Expanding on years of mass spectrometry-based fingerprint detection technology and research, Melanie Bailey and her team have now proven a single fingerprint left at a crime scene can be used to determine whether someone has ingested cocaine or simply touched it.
In 2017, Bailey, a forensic scientist at the University of Surrey, utilized a high-resolution mass spectrometry (HRMS) method to detect cocaine in the fingerprints of drug users, but did not consider the fact that 1 in 10 non-drug users are exposed to cocaine through environmental factors. Her next study, published in February of last year, showed the HRMS method can tell the difference between contact and ingestion of cocaine but only if the person has washed their hands prior to leaving a fingerprint—an unlikely situation for fingerprints found at crime scenes.
Now, in the latest study published this month, Bailey and her team successfully removed the limitations of their previous research, demonstrating how three different mass spectrometry imaging techniques can detect the difference between cocaine contact versus ingestion in unwashed hands.
"In forensic science, being able to understand more about the circumstances under which a fingerprint was deposited at a crime scene is important,” said Bailey. “This gives us the opportunity to reconstruct more detailed information from crime scenes in the future. The new research demonstrates that this is possible for the first time using high-resolution mass spectrometry techniques."
Acquiring one fingerprint sample from four individuals who touched cocaine and four who ingested it in the prior 24 hours, the research team imaged the samples using desorption electrospray ionization (DESI), matrix assisted laser desorption ionization (MALDI) and time-of-flight secondary ion mass spectrometry (ToF-SIMS).
DESI and MALDI both demonstrated the ability to detect differences in fingerprints deposited after use versus touch of cocaine. According to the imaging results, ingestion can be characterized by a higher intensity of benzoylecgonine (BZE)—the molecule produced in the body when cocaine is ingested—relative to the drug) in the area of fingerprint deposition. Additionally, ingestion showed an even distribution of these analytes across the fingerprint and on the ridges.
Meanwhile, a higher intensity of cocaine relative to BZE in the area of fingerprint deposition can be seen for users that touched cocaine, but did not ingest it. In this case, the researchers noted analyte distribution that does not directly correlate with the flow of fingerprint ridges.
While the results of ToF-SIMS imaging did eventually match DESI and MALDI, it wasn’t without modification. At first, the technique lacked the sensitivity needed to provide acceptable images of either cocaine or BZE distribution. So, the team turned to a new innovation.
“One of the limitations of conventional ToF-SIMS primary beam analysis is low ion yield from organic molecules due to high levels of fragmentation,” the authors explain in the study. “Advances in ion gun technology have recently resulted in the introduction of water cluster guns, which have shown promising results for analysis of organic and biological molecules.”
Employing a water cluster SIMS source, the same results for contact—a high BZE-to-cocaine intensity ratio over the area of the fingerprint—were generated. This is the first application of water cluster SIMS to a fingerprint sample.
“To image these metabolites excreted through the skin requires very powerful analytical tools such as the unique Water Cluster Source that Ionoptika has been developing for over a decade,” said Allen Bellew, applications manager at Ionoptika. “It's clear that this new technique will be important for forensic science in the future, and as a small business in the UK, it's very exciting to see the role that our J105 SIMS instrument has played in its development."
Photo: The J105 SIMS instrument. Credit: Ionoptika