Forensic Applications
of
Infrared Spectroscopy
Infrared spectroscopy is useful for identifying substances and confirming their identity. With the technology of FTIR spectroscopy (Fourier Transform Infrared spectroscopy) machines and integrated computer databases of known IR absorbance graphs, nearly any substance can be identified. For example, IR can be used to locate the make, model, and year of car by analyzing a paint chip. Click on the links below to see how IR is used to help solve crimes.
Analyzing Alcohol
Analyzing Drugs
Analyzing Fibers
Analyzing Paint
Analyzing Alcohol
with Infrared Spectroscopy
THE ANALYTE: Breath (suspected to contain alcohol)
THE IDENTITY TEST: The breath is tested with a mechanism similar to a breathalyzer. The diffference is that a breathalyzer relies on chemical oxidation, and therefore requires a dangerous reangent, while a mechanism using an IR test (for example, a CMI INTOXILIZER), uses the unique response of alcohol to infrared radiation.
Different compounds within alcohol (and other substances) resonate and absorb radiation only at specific wavelengths. The intoxilizer or other IR mechanism compares the resonance and absorption of infrared radiation from the breath in one chamber to the same properties in a second chamber, which contains air.
THE IDENTITY AND THE CRIME: An officer pulls over a driver who is breaking the speed limit. Suspecting the driver of being intoxicated, the officer tests the driver's breath with an intoxilizer, which gives the officer immediate result. Finding that the driver is under the influence of alcohol and is over the legal limit of alcohol. The officer suspends the driver's license, and the driver is ticketed and fined for driving under the influence of alcohol and putting other drivers and passengers at risk.
Photo from Eden Prairie Police Website
Analyzing Drugs
with Infrared Spectroscopy
THE ANALYTE: Drugs (perhaps heroine, LSD, or cocaine...)
THE IDENTITY TEST: A tiny sample of the drug is put into an IR testing machine. The drug's various chemical components respond uniquely to the infrared light. The machine prints out a graph, which a chemist compares to known graphs, either by using knowledge of peak properties or using a computer database, probably both. The identity of the drug is confirmed.
THE IDENTITY AND THE CRIME:
SCENARIO #1: A victim of assault claims to have drugged and raped. Samples tested with IR are identified as rohypnol, a 'date-rape drug'. After obtaining a warrant, investigators find that the suspect is in possession of the drug. The suspect is arrested and sent to trial.
SCENARIO #2: A public figure is suspected of possessing illegal drugs. A mysterious white powder found in the public figure's kitchen is tested with IR and proven to be cocaine. The public figure is arrested and tried for illegal possession.
Read about Toxicological Investigations of Drug-Facilitated Sexual Assaults.
Pictures from The Detroit News and Drug Detection.com
Analyzing Fibers
with Infrared Spectroscopy
THE ANALYTE: Synthetic Fibers (possibly polyester, nylon, or acrylic...)
THE IDENTITY TEST: The expected identity of the fiber has been established by observing it under a microscope. Now it is tested with IR to confirm the suspected identity.
Single threads of the fiber are put into a machine that shines infrared light on them. The unique chemical structure of compounds within the fiber react to the light; different compounds absorb different wavelengths.
The machine prints out a graph, which the chemist analyzes by comparing the graph to known graphs. The known graphs may be a list of wavelengths in a book, or an integrated, international computer database the matches the graph to an identical or very similar known graph.
THE IDENTITY AND THE CRIME: The fiber has been confirmed as matching fibers found on the crime scene, the victim, or the suspect. Combined with other evidence, the forensic scientist testifies to a judge, district attorney, or jury, and the culprit is prosecuted.
Read about the FBI's Forensic Fiber Guidelines
Chapter 1 (Introduction)
Chapter 2 (Microscopy Textile Fibers)
Chapter 3 (Visible Spectroscopy of Textile Fiber)
Chapter 4(TLC of Dyes in Textile Fibers)
Chapter 5 (PGC of Textile Fibers)
Chapter 6 (IR Analysis of Textile Fibers)
Photo from the Handbook of Analytical Methods
Analyzing Paint with Infrared Spectroscopy
THE ANALYTE: Paint (from a car, or on a weapon or someone's clothing...)
THE IDENTITY TEST: Paint has been recovered from a crime scene. Investigators must record the evidence and keep track of it. Since there is a limited amount of paint, the first tests to be done should be nondestructive. Colors, layers, texture, and other physical properties are recorded, using a microscope as well as the naked eye.
Finally, the individual layers of paint are analyzed by infrared spectroscopy. The results can be compared to IR results of known paint.
THE IDENTITY AND THE CRIME:
SCENARIO #1: There has been a hit-and-run accident. The victim has paint smears on their clothing. A witness remembers part of the license plate. There are several possibilities.
After narrowing the cars down to those with a overcoat of paint the same color as the smear, the sample is analyzed with IR. Using a computer database of car paint processes, the paint is identified with a specific make, model, and year of vehicle.
The possible cars are narrowed down to one. The investigators use a combination of the IR results and other evidence to prosecute the owner of the vehicle.
SCENARIO #2: A robber has broken into a home using a blunt object. After obtaining a warrant, investigators find that a suspect is in possession of a crowbar that has some crushed paint chips on the end.
Using infrared spectroscopy, a forensic chemist matches the paint on the crowbar to paint on the door of the house that was burglarized. The suspect is convicted and sent to trial.
Read the FBI's Forensic Paint Analysis and Comparison Guidelines.
Photo from the Photo Index at a Texas Hospital Website