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Reading Selection, Lesson 17

"Separation Science" at the FBI

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FBI headquarters in Washington, DC

What does this building have to do with chromatography?

Two thieves rob a small-town bank of thousands of dollars. . . . A bomb explodes in Oklahoma City, killing more than 150 people. . . .

These two crimes are very different, but one of the techniques that crime detection experts use to investigate them is the same. It's called chromatography. One of the largest chromatography laboratories for crime detection is located in Washington, D.C., at the headquarters of the Federal Bureau of Investigation (FBI). Samples from crime scenes across the nation are sent to the FBI for forensic analysis. (Forensic analysis is a special type of scientific analysis that is performed in conjunction with legal or court proceedings.)

Many experts in forensic analysis work at the FBI. Some spend their time comparing fibers and hairs from crime scenes; others compare the soil from a suspect's shoes with the soil found at a crime scene. There are many rooms full of special equipment. Two of those rooms are dedicated specifically to chromatography. It is here that FBI Special Agent Kelly Mount works.

"Chromatography," explains Kelly Mount, a specialist in chromatography, "is a "separation science." It's the technique we use in our labs to separate the components of a mixture."Chromatography has many uses; it's not limited to crime detection. In fact, the first chromatography was done by a Russian botanist in 1906. He discovered that chlorophyll pigment could be separated from green leaves by passing an ether solution through a tube containing powdered calcium carbonate (chalk).

Mount uses a variety of substances to do this separation. Gases and liquids are the most common. Separating the substances, Mount says, is just the beginning of forensic analysis. "The purpose of chromatography is not to identify, but to separate," she emphasizes. Once scientists have their results, they have to compare them with the results of other analytical techniques before they can be admitted into evidence.

"Let's go down the hall and take a look at two ways in which we use chromatography," Mount says. "The first one is simple and inexpensive—students do something similar to this in science class. The other one needs some sophisticated equipment.

"First, we'll take a look at how we use a simple form of chromatography, called thin-layer chromatography, to help track down bank robbers," she says. "Here's how it works. When banks bundle paper currency together, they routinely include a special security device inside some of the packs. This device has a miniature"bomb" inside of it. When triggered, the bomb explodes. It doesn't do any damage to humans, but it does release a bright red liquid. The liquid is impossible to wash out."

When a robber says, "Hand over the cash," the bank teller obligingly turns it over, making sure to include a bundle of the specially packaged money. Soon after the robber leaves the bank, the device explodes, showering his or her car, clothing, or bag with the distinctive red dye.

The suspected criminal is sometimes caught "red handed." A sample of clothing or other material stained with the dye is sent to Kelly Mount's unit for analysis. Is it the dye from the security device? Or did the red color come from another source?

The chemical composition of the red dye used by banks is unique. No other dye has the same composition. Mount compares this dye of known composition with the dye found on a suspected criminal. She takes a crime scene sample of the material containing the dye and puts it in a solvent to extract the dye from the material. Mount then places several drops of the dissolved sample along a line at the bottom of a small, specially coated plate. (The thin coating on the plate is what gives thin-layer chromatography its name.) She puts the plate in a liquid. As the dots become moist and interact with the coating on the plate, they begin to move up the plate at different rates, depending on the solubility of the components. All dyes made from the same components will form the same pattern on the plate. So, if the crime scene sample matches that of the dye used by banks, another crime has been solved.

Site of the crash in Lockerbie, Scotland

Chromatography is used by the FBI as a weapon against terrorism. Residues from terrorists’ bombs, like the one that destroyed this aircraft, can be analyzed to determine the type and origin of the explosive used.

Bombs and Explosives
Chromatography also comes in handy for analyzing the materials used in bombs and explosive devices. The FBI analyzes samples from all major bombings involving the United States, including the one at the Murrah Federal Building in Oklahoma City and others causing airline crashes. The technique is called high-pressure liquid chromatography—HPLC, for short.

The first step is examination under a microscope. "Most bomb samples look pretty much alike. They look like black powder," says Mount. Even so, this first step is important. The scientists might, for example, be able to sort out small pieces of material from the residue.

The next step is extraction. The chemists place the sample in a solvent such as water. Once in solution, the particles in the sample may, depending on the composition of the sample, separate into smaller particles that carry positive or negative charges.

A small amount of the solution is placed in the HPLC machine. It moves up to the top, where it mixes with another liquid, and is then forced downward under pressure through a narrow glass column that is filled with a porous substance.

What happens in the column is the critical step. "Some of the [particles]," explains Mount, "seem to like it better in the tube than others. They stay longer."

The speed at which the particles leave the column is recorded by a detector, which then prints out the information. By comparing the time that the particles have stayed in the column with known retention times, Mount and her colleagues are able to distinguish the various types of particles in the test sample.

Still a Lot To Learn
Does it always work? "No," says Mount.  "Sometimes we find nothing. And other times, we find nothing conclusive. It's also important to note that when it comes to explosive materials, HPLC is only a qualitative analysis technique. It helps us identify what materials are in an unknown powder. It doesn't provide quantitative information; in other words, we can't tell how much of each substance is in the powder."

Kelly Mount loves her work. To prepare for her career, she earned a bachelor's degree in chemistry and then went on to get a master's degree in forensic science. Life in the lab is never routine—this means getting called in to the lab on weekends or even at night when there is an emergency. Whether the problem is bombs or banks, Mount has the expertise to help the FBI solve its mysteries.

Question

1. What are the two types of chromatography mentioned in this reader? Use an encyclopedia, other library resources, and the Internet to find out more about these and other chromatography techniques.

2. Luminol is a chemical used by forensic scientists to reveal traces of blood not readily visible to the human eye. Do a search on the Web site http://www.howstuffworks.com to find out how Luminol works.

 

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