Introduction

Trace Evidence

Trace evidence is evidence that is created when objects come into contact with other objects. This means that when people touch objects with any part of their body evidence can be left behind. It also means that when any object touches another object there is a chance that evidence is left behind. Trace evidence is often small or present in small amounts. Trace evidence includes things like hairs, fibers, fingerprints, DNA, soil, paint, and glass. It can also include marks made by tools or firearms.  In this unit we will learn about several different types of trace evidence and how it is analyzed in the lab.

Since trace evidence is often very small or sometimes even microscopic, it often requires the help of microscopes in order to analyze the evidence in the lab. Before we learn about each type of trace evidence, we will learn about the different types of microscopes used to analyze that evidence. There are several different types of microscopes that are used for different types of evidence.

Introduction

 Microscopes provide a direct image of a small object of interest that often would not be seen by the naked eye. There are numerous types of microscopes. Each type offers the forensic scientist a unique opportunity to view and analyze different types of forensic evidence. Whether the Forensic Scientist is analyzing paint chips or examining for gun shot residue, microscopes make the seemingly ordinary minute evidence come alive in very detailed images. Today, all modern crime labs are equipped with multiple microscopes due to their importance in evidence analysis.

Essential Questions

1. What are the parts of a microscope?
2. Are some types of microscopes better than others for certain types of evidence analysis?
3. When is a microscpectrophotometer used in Forensic Science?
4. Why are microscopes important to Forensic Science?

Module Minute

Microscopes have been used for hundreds of years to make the seemingly invisible, visible. Galileo designed a compound microscope in 1625 that is quite similar in function to compound microscopes that are still widely used today! The "Father of Microscopy", Anthony Leeuwenhoek, made his own microscope lenses to magnify threads that he counted. Among his many contributions to the science of microscopy, he documented bacteria, yeast, blood cells and small water animals with his handmade lenses. Microscopes today are used in Forensic Science to analyze trace evidence that is too small to discern with the naked eye. Microscopes help Forensic Scientists to analyze evidence such as paint chips, items with suspected gunshot residue, glass, soil, blood evidence, tool marks, bullets, illicit drugs, fibers and many other items of evidentiary significance.

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How and Why Microscopes are Used

How and Why are Microscopes Used?

Though microscopes have been used for hundreds of years to investigate scientific queries, they were not used in Forensic Science in any great amount until the early 1900s. Today, microscopes are a very important part of the crime lab; whether examining skin cells, hair or fiber evidence, microscopes enable the scientist to see details not visible to the naked eye. Microscopes are also used to locate, isolate, identify, and compare samples. In general, there are six types of microscopes used in Forensic Science:

Each of the microscopes used in Forensic Science has different features which lend themselves to various functions and capabilities in the crime lab. While some of the microscopes are used as a preliminary tool to locate or survey evidence, others are used to identify crystalline structures or to view the minute surface details on a specimen of evidence. Without microscopes, Forensic Science would not be able to identify and compare many pieces of evidence due to their size or uniform appearance to the unaided eye. Microscopes are incredibly versatile and useful in detecting these tiny characteristics that would go unnoticed otherwise.

Though microscopes are very useful in the crime lab, one must always be very careful to remain objective when evaluating microscopic evidence. Being objective simply means that any observations made are based only upon facts. A forensic scientist must make careful observations of the evidence that are without opinion or assumption and always remain objective throughout analysis of evidence. If a scientist does not remain objective while evaluating evidence, he or she risks the evidence becoming inadmissible to court, reaching the wrong conclusion or even implicating an innocent person in a crime.

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Compound Microscope

Compound Microscope

 The compound microscope uses a series of lenses in order to magnify an image so that the subtle characteristics of that object are more clearly seen.  The most famous and accepted history of the modern compound microscope is that of Galileo Galilei who is said to have developed a compound microscope with adjustable focus in 1609. It was given the name of "microscope" in 1625. The term microscope comes from the Greek words micron and skopein which mean "to look at small things".

The compound microscope works by gathering light, redirecting it through a condenser lens and into the path of the specimen. The condenser lens focuses or condenses the light onto the specimen and is needed for higher magnification because it increases the illumination of the light and the resolution. The image of the specimen is then directed to the back portion of the microscope, called the focal plane, by the objective lens. The image from the focal plane is then received by the ocular lens and the image is redirected to the eye. Once the image reaches the eye, it is actually viewed in reverse of its orientation on the slide; essentially the image is upside down and backwards from the orientation on the stage. A compound microscope can generally magnify a specimen in a range of about 40X to 400X, but could be magnified up to 1000X in some compound microscopes.

While the compound microscope is very useful, it is also limited by the constructs of resolution and magnification. Resolution is the shortest distance between two separate points in a microscope's field of view that can still be distinguished as distinct entities. It directly relates to the clarity of the image when viewed. If the image lacks resolution, it will appear "fuzzy" and individual components or characteristics of the image may be obscured. Magnification is the act or process of enlarging the physical appearance or image of something. Magnification does not affect the clarity of the image; it merely makes the image appear larger to the eye so that more qualities or characteristics of the image can be observed. The total magnification of a microscopic image can be calculated by multiplying the magnification of the objective lens by the magnification of the ocular lens.

For example, what would be the total magnification on a microscope with a 10X ocular lens and a 40X objective lens?

A simple but powerful tool in the Forensic Scientist's arsenal, the compound microscope is an essential part of crime labs for very small or dense pieces of evidence. Compound microscopes are most useful when high magnification is needed, but are limited by the size of the object to be viewed. The item must be small enough to fit on slides on the stage while still fitting under the objective lenses. Most compound microscopes used in crime labs have high quality cameras integrated so that microscopic images can be documented photographically at the touch of a button or click of a mouse! The cameras are often programmed so that each photo document is stamped automatically with a date, time, magnification and other important information. In addition, photo documentation of microscopic evidence is stored in multiple places on computers, back-up files and in the case file to prevent loss of important evidence documentation.

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Comparison Microscope

Comparison Microscope

 The comparison microscope is another essential tool in the crime lab. It allows for comparison of various artifacts of evidence simultaneously and is the primary microscopy tool used in trace evidence investigations of bullet, hair, fiber, and tool mark evidence. It was developed by Phillip Gravelle in the 1920s and first used by Calvin Goddard to examine evidence. In 1927, Goddard was called to examine evidence from the Sacco and Vanzetti case due to a huge outcry from the public at the time of their execution. Goddard was able to ascertain that the bullet that killed the victim was fired from Sacco's gun by using the comparison microscope to compare the spent bullet casings. Goddard was called to analyze bullet evidence and testify in another famous case involving the comparison microscope, the St. Valentine's Day Massacre, in 1929. He examined bullets and cartridge evidence and ultimately absolved the Police Department of Chicago from any involvement in the massacre based upon his findings using the comparison microscope.

A comparison microscope is basically two compound microscopes that can be viewed simultaneously on a split screen. The split screen view can be rotated so that the two items being compared are oriented in the same direction for easier comparison. The split screen can also be changed to an overlay setting where one field of view is placed on top of the other as an overlay to see how closely the evidence items align or confirm a match. Often each piece of evidence is given a different color filter when using the overlay technique so that the segments that overlay are darker than the areas that do not match up. The comparison microscope also ensures that the items of evidence being analyzed are viewed under the same lighting and magnification. The images from the microscope are often projected on to a television screen or computer monitor. Photo documentation of the magnified views are documented either by screenshot or directly from a camera mounted within the microscope. These images are very important documentation of what was seen at the time of microscopic analysis and are frequently displayed in court as evidence when an expert is testifying.

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Stereoscopic Microscope

Stereoscopic Microscope

 A stereoscopic microscope views particles of evidence in three dimensions and allows the investigator to turn over a tiny object such as a seed or a hair beneath the microscope. When using a stereoscopic microscope, the light is reflected from the surface of the specimen rather than through the specimen as the compound microscope does. This is especially useful when the specimen for analysis is too dense or thick for the light to properly transmit. Unlike the compound microscope, the image formed on a stereoscopic microscope is viewed in the original orientation rather than "upside down and backwards".

Stereoscopic microscopes are often used in Forensic Science because of their versatility in viewing evidence of larger or irregular size. While a stereoscopic microscope does not offer the higher magnification found in other types of microscopes, it does allow samples that would not normally fit under a conventional microscope to be viewed microscopically. It is also used in cases with live animals or organisms being dissected, such as in entomology, because the specimens can be rotated, manipulated and viewed freely without being mounted on slides. These microscopes are often used on the majority of trace evidence before being examined in other more intrusive manners. This is because the stereoscopic microscope allows for observation and analysis without damaging or corrupting the integrity of the evidence. The evidence is observed under the stereoscopic microscope to identify any distinguishing marks or other observations and then is often sent for further analytical procedures which are more intrusive.

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SEM

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.

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