Database sheds light on crime

Database sheds light on crime

FBI app uses X-ray technology to identify most any element


Imagine a murder victim bound and gagged with duct tape and smeared with lipstick. Now imagine the prosecutor convicting the murderer with spectral proof of the brands of duct tape and lipstick.

FBI chemist Dennis Ward is developing a database application that will help investigators identify trace substances found at crime scenes.
An FBI database now under development would give police a repository built up from X-ray spectroscopy to positively identify the trace substances found at crime scenes.

Dennis C. Ward, a chemist with the FBI Laboratory in Washington, is working with Phoenix consultant John Colby to develop a searchable database called the Spectral Library Identification and Classification Engine, or SLICE.

The prototype resides on Ward's IBM ThinkPad notebook computer at present, but he hopes to turn SLICE into a large database for forensic scientists worldwide. Eventually, he said, it might contain tens of thousands of X-ray spectra of metal alloys, gunshot residues, adhesives, cosmetics and paints.

Forensic labs use X-ray spectroscopy to identify inorganic substances, Ward said, and longer-wave-length spectroscopy for organic compounds. The X-ray technique works better on trace amounts of material or tiny paint chips.

To study materials at the FBI Lab, Ward uses a JEOL JSM-6300 scanning electron microscope from JEOL USA Inc. of Peabody, Mass., with a built-in, energy-dispersive spectrometer from Oxford Instruments PLC of Oxford, England.

He controls the setup with Link ISIS Series 3000 software from Oxford Instruments, running on a Hewlett-Packard Vectra XM Series 4 workstation.

An electron microscope illuminates tiny objects with a focused beam of electrons instead of visible light. It reveals such things as tool marks, pits or fracture patterns.

While the target item is in the electron beam, oncoming electrons interact with the atoms of the material, causing them to give off X-rays that a solid-state detector can pick up.

Because each element emits X-rays at a unique set of energies, a plot of the number of X-rays emitted at each energy level reveals the chemical composition.

Coin toss

Scanning a dime to demonstrate the setup, Ward pointed out the spectral peaks for nickel and copper in the coin. He said comparing the heights of the peaks shows the relative proportions of the elements within.

FBI's Dennis Ward has collected spectra data on 1,000 substances and hopes to add more from crimefighters around the world. Such a database is 'only as good as the number of entries in it,' he says.
Unlike some other types of spectroscopy, X-ray spectra have no logically organized, searchable databases. Off-the-shelf software is available to control the instruments and store the spectra, but it lacks search functions and categorization.

About five years ago Ward built a prototype dBase database, but it lacked functions, he said. SLICE, which Colby built with Microsoft Access, displays the spectrum of each material with the common functions that spectroscopists use: peak identification, labels, scale expansions 'and all those little goodies that you need to characterize the material,' Ward said.

The FBI database attaches to each spectrum the manufacturer, model, product code, laboratory and operator that created the spectrum, chemical composition, and an image of the product or its packaging.

SLICE categorizes material by overall color and by product category. Categories are organized in a tree structure. For example, a user would drill down from manufactured products to personal products, cosmetics and lipstick.

'This is intended to be all the information you would ever want or need about the material, attached to the spectrum,' Ward said.

Peaks and valleys

The power of the prototype database lies in its ability to search by multiple criteria. For example, SLICE can compare the peaks in an unknown spectrum against other spectra in the database to see what fits best.

'That's very simple, but we've never had that capability before,' Ward said.
Users can limit searches by material or color category. They can also initiate searches by keyword or by chemical composition.

'It's designed for the person who doesn't have necessarily a good materials sense,' Ward said.
A best-fit search performs statistical tests to find the most probable matches between known and unknown spectra. But the X-ray spectral database is 'only as good as the number of entries in it,' Ward said.

SLICE contains entries for about 1,000 substances so far. Ward has been building the database by scanning the spectra of used cosmetics donated by coworkers.

Ultimately he hopes that local, state and international crimefighters will contribute spectra for materials that they come across in investigations.

For a multiuser database to be accurate, however, it must take into account the variations between instruments that collect the spectra. Every spectrometer responds slightly differently to incoming signals, Ward said.

To eliminate that difference, the database administrator would have to require each member lab to contribute a spectrum of a pure standard material, such as nickel. FBI researchers would then compare the spectrum against their own reference spectrum and mathematically compensate for the differences.

John J. Behun, chief of the FBI Lab's Forensic Science Systems Unit, said the bureau has had past success in building comprehensive databases for law enforcement, most notably the Combined DNA Index System [GCN, May 15, 2000, Page 3].

Once an X-ray spectral database is open to the law enforcement community, it could help with cases where the same material shows up at multiple crime scenes, Behun said.


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