ASTM E3501-25

1.1 This guide covers recommended techniques and procedures for the use of Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry (SEM/EDS) for the forensic analysis of geological materials to include soils, rocks, sediments, and materials derived from them (for example, concrete).

1.2 This guide describes some options for sample handling and preparation of specimens, instrument operating conditions, spectral and image data collection, evaluation of data quality, interpretation of EDS spectra for the identification of inorganic geological materials, documentation of morphology by SEM imaging, and criteria for sample comparison.

1.3 This guide is intended for use by competent forensic science practitioners with the requisite formal education, discipline-specific training (refer to Practice E2917) and demonstrated proficiency to perform forensic casework.

1.4 The values stated in SI units are to be regarded as standard. Other units are avoided, but photon energy is commonly reported in units of kiloelectronvolts (keV), data are collected as counts per second (cps), and compositional data based on EDS are reported in elemental weight percent (wt. %).

1.5 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.

1.6 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.

4.1 This guide is intended to advise and assist analysts in the effective application of SEM/EDS to the analysis of geological materials. It is intended to be applicable to most modern SEM/EDS systems typically available in the forensic laboratory. The theoretical foundation of SEM/EDS is covered in numerous texts such as Scanning Electron Microscopy and X-ray Microanalysis (1).

4.2 The goals of a forensic examination of geological evidence include identification of an unknown substance, comparison of two or more items for possible common origin, or estimation of provenance. SEM/EDS analysis is a component of an overall examination scheme and is not typically used on its own.

The presence, absence, and relative abundance of mineral species or morphotypes determined by SEM/EDS in two samples can be used to evaluate whether there are exclusionary differences during a forensic comparison.

4.3 SEM/EDS can be used to determine the elemental composition of geologically derived materials with high spatial resolution (sub-micrometer).

4.4 SEM/EDS is advantageous because it allows for the simultaneous imaging of individual particles to determine morphology and the measurement of the elemental composition of particles or discrete domains within particles.

4.5 The elemental compositions derived from SEM/EDS can be used to assign provisional mineral identifications by visual comparison of specimen EDS data to the following:

• reference EDS data published in mineralogy textbooks (2);
• empirically collected EDS data from known mineral standards;
• published mineral elemental composition data; or
• simulated EDS spectra (DTSA-II (3)).

4.6 Qualitative or semi-quantitative EDS data can provide additional information to test provisional mineral identifications made using other techniques (for example, polarized light microscopy (PLM), Raman spectroscopy). In this guide, semi-quantitative EDS analysis consists of attributing peaks to elements and observing the peak height and area.

4.7 Imaging by SEM can provide additional information to test and provide support for provisional mineral identifications and morphometric evaluation.

4.8 Characterization of building materials by SEM/EDS to assess engineering properties is beyond the scope of this guide. Refer to Guides C1723 and C295, and Practice C856.

4.9 Procedures for quantitative methods using EDS are not covered in this guide. Refer to Guide E1508, ISO 22309:2011, or Ref (1).

4.10 Limitations to SEM/EDS:

• 4.10.1 Detection limits depend on elements and matrices, and are typically around 0.1 weight % concentration for elements with atomic numbers above fluorine. For lighter elements, detection limits are typically around 1 weight % concentration. Alternative analytical techniques, such as inductively coupled plasma mass spectrometry or micro X-ray fluorescence, can provide lower detection limits.
• 4.10.2 Some samples and SEM systems require covering the specimen with a conductive coating.
• 4.10.3 Electron beam irradiation can irrevocably change some specimens (for example, discoloration, loss of water, or migration of light elements).
• 4.10.4 EDS only provides elemental information, which could limit its ability to identify certain minerals on its own.
• 4.10.5 EDS data obtained from a compositional domain (zone) of a particle might not be representative of the whole particle.