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Analytical Chemistry

Chemistry's Crime Fighters

ACS MEETING NEWS: Forensic scientists tell how they help solve crimes using a variety of analytical techniques

by BETHANY HALFORD, C&EN WASHINGTON
April 25, 2005 | A version of this story appeared in Volume 83, Issue 17

Like most people, when american Chemical Society members analyze evidence from a crime scene, they usually do so from the comfort of their couch during an episode of the TV series "CSI: Crime Scene Investigation." Thanks to the Committee on Science, a roomful of armchair criminalists at last month's ACS national meeting in San Diego got to vicariously duck behind the yellow police tape and see how chemistry clarifies criminal investigations.

"Maybe we could call this 'CSI: ACS' or something like that," joked Luis A. Echegoyen, a chemistry professor at South Carolina's Clemson University, during his opening remarks at the symposium "Finding Criminals with Forensic Chemistry." Echegoyen co-organized the symposium, which was cosponsored by the Divisions of Analytical Chemistry and of Chemistry & the Law. It gave attendees an idea of the current and emerging analytical techniques that forensic chemists use.

Today, "science is playing a much greater role overall in criminal investigations" than in the past, remarked Ronald L. Kelly, a forensic chemist with the Federal Bureau of Investigation's Explosives Unit. Kelly often travels to the sites of deadly bombings and explosions to help investigators gather trace evidence. He then analyzes this evidence to figure out what type of explosives and ignitable liquids were used in the attack. The information can point to the material's source and sometimes link the incidents to specific terrorist organizations.

In 1998, for example, Kelly was part of the team that investigated the bombings of U.S. Embassies in Nairobi, Kenya, and Dar es Salaam, Tanzania. The nature of the simultaneous attacks suggested that they were the work of the same terrorist organization, but it was up to Kelly and his coworkers to determine whether the two events were related forensically.

A CRIME SCENE
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Credit: NEWSCOM/AGENCE FRANCE PRESSE/ALEXANDER JOE
After the bombing of the U.S. Embassy in Nairobi, Kenya (pictured here), forensic chemist Kelly gathered trace evidence that enabled him to link the bombing to a simultaneous terrorist attack in Dar es Salaam, Tanzania.
Credit: NEWSCOM/AGENCE FRANCE PRESSE/ALEXANDER JOE
After the bombing of the U.S. Embassy in Nairobi, Kenya (pictured here), forensic chemist Kelly gathered trace evidence that enabled him to link the bombing to a simultaneous terrorist attack in Dar es Salaam, Tanzania.

After visiting both scenes and processing thousands of pieces of trace evidence, the investigators were able to physically prove that the attackers worked in harmony. They were also able to use the evidence to eliminate potential suspects.

Tracking down evidence from an explosion is no simple task, Kelly noted. "You have evidence going all over the place," he said. "Half the challenge is just locating the evidence." In 1995, when Kelly investigated the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, the assessment took more than three weeks and covered 20 sq blocks. Evidence from the bombing of Pan Am Flight 103 over Lockerbie, Scotland, was gathered over more than 50 sq miles, according to Kelly.

Investigators rely on handheld and portable devices when looking for trace evidence over such a large area, Kelly said, adding that "our old pal, the dog," is still one of the best detection systems that crime-scene investigators use. They also employ field instruments such as portable infrared and Raman spectrometers.

Naturally, instrumental analysis isn't limited to fieldwork. At his lab in Quantico, Va., Kelly has access to an entire battery of high-tech tools such as thermal energy analysis, ion-mobility spectrometry, ion chromatography, stable-isotope ratio analysis, and direct analysis in real time--to name just a few. "You really have to be well-versed in instrumental analysis to do this kind of work," he told the crowd.

SYMPOSIUM ATTENDEES also got a lightning-speed primer on the future of technology for detecting explosives and chemical warfare agents in the field, courtesy of University of Arizona chemistry professor M. Bonner Denton.

Denton discussed how instruments once considered too large and expensive for practical use by law enforcement officers are becoming cheaper, more portable, and more sensitive. In particular, Denton spoke about a device his group has developed that he compared to a Star Trek-style tricorder, which could be used by airport baggage screeners to scan luggage for illicit drugs, explosives, or other dangerous chemicals.

The device is a pocket-sized ion-mobility spectrometer. Airports already use ion-mobility spectrometers to detect explosives--it's that microwave-sized device into which transportation safety officers will insert swabs that they use to mop up residues on a briefcase or camera.

Denton said that, by improving the instrument's electronics, his group has managed to shrink it and make it 1,000 times more sensitive than the devices used at airports. The device is so sensitive, Denton said, that it could detect traces of explosives in the air that passes over a person who's handled them.

Taking circuitry originally developed for infrared astronomy, Denton managed to boost the spectrometer's readout circuitry. "This change in readout electronics is key to the vastly improved sensitivity. It boosts the signal while lowering the noise," he said. "This is the first radical change in ion detection since the 1930s."

University of Utah biology professor James R. Ehleringer spoke about his work using stable-isotope analysis to determine where people and substances originate. The technique can also be used to determine if two identical chemical substances come from the same source.

Ehleringer uses mass spectrometry to examine the ratio of stable isotopes, such as carbon-12 and carbon-13, in a compound. Depending upon the element he looks at, the ratio can point to a substance's source. For instance, the food industry uses stable-isotope testing to see if an unscrupulous juice maker has swapped corn syrup for the grape juice sweetener listed in the ingredients. The sugar from both is chemically identical, but molecules have different stable-isotope ratios depending on whether they come from grapes or corn.

The Drug Enforcement Administration already employs Ehleringer's analytical technique to find common sources for narcotics.

AT THE ACS MEETING, Ehleringer said that by analyzing the ratio of stable oxygen isotopes in a person's hair, he can often tell where that person lives. That's because the ratio of oxygen-18 to oxygen-16 in drinking water varies substantially from region to region. The water we drink is incorporated into our bodies, leaving the telltale signature of our hometown in our hair. Ehleringer's technology isn't easily thwarted either: Hair dye has almost no effect on the analysis, and limiting your drinking water to the bottled variety won't fool the technology because the source of most bottled water is close to where the water is ultimately consumed.

The oxygen-based analytical method can be used to help identify a John Doe in the morgue or to determine where a biological agent such as anthrax was grown, Ehleringer said.

No symposium on forensic chemistry would be complete without a discussion of DNA. During the afternoon session, San Diego County Superior Court Judge George W. Clarke noted that the ubiquitous molecule has become such a staple of TV court trials--both real and fictional--that in their deliberations, juries will often ask why DNA evidence wasn't presented, even if DNA has no bearing on the case.

Two chemists, John M. Butler of the National Institute of Standards & Technology and Bruce R. McCord of Florida International University, Miami, spoke about historical and current forensic DNA research. "DNA analysis is both sensitive and specific," McCord said, "and that warms an analytical chemist's heart." Nevertheless, he and Butler said scientists still face challenges when analyzing certain types of DNA evidence.

According to Butler, more than two-thirds of cases based on DNA evidence involve sexual assault. And one of the biggest challenges that criminalists face in these investigations, he said, is separating epithelial cells from a female victim from the sperm cells of the assailant.

Butler and his colleagues have developed a technique that allows them to amplify only the Y chromosome (originating from the male) in these mixed-DNA samples. To do this, the researchers developed primers that copy and amplify regions of DNA that are located exclusively on the Y chromosome. Butler said that using this method, his group can easily pick out the male DNA in a sample where the concentration of female DNA is 800 times higher.

McCord spoke about the challenge that degraded DNA poses to forensic chemists. If a body has been exposed to the elements for a long period of time or if a bloodstain is old, it's likely that the DNA has degraded. At a certain point, the DNA becomes too fragmented for routine analyses to work well, McCord said.

In collaboration with Butler, McCord developed primers that amplify smaller pieces of DNA, thereby helping forensic chemists glean information from these degraded samples. The technology, he said, helped investigators identify the victims of the World Trade Center attacks in 2001.

McCord is also working to develop a tool kit that would help investigators determine what type of DNA analysis is best suited to their sample. DNA analyses consume both time and money, and McCord thinks his tool kit could streamline the huge backlog of DNA evidence waiting to be processed.

Almirall first got interested in glass when he worked at the Miami-Dade police department. He explained that broken glass from a crime scene--for example, when a thief breaks into a house--often ends up on the crime's perpetrator. Investigators can use those tiny bits of glass evidence to link a suspect to the scene of the crime.

Investigators used to rely on the glass's refractive index to determine whether or not shards from the suspect and shards from the crime scene were from the same piece of glass, or "cannot be distinguished" in CSI-speak. These days, glassmakers, particularly automotive glassmakers, have gotten much better at making the refractive index of glass uniform. This makes it harder to link glass trace evidence beyond a reasonable doubt, Almirall explained.

When Almirall moved from the police department to academia, he thought he might be able to make the forensic study of glass more precise using elemental analysis. "I got interested in characterizing glass as a material," he said. By analyzing glass using solution-based, inductively coupled plasma mass spectrometry (ICP-MS), Almirall found that he could pick up trace-metal impurities in the glass.

Trace metals are introduced during the manufacturing process, often from the equipment or raw materials. They vary over time and consequently tend to be specific to a particular product and often a certain time frame, Almirall said. To prove the specificity of the elemental analysis, Shirly Montero, a graduate student of Almirall's, examined more than 700 different glasses from a variety of sources and more than 200 samples from a glass factory in Wisconsin. All but a handful of the possible comparison pairs were distinguishable by the technique. Furthermore, when Montero and Almirall analyzed 45 different headlights, they found that all of them could be distinguished from each other.

A TRACE EVIDENCE
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Credit: COURTESY OF JOSÉ ALMIRALL
Almirall used a specialized elemental analysis technique to link the glass at this crime scene to shards of glass found on the person suspected of breaking into the car.
Credit: COURTESY OF JOSÉ ALMIRALL
Almirall used a specialized elemental analysis technique to link the glass at this crime scene to shards of glass found on the person suspected of breaking into the car.

The solution-based method is not without its drawbacks, Almirall noted. "Dissolving the glass is very cumbersome. You need to use hydrofluoric acid and nitric acid," he said. Also, the analysis requires at least a few milligrams of glass that are ultimately destroyed.

Recently, Almirall has figured out a way to get around these problems by using laser ablation to sample the glass fragments--a technique he borrowed from geochemists. "Now, we can directly sample the glass, and we're only removing 300 ng of material," he said. "That's a big advantage in quantitative analysis."

Almirall presented one case study in which ICP-MS analysis was used to link a suspect and a crime scene. At 10 AM one Sunday in Miami Beach, a woman was struck and killed during a hit-and-run incident. A witness identified a black BMW as the car involved. It did not take the police long to find a smashed-up, black BMW in a nearby parking lot. But when they tracked down the vehicle's owner, he told police that his car had been stolen the night before.

The police had no way to prove he wasn't telling the truth, with the exception of tiny bits of glass they found on the man's clothing, in his bathroom, and in his sink. Almirall was able to use ICP-MS to determine that the glass from the BMW and the glass from the suspect's apartment were indistinguishable. "The glass evidence was the only evidence that tied him to the incident," Almirall said.

Almirall is currently trying to use laser ablation ICP-MS to analyze trace metals in paint, ink, bone, and teeth. He joked that this technique must be ready for prime time, as it figured as a plot device on "CSI."

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