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How Forensic Lab Techniques Work

Murder Investigations

A forensic analyst holds DNA samples that will be used to identify and prosecute a suspected serial killer.
Mario Villafuerte/Getty Images
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.

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