Researchers are pushing the boundaries of infrared forensics with promising results for criminal and anti-terrorism investigators
Many a time DNA extracted from a single hair, a flake of skin or a trace of blood or saliva at a crime scene can help in securing convictions for criminals on the basis of DNA evidence. However, it is not so well known as to how it is achieved. Forensic science has hugely benefited from the ability to identify the presence of molecules in a sample using synchrotron radiation.
Synchrotron radiation offers a superior light source in comparison to many conventional techniques that could play a major role in evidence analysis. The technique allows for greater sensitivity and analysis of much lower concentrations and smaller samples than typically possible.
Synchrotron sources are relevant to forensic science in cases where primarily, conventional techniques fail due to limits of detection, secondly added scrutiny and discrimination are required such as in high-proﬁle cases, and lastly, to perform fundamental research into 'evidence material science'.
With infrared light, forensic investigators can tell you whether a document is a forgery or whether a paper currency is counterfeit. They can take a paint chip and tell you the make, model, and age of a car. Now the boundaries of infrared forensics are being pushed into uncharted territories by researchers, and the results are promising for criminal and anti-terrorism investigators, as well as for historians and archaeologists. Many of the materials encountered at crime scenes are, however, too small to be analysed using standard instrumentation, even at infrared wavelengths. However, the advent of "infrared spectromicroscopy" has led to significant advances in forensics.
The technique has become one of the sought-after tools in the standard forensics laboratory, examining everything from drugs, paints, fibres and explosives to polymers, inks and documents. And, it has proved a highly effective tool for analysing samples from crime scenes, such as blood, fabrics and soil particles.
The early start
The use of IR spectromicroscopy in forensics got its start back in 1949, but its applications were sharply limited because large sample size was required for analysis. This changed with the commercialisation of thermal IR sources such as Globars (TM) in the early 1990s. Globars are silicon-carbide filaments that radiate IR light when heated; using Globars, forensics researchers are able to work with samples as small as 75 microns (75 millionths of a meter). But even that can be excessive when dealing with criminal evidence.
In the past decade, however, spectromicroscopy has been revolutionised thanks to high-quality optics, inexpensive spectrometers and fast computers to process the data.
A murder case in Japan was cracked using synchrotron radiation. Four people were killed and 63 others became sick as curry poisoned by arsenic was served during a summer festival at Wakayama City on Japan, on 25 July 1998.
At an early stage of the case, it was believed that the curry was served with rotten ingredients in it. Then cyanide was erroneously detected. The detection of arsenic was a key analysis for the medical treatment of the victims.
An insurance saleswoman was arrested for murder and attempted murder, and after long trials from a district court to the Supreme Court, the saleswoman was sentenced to death without confession, based on key evidence of the synchrotron radiation X‐ray fluorescence (SR‐XRF) analysis.
Authenticity of lottery scratch card
So, how has infrared spectroscopy helped in the fight against crime? Back in 1989, the authenticity of an instant-win lottery scratch card was questioned when it was submitted as a winning ticket in Connecticut in the US. Computer records showed that the ticket was not a winner based on its pre-printed serial number. Attempts to forge or alter lottery tickets are fairly common, so forensic staff at the lottery agency analysed the scratch card in detail. They could find no alteration or forgery. Since a great deal of prize money was at stake, the ticket was sent to a private company for detailed investigation.
During the probe, scientists there obtained infrared spectra from various parts of the ticket – in particular around the scratch panels and the serial number – and compared them with spectra from valid tickets. When the two sets of data were shown to match, the problem was deemed to be a "computer error" and the victory money was paid out.
The technology has also proved a powerful tool in the crackdown on illegal drugs. The challenge for forensic chemists is to identify the compounds in the sample so that the evidence stands up in a court. The best results are achieved with pure samples but in many cases, other techniques are needed to purify or isolate the sample before synchrotron radiation is applied. This is often the case for illegal drugs sold on the streets that are usually mixed with other substances.
In one case in the late 1990s, the police at Ottawa in Canada identified a derivative of amphetamine from a small piece of evidence using a combination of infrared spectroscopy and other techniques. The police also discovered small amounts of another contaminant in the sample that suggested that the drug had been manufactured illegally.
For the judgments in the scientific criminal investigations, non-destructive and high-sensitivity analysis methods are often necessary to get information from tiny samples.
We all know that when we lay our hands on something we tend to leave behind a fingerprint whose pattern of loops, whorls, arches and "tents" is distinctly our own. What we, however, may not know is that we also leave behind a minute residue of chemicals - proteins, salts, and fatty acids - whose proportions to one another may also be distinctly our own. There are called sweat prints.
Although the forensic jury is out on whether chemical sweat prints are as unique as physical fingerprints, researchers at Berkeley Lab in the US were able to correctly distinguish the sweat prints of three individuals.
An IR spectromicroscopic profile of a sweat print might also reveal the age and gender of the person leaving the sweat and possibly even identify when the sweat was deposited if the appropriate chemical markers can be observed in the IR spectrum. In any case, since the technique is non-destructive, once an IR profile has been acquired the undamaged sweat print can be studied more minutely by using other forensic techniques.
Synchrotron-based IR spectromicroscopy is also applicable to the characterisation of trace amounts of biological fluids on cloth or blood on glass; tracing explosive chemicals, poisons, or illicit drugs to their manufacturers and suppliers; and even identifying the geographic origins of dust particles.
(Ritu Sharma is Editor, Nuclear Asia. The views expressed are strictly personal)