BERKELEY, Calif., August 28 -- With infrared light, forensic investigators can tell you whether a document is a forgery or whether paper currency is counterfeit. They can take a paint chip and tell you the make, model, and age of a car. Now the boundaries of infrared forensics are being pushed into uncharted territories by researchers at Lawrence Berkeley National Laboratory, and the results are promising for criminal and antiterrorism investigations, as well as for historians and archaeologists.
Working at the Advanced Light Source (ALS), anelectron synchrotron that's been optimized for the production of x-ray and ultraviolet light but also generates intense beams of photons in the infrared (IR) spectrum, the Berkeley Lab researchers have applied IR spectromicroscopy in a proof-of-principle study to characterize a variety of inks on paper with unprecedented sensitivity. They've also used these IR beams to obtain chemical "sweatprints" that may be every bit as unique and ubiquitous as physical fingerprints.
"The combination of IR spectroscopy and microscopy is an extremely powerful analytical technique," says Dale Perry, a chemist with Berkeley Lab's Earth Sciences Division and one of the members of the team doing this research. Other members of the team are Tom Wilkinson, Wayne McKinney and Michael Martin.
They've been conducting their proof-of-principle studies on the IR Spectromicroscope at ALS beamline 1.4.3 which was designed under the leadership of McKinney and is now managed by Martin. Beamline 1.4.3 is one of three experimental endstations operating off a single ALS bend-magnet that make use of IR photons.
IR forensics is derived from the fact that all molecules, because of the nonstop motion of their atoms, vibrate at a characteristic frequency which falls within the infrared spectrum. When an individual molecule is struck by an infrared photon that matches its vibrational frequency it will resonate, and this resonance, detected through a variety of spectroscopic techniques, can be used to precisely identify the molecule, much like a fingerprint can be used to identify an individual person.
The use of IR spectromicroscopy in forensics got its start back in 1949, but its applications were sharply limited because a large sample size was required for analysis. This changed with the commercialization of thermal IR sources such as Globars (TM) in the early 1990s. Globars are silicon-carbide filaments that radiate IR light when heated; using Globars, forensics researchers are able to work with samples as small as 75 microns (75 millionths of a meter). But even that can be excessive when dealing with criminal evidence or precious historical artifacts.
Says Perry, "At the ALS, we can focus IR light down to a 10 micron spot size [.0004 inches] or less. Thanks to the high brightness of the light, we can also get about 200 times the sensitivity of a Globar. This means we can work with much tinier sample sizes and see details that would otherwise be missed using a Globar or any other conventional IR source."
Says Martin, "Another advantage is that we can shine an ALS infrared beam on an unknown sample and determine its chemical composition without the need for elaborate sample preparations. Also, IR light is nondestructive, in that it does not break any bonds or change the chemical formula of a sample. In tests, we've shown that a focused IR synchrotron beam heats a sample by only .5 degrees Celsius."
In their most recent study, published in the June, 2002 issue of the Journal of Applied Spectroscopy, Perry, Martin, McKinney and Wilkinson worked with the U.S. Secret Service to demonstrate the effectiveness of synchrotron-based IR spectromicroscopy on inks.
"The Secret Service is interested because they can use IR data on ink to identify the possible origins of a document, verify that the document is as old as it is claimed to be, and check if the same ink is used throughout a document," says Perry. "IR data is also potentially effective for identifying chemical aspects in other ink-based items such as currency and stamps."
In the past, characterizing ink on paper has been a daunting task. Using a Globar as the IR source, a sample of the ink must be extracted from the paper before it can be analyzed. Typically, a hole is punched through the paper with a hypodermic needle and the ink is chemically separated. This approach is not only destructive, but may also alter the chemistry of the ink before it is analyzed. Further complications arise from the mere act of writing in ink. From the moment the ink is applied to the paper, subtle chemical changes begin taking place as a result of the interaction between ink and paper.
"We overcame these problems using ALS IR photons at wavelengths of 2.5 to 25 microns to characterize ink samples," says Perry. "Our light beam was so intense we could make rapid and direct spectromicroscopic measurements of the inks without having to chemically separate them from the paper."
Infrared spectra of ink on a paper fiber using both synchtrotron and Globar infrared sources. The difference in the two spectra is due to the much greater brightness of the synchrotron light impinging on the fiber.
Furthermore, because of the high spatial resolution, the Berkeley Lab researchers were also able to create IR spectroscopic profiles of ink and paper interfaces so they could determine where one ink ended and another began in the same signature or line of print.
"The superior sensitivity and resolution of our synchrotron-based approach demonstrates its nearly unlimited possibilities for looking at very small fragmentary samples of ink on paper," says Perry.
Another study done earlier holds important possibilities for criminal and antiterrorism investigations. This study involved the use of IR spectromicroscopy to identify chemical sweatprints.
Everyone knows that when you touch something you leave behind a fingerprint whose pattern of loops, whorls, arches and "tents" is distinctly your own. What you may not know is that you also leave behind a minute residue of chemicals --proteins, salts, and fatty acids -- whose proportions to one another may also be distinctly your own.
Although the forensic jury is out as to whether chemical sweatprints are as unique as physical fingerprints, the Berkeley Lab researchers were able to correctly distinguish the sweatprints of three individuals.
"All of the oil metabolites in sweat have an IR spectrum. The question is whether we can find and identify their spectra in the context of the overall IR print spectrum which may contain many additional compounds," says Perry. "However, since we can analyze a sample less than 10 microns across, we have an advantage in that we can work with a sweatprint that is smaller than a single ridge in a physical fingerprint."
An IR spectromicroscopic profile of a sweatprint might also reveal the age and gender of the person leaving the sweat and possibly even identify when the sweat was deposited, if the appropriate chemical markers can be observed in the IR spectrum. In any case, since the technique is nondestructive, once an IR profile has been acquired the undamaged sweatprint can be studied further by other forensic techniques.
Synchrotron-based IR spectromicroscopy should also be applicable to the characterization of trace amounts of biological fluids on cloth or blood on glass; tracing explosive chemicals, poisons, or illicit drugs to their manufacturers and suppliers; and even identifying the geographic origins of dust particles.
Says Perry, "In light of what we've already demonstrated at the ALS, synchrotron-based IR spectromicroscopy as a forensics tool has a bright outlook."
Download a PDF of the report, "Use of Synchrotron Reflectance Infrared Spectromicroscopy as a Rapid, Direct, Nondestructive Method for the Study of Inks on Paper"
For more information, visit www.lbl.gov