NIST unveils forensic technique to measure mechanical properties of evidence

November 01, 2016

You may have seen it on CSI: The star examines hair from a crime scene and concludes its color or texture looks like the defendant's hair, or maybe his dog's. Case closed.

But looks can be deceiving, as well as vague and subjective. In real life, the FBI is now reviewing thousands of cases involving hair comparisons going back to the 1980s because traditional identifications--often based on looks alone--have been called into question.

Instead, what if investigators could precisely measure a hair's mechanical properties--its stiffness and stickiness? In fact, they can, according to recent experiments at the National Institute of Standards and Technology (NIST), which is developing science-based methods to help ensure rigorous forensic practices.

"Lots of forensics is based on the how the evidence looks," NIST engineer Frank DelRio says. "We are trying to add another dimension, how things feel. How an object feels--its mechanical response--depends on the material and the object's history."

DelRio is an expert in atomic force microscopy (AFM), a precision tool with a mechanical probe typically used in basic sciences for imaging but also to measure responses to force, or pulling. He usually measures industrial materials like silicon. But he also watches a lot of CSI and thought his expertise could help answer national calls to enhance the accuracy, reliability and statistical rigor of forensics.

DelRio and NIST physicist Robert Cook recently used AFM to demonstrate quantitative methods for measuring--nondestructively and at the nanometer size scale--the mechanical properties of four types of evidence: hair, documents, fingerprints and explosives.

The researchers measured the stiffness and pull-off force (stickiness) for hair as a function of treatment, specifically conditioning and bleaching. They also measured these properties for test documents made to mimic forgeries marked with both ballpoint ink and printer ink, impression and pattern evidence such as how fingerprints change over time, and interactions of explosive particles and surfaces as a function of fabric type, rayon versus cotton.

The measurement results clearly distinguished various treatments of hair, types of ink, age of fingerprints and composition of fabrics, and related these data to the structure of the sample such as broken bonds in the hair and the smooth ballpoint ink versus the rough printer ink. Importantly, the measurements were rigorous--that is, precise enough to allow for tests and quantitative specifications of the statistical significance of the similarities or differences in properties. DelRio imagines that someday AFM might be used, for example, to measure old hair evidence and determine the probability that a criminal used a certain shampoo.

"This is all theoretical at this point," DelRio notes. "For this to be an effective practical tool, a lot of baseline measurements and in-depth studies would need to be done to develop a good sense of how these properties change over time."

In addition, DelRio notes that AFM calibration methods and standard samples or other methods for specifying accuracy would need to be developed to enable accurate comparison of measurements across laboratories. Also crucial would be the development of an experience base to build trust in AFM techniques, requiring widespread availability of instruments, training, protocols and standards for forensics labs, the paper notes.
-end-
Paper: F.W. DelRio and R.F. Cook. Quantitative Scanning Probe Microscopy for Nanomechanical Forensics. 2016. Experimental Mechanics. Posted online Oct. 31, 2016. DOI: 10.1007/s11340-016-0238-y

National Institute of Standards and Technology (NIST)

Related Fingerprints Articles from Brightsurf:

Pofatu: A new database for geochemical 'fingerprints' of artefacts
Due to the improvement and increased use of geochemical fingerprinting techniques during the last 25 years, the archaeological compositional data of stone tools has grown exponentially.

Scientists recreate DNA damage caused by toxins from smoking
Researchers from the University of York have recreated how toxins from smoking cause unique patterns of DNA damage.

Chocolate 'fingerprints' could confirm label claims
The flavor and aroma of a fine chocolate emerge from its ecology, in addition to its processing.

Genetic 'fingerprints' implicate gut bacterium in bowel cancer
A common type of bacteria found in our guts could contribute to bowel cancer, according to research funded by a £20 million Cancer Research UK Grand Challenge award and published in Nature today (Thursday).

Chemists use mass spectrometry tools to determine age of fingerprints
Chemists at Iowa State University may have solved a puzzle of forensic science: How do you determine the age of a fingerprint?

Cancer-causing culprits could be caught by their DNA fingerprints
Researchers from University of California San Diego School of Medicine have defined the most detailed list of genetic fingerprints of DNA-damaging processes that drive cancer development to date.

Experimental fingerprint test can distinguish between those who have taken or handled cocaine
An experimental fingerprint detection approach can identify traces of cocaine on human skin, even after someone has washed their hands -- and the test is also smart enough to tell whether an individual has actually consumed the class A drug, or simply handled it.

Study catalogues cancer 'fingerprints' in decade-long global effort to map cancer genomes
A global research collaboration, led by world class institutions in Singapore, the UK and the USA, has developed the most detailed catalogue of mutational fingerprints found in most types of cancers that could help clarify their developmental history and lead to new prevention and treatment strategies.

Residues in fingerprints hold clues to their age
Police have long relied on the unique whorls, loops or arches encoded in fingerprints to identify suspects.

Low power metal detector senses magnetic fingerprints
Recent studies have shown metallic objects have their own magnetic fingerprints based on size, shape and physical composition.

Read More: Fingerprints News and Fingerprints Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.