4.1 The SEM can be used to define and compare the layer structure of multilayered samples, the structure of individual layers, the bulk elemental composition of individual layers, and the elemental composition of individual particulate components within paints and coatings.

4.2 The test methods described in this guide may have some limitations. They include the inability to detect elements in trace concentrations, the need for a conductive coating of the sample, the inability to remove a sample from most embedding materials after analysis, and the discoloration of materials by irradiation.

4.3 Although quantitative and semiquantitative methods are available for EDS (see Guide E1508), they are not appropriate for most paint analyses because of the typical heterogeneity of paint. Application of quantitative methods is further complicated by an inability to predict what compounds may be present (see 7.12.1).

4.4 The information available from a specimen may diminish as its size is reduced and its condition degrades. The smaller a specimen is, the less valuable it becomes for association with a known because it may contain fewer characteristics of the original material. As specimen size is reduced, it may no longer be representative of the original material. This may also be true of a degraded sample.

4.5 This guide is intended to advise and assist laboratory analysts in the effective application of scanning electron microscopy to the analysis of paint evidence. It is intended to be applicable to most modern scanning electron microscopes typically used in the forensic laboratory.

4.6 It is not the intention of this guide to present comprehensive methods of SEM. It is necessary that the analyst have an understanding of SEM operation and general concepts of specimen preparation before using this guide. This information is available from manufacturers' reference materials, training courses, and references such as Scanning Electron Microscopy and X-ray Microanalysis: A Text for Biologists, Materials Scientists, and Geologists (1).³