INSTRUMENTAL METHODS
Given the complex chemistry of paints and related coatings, it is not surprising that many instrumental methods are available for their analysis. Rarely will all of the instruments listed in this section appear in one laboratory—even if they did, the laboratory’s analytical scheme would probably not include all of them—and the order of examination will be keyed to the instrumentation on hand. Infrared (IR) spectroscopy can identify binders, pigments, and additives used in paints and coatings. Most IRs used in forensic science laboratories employ a micro scope bench, as shown in Figure 1, to magnify the image of the sample and focus the beam on the sample. The bench is a microscope stage attached to the instrument chassis with optics to route the beam through the microscope and back to the detector. Most modern IRs will also be Fourier transform infrared spectroscopy (FT-IR) spectrometers, which employ a mathematical transformation (the fast Fourier transform) that translates the spectral frequency into wavelength. The analysis of paints by FT-IR can be done in transmittance (where the beam passes through a very thin sample and then onto the detector) and reflectance (where the beam is bounced off the sample and then to the detector), but transmittance is preferred because it equalizes the signal as well as the sample geometry; also, and probably more importantly, most of the reference information available from publications and instrument vendors is in transmittance.
FIGURE 1 An IR spectrometer with a microscope attachment or “bench.” Normal IRs require a sample to be pressed into a pellet or placed on a special specimen card to which the instrument is “blind.” The microscope attachment allows for the handling and analysis of microscopic samples too small for either pellets or cards. This also provides for positional information about the sample to be analyzed; in the case of paints, individual layers or particles can be analyzed in place with no additional preparation.
FIGURE 2 Cross-section of chips from hit-and-run case.
FIGURE 3 Spectrum of paint samples from hit-and-run case.
An IR-related technique that is gaining application in forensic science is Raman spectroscopy, which is based on light scattering rather than absorption. Because of this, Raman spectra provide complementary information to that obtained from IR spectroscopy. Raman spectroscopy shows great promise for a number of evidence types, but, for budget and training reasons, it will be some time before Raman spectroscopy becomes a standard method in forensic science laboratories. Pyrolysis-gas chromatography (PGC) disassembles molecules through heat (pyrolysis). This destructive technique uses the breakdown products for comparison of paints and identification of the binder type. PGC is influenced by the size and shape of the samples and instrument parameters, such as rate of heating, the final temperature, the type of column, and gas flow rates. This can make PGC vary from day to day and sample to sample; this has several methodological implications. The conditions from one analysis to the next should be the same and should be run very close in time to each other. It is important to select the known samples as carefully as possible because of the influence of size and shape on the final chromatographic results. As little as 5–10 µg of sample are required for PGC. The patterns of peaks in the known and questioned sample chromatograms (also called “pyrograms”) are compared and the peaks must coincide for the identification to be determined.
If the instrumentation is available, pyrolysis products may be identified by pyrolysis-gas chromatography-mass spectrometry (P-GC-MS). The resulting recon structed total ion chromatogram may help to identify additives, organic pigments, and impurities in addition to binder components. Because one of the major purposes of paints and coatings is to impart color to an object, the analysis of color has been integral to the coatings industry nearly since its inception. The gross visual color of paints can be categorized systematically by one of many color systems currently in use. Two of the main systems traditionally used are the Munsell system (developed in 1915 by Alfred Munsell, an artist) and the Commission Internationale de l’Eclairage (CIE) system, which is described in the ASTM International Standard Method D 1535 and Test Method E 308. Color systems are used to classify colors for description and communication of color infor mation and for databases only; absorption spectra of any known and questioned paint samples are compared in forensic paint comparisons. Absorption spectroscopy, using a microspectrophotometer (or MSP for short), has been used to categorize and discriminate between otherwise visually similar paints. MSP can also differentiate between metameric samples. Metamerism is the condition in which two colors appear similar under one set of conditions but differ ent under others. One of the benefits of employing MSP is that it adds an objective method to the analysis of color. The instrumental parameters can be easily reproduced between instruments or laboratories and this provides a basis for interlaboratory testing and quality control. Careful reference sampling is essential to the success of color comparisons of such surfaces. Comparison of paint layers by transmission MSP of paint thin sections is a more definite method of color analysis than reflectance techniques, but transmission MSP demands more careful preparation. The sample thickness and measurement location, for example, are critical for significant analytical comparisons, as illustrated by Figure 4. One of the most generally useful instruments in forensic paint analysis is the scanning electron microscope outfitted with an energy dispersive X-ray spectrometer (SEM/EDS). SEM/EDS can be used to characterize the structure and elemental composition of paint layers. The SEM uses an electron beam rather than a light beam and changes the nature of the information received from the paint. The electron beam rasters over the area of interest; the electrons interact with the sample and generate a variety of signals, including surface information (secondary electrons), atomic number (backscattered electrons), and elemental information (X-rays). Secondary electrons impact the surface of the sample and are reflected to the detector, providing a visual representation of that surface, pictured in Figure 5. Backscattered electrons penetrate the surface of the sample and are kicked back out of the sample, with more being kicked out from the atomically denser regions. Therefore, backscattered electrons create an image where brightness is proportional to atomic number. These types of imaging can be of great assistance in distinguishing paint layers and structures within the layers.
FIGURE 4 Microspectrophotometry (MSP) of paint layers by transmission of thin paint sections is an excellent method of discriminating between paint colors but demands more careful preparation. The sample thickness and measurement location, for example, are critical for significant analytical comparisons. In this figure, the small black square is the sampling area for the spectrum; it is located off the sample to collect a background spectrum.
FIGURE 5 20mm EL9k Detector = QBSD Date :20 Oct 2000 Secondary electrons impact the surface of a paint sample and are reflected to the detector, pro viding a visual representation of that surface. Electrons carry no color information; their resolution, however, is very good and can provide images magnified thousands of times. Samples can also be imaged by backscattered electrons (BSEs). Here, brightness is proportional to atomic number with larger numbered elements (iron, titanium) being brighter than smaller ones (silicon, oxygen). BSE images are useful for delineating paint layer structures and pigments. This SEM image shows a sample of paint taken from the entrance lobby of the Osborne House on the Isle of Wight, bought by Queen Victoria in 1845. The paint samples were mounted in resin and polished to show the successive layers of paint in cross-section. Since the raw materials used to produce paints have varied with time, the analytical information can be used to work out which paint schemes are contemporary. The image was taken in BSE mode.
The primary reason for analyzing paint samples with an SEM/EDS system is to determine the elemental composition of the paint and its layers. When the electrons impact the surface, X-rays are produced as a result of high-energy electrons creating inner shell ionizations in sample atoms, with subsequent emission of X-rays unique to those atoms. The minimum detection limit under many conditions is 0.1%. Elements with atomic numbers ≥11 are customarily detectable. Detection of elements with atomic numbers ≥4 is possible using a detector with an organic film window or a windowless detector. Analysis can be performed in a rastered beam mode for bulk layer analysis or static beam (spot) mode for individual particle analysis. Goldstein et al. (2003) present a general treatment of all aspects of SEM and X-ray microanalysis. Mapping of elements across the cross-section of a multilayer paint can be useful for explaining or demonstrating elemental distributions and elemental associations. Another technique that provides good visualization of elemental differences is atomic number contrast with backscattered electrons. These images can be used to characterize and compare the structure of paints, including layer number, layer thick ness, distribution and size of pigment particles, and the presence of contaminants.
