Time-of-Flight Mass Spectrometry is a technique for separating ions of different masses by measuring the time taken to traverse a fixed distance through a magnetic field. Sounds a bit arcane? The technique is used daily by forensic investigative teams to research criminal profiling and provide reliable evidence for the prosecution…
Ever since the late 1800s, fingerprints have been at the heart of criminal forensic investigations. Fingerprints, or finger marks, are made when the tip of the human finger comes into contact with a surface and leaves an impression of the friction ridges. These ridges contain secretions of sweat from the eccrine glands in the fingertip, and the natural complexity of their patterns ensures that no two individuals’ sets of prints are ever identical.
Invisible prints, termed ‘latent’, can be developed by a number of techniques that usually rely on a chemical reaction with a molecular component within the fingerprint. By far, the most frequent type is the latent print.
Latent prints are formed by sweat, either from the hands themselves or by unconscious contact between the fingers and the face or other parts of the body. Even the swiftest of criminals find it difficult to escape without leaving behind the trace of a single fingerprint.
Often those fingerprints are difficult to develop, depending on their age or the surface upon which they have been deposited, and forensic scientists are constantly looking for new and better methods to enhance them. Researchers are also finding ways to interrogate the molecular composition of the residues within fingerprints to discover information about the person who deposited them and whether they have handled or consumed particular drugs like cannabis, for example.
Dusting for Prints
Dusting the crime scene is the commonest, simplest and oldest technique for developing latent prints. A powder, usually gray or black, is dusted on wood, metal, glass and other such surfaces to expose the hidden prints, quite simply to make the prints apparent to the naked eye.
The powder is made of resinous polymers that can be mixed in literally hundreds of ways and adheres to the skin oils that invariably are left behind on anything touched directly with the fingers.
Fluorescent and phosphorescent fingerprint powders have been created to solve the contrast problem of developing fingerprints on many coloured surfaces.
The main components of fingermarks are water, amino acids, salts and fatty acids.
Iodine fuming is one of the earliest known techniques for investigating latent fingerprints. Iodine crystals vaporise rapidly when subjected to heat and produce violet fumes that are absorbed by skin oils. Latent prints absorb the iodine fumes and become visible. The prints remain visible for as long as the fumes last.
Porous Substrate Surface
Nowadays, the standard test for latent marks on a porous substrate involves exposing the surface to a solution of ninhydrin (2,2-dihydroxyindane-1,3-dione), which reacts with the terminal amines of lysine residues within the sweat to create a purple product.
Non-Porous Substrate Surface
For non-porous substrates, another popular method of chemical fuming is cyanoacrylate fuming of the sample – using Super Glue vapour. For reasons not fully understood, this polymerises when it comes into contact with the fingermark, thus becoming visible. So, for a fresh finger print on metal, the all-to-familiar Super Glue normally works very well.
But these methods do not always work.
One example of a ‘difficult’ surface is spent ammunition cartridges or shells. On a spent shotgun cartridge casing, it is extremely difficult to recover sweat using conventional techniques. The main reason being, the extremely high temperatures that the shells are exposed to simply vaporises the sweat.
Back in their laboratory, the CSI team uses an analytical technique called mass spectroscopy to find traces of substances, no matter how small, on or within the ridges of the print.
Mass Spectrometry uses a Magnetic Field
Mass spectrometry (MS) is an analytical technique that produces spectra of the masses of atoms or molecules in a sample of material. The spectra are used to determine the elemental or isotopic signature of the sample. Additionally, the technique helps determine the masses of particles or molecules and elucidate the chemical structures of molecules, such as peptides and other chemical compounds.
Time-of-flight mass spectrometry (TOFMS) is a method of mass spectrometry in which the mass-to-charge ratio of an ion is determined via a time measurement.
The ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has the same charge.
Mass spectrometry works by ionising chemical compounds to generate charged molecules or molecule fragments. The velocity of the ion depends on the mass-to-charge ratio (m/Q) – a physical quantity, widely used in the electrodynamics of charged particles. Some fields use the charge-to-mass ratio (Q/m) instead…
Subsequently, the time it takes for the particle to reach a detector at a known distance is measured. This time will depend on the mass-to-charge ratio of the particle (heavier particles reach lower speeds). From this time “of flight” and the known experimental parameters, one can find the mass-to-charge ratio of the ion.
The method works by vaporising the sample, and then firing it through an electric and magnetic field. Particles of different mass behave differently under these conditions and this means the team can identify molecules found within the print.
Back to the CSI lab…
Gas Chromatography-Mass Spectrometry
At the University of Lincoln in the United Kingdom, Ruth Croxton’s lab team investigates the deep chemistry of latent finger marks, identifying and quantifying the range of components with gas chromatography-mass spectrometry. They found that the most abundant fatty acids are hexadecanoic, octadecanoic and cis-9-octadecenoic acids. Triterpene squalene was also present.
The most abundant amino acids were serine, glycine, alanine, and aspartic acids.
Print interrogation with MALDI-MSI
At Sheffield Hallam University in the UK, Dr Simona Francese explains: ‘If a fingerprint is not present on the police database to provide a match, its evidential power is limited. We want to use and develop technology that will enable us to add intelligence in the case under investigation.’
And that is just what Francese’s team does.
“The kind of information we can find is very diverse. For example, by looking at the proteins found in the mark, we can find out if the suspect is a male or female,” says Dr Francese.
All kinds of exogenous substances can link to the lifestyle of the person or their activities. As well as providing information about an individual’s identity, finger marks can also reveal information about the person who left them, such as whether he or she had consumed or handled drugs of abuse like cocaine, or other substances.
The system produces both an enhanced image of the pattern of the fingerprint ridges, as well as detecting various chemical species that are present in the print. Multiple ions can be detected in a single analysis, and the print is not damaged by the technique.
They can tell who you are, where you’ve been and what you’ve had for breakfast…
The researchers can detect a broad range of molecules, including drugs and caffeine, originating from illegal substance abuse to ingested coffee.
‘We can understand whether or not a person has dealt drugs or actually taken drugs. We can detect ingested substances, so we may be able to reconstruct what that person has been eating just before committing the crime.’
The power of the method has been neatly illustrated with a study on condom lubricants. In many cases of sexual assault, the perpetrator uses a condom for protection and to avoid leaving DNA evidence.
‘We have carried out experiments looking at condom lubricant residues in fingermarks, and our method can detect a variety of lubricants, such as PDMS [polydimethylsiloxane] and PE [polyethylene glycol]. Different brands of condoms have different lubricant profiles, so the technique could be used to identify a particular brand of condom that was used,’ says Francese.
The team is currently refining the technique and drawing up protocols for its use in forensic laboratories.
So it’s Another Brick in the Whorl for Forensic Science…