Example of using the new method: (A) shows a digital photo of fingerprints on a gelatin lifter (the prints are developed with white powder and lifted from a glass surface). It is clear that there are two overlapping prints, but the pattern details of the individual prints in the overlapping area are not immediately visible in the digital photo. With the new method, we can create hundreds of images of the chemical compounds present in the fingerprints. As shown in (B), some of these compounds are unique to each fingerprint, allowing us to visually separate the prints.
A two-decade old mass spectrometry method has breathed new life into forensic fingerprinting, seemingly solving a common problem—analysis of overlapping fingerprints.
By their very nature crime scenes are messy, so it’s not surprising that investigators often collect overlapping and weak fingerprints rather than perfectly placed whorls and swirls. Unfortunately, traditional photography cannot separate overlapping fingerprints, leaving the evidence virtually useless.
In a new study, however, researchers at Aarhus University demonstrate it is possible to use desorption electrospray ionization mass spectrometry (DESI-MS) to analyze overlapping and weak fingerprints lifted using gelatin lifters. This could be crucial in criminal cases where current methods fall short.
"We are presenting a method that has the potential to be integrated into the police's traditional workflow. If this happens, more fingerprints from crime scenes could be used and evaluated both visually and chemically," says postdoc Kim Frisch, lead author of the study.
Gelatin lifters and DESI-MS
Gelatin lifters are easy to use and are suitable for lifting fingerprints from delicate surfaces, such as peeling wall paint, and irregular objects like door handles. DESI-MS works by measuring the chemical compounds in these collected fingerprints based on their mass.
"We send a very fine spray of solvent, consisting of electrically charged droplets of methanol. This releases and ionizes substances on the surface of the fingerprint on the gelatin lifter. The substances are then drawn into the instrument, where their masses are measured individually," explains Frisch.
DESI-MS was invented about 20 years ago, initially for general surface analysis. In 2008, it was shown that the technique could be used for chemical imaging of fingerprints on glass surfaces and tape.
“Now we show that the technique can also be used to analyze fingerprints collected on gelatin lifter, which are used by police in many countries,” said Frisch.” This is analytical chemistry used in a forensic context, and it has great potential.”
Frisch and team tested their new method on fingerprints in the laboratory before moving on to fingerprints from crime scenes, supplied by the National Special Crime Unit of the Danish Police. That work is currently in progress.
More than a pattern
The researchers are also working to leverage the chemical imaging method for profiling. Fingerprints can reveal much more than a pattern, including the use of nicotine, drugs and caffeine, as well as potentially incriminating substances such as lubricant from condoms. Many researchers across the globe are working on this utilizing a variety of methods, in addition to using fingerprints to determine individuals' gender, age and lifestyle.
Frisch and her team at Aarhus University will continue to work on the 2.5-year-long study—supported by the Danish Victims Fund—in an effort to maximize the information that can be obtained from fingerprints.
Ultimately, the results suggest the method could be used in practice—in specific situations.
“The scanning process is time consuming, which means that we would not be able to analyze samples in the hundreds, as we do with, for example, blood samples. We expect that the method will be used in the future as a special analysis in more serious cases such as murder and rape," said Frisch.