READ: Trace Evidence and Microscopes

Scanning Electron Microscope

 The Scanning Electron Microscope, or SEM, uses beams of electrons rather than light to look at elements of tiny specimens such as gunpowder residue from bullets or ink from a ransom note. Capable of magnifying up to 300,000X, the scanning electron microscope is another versatile and crucial tool in the crime lab. It is used for analyzing many types of evidence including:

• hair

• diatoms

• pollen

• gunshot residue

• bullet fragments, markings and cartridges

• paint particles

• gems and jewelry

• fibers

• handwriting and document examination (forgeries and counterfeit)

• trace evidence comparison

An SEM can analyze the elemental composition of even the smallest features on a specimen as well as identify the origin of materials because it can see past the surface of the object. It has a greater depth of field than that of other types of microscopes and generates an almost 3D view of the sample being analyzed. Compare the images of the frost crystals (to the left); they are of the same crystal viewed on two different microscopes. The image on the left is from a compound light microscope while the image on the right is from a scanning electron microscope. Notice how much more detail is apparent in the SEM image and how it looks almost 3D!

Scanning Electron Microscope Components Interactivity

A scanning electron microscope produces a magnified image by tracing the shape of the object in electrons so that an exact copy of the image is transmitted to a computer monitor attached to the microscope. The detectors detect electrons that have been reflected from the surface in various patterns. The pattern helps define the contours and brightness of the image. The entire sample is scanned piece by piece like this creating a high definition image. This extensive scan process used is why the microscope is called a scanning electron microscope. The microscope has controls to move the electron beam over the sample so that multiple areas of the sample can be scanned separately as needed. An interesting fact about using the SEM is that samples must be coated in a conductive coating such as gold or platinum before being placed in the chamber to be analyzed. This coating, known as sputter coating, allows the specimen to be grounded. Grounding prevents damage to the specimen by the electron beam.

Georgia Virtual, Microscopes, CC BY-NC-SA 3.0