ASTM E3295-23

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
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Designation: E3295 − 22 E3295 − 23 An American National Standard
Standard Guide for
Using Micro X-Ray Fluorescence (μ-XRF) in Forensic
Polymer Examinations
This standard is issued under the fixed designation E3295; 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.
INTRODUCTION
Micro X-ray fluorescence spectrometry (μ-XRF) is one technique in an analytical scheme that can
provide information regarding potential relationships between the sources of polymeric materials.
1. Scope
1.1 This guide covers recommended techniques and procedures intended for use by forensic laboratory personnel that perform
μ-XRF analysis of polymer samples.
1.2 This guide describes various techniques and procedures used in the μ-XRF analysis of polymers that include sample handling
and preparation, instrument operating conditions, and spectral data collection, evaluation and interpretation.
1.3 This guide describes the application of μ-XRF systems equipped with either mono- or poly- capillary optics and an energy
dispersive X-ray detector (EDS).
1.4 This guide is intended to be applied within the scope of a broader analytical scheme (for example, Guide E1610, Guide E3260)
for the forensic analysis of a polymer sample (1-6). A μ-XRF analysis can provide additional information regarding the potential
relationships between the sources of polymeric materials.
1.5 The fundamental aspects of the composition and manufacture of polymeric materials or theory of X-ray fluorescence can be
found in various texts (7-18).
1.6 This standard is intended for use by competent forensic science practitioners with the requisite formal education,
discipline-specific training (see Practices E2917, E3233, E3234), and demonstrated proficiency to perform forensic casework.
1.7 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.8 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
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 Nov. 1, 2022Aug. 1, 2023. Published February 2023August 2023. Originally approved in 2022. Last previous edition approved in 2022 as
E3295 – 22. DOI: 10.1520/E3295-22.10.1520/E3295-23.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E3295 − 23
1.9 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.
2. Referenced Documents
2.1 ASTM Standards:
E620 Practice for Reporting Opinions of Scientific or Technical Experts
E1492 Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory
E1610 Guide for Forensic Paint Analysis and Comparison
E1732 Terminology Relating to Forensic Science
E2917 Practice for Forensic Science Practitioner Training, Continuing Education, and Professional Development Programs
E2926 Test Method for Forensic Comparison of Glass Using Micro X-ray Fluorescence (μ-XRF) Spectrometry
E3233 Practice for Forensic Tape Analysis Training Program
E3234 Practice for Forensic Paint Analysis Training Program
E3260 Guide for Forensic Examination and Comparison of Pressure Sensitive Tapes
2.2 SWGMAT Documents:
SWGMAT Trace Evidence Recovery Guidelines
SWGMAT Trace Evidence Quality Assurance Guidelines
2.3 ISO Standard:
ISO/IEC 17025 Laboratory Competence
3. Terminology
3.1 Definitions—The terms defined relate specifically to μ-XRF as described in this document. For additional terms commonly
employed for general forensic examinations, see Terminology E1732.
3.2 Definitions:
3.2.1 background radiation, n—X-rays resulting from scattered Bremsstrahlung and coherently and incoherently scattered tube
target peaks.
3.2.2 characteristic X-ray, n—X-ray emission resulting from de-excitation of an atom following inner shell ionization.
3.2.2.1 Discussion—
The energy of a characteristic X-ray is related to the atomic number of the atom, providing the basis for energy dispersive X-ray
spectroscopy.
3.2.3 coherent (Rayleigh) scatter peaks, n—spectral artifacts that result from elastic scattering of the tube target characteristic
X-rays by the sample.
3.2.3.1 Discussion—
Because no energy is lost in elastic scattering, coherent scatter peaks occur at the same energies as the tube target characteristic
X-rays.
3.2.4 dead time, n—the time (expressed as a percentage of real time) during which the energy dispersive X-ray spectrometer is
not able to process X-rays.
3.2.5 diffraction peaks, n—spectral artifacts that result from preferential diffraction of tube X-rays into the detector as a result of
striking a crystalline sample.
3.2.5.1 Discussion—
Diffraction peaks vary in energy and intensity depending on orientation of the crystalline planes with respect to the beam angle.
3.2.6 escape peak, n—a spectral artifact resulting from incomplete deposition of the energy of an X-ray entering the energy
dispersive X-ray spectrometer detector.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
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https://www.iso.org.
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3.2.6.1 Discussion—
An escape peak is produced when an incoming X-ray excites a silicon atom within the detector crystal, and the resulting Si Kα
fluorescence X-ray exits the detector crystal. It occurs at the energy for the original X-ray minus the energy of the Si Kα
fluorescence X-ray (1.74 keV). The escape peak intensity is about 1-2 % of the parent peak.
3.2.7 exclusionary difference, n—a difference in a feature or property one or more characteristics between compared items that is
substantial enough sufficient to determine that they the compared items did not originate from the same source.source, are not the
same source, or do not share the same composition or classification.
3.2.8 incoherent (Compton) scatter peaks, n—spectral artifacts that result from inelastic scattering of the tube target characteristic
X-rays by the sample.
3.2.8.1 Discussion—
Because energy is lost in inelastic scattering, incoherent scatter peaks occur at a lower energy than the tube target characteristic
X-rays.
3.2.9 KLM reference lines, n—the energies associated with the transitions of the K, L, and M shell electrons.
3.2.9.1 Discussion—
Each element has characteristic energies of transitions of electrons between shells.
3.2.10 live time, n—the time during which an energy dispersive X-ray spectrometer is available to accept and process incoming
X-rays.
3.2.10.1 Discussion—
Live time is often expressed as a percentage of real time, in seconds.
3.2.11 pulse processor time constant, n—operator-selected value for the time designated to record a response by the detector. A
higher value (longer time) results in a more accurate determination of the detector amplifier pulse height (better spectral
resolution). A lower value results in a higher count rate but with reduced spectral resolution.
3.2.12 spectral artifacts, n—spectral peaks other than characteristic peaks from the sample, produced during the energy dispersive
detection process. Examples include escape peaks, sum peaks, tube target coherent and incoherent scatter peaks, system peaks, and
diffraction peaks.
3.2.13 spectral resolution, n—measure of the ability to distinguish between adjacent peaks in a spectrum; it is usually determined
by measuring peak width at half the maximum value of the peak height or full-width half-maximum (FWHM).
3.2.13.1 Discussion—
This value is usually quoted for the FWHM of Mn Kα.
3.2.14 sum peak, n—a spectral artifact that results from the simultaneous detection of two X-rays, manifested as a peak at the
combined energy of the detected X-rays.
3.2.15 system peaks, n—spectral artifacts that result from the production of characteristic X-rays from structural components of
the XRF instrument.
4. Significance and Use
4.1 μ-XRF is a nondestructive qualitative elemental analysis technique used for polymers. It involves excitation of a sample by
an X-ray source resulting in the emission of characteristic X-rays detected using an energy dispersive X-ray detector. Results are
displayed simultaneously as a spectrum of intensity as a function of energy for elements of atomic number 11 or greater.
4.2 μ-XRF enables the determination of the elemental composition of a specimen and can be utilized for comparisons of
components of polymeric materials (for example, tape backings, tape adhesives, paint layers).
4.3 Comparisons of X-ray spectra acquired from polymer samples are conducted for source discrimination or potential association.
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4.4 Quantitative processes for μ-XRF analysis are available but are not used for polymer analyses because of the lack of prepared
polymer standard reference samples.
4.5 In general, information available from a heterogeneous specimen diminishes as its size is reduced or its condition degrades,
which lessens its likelihood of being representative of the source material.
4.6 μ-XRF data collected from polymers is limited to specific information (for example, elements detected, relative elemental
abundance); additional analytical procedures are required to further characterize and identify the chemical composition of the
polymer sample.
4.7 Limitations of μ-XRF include the inability to detect some elements in trace concentrations, the inability to analyze individual
particles, the potential interference related to the penetration depth of the beam relative to the sample thickness, the inability to
resolve the peaks of some elements (for example, Ba Lα / Ti Kα), and the potential for discoloration of some materials due to
exposure to radiation.
5. Calibration and Standardization
5.1 Performance Checks—The instrument is optimized in accordance with manufacturer’s instruction.
5.1.1 Prior to sample analysis, operating tolerances for each of the following parameters and the minimum frequency at which they
are monitored are established and recorded:
5.1.1.1 Energy Calibration—The X-ray energy scale is calibrated to characteristic X-ray emission lines by either measuring the
centroid energy of a low- (<2 keV) and high- (>6 keV) energy peak or by using software provided by the instrument manufacturer.
For example, the aluminum (1.486 keV) and copper (8.040 keV) Kα -X-ray energy lines can be used.
5.1.1.2 Stage Calibration—For automated or multiple point analysis, the stage position is initialized to ensure that the stage
coordinates accurately reflect the stage position.
5.1.1.3 Optical Alignment—X-ray optics are aligned to obtain the maximum count rate. Align visualization optics to ensure that
the visual target area coincides with the X-ray beam position.
5.1.1.4 Reference Standard—A reference sample (for example, NIST SRM 1831, Mn, and Zr) is analyzed to verify calibration of
X-ray energy lines for elements present and determine if the instrument response is within acceptable limits.
5.1.1.5 Blanks—A spectrum is collected of a sample devoid of elements having an atomic number of 11 or greater, such as the
plastic stage plate or an area of the support material having no sample present. System peaks present are recorded for future
reference.
6. Specimen and Sample Handling
6.1 Practice E1492 and relevant portions of the SWGMAT Trace Evidence Quality Assurance Guidelines and Trace Evidence
Recovery Guidelines are followed for the collection, handling, and tracking of samples and specimens.
6.2 In developing an analytical scheme, consider the following:
6.2.1 Presence of extraneous materials and a strategy for removal,
6.2.2 Means of attachment to an inert XRF mount,
6.2.3 Procedure(s) for producing a uniform geometry, and
6.2.4 Determination of the presence of surface features of analytical interest.
6.3 Sample Preparation:
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6.3.1 The specimen is examined to determine an appropriate sample preparation approach. The choice for sample preparation
depends on the size, nature, and condition of the specimen, as well as the analytical objective. Multiple preparation procedures are
described below for use as necessary.
6.3.2 Samples for comparison are prepared under the same conditions whenever possible. The preparation approach, any details
required to reproduce the preparation process, and differences between sample preparations are recorded.
6.3.3 Samples for μ-XRF analysis are prepared in a manner that permits resolution and analysis of individual layers, using one
or more of the following:
6.3.3.1 A portion or all of the specimen is cleaned to remove any contamination or extraneous materials that might interfere with
the analysis. This can be accomplished using physical means (for example, particle picking with a probe, exposing underlying
polymer with a scalpel) or chemical means (for example, washing in a solvent).
6.3.3.2 Cross-sections of multiple layers or thin sections within a single layer are removed using a scalpel, microtome, or other
separation technique.
6.3.4 The sample mounting technique depends on the sample size and shape, beam size, X-ray fluorescence spectrometer chamber
design, and purpose of the examination.
6.3.5 Samples are elevated off the surface of the analysis stage using an X-ray transparent sample holder or supportive X-ray film,
or both. This sample mounting improves the spectrum signal-to-noise ratio by reducing X-ray scatter from the surface of the stage.
Because analysis is performed under vacuum, samples are secured with adhesive to retain their position on the sample holder.
Several different thin polymer support films are commercially available (for example, mylar, polypropylene, polycarbonate,
Kapton). polycarbonate). These films vary in their X-ray transmission characteristics and therefore should be evaluated and
selected based upon their transmission characteristics.
6.3.6 The penetration depth of the X-ray beam in a polymer sample is difficult to predict since it is dependent on the elemental
composition of the sample and, hence, the absorption characteristics of that sample. Because beam penetration can reach several
millimeters below the top surface, X-ray emissions from underlying layers, substrate, or both can be detected. In addition, the
intensity of the X-ray emissions from an element in the uppermost layers can be enhanced. These effects will generally be confined
to the higher energy X-rays in each layer and will be affected by the total elemental composition of each layer.
6.3.7 Samples removed from individual layers of multi-layered items (for example, paint layers, tape backing and adhesive)
should be mounted separately.
6.3.8 The geometry of compared samples, including flatness, thickness, and take-off angle, should be similar.
6.3.9 A small amount of the mounting adhesive and mounting film is also analyzed to determine the presence of any elements in
the sample X-ray spectrum that can be attributed to these materials.
7. Instrument Operating Conditions
7.1 The following are recommended operating parameters that can be altered to optimize conditions for v
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