Learn More About the Laboratory

 

Welcome to the kids’ page of the DuPage County Sheriff’s Office Crime Laboratory web site!

 

Here at the crime laboratory our Forensic Scientists perform many different tests to help the police solve crimes.  One of the basic ideas behind forensic science is that a criminal always takes something from the scene of a crime and always leaves something behind.  It is our job to find what the suspect may have taken with him from the scene or what he may have left behind.

 

Unlike shows you may have seen on television our Forensic Scientists only work in the laboratory.  At the DuPage County Sheriff’s Office we have a highly trained unit of Police Officers who work in our Forensic Investigations Unit.  Their job is to go to crime scenes and collect evidence which they bring to the Crime Laboratory for testing.

 

The following sections talk about the different types of evidence that is tested at the Crime Laboratory.

 

Drug Chemistry

Forensic scientists in the Drug Chemistry section are chemical detectives.  They receive unknown samples of powders, plant material, tablets, capsules, paper and liquids for testing.  They have to determine if these samples contain illegal drugs.  Illegal means that it’s against the law, and a drug is anything that causes your body to change the way it works.

 

First, electronic scales are used to weigh the sample because the more illegal drug you have, the more trouble you are in.  Next, the sample is tested.  There are a variety of tests they can use to determine if a sample contains an illegal drug.  One type of test they perform is called a color test. This involves taking a small amount of a sample and adding a drop of liquid chemical.  If the sample changes color it could mean that an illegal drug is present.

Color tests

 

Another test they perform is using an instrument, called the gas chromatograph-mass spectrometer or the GC/MS.  A small amount of the sample is dissolved in liquid, just like when you dissolve Kool-Aid^® in water.  Then the liquid sample is injected into the GC/MS where it is tested.  This instrument will determine how many drugs are in a sample and give us a chemical fingerprint for each one.  Using the chemical fingerprints, which are unique to each chemical, they can determine what is in a sample.

 

 

Trace Chemistry

 

Some forensic scientists study tiny bits of stuff such as hairs, fur and fibers.  They also examine splinters of glass, paint chips, explosive residues and an almost endless list of other small things called ‘trace evidence’.  These are just a few common examples of the tiny clues that can be left behind at a crime scene or may get stuck on a suspect.  These different specks of stuff transfer from the victim onto the suspect and from the suspect onto the victim.  This transfer of clues is called the Locard Exchange Principle.

 

Forensic scientists use all sorts of different microscopes, chemical tests and different high tech instruments to study these tiny clues.  The following sections describe different types of trace evidence that can be used to link a suspect to a crime scene.

A forensic scientist using the comparison microscope

 

Glass

When a criminal breaks into a building or a car to steal things, they usually break a glass window so that they can get in.  Some splinters of broken glass will fall onto the criminal’s clothes and may get stuck in their shoes.  The police will collect some of the broken glass from the window.  When they catch the suspect, the police will also collect their clothes and shoes.  The forensic scientist will then look for the glass on the suspect’s shoes and clothes.  If the glass from the crime scene matches up with the glass on the suspect’s clothes, this ‘glass link’ may place the suspect at the crime scene.

 

Paint

In a hit-and-run car accident, a careful search of a red car that was hit may show a blue paint chip.  The police find a car nearby that looks like it has been in an accident and it is blue in color.  Car paint usually contains lots of paint layers of different colors.  The forensic scientist will study the foreign paint chip found on the red car and compare it to the paint on the blue car to see if all of the layers of paint have the same color and that they are made of the same chemicals.  This helps prove that the blue car may have been involved in the accident.

A cross section of multiple paint layers at 60x magnification

 

Hairs

Hair evidence is often found at crime scenes and submitted to the crime laboratory.  Hairs are examined under a microscope, and the following questions may be answered:  Is this a hair or fiber?  Is the hair human or animal?  What part of the body did the hair come from?  What is the race of the hair donor?  Has the hair been exposed to cosmetic treatment?  Has the hair been damaged in some manner?

 

Once these questions have been answered, the forensic scientist can then compare questioned hairs against known hairs using a microscope.  Sometimes animal hairs are also of forensic value.  Depending upon the quality and quantity of an animal hair sample, it may be possible to determine the species of animal that a hair originated from.

To collect hairs and fibers, the analyst uses a piece of clear tape, sticky side down

 

Fibers

You might think if you have seen one fiber, you’ve seen them all.  But there are so many different kinds that fiber detection can yield a lot of clues!  There are fibers of natural materials like cotton, wool or silk.  Man-made fibers include nylon, polyester, acetate and acrylic, and some glow under special lights.

 

Some fibers, like white cotton or blue denim, are very common.  Other fibers, such as those from certain carpets, are very distinctive and uncommon.  If unusual fibers from a victim’s living room carpet are found on the suspect, this is a clue that will help prove that he may have been in the victim’s living room.

Cotton fibers (left) and polyester fibers (right) at 200x magnification

 

Arson

When a suspect tries to burn down a building he is called an arsonist.  Arsonists usually use gasoline or other flammable liquids like kerosene or paint thinner to help start the fire.  The forensic scientist will examine debris collected by the Fire Investigator from the burnt building for tiny amounts of these flammable liquid clues to help prove that the fire was caused by arson.  If the police catch a suspect soon after the fire, they will place the suspect’s clothes into airtight metal cans.  If the forensic scientist finds the same kind of flammable liquid on the suspect’s clothes and in the fire debris, this is a clue that the suspect may have started the fire.

 

 

Latent Prints/Other Impressions

 

If you look closely at your fingertips, you can see tiny ridges covering the skin and forming different patterns.  These patterns make up our fingerprints.  Each of us is born with these ridges already in place and they stay there in the same pattern for the rest of our lives.  These ridges are what help us hold on to things, like pencils and paper.  If our fingertips were smooth, those things would slide right out of our grip!  These ridges that make up our fingerprints are also a good way to identify who we are, because no two people in the world have the same fingerprints – not even identical twins!  If you look at your fingerprints and then look at someone else’s, you might see some of the same basic patterns – things that look like circles, loops, or hills.  These basic patterns appear on a lot of people’s fingers, but the tiny details within each pattern are different for every single fingerprint!  You can’t see these very well unless you use a magnifying glass, but it is these details that allow us to say that a fingerprint belongs to one specific person and no one else.

Typical fingerprint

 

The ridges on our fingers have thousands of sweat pores.  You need a magnifying glass to see these too.  The sweat on our fingers has some of your natural body fluids in it and can leave an outline of your fingerprint on an object after you touch it.  Sometimes we can see fingerprints left behind on an object very easily, like when your parents tell you not to touch the walls and windows because you are leaving behind dirty fingerprints.  Other times, these prints are difficult or impossible to see without using special methods to make them visible.  These types of fingerprints are called latent prints.  In the Crime Laboratory, forensic scientists have a lot of ways to make latent prints visible.  Special powders or chemicals are used to help find fingerprints.  Some of these turn the fingerprints a different color so that they can be seen, while others make the fingerprints glow brightly when a laser light is shined on them. 

 

Some of you may have had ink put on your fingers and had your fingers rolled onto a blank card, so that your parents can have a copy of your fingerprints.  Those are fingerprint standards or record fingerprints.  If the police have a suspect for a crime, they can get fingerprint standards from that person and bring those standards to the Crime Laboratory.  Using magnifying glasses, the standards can be compared to any latent prints found at the crime scene to see if they match.  If they do match, then we know that person was at the scene and touched that object.

 

Comparison of a latent print to a fingerprint standard

 

 

If the police don’t have any idea who might have committed the crime, then the AFIS computer system is used.  AFIS stands for Automated Fingerprint Identification System.  A latent print from a crime scene is scanned into the computer.  The computer can search a database of fingerprint standards and print out a list of the fingerprints that look the most like the latent print.  Then the forensic scientist uses a magnifying glass again to see if any of the fingerprints actually match.  If they do match, then the police have a suspect for the crime!  The computer database has fingerprint standards from people who have been fingerprinted before, either because they committed a crime in the past or because they had to be fingerprinted for their job.  The FBI database has over 250 million sets of fingerprint standards!

 

Fingerprint patterns are not the only patterns forensic scientists examine.  The palms of your hands and the bottoms of your feet also have ridges on them with unique patterns and footprints and palm prints are often left at crime scenes.  Other items can leave unique prints too, like shoes and tires.  A criminal’s shoe can be compared to a shoeprint left at a crime scene, or a tire on a car can be compared to a tire track from the scene. 

 

Comparison of a suspect’s shoe (known) to the questioned print left at the crime scene and test print made in the laboratory

 

                  

Firearms

 

Firearms examination involves the examination of fired ammunition parts, such as bullets and cartridge cases, to determine if they were fired from a specific gun.  Firearms examination is actually a special form of toolmark examination because a gun is a tool that leaves marks on bullets and cartridge cases when they are fired.

On the inside of a rifled gun barrel there are lands and grooves.  The lands are raised ridges that spiral down the length of the barrel.  The lands are used to make a bullet spin as it travels down the barrel of the gun, because a bullet that spins will go further and straighter than a bullet that doesn’t spin.  A bullet travels very fast in the barrel of a gun and is “scratched” by the spiral ridges.  Every gun leaves its own unique pattern of scratches.  Bullets have patterns of scratches that can be matched to the guns that fired them.

In the laboratory, shooting into a large tank of water is a safe way to fire bullets through the barrel of a gun so the bullets can be collected by the scientist.  Under the comparison microscope, a firearms examiner can examine two objects side-by-side at the same time.  The scientist looks at the scratches on the bullets collected in the laboratory and compares them to the toolmarks on a bullet found at a crime scene. If the scratches line up in the same pattern, that’s proof to the scientist that the police submitted the right gun!    

Comparison of two bullets

                      

Comparison of two cartridge cases

 

Toolmarks

When a hard object comes in contact with a softer object it usually leaves a mark.  The mark left on the softer object is a toolmark.  Toolmarks can be important clues to what may have happened at a crime scene.  Some examples of objects that leave toolmarks are screwdrivers, hammers, bolt cutters, guns and even teeth.

 

Imagine a thief used a tool to force open a cabinet and left a scrape mark on the cabinet.  The police conduct an investigation and find a screwdriver in a suspect’s car.  The suspect’s screwdriver is submitted to the crime laboratory so that test marks can be made.  A special type of microscope, called a comparison microscope, is used to examine marks left by tools.  Using the comparison microscope, a forensic scientist can examine two objects side-by-side at the same time.  Under the microscope, test marks from the screwdriver are compared to the scrape made on the cabinet at the crime scene.  Every screwdriver makes a different toolmark because dents and nicks in the tool leave one-of-a-kind marks.  If the scientist matches the marks then the police have their tool!   

 

 

Forensic Biology/DNA

 

Body fluids like blood or spit can often be found on items of evidence.  These body fluids contain cells and those cells contain DNA (deoxyribonucleic acid).  The DNA is like an instruction book for our body.  Because of DNA our hearts beat, our brain thinks, and our lungs breathe.  Our DNA tells whether we will have brown hair or blue eyes or whether we are tall or dark skinned.  Because everyone is unique in these characteristics, DNA must also be unique in every individual.  The only people who have identical DNA are identical twins.  There are certain bits of the DNA that we examine to determine a DNA profile. 

 

For example, imagine that someone is walking past a jewelry store.  They see some valuable items on display in the window.  They smash the window with their fist, break the glass and reach in to steal some of the jewelry.  While reaching in, they cut their arm on the jagged glass and leave some blood on the glass.  Running away, their blood drips onto the pavement along the trail.  When the police arrive on the scene, they collect some of the blood on the glass and some of the blood from the trail.  Maybe there is enough blood to follow the trail all the way to a house a few blocks away, where they find a man with a bandage on his arm.  The police can get a sample of the man’s saliva by swabbing the inside of his mouth with a cotton swab. 

 

The police bring the swab of saliva from the suspect and the swabs of the blood from the glass and the trail to the laboratory.  The first thing the scientist would do would be to examine the crime scene swabs and confirm that there is blood on them.  Then the scientist would examine some of the unique features of the DNA in the blood.  Those unique features can be compared to the DNA of the suspect from the swab of his spit.  If both of the DNA profiles match, we can be sure that the suspect is the person who bled on the glass and ran away from the store.

 

Now imagine if there was not a long trail of blood, and the police were never able to find the crook with the cut on his arm.  The biologists would still analyze the blood and determine the DNA profile from the crime scene.  The profile would be entered into a DNA database of people who are in jail or who have been in jail for serious crimes.  If the crime scene profile matches one of the bad guys in the database, we will be able to provide his name to the police to help solve the crime.

 

Some tools that forensic biologists use are microscopes, computers and a special light that helps us to find stains on clothing.  A bloodstain is usually visible to the eye, but many of the other body fluids we look for are not visible to the naked eye.  Also, once we break open the cell and take out the DNA, DNA is not visible. We must use chemicals and special instruments to detect the differences in the DNA profiles.