ELECTRON MICROSCOPY
A completely different type of microscopy is well known to many forensic scientists which does not use light; instead, it uses electrons. Electron microscopy employs a particle beam of electrons focused by magnetic lenses. Electron micro scopes have a much higher resolving power and greater depth of field (i.e., more of the image is in focus at one time) than light microscopes and can magnify a specimen hundreds of thousands of times (see Figure 4.15). Electron microscopes are either transmission or scanning. In a transmission electron microscope, or TEM, the electron beam passes through a specimen that has been very thinly sectioned and projects the beam onto a specially treated plate that transmits the image to a monitor. In this sense, TEM images are much like light microscope images in that they provide information about the internal structure of the specimen. A TEM can resolve extremely fine details, down to the atomic scale. Sample preparation for a TEM is complicated, and the samples must be very small to fit on the support grid for viewing (see Figure 4.16), which is only a few millimeters in diameter. A scanning electron microscope, or SEM, rasters a beam of electrons across a specimen and provides a noncolored image of its surface (see Figure 4.17). SEMs can reveal details only a few nanometers in size; their magnification ranges from very low (around 2×) to up to 250,000× or more. SEMs are used in forensic laboratories to analyze a wide variety of samples, including paint, particles, fractures, toolmarks, and gunshot residue (see Figure 4.18). This analysis is enhanced by additional analytical instrumentation that can be attached to an SEM (or, in many cases, a TEM), such as:
• Back-scattered electron detector (BSED): Not all electrons bounce off the surface of the sample: Many penetrate the sample and are ejected at a rate and energy proportional to the material’s average atomic number. Thus, lead would appear brighter than aluminum in a BSED image. BSEDs assist analysts in detecting materials of interest, like gunshot residue particles.
• Energy-dispersive spectrometer (EDS): Electrons from the beam are not the only thing ejected from the sample. The electron beam excites the sample creating electromagnetic radiation from atomic exchanges in the various bands of electrons. Because each element has a particular atomic structure, the X-rays emitted are characteristic of that element. Nearly all the elements in a sample and their respective weight percentages can be determined with a high degree of accuracy (around ±0.5%).
• Wavelength-dispersive spectrometer (WDS): Unlike EDS, wavelength-dispersive spectroscopy measures the wavelength of the emitted radiation using a tightly spaced crystal lattice. The wavelength and the lattice spacing are related using Bragg’s law. WDS, unlike EDS, can detect only one wavelength at a time (EDS measures all elements in a sample simultaneously). This limitation is moderated by WDS’s greatly increased detection limit of 100 parts per million (ppm) in most cases, lower in others.
SEMs are fairly common in many forensic laboratories; electron microscopy centers at universities are good sources of information about the instruments and their applications.
FIGURE 4.15 Pollen from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), hollyhock (Sidalcea malviflora), lily (Lilium auratum), primrose (Oenothera fruticosa), and castor bean (Ricinus communis). The image is magnified some 500×; the grain in the bottom left corner is about 50-µm long.
FIGURE 4.16 A transmission electron microscope sample grid. Samples are mounted in supporting material and sectioned on an ultramicrotome down to only a few nanometers in thickness. A rectangular sample can be seen faintly on the grid.
FIGURE 4.17 A modern scanning electron microscope is a highly computerized instrument. The sample would be introduced to the chamber through the airlock at the base of the blue column.
FIGURE 4.18 A particle of gunshot residue is formed in the discharge of ammunition in a firearm. The molten materials congeal into rough spheres with a particular elemental content.
