Scanning Electron Microscopy

Scanning Electron Microscopy

The scanning electron microscope (SEM) is an important tool in modern forensic science due to its wide range of applications. SEM allows the rapid analysis of elements that compose very small specimens and the conclusive determination of the origin of many materials that are crucial to the chain of evidence . Paint particles, fibers (both natural and artificial), bullet fingerprints, gunshot residue , counterfeit bank notes, forged documents, and trace evidence are all examples of specimens that can be analyzed using the scanning electron microscope. Scanning electron microscopy also renders detailed three-dimensional (3-D) images of extremely small microorganisms, 3-D anatomical pictures of insect, worm, spore, or other organic structures, and the analysis of gems and gem fragments.

Conventional microscopes use light and several lenses to magnify images, whereas SEM uses electron beams to sweep the surface of specimens, producing magnified images in black and white. In most SEMs, samples are placed in a vacuum chamber after being adequately prepared to conduct electricity. Once the sample is in the chamber, the air is extracted and an electron gun at the top of the chamber emits a beam of electrons, which passes through a series of magnetic lenses that condense the beam into an extremely fine focus, capable of sweeping nano spots on the sample surface. A scanning device near the bottom of the vacuum chamber controls the movement of the electron beam across the specimen, row by row. As the electron beam sweeps the surface, it excites electrons present in the atomic structure of molecules, causing some of them to escape from the surface. These escaping electrons, known as deflected secondary electrons, have specific energies that can be measured. As they are released from each area of the sample, they are collected and counted by a detector that sends their amplified signals. The various electronic energies produced are analyzed by computer software, and the resulting image is displayed on a computer monitor.

Some modern SEMs offer an additional advantage for forensic purposes because of new methods of biological sample analysis that do not corrupt the specimen, a major drawback with conventional SEMs. In conventional electron microscopy, biological samples have to be dehydrated and then coated with a material that conducts electricity, such as a thin layer of gold or carbon. Modern SEMs allow the adjustment of the internal pressure in the chamber to dissipate the electric charge that would otherwise charge the sample, thus dispensing with coating and dehydration. Examples of non-conductive materials that require special preparation in conventional SEMs are paper, paint, textiles, bone, hair, and glass .

Each chemical element consists of an atomic structure composed by a given number of particles in the nucleus and of electrons vibrating in different levels or shells around the nucleus, each at a specific distance from the nucleus. Electrons in different shells ("orbits") have different energies and the atomic weight of the nucleus determines the quantity of electrons of an atom. Atoms are usually neutral because all their positive and negative particles are in a state of dynamic electrical balance. However, when free atoms collide or when they are bound through molecular chemical reactions, some atoms gain or lose an extra electron, thus becoming positive or negatively charged (cations or anions). When the electron beam of a SEM hits the sample, it deflects two types of electrons from the sample: inelastic electrons and elastic electrons. Inelastic electrons are low-energy particles that give information about the topographic variations on the sample surface and are responsible for 3-D black and white images. They are also known as secondary electrons and most of them have charges inferior to ten electron Volts (<10 eV). Elastic electrons are those that collide with the electrons generated by the SEM (that are present in the beam). The collision of electrons produces specific energy quanta that are retained by the elastic electrons. By calibrating the microscope to different beam intensities, analysts can study several types of data provided by elastic electrons, such as the sample composition and crystallographic structure of the surface, the internal structure of semi-conductor materials, the distribution and energy levels of phosphorous compounds, and information about the elements and chemicals present in the several layers of the surface.

Forensic analytical tests such as scanning electron microscopy, spectrometers, chromatography , and x-ray dispersion aim at producing individualized evidence that allows the identification and origin of samples and the accurate interpretation of data in relation to a crime or a suspect investigation or to help explain an explosion, arson , or airplane crash. Modern scanning electron microscopy provides nondestructive analysis of both organic and inorganic samples. Another application of this method in forensics is the analysis and identification of dust particles in the air of indoor environments to either assess the air quality or to detect possible pathogens (disease-causing organisms) or hazardous substances. Mineral grains (such as carbonates, glass, quartz, or mica), biological materials (such as mold spores , pathogen spores, insect particles, skin cells, and rodent fecal dust), fibers (such as hair, textile fibers, carpet fibers, cellulose, and asbestos), and miscellaneous particles (such as metallic particles, paint, soot, rubber, and plastic) are all materials that have been used in forensic analysis done with scanning electron microscopy.

see also Accelerant; Aircraft accident investigations; Analytical instrumentation; Arson; Artificial fibers; Ballistic fingerprints; Bomb (explosion) investigations; Document forgery; Fibers; Filaments; Hair analysis; Handwriting analysis; Ink analysis; Isotopic analysis; Minerals; Organic compounds; Paint analysis; Point-by-point analysis.