New generation SEMs incorporate computer automation and detector technology that allows for rapid elemental analyses of small particles. Already the gold standard for GSR analysis, the technique is being evaluated for forensic soil comparisons.
Speed, accuracy, and consistency are keys for moving forensic caseloads from the lab to the courtroom. Crime laboratories have long employed scanning electron microscopes (SEM) for use in trace element cases, including analysis of glass, paint, fibers, and gunshot residue (GSR). The newest generation of SEM, utilizing energy dispersive x-ray spectrometers (EDS), incorporates computer automation that allows for rapid automated elemental analyses of small particles or inclusions (Figure 1). These automated analyses yield chemical and morphological information which can be used to characterize specific populations of materials present in a forensic sample.This technique removes operator subjectivity inherent in time intensive manual SEM or optical characterization techniques, and allows rapid characterization of all the materials present in a sample. Currently the gold standard for gunshot residue analysis, the usefulness of automated SEM-EDS is being evaluated for other trace evidence cases, specifically forensic soil comparison.
By performing an automated SEM-EDS analysis,more particles can be analyzed per unit time, allowing for efficient and more complete sample characterization. For example, specialized SEM-EDS systems, utilizing new software algorithms, can now analyze and classify between 500-1,500 particles per hour, and current trends in detector technology promise to further improve this rate. Automated SEM-EDS also increases the probability of finding and identifying rare materials that are characteristic of forensic and environmental samples. For these reasons, automated SEM-EDS analysis is becoming a fundamental tool used by crime labs.
Automated SEM Analysis
Whereas a light microscope uses visible light and lenses to magnify a sample for analysis, an elemental analysis in an SEM, whether manual or automated, involves the interaction of a high-energy electron beam with the sample of interest, resulting in the generation of back scattered electrons (for imaging), and characteristic x-rays (for elemental analysis). Back scattered electrons are high-energy electrons that are elastically scattered from within the specimen. Higher atomic number (Z) elements tend to back scatter more electrons, and therefore appear brighter (higher gray-level value) than lower Z materials in back scattered electron images (BEI) (Figure 2).