Standard Guide for Using Scanning Electron Microscopy/x-Ray Spectrometry in Forensic Paint Examinations: Scientific Working Group on Materials Analysis (SWGMAT)

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1. Scope

This document is an outline of methods for scanning electron microscopy intended for use by forensic paint examiners. The methods employed by each examiner and/or laboratory depends upon sample size, sample suitability, and laboratory equipment. The term scanning electron microscopy occasionally refers to the entire analytical system, including energy dispersive X-ray spectrometry and/or wavelength dispersive X-ray spectrometry. This guide does not cover the theoretical aspects of many of the topics presented.

This guide does not purport to address all of the safety concerns, if any, associated with this technology. It is the responsibility of the analyst to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to its use.

2. Referenced

2.1. ASTM. Guide for Forensic Paint Analysis and Comparison, ASTM E1610-94, 1994.

2.2. ASTM. Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Laboratory, ASTM E1492-92, 1992.

2.3. Scientific Working Group on Materials Analysis. Forensic paint analysis and comparison guideline, Forensic Science Communications [Online]. (July 1999). Available: www.fbi.gov/hq/lab/fsc/backissu/july1999/painta.htm

2.4. Scientific Working Group on Materials Analysis. Trace evidence quality assurance guidelines, Forensic Science Communications [Online]. (January 2000). Available: www.fbi.gov/hq/lab/fsc/backissu/jan2000/swgmat.htm

2.5. Scientific Working Group on Materials Analysis. Trace evidence recovery guidelines, Forensic Science Communications [Online]. (October 1999). Available: www.fbi.gov/hq/lab/fsc/backissu/oct1999/trace.htm

3. Definitions

Background X-rays (Bremsstrahlung, braking radiation, continuous spectrum): Nonspecific X-ray radiation with a continuous energy range from zero up to the beam voltage. Background radiation results from the deceleration of beam electrons in the atomic Coulombic field. A typical X-ray spectrum consists of both a continuous background and peaks from characteristic X-rays.

Backscattered electrons (BE): Primary beam electrons that are scattered from the sample after undergoing few inelastic interactions. The probability of backscattering is proportional to the atomic number.

Bulk analysis: A type of scanning electron microscopy analysis that determines the average elemental composition of a material. The area of analysis is as large as possible and may be achieved by a single large area raster or the summed results from multiple smaller area rasters.

Cathodoluminescence: Emission of photons in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum as a result of electron beam interaction with certain materials.

Characteristic X-rays: X-ray emission resulting from de-excitation of an atom following inner shell ionization. The energy of the X-rays is related to the atomic number of the atom, providing the basis for energy dispersive X-ray spectrometry. A typical X-ray spectrum consists of both a continuous background and peaks from characteristic X-rays.

Charging: Negative charge accumulation on either a nonconductive sample or a sample that is not properly grounded. This effect may interfere with image formation and X-ray analysis because of beam deflection. It can usually be eliminated by the application of a conductive coating.

Concentration: For the purpose of this guide, the following ranges shall apply: Major: greater than 10 percent; Minor: 1 to 10 percent; Trace: less than 1 percent.

Detector fluorescence peak (dead-layer peak, silicon internal fluorescence peak): A peak resulting from the emission of characteristic X-rays in a thin layer of inactive crystal area in the front of an energy dispersive X-ray spectrometer detector. The peak is characteristic of the type of detector, such as silicon for a lithium-drifted silicon detector. …