ASTM E2937-13
(Guide)Standard Guide for Using Infrared Spectroscopy in Forensic Paint Examinations
Standard Guide for Using Infrared Spectroscopy in Forensic Paint Examinations
SIGNIFICANCE AND USE
5.1 FTIR spectroscopy may be employed for the classification of paint binder types and pigments as well as for the comparison of spectra from known and questioned coatings. When utilized for comparison purposes, the goal of the forensic examiner is to determine whether any significant differences exist between the known and questioned samples.
5.2 This guide is designed to assist an examiner in the selection of appropriate sample preparation methods and instrumental parameters for the analysis, comparison or identification of paint binders and pigments.
5.3 It is not the intent of this guide to present comprehensive theories and methods of FTIR spectroscopy. It is necessary that the examiner have an understanding of FTIR and general concepts of specimen preparation prior to using this guide. This information is available from manufacturers’ reference materials, training courses, and references such as: Forensic Applications of Infrared Spectroscopy (Suzuki, 1993) (4), Infrared Microspectroscopy of Forensic Paint Evidence (Ryland, 1995) (5), Use of Infrared Spectroscopy for the Characterization of Paint Fragments (Beveridge, 2001) (6), and An Infrared Spectroscopy Atlas for the Coatings Industry (2).
SCOPE
1.1 This guide applies to the forensic IR analysis of paints and coatings and is intended to supplement information presented in the Forensic Paint Analysis and Comparison Guidelines (1)2 written by Scientific Working Group on Materials Analysis (SWGMAT). This guideline is limited to the discussion of Fourier Transform Infrared (FTIR) instruments and provides information on FTIR instrument setup, performance assessment, sample preparation, analysis and data interpretation. It is intended to provide an understanding of the requirements, benefits, limitations and proper use of IR accessories and sampling methods available for use by forensic paint examiners. The following accessory techniques will be discussed: FTIR microspectroscopy (transmission and reflectance), diamond cell and attenuated total reflectance. The particular methods employed by each examiner or laboratory, or both, are dependent upon available equipment, examiner training, specimen size or suitability, and purpose of examination. This guideline does not cover the theoretical aspects of many of the topics presented. These can be found in texts such as An Infrared Spectroscopy Atlas for the Coatings Industry (Federation of Societies for Coatings, 1991) (2) and Fourier Transform Infrared Spectrometry (Griffiths and de Haseth, 1986) (3).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
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 appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E2937 − 13
Standard Guide for
Using Infrared Spectroscopy in Forensic Paint
Examinations
This standard is issued under the fixed designation E2937; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Infrared (IR) spectroscopy is commonly used by forensic laboratories for the analysis of paints and
coatings received in the form of small chips, residues, particles, or smears, and serves as a staple
comparative technique in the assessment of whether or not questioned paint could have come from a
particular source. IR spectroscopy provides molecular structure information on many of the organic
and inorganic constituents contained within a single paint layer. This information can be used to
classify both binders and pigments in coating materials. The classification information may then be
utilized to identify probable types of paint such as architectural, automotive, or maintenance.
Additionally, the use of automotive paint databases may allow the determination of information such
as potential vehicle year, make and model. Databases may also aid in the interpretation of the
significance (for example, how limited is the group of potential donor sources) of a questioned paint.
1. Scope (Federation of Societies for Coatings, 1991) (2) and Fourier
Transform Infrared Spectrometry (Griffiths and de Haseth,
1.1 This guide applies to the forensic IR analysis of paints
1986) (3).
and coatings and is intended to supplement information pre-
sented in the Forensic Paint Analysis and Comparison Guide- 1.2 The values stated in SI units are to be regarded as
lines (1) written by Scientific Working Group on Materials standard. No other units of measurement are included in this
Analysis (SWGMAT). This guideline is limited to the discus- standard.
sion of Fourier Transform Infrared (FTIR) instruments and
1.3 This standard does not purport to address all of the
provides information on FTIR instrument setup, performance
safety concerns, if any, associated with its use. It is the
assessment, sample preparation, analysis and data interpreta-
responsibility of the user of this standard to establish appro-
tion. It is intended to provide an understanding of the
priate safety and health practices and determine the applica-
requirements, benefits, limitations and proper use of IR acces-
bility of regulatory limitations prior to use.
soriesandsamplingmethodsavailableforusebyforensicpaint
examiners. The following accessory techniques will be dis- 2. Referenced Documents
cussed: FTIR microspectroscopy (transmission and
2.1 ASTM Standards:
reflectance), diamond cell and attenuated total reflectance.The
D16TerminologyforPaint,RelatedCoatings,Materials,and
particular methods employed by each examiner or laboratory,
Applications
or both, are dependent upon available equipment, examiner
E131Terminology Relating to Molecular Spectroscopy
training, specimen size or suitability, and purpose of examina-
E1421Practice for Describing and Measuring Performance
tion. This guideline does not cover the theoretical aspects of
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
many of the topics presented.These can be found in texts such
eters: Level Zero and Level One Tests
as An Infrared Spectroscopy Atlas for the Coatings Industry
E1492Practice for Receiving, Documenting, Storing, and
Retrieving Evidence in a Forensic Science Laboratory
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 Sept. 1, 2013. Published October 2013. DOI: 10.1520/ For referenced ASTM standards, visit the ASTM website, www.astm.org, or
E2937-13. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2937 − 13
E1610Guide for Forensic Paint Analysis and Comparison 3.2.16 mercury cadmium telluride (MCT) detector—aquan-
tum detector that utilizes a semi-conducting material and
requires cooling with liquid nitrogen to be operated; this type
3. Terminology
ofdetectoriscommonlyusedinmicroscopeaccessoriesdueto
3.1 Definitions—For definitions of terms used in this guide
its sensitivity.
other than those listed here, see Terminologies D16 and E131.
3.2.17 paint—a pigmented coating.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 100 % line—calculated by ratioing two background
3.2.18 pigment—a finely ground, inorganic or organic,
spectratakenunderidenticalconditions;theslopeandnoiseof insoluble, and dispersed particle; besides color, a pigment may
100 % lines are used to measure the performance of the
provide many of the essential properties of paint such as
instrument. opacity,hardness,durability,andcorrosionresistance;theterm
pigment includes extenders.
3.2.2 absorbance (A)—the logarithm to the base 10 of the
reciprocal of transmittance T, written as A = log 10 (1/T) =
3.2.19 representative sample—a portion of the specimen
–log T.
selected and prepared for analysis that exhibits all of the
characteristics of the parent specimen.
3.2.3 absorbance spectrum—arepresentationoftheinfrared
spectrum in which the ordinate is defined in absorbance units
3.2.20 significant difference—a difference between two
(A);absorbanceislinearlyproportionaltoconcentrationandis
samples that indicates that they do not share a common origin.
therefore used in quantitative analysis.
3.2.21 smear—a transfer of paint resulting from contact
3.2.4 additive (modifier)—any substance added in a small
betweentwoobjects;thesetransfersmayconsistofco-mingled
quantity to improve properties; additives may include sub-
particlesfromtwoormoresources,fragments,orcontributions
stances such as driers, corrosion inhibitors, catalysts, ultravio-
from a single source.
let absorbers, and plasticizers.
3.2.22 specimen—a material submitted for examination;
3.2.5 attenuated total reflectance (ATR)—a method of spec-
samples are removed from a specimen for analysis.
trophotometricanalysisbasedonthereflectionofenergyatthe
interfaceoftwomediathathavedifferentrefractiveindicesand
3.2.23 transmittance (T)—the ratio of the energy of the
are in intimate contact with each other. radiation transmitted by the sample to the background, usually
expressed as a percentage.
3.2.6 aperture—an opening in an optical system that con-
trols the amount of light passing through a system.
3.2.24 transmittance spectrum—a representation of the in-
frared spectrum in which the ordinate is defined in %T;
3.2.7 background—the signal produced by the entire ana-
transmittance is not linearly proportional to concentration.
lytical system apart from the material of interest.
3.2.8 beam condenser—a series of mirrors that focus the
3.2.25 wavelength—thedistance,measuredalongthelineof
infrared beam in the sample compartment to permit the propagation, between two points that are in phase on adjacent
examination of smaller specimens. waves.
3.2.9 beam splitter—an optical component that partially
3.2.26 wavenumber—the inverse of the wavelength; or, the
reflects and partially transmits radiation from the source in
number of waves per unit length, usually conveyed in recip-
-1
such a manner as to direct part to a fixed mirror and the other
rocal centimeters (cm ).
part to a moving mirror.
4. Summary of Practice
3.2.10 binder—anonvolatileportionoftheliquidvehicleof
acoating,whichservestobindorcementthepigmentparticles
4.1 The film forming portion of a paint or coating is the
together.
organic binder, also referred to as the resin. The binder forms
3.2.11 coating—agenerictermforpaint,lacquer,enamel,or
a film that protects as well as displays the organic and
other liquid or liquefiable material that is converted to a solid,
inorganic pigments that make a coating both decorative and
protective,ordecorativefilmoracombinationofthesetypesof
functional. Infrared spectroscopy is commonly employed for
films after application.
the analysis of paint binders, pigments and other additives that
are present in detectable concentrations.
3.2.12 deuterated triglycine sulfate (DTGS) detector—a
thermal detector that operates at room temperature but lacks
4.2 Paints and coatings absorb infrared radiation at charac-
the sensitivity for use with microscope accessories.
teristic frequencies that are a function of the coating’s compo-
3.2.13 extraneous material (contaminant, foreign
sition. These absorption frequencies are determined by vibra-
material)—material originating from a source other than the
tions of chemical bonds present in the various components.
specimen.
4.3 Theanalysisofcoatingsusinginfraredspectroscopycan
3.2.14 interferogram—a plot of the detector output as a
be carried out using either transmission or reflectance tech-
function of retardation.
niques. These measurements can be taken with a variety of
3.2.15 microtomy—a sample preparation method that se- equipment configurations and sampling accessories, the most
quentially passes a blade at a shallow depth through a common being the use of an infrared microscope.Avariety of
specimen, resulting in sections of selected thickness. accessories can also be utilized in the system’s main bench.
E2937 − 13
However, the use of a nonmicroscope accessory typically Applications of Infrared Spectroscopy (Suzuki, 1993) (4),
requires a larger sample size than those that can be analyzed Infrared Microspectroscopy of Forensic Paint Evidence
using a microscope. (Ryland, 1995) (5), Use of Infrared Spectroscopy for the
Characterization of Paint Fragments (Beveridge, 2001) (6),
4.4 For transmission FTIR, a thin-peel of each paint layer,
and An Infrared Spectroscopy Atlas for the Coatings Industry
orathincross-sectionofapaintsampleismadeeitherbyhand
(2).
with a sharp blade or using a microtome. It is then analyzed
using either a microscope attachment or other suitable
6. Sample Handling
accessory, such as a diamond anvil cell. When thin samples
suitable for transmission FTIR are not obtainable, reflectance
6.1 The general collection, handling, and tracking of
techniques (ATR, reflection) may be employed using micro-
samples shall meet or exceed the requirements of Practice
scope objectives or bench accessories.
E1492 as well as the relevant portions of the SWGMAT’s
Trace Evidence Quality Assurance Guidelines (7).
4.5 Basic Principles:
4.5.1 Infrared spectroscopy (mid-range) is capable of utiliz-
6.2 The work area and tools used for the preparation of
-1
ingaspectralrangebetween4000andapproximately400cm .
samples shall be free of all extraneous materials that could
Extended range instruments are needed to take measurements
transfer to the sample.
-1
down to approximately 200 cm . The actual spectral cutoff
6.3 As stated in Guide E1610, a paint specimen should first
depends upon the type of detector and optics used.
be examined with a stereomicroscope, noting its size,
4.5.2 An FTIR spectrometer measures the intensity of
appearance, layer sequence, heterogeneity within any given
reflected or transmitted radiation over a designated range of
layer,andpresenceofanymaterialthatcouldinterferewiththe
wavelengths.Thespectrumofasampleisproducedbyratioing
analysis (for example, traces of adhesive, surface abrasion
the transmitted or reflected infrared spectrum to a background
transfers, or zinc phosphate conversion coating residue on the
spectrum.
underside of the base primer on electro-coated parts). Some
4.5.3 Transmission spectra may be plotted either in percent
surface materials may be of interest and therefore may be
transmittance (%T) or in absorbance (A). Reflectance spectra
worthy of analysis before removal.
may be plotted either in percent reflectance (%R) or in
absorbance (A).
6.4 Each layer of a multi-layered paint should be analyzed
individually.
4.6 Instrumentation:
4.6.1 An FTIR instrument consists of a source to produce
6.5 When analyzing difficult items (for example, smears,
infrared radiation, an interferometer, a detector and a data
dirty or heterogeneous specimens) care shall be taken when
processing device.Amicro-FTIR instrument also has a micro-
sampling the paint and in choosing appropriate analytical
scope equipped with a detector and infrared compatible optics.
conditions. An attempt should be made to remove any extra-
4.6.2 Most FTIR systems are equipped to collect data using
neous material from the exhibit before sampling. It may be
-1
the main bench in the range of 4000 to 400 cm . Extended
necessary to analyze a number of samples to ensure reproduc-
range systems are equipped with a beamsplitter and optics that
ibility and understand inter/intrasample variation.
-1
allow transmission down to approximately 200 cm . Systems
6.6 Extraneous material should be removed either by scrap-
equipped with an FTIR microscope utilize a more sensitive
ingwithasuitabletoolsuchasascalpelorwashingwithwater.
detector type. Depending on the specific detector type, micro-
If needed, alcohols or light aliphatic hydrocarbons can be
scopic samples can be analyzed in the range of approximately
-1
usefulforcleaning.However,itshouldbenotedthattheuseof
4000 to 450 cm .
organicsolventsforcleaningpaintcanalterthecompositionby
5. Significance and Use extracting soluble components such as plasticizers or dissolve
the paint binder. If solvents are used, known and unknown
5.1 FTIR spectroscopy may be employed for the classifica-
samples should be treated the same, making sure no residual
tion of paint binder types and pigments as well as for the
solvent remains.
comparison of spectra from known and questioned coatings.
When utilized for comparison purposes, the goal of the
6.7 For the accurate comparison of paint evidence, samples
forensic examiner is to determine whether any significant
should be prepared and analyzed in the same manner.
differences exist between the known and questioned samples.
7. Analytical Techniques and Operating Conditions
5.2 This guide is designed to assist an examiner in the
selection of appropriate sample preparation methods and in-
7.1 Paints may be analyzed by transmission or reflectance
strumental parameters for the analysis, comparison or identi-
utilizing the microscope accessory or the bench accessories.
fication of paint binders and pigments.
Thetechniquechosenisdependent
...
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