spectrometry compared with inductively coupled
plasma-optical emission spectrophotometry are a smaller and
better-defined set of spectral interferences, detection limits
roughly 1,000-fold lower, longer linear working ranges, and
more rapid scanning capability. The better sensitivity of mass
spectrometry compared to optical emission spectrophotometry
permits use of a smaller sample size and the quantitative determination
of some trace elements not detectable by inductively coupled
plasma-optical emission spectrophotometry. Using solution inductively
coupled plasma-mass spectrometry, approximately 40 elements
can be determined in a 1mg glass fragment. |
|
|
7.4.1.3. |
Another characteristic of inductively
coupled plasma-mass spectrometry is that samples need not be
introduced as solutions. The rapid scanning speed of the mass
spectrometer allows measurement of transient signals, such as
those produced with laser ablation of solid samples used to
sweep a vapor into the plasma for analysis (Montaser et al.
1998). |
|
|
|
|
7.4.2.
|
Uses of inductively coupled plasma-mass
spectrometry in glass examinations
|
|
|
7.4.2.1. |
The first reported studies of the
use of inductively coupled plasma-mass spectrometry for the
forensic comparison of glass fragments indicated an ability
to quantitatively determine the concentrations of 40 to 62 elements
in glass fragments as small as 500µg (Parouchais et al.
1996; Zurhaar and Mullings 1990). For any given glass, approximately
40 of these elements are likely to be present at detectable
concentrations. |
|
|
7.4.2.2. |
More thorough studies have been aimed at developing
reliable analytical procedures and determining which elements
provide the best discrimination capability. Reported analytical
precisions for sample introduction in solution are typically
better than 10 percent relative standard deviation for elements
present at concentrations greater than a few parts per billion
(Duckworth et al. 2000).
|
|
|
|
|
7.4.3. |
Analytical considerations |
|
|
7.4.3.1. |
The dissolution procedures for inductively
coupled plasma-mass spectrometry are similar to the procedures
used for inductively coupled plasma-optical emission spectrophotometry,
except that HNO3 is preferred
over HCl for the final analytical solution to minimize or eliminate
interferences from polyatomic chlorides. Because of the high
sensitivity and low detection limits of inductively coupled
plasma-mass spectrometry, it is essential that contamination
from such sources as digestion reagents and the laboratory environment
be controlled. The range of major, minor, and trace element
concentrations in glass digest solutions is so great that most
inductively coupled plasma-mass spectrometry protocols require
analysis of multiple sample dilutions to measure the concentrations
of all analyte elements. |
|
|
7.4.3.2. |
Internal standards must be added to all sample
and standard solutions and used in the calculation of element
concentrations when making inductively coupled plasma-mass
spectrometry measurements. Internal standards consist of 10
to 100µg/L concentrations of elements not present at
high concentrations in the glass (e.g., Rh, In, Y, Tl).
|
|
|
7.4.3.3.
|
Standard concentrations are different than
those used with inductively coupled plasma-optical emission
spectrophotometry because of the higher sensitivity of mass
spectrometry over optical emission spectrophotometry and the
greater number of elements measured. Although the linear dynamic
range of inductively coupled plasma-mass spectrometry is greater
than that of inductively coupled plasma-optical emission spectrophotometry,
the linearity is so good that only two or three standard concentrations
are typically required for each element. The number and combination
of elements present in each standard solution must be selected
based on chemical compatibility at the mg/L to ng/L level,
the presence of contaminants in stock solutions, and ultimately,
on the discrimination power afforded by each element for the
comparison of glass fragments.
|
|
|
7.4.3.4 |
Sample introduction for inductively coupled
plasma-mass spectrometry may also be made by direct vaporization
of a solid sample, for instance, by laser ablation and passing
the vapors directly into the plasma. Problems associated with
the solubilization of glass fragments, such as reagent contamination,
dilution of the sample, and incomplete digestion are avoided
when using laser ablation-inductively coupled plasma-mass
spectrometry.
|
|
|
|
|
7.4.4. |
Advantages of inductively coupled
plasma-mass spectrometry |
|
|
7.4.4.1 |
Inductively coupled plasma-mass spectrometry
can be used to perform fast, multielemental analysis similar
to inductively coupled plasma-optical emission spectrophotometry
but with greater sensitivity and better detection limits,
allowing for the analysis of smaller fragments and additional
elements. The additional elements can provide more points
of comparison, potentially improving discrimination capability,
and can be used to detect specific elements present in some
products, such as Ce when it is added as a decolorizer.
|
|
|
7.4.4.2.
|
Inductively coupled plasma-mass spectrometry
can also be used for measuring multiple isotopes of the same
element. Isotopic measurements may potentially be used for
improved accuracy of element quantitation (isotope dilution
method) or for detection of isotopic variations among sources.
|
|
|
7.4.4.3. |
Laser ablation-inductively coupled plasma-mass
spectrometry provides a means for rapid, direct determination
of element concentrations in solid samples with minimal sample
destruction and contamination.
|
|
|
|
|
7.4.5.
|
Limitations of inductively coupled
plasma-mass spectrometry |
|
|
7.4.5.1.
|
The higher sensitivity and multielement capabilities
of inductively coupled plasma-mass spectrometry relative to
other elemental analytical techniques requires that strict
contamination control of analyte elements be maintained during
sample preparation and analysis.
|
|
|
7.4.5.2.
|
Interferences when using a quadrupole
instrument, though generally well known, in some instances are
not easily corrected and prevent accurate quantitation of a
few elements. Removal of some interference effects, particularly
isobaric mass overlaps, can be accomplished using a high-resolution
spectrometer or by collision cell technology, both at added
cost and complexity of instrument operation. |
|
|
7.4.5.3.
|
Limitations to the implementation of inductively
coupled plasma-mass spectrometry technology in forensic laboratories
are the high cost of instrumentation and laboratory support,
the high level of operator training required, and the large
time requirements for sample and standard preparation and
analysis.
|
|
|
7.4.5.4.
|
Historically, it has been difficult to obtain
quantitative results using solid sample introduction because
of the inability to add an internal standard to a solid material.
However, for some glass types, such as soda-lime glass, this
limitation has been overcome by using a matrix element (silicon)
as the internal standard for quantification. An external standard
(e.g., NIST SRM) should be used for quantitative analysis
of solid sampling by laser ablation.
|
|
|
7.4.5.5. |
The precision of inductively coupled plasma-mass
spectrometry is poorer than that of inductively coupled plasma-optical
emission spectrophotometry under optimum conditions. The inductively
coupled plasma-mass spectrometry precision measurements for
all elements when present in the middle of their respective
concentration ranges are generally 10 percent or better relative
standard deviation.
|
8. Considerations
|
8.1.
|
Elemental analysis methods are used when other methods of
comparison fail to distinguish two glass fragments as having
different sources. The amount of additional discrimination provided
by element concentration regardless of the method of determination
depends upon the number of elements measured and the precision
of the measurements. |
|
8.2.
|
Replicate measurements must be taken to assess
the extent of element concentration variations within the specimens. |
9. References
Almirall, J. Elemental analysis of glass fragments. In: Trace
Evidence Analysis and Interpretation: Glass and Paint. B. Caddy,
ed. Taylor and Francis, London, 2001, pp. 65-83.
Andrasko, J. and Maehly, A. C. The discrimination between samples of window glass
by combining physical and chemical techniques, Journal
of Forensic Sciences (1978) 23:250-262.
Buscaglia, J. Elemental analysis of small glass fragments in forensic
science, Analytica Chimica Acta (1994) 288:17-24.
Catterick, T. and Hickman, D. A. The quantitative analysis of glass by inductively
coupled plasma-atomic emission spectrometry: A five element survey, Forensic
Science International (1981) 17:253-263.
Coleman, R. F. and Goode, G. C. Comparison of glass fragments by neutron activation
analysis, Journal
of Radioanalytical Chemistry (1973) 15:367-388.
Duckworth, D. C., Bayne, C. K., Morton, S. J., and Almirall, J.
Analysis of variance in forensic glass analysis by ICP-MS: Variance
within the method, Journal of Analytical Atomic Spectrometry
(2000) 15:821-828.
Dudley, R. J., Howden, C. R., Taylor, T. J., and Smalldon, K. W.
The discrimination and classification of small fragments of window
and nonwindow glasses using energy-dispersive X-ray fluorescence
spectrometry, X-Ray Spectrometry (1980) 9:119-122.
Haney, M. A comparison of window glasses by isotope dilution spark source mass
spectrometry, Journal
of Forensic Sciences (1977) 22:534-544.
Hickman, D. A. A classification scheme for glass, Forensic
Science International (1981) 17:265-281.
Hickman, D. A. Elemental analysis and the discrimination of sheet
glass samples, Forensic Science International (1983) 23:213-223.
Hickman, D. A. Glass types identified by chemical analysis, Forensic
Science International (1987) 33:23-46.
Hickman, D. A., Harbottle, G., and Sayre, E. V. The selection of the best elemental
variables for the classification of glass samples, Forensic
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Howden, C. R., German, B., and Smalldon, K. W. The determination of iron and
magnesium in small glass fragments using flameless atomic absorption spectrophotometry, Journal
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Hughes, J. C., Catterick, T., and Southeard, G. The quantitative analysis of
glass by atomic absorption spectroscopy, Forensic
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Keeley, R. H. and Christofides, S. Classification of small glass
fragments by X-ray microanalyisis with the SEM and a small sample
XRF spectrometer. In: Proceedings of Scanning Electron Microscopy,
SEM, AMF O'Hare, Illinois, 1979, Part I, pp. 459-464.
Koons, R. D. and Buscaglia, J. The forensic significance of glass composition
and refractive index measurements, Journal
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Koons, R. D., Fiedler, C., and Rawalt. R. C. Classification and discrimination
of sheet and container glasses by inductively coupled plasma-atomic emission
spectrometry and pattern recognition, Journal
of Forensic Sciences (1988) 33:49-67.
Koons, R., Peters, C., and Rebbert, P. Comparison of refractive
index, energy dispersive X-ray fluorescence and inductively coupled
plasma atomic emission spectrometry for forensic characterization
of sheet glass fragments, Journal of Analytical Atomic Spectrometry
(1991) 6:451-456.
Montaser, A., Minnich, M. G., McLean, J. A., Liu, H., Caruso, J.
A., and McLeod, C. W. Sample introduction in ICPMS. In: Inductively
Coupled Plasma Mass Spectrometry. A. Montaser, ed. Wiley-VCH,
New York, 1998, pp. 194-218.
Parouchais, T., Warner, I. M., Palmer, L. T., and Kobus, H. The analysis of small
glass fragments using inductively coupled plasma mass spectrometry, Journal
of Forensic Sciences (1996) 41:351-360.
Reeve, V., Mathiesen, J., and Fong, W. Elemental analysis by energy dispersive
X-ray: A significant factor in the forensic analysis of glass, Journal
of Forensic Sciences (1976) 21:291-306.
Ryland, S. Sheet or container?
— Forensic glass
comparisons with an emphasis on source classification, Journal of Forensic
Sciences (1986) 31:1314-1329.
Terry, K. W., van Riessen, A., and Vowles, D. J. Elemental analysis
of glasses in a SEM, Micron (1982) 13:293-294.
Wolnik, K. L., Gaston, C. M., and Fricke, F. L. Analysis of glass in product
tampering investigations by inductively coupled plasma atomic emission spectrometry
with a hydrofluoric acid resistant torch, Journal
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Zurhaar, A. and Mullings, L. Characterisation of forensic glass
samples using inductively coupled plasma mass spectrometry, Journal
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10. Bibliography
Bertin, E. P. Principles and Practice of X-Ray Spectrometric
Analysis. 2nd ed. Plenum, New York, 1975.
Goldstein, J. I., Newbury, D. E., Echlin, P., Joy, D. C., Romig,
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Janssens, K. H., Adams, F. C. V., and Rindby, A. Microscopic
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Montaser, A. Inductively Coupled Plasma Mass Spectrometry.
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Newbury, D. E., Joy, D. C., Echlin, P., Fiori, C. E., and Goldstein,
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Microanalysis, Plenum, New York, 1986.
Thompson, A. and Walsh, J. N. Handbook of Inductively Coupled
Plasma Spectrometry, 2nd ed. Blackie, Glasgow, 1989.
|