ASTM E3423-24 Standard Guide for Forensic Analysis of Explosives By Polarized Light Microscopy

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Designation: E3423 24 An American National Standard
Standard Guide for
Forensic Analysis of Explosives By Polarized Light
Microscopy
1
This standard is issued under the fixed designation E3423; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide addresses the use of polarized light micros-
copy (PLM) to identify explosive-related compounds from
intact explosives and post-blast residues containing uncon-
sumed explosive compounds or their solid reaction products,
and to isolate them for further analysis.
1.2 This guide is intended for use by competent forensic
science practitioners with the requisite formal education,
discipline-specific training (see Practice E2917), and demon-
strated proficiency to perform forensic casework (refer to the
T/SWGFEX Suggested Guide for Explosives Analysis Train-
ing).
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
2
E620 Practice for Reporting Opinions of Scientific or Tech-
nical Experts
E860 Practice for Examining and Preparing Items That Are
or May Become Involved in Criminal or Civil Litigation
E1732 Terminology Relating to Forensic Science
E2917 Practice for Forensic Science Practitioner Training,
Continuing Education, and Professional Development
Programs
E3196 Terminology Relating to the Examination of Explo-
sives
E3253 Practice for Establishing an Examination Scheme for
Intact Explosives
E3255 Practice for Quality Assurance of Forensic Science
Service Providers Performing Forensic Chemical Analysis
E3329 Practice for Establishing an Examination Scheme for
Explosive Residues
2.2 Other Resources
T/SWGFEX Suggested Guide for Explosive Analysis Train-
ing
3
3. Terminology
3.1 Definitions—For definitions of terms used in this guide
other than those listed in 3.2, see Terminologies E1732 and
E3196.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 alpha (α)—symbol representing the lowest of the three
principal refractive indices of a biaxial crystal.
3.2.2 anomalous interference colors, n—atypical polariza-
tion colors which occur due to variation with wavelength of the
refractive index (very few substances display this characteris-
tic).
3.2.3 Becke line, n—a halo observed near the boundary of a
transparent particle that is mounted in a medium that differs
from the particle’s refractive index.
3.2.4 Becke line method, n—method for determining the
refractive index of a transparent particle relative to its moun-
tant by noting the direction in which the Becke line moves
when the focus is changed.
3.2.4.1 Discussion—The Becke line always moves toward
the higher refractive index medium (particle or mountant)
when the focus is raised, and towards the lower refractive
index medium when the focus is lowered. At the point where
the index of the particle matches the index of the mounting
medium, the Becke line is no longer visible. The Becke line is
generally viewed at a wavelength of 589 nm (the D line of
sodium [n
D
]).
1
This guide is under the jurisdiction of ASTM Committee E30 on Forensic
Sciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics.
Current edition approved July 15, 2024. Published August 2024. DOI: 10.1520/
E3423-24.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at www.astm.org/contact. For Annual Book of
ASTM Standards volume information, refer to the standard’s Document Summary
page on the ASTM website.
3
Available from the Technical/Scientific Working Group for Fire and Explosion
Analysis (T/SWGFEX), https://www.nist.gov/system/files/documents/2018/09/21/
twgfex_suggest_guide_for_explosive_analysis _training.pdf.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
1
3.2.5 beta (β)—symbol representing the intermediate prin-
cipal refractive index of a biaxial crystal.
3.2.6 biaxial, adj—an anisotropic crystal in the
orthorhombic, monoclinic, or triclinic system with three prin-
cipal refractive index directions (α,β,γ) and two optic axes
that are isotropic.
3.2.7 birefringence, n—the numerical difference between
the maximum and minimum refractive indices of anisotropic
substances.
3.2.8 crystal system, n—crystals are classified according to
their crystallographic axes length and the angles between them.
There are six crystal systems: cubic, tetragonal, hexagonal,
orthorhombic, monoclinic, and triclinic. All crystal systems are
anisotropic except for the cubic system, which is isotropic.
3.2.9 epsilon (ε) —any vibration direction in the plane of the
c axis for uniaxial crystals.
3.2.10 euhedral, adj—well-formed crystals bounded by
plane faces.
3.2.11 extinction, n—condition when an anisotropic sub-
stance appears dark when observed between crossed polarizers
and occurs when the vibration directions of the specimen are
parallel to the vibration directions of the polarizer and analyzer.
3.2.12 extinction, oblique, n—a type of extinction in which
the vibration directions are inclined at an angle relative to a
crystal face.
3.2.13 extinction, parallel, n—a type of extinction in which
the vibration directions are parallel to the crystal faces.
3.2.14 extinction, symmetrical, n—a type of extinction in
which the vibration directions bisect a prominent interfacial
angle of the crystal.
3.2.15 fusion methods, n—A process in which compounds
are heated on a microscope slide and observed via PLM during
heating, melting, and, upon cooling, recrystallization of the
melt.
3.2.15.1 Discussion—Fusion methods frequently employ a
temperature controlled hot stage capable of at least 300 °C
placed on the stage of a polarizing light microscope.
3.2.16 gamma (γ)—symbol representing the highest of the
three principal refractive indices of a biaxial crystal.
3.2.17 interference figure, n—pattern observed during cono-
scopic observation of an anisotropic material which consists of
a combination of extinction positions and interference colors
corresponding to the full cone of directions by which the
sample is illuminated.
3.2.17.1 Discussion—Conoscopic observations are typically
made by inserting a Bertrand lens into the body tube or by
removing an eyepiece and viewing down the body tube.
3.2.18 omega (ω)—any vibration direction in the plane of
the a axis for uniaxial crystals.
3.2.19 optic axial angle (2V), n—the acute angle between
two optic axes of a biaxial crystal.
3.2.20 optic axis, n—a direction of isotropic light propaga-
tion in an anisotropic crystal. Uniaxial crystals have one optic
axis; biaxial crystals have two optic axes.
3.2.21 optic sign, n—determined by the relationship of the
refractive indices of a material. For uniaxial crystals, if ε>ω,
the crystal is positive (+); if ω>ε, the crystal is negative (-).
For biaxial crystals, if γ-β>β-α, the crystal is positive (+); if
γ-β<β-α, the crystal is negative (-).
3.2.22 polymorphism, n—crystallization into two or more
chemically identical but crystallographically distinct forms.
3.2.23 relief, n—contrast between a particle or crystal and
its media due to the difference between their refractive indices.
The greater the numerical difference in refractive indices, the
greater the relief.
3.2.24 subhedral, adj—crystals with imperfectly developed
faces.
3.2.25 uniaxial, adj—an anisotropic crystal in the tetragonal
or hexagonal system having one optic axis (isotropic direction)
and either two (tetragonal) or three (hexagonal) directions
which are alike and perpendicular to the direction of the optic
axis.
4. Summary of Practice
4.1 Solid chemical components in samples are analyzed
using a polarized light microscope (PLM). The optical crystal-
lographic properties such as refractive index, birefringence,
and morphology are determined. Determination of these
properties, which are different for various explosive compo-
nents and other chemical compounds, can aid in the identifi-
cation of explosives and explosive residues submitted to the
forensic laboratory.
4.2 Optical crystallographic properties of an unknown com-
ponent can be determined by either mounting the component in
a refractive index liquid, recrystallizing the component from a
small drop of solvent, or recrystallizing the component from its
melt, that is, recrystallization occurring when a melted com-
pound is cooled. Recrystallized components can be isolated for
further analysis using other analytical techniques such as
Fourier transform infrared spectroscopy (FTIR), Raman
spectroscopy, or scanning electron microscopy-energy disper-
sive x-ray spectroscopy (SEM-EDS).
4.3 Water-soluble components, such as inorganic salts used
as oxidizers, are dissolved in a drop of water and tested using
reagents to form characteristic microcrystals. Microcrystal
tests can aid in the determination of anions and cations present
in oxidizers and other components of explosives.
4.4 The optical crystallographic properties of an inorganic
compound and microcrystal testing for its ions are independent
techniques that provide complementary information. A combi-
nation of tests that includes the optical crystallographic char-
acterization of a compound and microchemical tests to confirm
the anion and cation are used to identify oxidizers and other
salts present in explosives.
4.5 A combination of tests that include optical crystallo-
graphic characterization or microchemical tests in conjunction
with other independent techniques are used to identify chemi-
cal components present in explosives. Refer to Practices E3253
and E3329 for the requirements for the identification of
E3423 − 24
2
commonly encountered explosives and chemical components
present in explosive materials.
5. Significance and Use
5.1 This guide is designed to assist the analyst using
polarized light microscopy for the examination of test samples
for the presence of explosives.
5.2 Intact explosives and post-blast residues that contain
unconsumed explosive components or their solid reaction
products are suitable for this guide (Practices E3253 and
E3329). Particles only a fraction of a millimeter in size can be
examined using this guide.
5.3 This guide is not suitable for the examination of
smokeless powder or liquid explosives.
5.4 Some of the microscopical techniques described in this
guide allow for the recovery of the specimen for subsequent
analysis, but others, such as microcrystal tests, are destructive.
5.5 Identifications of explosive-related components based
on these properties shall be part of an analytical scheme as
described in Practices E3253 and E3329.
6. Apparatus
6.1 Polarized Light Microscope—A compound microscope
with a rotating stage, polarizing filters, substage condenser, and
compensator slot.
NOTE 1—Most commercially available polarized light microscopes
should be adequate for the forensic analysis of explosives and appropriate
for use following this guide.
6.2 Objectives—Strain-free typically 10×, 20×, and 40×.
6.2.1 A 40× objective with a numerical aperture of 0.65 or
higher is recommended to provide the widest view of the back
focal plane for conoscopic observations.
6.2.2 A 10× dispersion staining objective is required if
performing dispersion staining.
6.3 Compensator—A full wave compensator for use with
the microscope. Optional compensators include quarter-wave
plate and quartz wedge.
6.4 Hot Stage—A temperature controlled hot stage for use
with the microscope with a range of approximately 30 °C to
300 °C.
6.5 Alcohol Lamp.
7. Reagents and Materials
7.1 Purity of Reagents—Reagent grade chemicals shall be
used in all tests. It is intended that all reagents conform to the
specifications of the Committee on Analytical Reagents of the
American Chemical Society. Other grades may be used,
provided it is first ascertained that the reagent is of sufficiently
high purity to permit its use without lessening the accuracy of
the determination.
4
7.2 10 % Platinum Chloride (H
2
PtCl
6
) in water.
7.3 Zinc Uranyl Acetate.
7.4 Ammonium Molybdate ((NH
4
)
6
Mo
7
O
24
·4H
2
O).
7.5 Squaric Acid.
7.6 Potassium Iodide (KI).
7.7 Nitron Sulfate.
7.8 Strychnine Sulfate.
7.9 0.3 % Methylene Blue in deionized water (Methylene
Blue Reagent).
7.10 20 % Potassium Nitrate (KNO
3
) in deionized water.
7.11 Saturated Zinc Sulfate (ZnSO
4
) in deionized water.
7.12 Toluene or Methylene Chloride.
7.13 Sodium Hydroxide.
7.14 Deionized water.
7.15 Reference Materials, including reagent grade chemi-
cals and explosive materials, can be obtained from commercial
and retail sources or directly from the distributor or manufac-
turer.
7.16 Refractive Index Liquids with known temperature co-
efficients (dn/dt) and tolerance at n
D
.
7.17 Microscope Slides and Coverslips.
7.18 Glass Rod with a tapering tip of approximately 1 mm
or less in diameter.
7.19 Wood Toothpicks.
7.20 Glass Ring with an approximate diameter of 10 mm to
12 mm and 5 mm to 10 mm in height.
7.21 Tungsten Needles.
7.22 589 nm Light Filter.
8. Initial Microscopical Examination
8.1 Preliminary assessment of the samples using visual and
stereo microscopical examinations are conducted following
Practices E3253 and E3329. If preliminary examinations indi-
cate that the sample is an explosive mixture or a component
thereof, a portion of the sample is examined by PLM in order
to obtain further information regarding the composition of the
sample.
4
ACS Reagent Chemicals, Specifications and Procedures for Reagents and
Standard-Grade Reference Materials, American Chemical Society, Washington,
DC. For suggestions on the testing of reagents not listed by the American Chemical
Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
copeial Convention, Inc. (USPC), Rockville, MD.
E3423 − 24
3
8.2 One to three particles or grains (approximately 1 mg or
less) are mounted on a glass microscope slide with a cover slip
and dispersed in a refractive index oil. For post-blast evidence
a similar quantity of residue material is mounted in the same
manner. Any refractive index oil in the 1.400 to 1.800 range
should be used, however, if preliminary examinations indicate
a particular explosive material (for example, black powder,
ammonium nitrate-fuel oil (ANFO), or Composition C-4), the
examiner should select a refractive index oil that will enable
confirmation of characteristic optical crystallographic data
based upon Tables 1 and 2.
8.3 The mounted sample is examined with a polarized light
microscope at low magnification (for example, 10×). The
magnification is increased as needed to better view particles for
examination.
8.4 Charcoal, if present, will appear as irregularly shaped,
black and opaque particles, and as brown flakes often with
cellular features such as pits or cell walls (1).
5
8.5 Most common explosives and oxidizers are colorless
when viewed microscopically with transmitted plane polarized
light. Crossed polarizers are used to determine if the crystals
are isotropic or anisotropic. Interference colors are assessed to
determine if the birefringence is low (less than ~0.01), mod-
erate (~0.01 to 0.10) or high (above ~0.10).
8.6 The relative refractive indices of the crystals present in
the specimen are determined using the Becke line method or
dispersion staining.
5
The boldface numbers in parentheses refer to a list of references at the end of
this standard.
TABLE 1 Optical Crystallographic Properties of Common Fuels & Oxidizers
Compound Crystal System Refractive Indices/ Optic Angle Habit Comments
Sulfur (S) (1)orthorhombic 1.958
2.038
2.245
2V = 68 °(+)
dipyramids High relief in solvent. Larger
crystals appear yellowish in
color.
Ascorbic Acid (4)monoclinic 1.474
1.595
1.746
2V = 88 °(+)
High order interference colors.
Sucrose (1,4)monoclinic 1.540
1.567
1.572
2V = 48 °(-)
Sodium chlorate (NaClO
3
)(3)cubic 1.518 squares, rectangles
Barium nitrate (Ba(NO
3
)
2
)(3)cubic 1.571 cubes, octahedra
Strontium nitrate (Sr(NO
3
)
2
)(2)cubic 1.586 octahedra
Lead nitrate (Pb(NO
3
)
2
)(3)cubic 1.781 octahedra, cubes High relief in water.
Potassium perchlorate (KClO
4
)
(3)
orthorhombic 1.4731
1.4737
1.4769
2V = 50 °(+)
rectangular prisms, rhombs Low order interference colors.
Ammonium perchlorate
(NH
4
ClO
4
)(3)
orthorhombic 1.4818
1.4833
1.4881
2V = 70 °(+)
six-sided prisms Low order interference colors.
Crystals lying on a {110} face
will exhibit an off-centered
biaxial interference figure.
Sodium perchlorate (NaClO
4
)
(3)
orthorhombic 1.4606
1.4617
1.4730
2V = 69 °(+)
See NaClO4.2H20 Sodium chlorate recrystallizes
as the hydrate. Optical data is
for the anhydrate.
Sodium perchlorate dihydrate
(NaClO
4
·2H
2
O) (3)
monoclinic diamonds, rhombs Low relief in water. Some
orientations exhibit oblique
extinction. Refractive indices
not known.
Potassium chlorate (KClO
3
)(3)monoclinic 1.415
1.517
1.523
2V = 28 °(-)
diamond-shaped tablets, often
truncated
Moderate order interference
colors, symmetrical extinction.
Potassium nitrate (KNO
3
)(3)hexagonal & orthorhombic 1.3346
1.5056
1.5064
2V = 7 °(-)
rhombohedra & chevrons early;
prisms late
The hexagonal forms with high
order interference colors
appear first at the edge of the
drop. Moving them to the
middle of the drop with a glass
rod or toothpick causes the
stable orthorhombic forms to
appear.
Ammonium nitrate (NH
4
NO
3
)
(3,5)
orthorhombic 1.413
1.611
1.637
2V = 35 °(-)
prisms, as blades & rods Crystal formation is slow due to
the high solubility of ammonium
nitrate. The resulting crystals
exhibit high order interference
colors. As a fusion preparation
cools, three to four polymorphs
are observed. MP = 169 °C.
Sodium nitrate (NaNO
3
)(3)hexagonal 1.5874 (ω)
1.3361 (ε)
rhombohedra High order interference colors
and symmetrical extinction.
E3423 − 24
4
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