Optical Technique Could Improve Forensic Information
Gained from Fingerprints
Fingerprint analysis is one of the oldest techniques used by forensic scientists and police to identify suspects. However, the method is not always reliable and gives information only about what the fingerprint looks like, even though fingerprints have the potential to tell a lot more. With advances in modern technology, researchers have begun to examine whether fingerprints as well as ridge patterns could give specific chemical information about a suspect.
These Fourier transform infrared spectroscopy images of a latent fingerprint lifted from the door handle show the distribution of lipids (left) and the distribution of the gelatin tape (right).
A fingerprint contains a complex mixture of natural bodily secretions as well as contaminants from the environment. The primary component of fingerprints is sweat from glands on the finger ridges; analysis of this sweat can reveal information about the fingerprint’s owner.
“The makeup of someone’s sweat depends on a variety of factors, including age, gender, race and habits,” said Sergei G. Kazarian, whose group at Imperial College London has developed a technique for chemically imaging fingerprints. “It could even tell you if someone is a vegetarian because their sweat would contain different proteins and amino acids when compared with someone who eats meat.”
To identify a crime suspect, the police are interested also in information such as how long the fingerprint has been in place and what substances the suspect has handled. Conventional techniques for lifting fingerprints, such as dusting the print with powder, make the print unusable for chemical analysis.
Kazarian and his colleague Camilla Ricci have developed an optical technique for performing chemical analysis of a fingerprint without powders or chemicals. Based on Fourier transform infrared spectroscopy imaging, this noncontact method involves lifting the fingerprint with a gelatin tape and imaging it. The chemical constituents of the fingerprint can be analyzed; the results could in the future be used to provide “lifestyle clues” about the person depositing it — for example, about smoking and drug use habits.
Standard Fourier transform infrared spectroscopy typically has been used only in academic studies because the technique relies on depositing the fingerprint on the sensor. Gel lifting also is an established technique for lifting both latent and powdered marks from a range of surfaces.
However, the gelatin tape contributes interference. “These tapes have a large number of bands in the mid-infrared region that can cover the spectral features of the finger mark residue,” Kazarian said.
To minimize this interference, the researchers adapted a Varian Inc. attenuated total reflection Fourier transform infrared spectrometer by adding a single reflection accessory from Pike Technologies Inc. of Madison, Wis.
Because the penetration depth at any specific wavelength is strongly affected by the incident angles and refractive indices of the attenuated total reflection crystal, using a variable-angle accessory makes it possible to change the angle of incidence and to obtain infrared spectra from various depths within the sample. This means that the researchers were effectively imaging in the Z-direction, which is not limited by diffraction and, therefore, lessens interference from the tape.
“We [minimized contribution from the tape] by using an accessory which was not designed for imaging applications, so this was difficult,” Kazarian said. “The approach required some optical calibration and assessment of the imaged area, and this was successfully achieved.”
Kazarian said that developing a lifting medium with absorption peaks outside the regions characteristic of finger mark residues would improve the performance of the technique even further.
Contact: Sergei G. Kazarian, Imperial College London; e-mail: email@example.com.
MORE FROM PHOTONICS MEDIA