How Forensic Lab Techniques Work

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A scientist at Preston Forensic Science Laboratory removes a hair from a hat left at the scene of a shooting in the 1940s.

The history of forensic science dates back thousands of years. Fingerprinting was one of its first applications. The ancient Chinese used fingerprints to identify business documents. In 1892, a eugenicist (an adherent of the often prejudiced system of scientific classification) named Sir Francis Galton established the first system for classifying fingerprints. Sir Edward Henry, commissioner of the Metropolitan Police of London, developed his own system in 1896 based on the direction, flow, pattern and other characteristics in fingerprints. The Henry Classification System became the standard for criminal fingerprinting techniques worldwide.

In 1835, Scotland Yard's Henry Goddard became the first person to use physical analysis to connect a bullet to the murder weapon. Bullet examination became more precise in the 1920s, when American physician Calvin Goddard created the comparison microscope to help determine which bullets came from which shell casings. And in the 1970s, a team of scientists at the Aerospace Corporation in California developed a method for detecting gunshot residue using scanning electron microscopes.

In 1836, a Scottish chemist named James Marsh developed a chemical test to detect arsenic, which was used during a murder trial. Nearly a century later, in 1930, scientist Karl Landsteiner won the Nobel Prize for classifying human blood into its various groups. His work paved the way for the future use of blood in criminal investigations. Other tests were developed in the mid-1900s to analyze saliva, semen and other body fluids as well as to make blood tests more precise.

With all of the new forensics techniques emerging in the early 20th century, law enforcement discovered that it needed a specialized team to analyze evidence found at crime scenes. To that end, Edmond Locard, a professor at the University of Lyons, set up the first police crime laboratory in France in 1910. For his pioneering work in forensic criminology, Locard became known as "the Sherlock Holmes of France."

August Vollmer, chief of the Los Angeles Police, established the first American police crime laboratory in 1924. When the Federal Bureau of Investigation (FBI) was first founded in 1908, it didn't have its own forensic crime laboratory -- that wasn't set up until 1932.

By the close of the 20th century, forensic scientists had a wealth of high-tech tools at their disposal for analyzing evidence from polymerase chain reaction (PCR) for DNA analysis, to digital fingerprinting techniques with computer search capabilities.

Next, we'll see some of the applications of these modern forensic technologies.

Forensic labs are often called in to identify unknown powders, liquids and pills that may be illicit drugs. There are basically two categories of forensic tests used to analyze drugs and other unknown substances: Presumptive tests (such as color tests) give only an indication of which type of substance is present -- but they can't specifically identify the substance. Confirmatory tests (such as gas chromatography/mass spectrometry) are more specific and can determine the precise identity of the substance.

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Forensic technicians are often called to identify unknown drugs. A beauty student allegedly tried to smuggle more than 10,000 amphetamine tablets into Australia.

Color tests expose an unknown drug to a chemical or mixture of chemicals. What color the test substance turns can help determine the type of drug that's present. Here are a few examples of color tests:

Other drug tests include ultraviolet spectrophotometry, which analyzes the way the substance reacts to ultraviolet (UV) and infrared (IR) light. A spectrophotometry machine emits UV and IR rays, and then measures how the sample reflects or absorbs these rays to give a general idea of what type of substance is present.

A more specific way to test drugs is with the microcrystalline test in which the scientist adds a drop of the suspected substance to a chemical on a slide. The mixture will begin to form crystals. Each type of drug has an individual crystal pattern when seen under a polarized light microscope.

Gas chromatography/mass spectrometry isolates the drug from any mixing agents or other substances that might be combined with it. A small amount of the substance is injected into the gas chromatograph. Different molecules move through the chromatograph's column at different speeds based on their density. For example, heavier compounds move more slowly, while lighter compounds move more quickly. Then the sample is funneled into a mass spectrometer, where an electron beam hits it and causes it to break apart. How the substance breaks apart can help the technicians tell what type of substance it is.

Forensic scientists are sometimes called to help analyze evidence left from a hit-and-run or possible case of arson. They have special techniques to study what's often small or extremely damaged evidence.

Paint Analysis

Sometimes forensic scientists need to analyze a paint sample -- for example, if a paint chip is found on the body of a hit-and-run victim and investigators are trying to match it to a make and model of car.

First, the scientists look at the appearance of the sample -- its color, thickness and texture. They examine the sample under a polarized light microscope to view its different layers. Then they can use one of several tests to analyze the sample:

• Fourier transform infrared (FTIR) spectrometry determines the type of paint (chemicals, pigments, etc.) by analyzing the way in which its various components absorb infrared light.
• Solvent tests expose the paint sample to various chemicals to look for reactions such as swelling, softening, curling and color changes.
• Pyrolysis gas chromatography/mass spectrometry helps distinguish paints that have the same color, but a different chemical composition. The paint sample is heated until it breaks into fragments, and then is separated into its various components.

Arson Investigations

To light a fire, arsonists need a flammable material and an accelerant (such as kerosene or gas). Arson investigators look for these items when they're investigating the crime scene. Because all that's usually left of the evidence is charred remains, the investigators will collect fire debris and take it back to the forensics lab for analysis.

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Investigators look through the remains of the Morning Star Missionary Baptist Church on Feb. 8, 2006, near Boligee, Ala. Forensic technicians will examine the fire debris.

Samples are sealed in airtight containers and then tested for residues of accelerant liquid that might have been used to start the fire. These are the most common tests performed by forensics labs during an arson investigation:

• Static headspace heats the sample, causing the residue to separate out and vaporize into the top, or "headspace" of the container. That residue is then injected into a gas chromatograph, where it's broken apart to analyze its chemical structure.
• Passive headspace heats the sample and the residue collects onto a carbon strip in the container. Then the residue collected is injected into a gas chomatograph/mass spectrometer for analysis.
• Dynamic headspace bubbles liquid nitrogen gas through the sample and captures the residue onto an absorbent trap. The trapped compounds are then analyzed using gas chromatography.

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A forensic analyst holds
DNA samples.

Murder scenes can produce a wealth of evi­dence, from shell casings to human blood and hair. Investigators gather all of this evidence, and forensic technicians analyze it in various ways, based on the type of evidence:

Gunshot residue: When a gun is fired, residue exits the gun behind the bullet. Traces of this residue can land on the hands of the person firing the weapon or on the victim. Police use tape or a swab to lift residue off the hands of a suspected shooter. Then the forensics technician uses a scanning electron microscope to examine the sample. Because elements in gunpowder have a unique X-ray signature, examination under the electron microscope can help determine whether the substance is actually gunshot residue. Technicians will also use dithiooxamide (DTO), sodium rhodizonate or the Greiss test to detect the presence of chemicals produced when a gun is fired.

Fibers: Infrared spectrometry/spectroscopy identifies substances by passing infrared radiation through them and then detecting how much of the radiation they absorb. It can identify the structure and chemical components of various substances like soil, paint or fibers. With this technique, forensic technicians can match fibers found on a victim's body to those in a piece of clothing or furniture.

Fingerprints: Fingerprinting relies on the unique pattern of loops, arches and whorls that covers each person's fingertips. There are two types of fingerprints. Visible prints are made on a card, or on a type of surface that creates an impression, such as blood or dirt. Latent prints are made when sweat, oil and other substances on the skin reproduce the fingerprints on a glass, murder weapon or any other surface the perpetrator has touched. These prints can't be seen with the naked eye, but they can be made visible using dark powder, lasers or other light sources.

One method forensics labs use to make latent prints visible uses cyanocrylate -- the same ingredient in superglue. When it's heated inside a fuming chamber, cyanocrylate releases a vapor that interacts with the amino acids in a latent fingerprint, creating a white print. Technicians may also use a wandlike tool that heats up a mixture of cyanocrylate and fluorescent pigment. The tool then releases gases on the latent prints, to fix and stain them on the paper. Other chemicals that react with oils in fingerprints to reveal latent prints include silver nitrate (the chemical in black-and-white film), iodine, ninhydrin and zinc chloride.

Body fluids: A number of tests are used to analyze blood, semen, saliva and other bodily fluids:

• Semen: To test a sample to see whether it contains semen, technicians use acid phosphatase, an enzyme found in semen. If the test turns purple within a minute, it's positive for semen. To confirm the results, technicians look at stained slides of the sample under a microscope. The stain colors the heads of the sperm red and the tails green (which is why the test is referred to as the "Christmas tree stain").
• Blood: the Kastle-Meyer test uses a substance called phenolphthalein, which is normally colorless, but turns pink in the presence of blood. Another test for blood is luminal, which is sprayed over a room to detect even the tiniest droplets of blood.
• Saliva: The phadebas amylase test is used to detect a-amylase, an enzyme in human saliva. If amylase is present, a blue dye will be released.

DNA analysis: DNA is the unique genetic fingerprint that distinguishes one person from another. No two people share the same DNA (with the exception of identical twins). Today, forensic scientists can identify a person from just a few tiny blood or tissue cells using a technique called polymerase chain reaction (PCR). This technique can make millions of copies of DNA from a tiny sample of genetic material.