Forensic Paint Analysis and Comparison Guidelines by SWGMAT Paint Subgroup, Part 3 (FSC, July 1999)
July 1999 - Volume 1 - Number 2
Forensic Paint Analysis and Comparison Guidelines
Scientific Working Group on Materials Analysis
May 2000 Revision
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9.1. Fluorescence Microscopy
9.1.1. Fluorescence microscopy of thin or bulk cross sections, as an aid in differentiating samples or various layers within intact paint fragments, is discussed by Stoecklein and Tuente (10). When using an excitation wavelength of 365 nanometers, the technique may be sensitive to differences in organic pigments, additives, and film-forming components. Allen (53) reports it to be most useful with light-colored architectural coatings.
9.2. Low-Temperature Ashing
9.2.1. The low-temperature asher is a device in which an oxygen plasma is used to remove organic materials from a complex matrix. Materials that produce volatile oxides (principally organic components) are removed from the matrix with minimal elevation of the sample temperature in contrast to pyrolysis systems. Ashing usually continues until all such volatile oxides are removed.
9.2.2. Inorganic pigments, extenders, and some additives in the different layers of the ashed paint film will remain after the organic material is volatilized. The relative size and morphology of the different particles, noted during previous tests, serve to help identify and separate these residue components for additional analysis. A brief description of the technique is provided by McCrone and Delly (54) and Brown (55).
9.2.3. Ashing residues can be analyzed by a variety of methods, including PLM, SEM-EDS, analytical electron microscopy (AEM), or XRD techniques.
9.3. Solvent Extraction
9.3.1. Solvent extraction can be used to separate some of the organic components from paint films, depending on the paint system or systems in question. The objective of the procedure is to recover a solute that can be examined by IR, GC, or GC-MS techniques. It is especially useful for coatings where the volume of pigments or extenders is very high. The identification of the binders by FTIR is often only possible after separation of the pigments and extenders.
9.3.2. When using solvent extraction, separation of paint layers is very important. If this is impossible, it is important that identical conditions (e.g., time, and temperature) be applied to both the known and questioned samples.
9.4. Analytical Electron Microscopy
9.4.1. Analytical electron microscopy (AEM) is the term applied to the use of transmission electron microscopy (TEM) in conjunction with both selected area electron diffraction and EDS. The combination of techniques can provide more definitive identification of some pigment grains that cannot be identified conclusively by PLM because of their extremely small size or opacity.
9.4.2. AEM requires that the sample be sufficiently thin to permit transmission of the analytical electron beam. It is thus applied only to dispersions of extracted inorganic particulates, such as those recovered from low-temperature ashing, dissolved paint layers, or ultramicrotomed sections of a paint film (56).
9.5.1. Cathodoluminescence (CL) is the emission of radiation from the sample in the visible light region and neighboring wavelengths following excitation by electrons where these, originating from a cathode and accelerated in an electric field, strike upon an insulator or semiconductor (e.g., inorganic pigments or fillers). This phenomenon can be observed through the use of specially equipped optical or scanning electron microscopes.
9.5.2. Cathodoluminescence can be performed on embedded sections as an aid in differentiating samples or various layers within intact paint fragments. The technique has been found to be most useful for characterizing and comparing multilayered white or beige architectural and marine paint fragments (57).
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