One of my favourite TV series is Criminal Minds, an American police series set primarily at the FBI’s Behavioural Analysis Unit, Quantico (Virginia). It focuses on a skilled team of FBI profilers who catch serial killers through behavioural profiling. Besides Criminal Minds, also other series recently captured the interest of the audience, such as CSI, Law and Order and Cold Case. In all these TV programs the police solves the cases quickly and efficiently thanks to the talent of the agents but also thanks to the evidence that is collected on the crime scene and analysed by the chemists in a crime laboratory, who provide essential information to reconstruct the story. Although the role of forensic chemistry is fundamental in case solving, this is not often recognized. However, chemistry plays a key role to put together the pieces of the puzzle in all investigations.
WHAT DOES A FORENSIC CHEMIST DO?
In few words, a forensic chemist analyses physical evidence collected from the crime scene and provides the obtained information to the detective who is working on the case to solve the crime. The analysis of physical and chemical properties of substances is a crucial part of the process.
The physical properties can be observed and measured without altering the composition of the matter and they can be used to observe and describe it (e.g. appearance, texture, colour, solubility, polarity, odour). By contrast, the chemical properties may only be observed by performing a chemical reaction or a chemical change and they are characteristic of a particular substance. For example cocaine is a drug that can be described as a fine white powder (physical property); it also reacts with cobalt thiocyanate to give a blue-coloured product (chemical property). These different properties of cocaine help the investigators to identify it.
Chemical and physical properties, however, can be used only for presumptive analyses, as sometimes these characteristics are not enough by themselves to identify a substance. Methamphetamine, for example, gives a deep blue-coloured product when it reacts with sodium nitroprusside in the presence of sodium bicarbonate. Nevertheless, other substances with a similar molecular structure will react in the same way and will give the same product. For this reason, confirmatory analyses, which are based on unique properties of substances, are required to identify with certainty a sample.
Different techniques can be applied for this purpose and most of them are spectroscopic techniques. The ultraviolet-visible-near infrared spectroscopy, for instance, is used to test certain drugs of abuse to confirm their property to absorb UV light at a certain wavelength. This property depends on the presence of chromophores in the structure of the compound, functional groups that can absorb UV light and are responsible for its colour. Another technique is represented by Fourier Transform infrared spectroscopy (FTIR), which gives a characteristic spectrum for each compound depending on which functional groups are present in the molecule structure. The analysis of the obtained spectrum allows the recognition of these functional groups in the molecule thus allowing its identification. The cited techniques are often used in combination with others, especially if the analysed sample is in reality a mixture of two or more compounds. In this case, the compounds in the mixture can be separated and individually analysed using a gas chromatograph-mass spectrometer (GC-MS). This instrument allows the separation of the different components in the mixture and their recognition by breaking them into fragments whose mass can be measured. The generated fragments are exclusive for each compound and allow their identification.
Another technique used to separate different compounds in a mixture is thin layer chromatography (TLC), which can be applied to separate drugs, fibre dyes, poisons and inks.
This method exploits the different polarity of the compounds in the mixture allowing their separation by the different interaction with a solvent system (mobile phase) and a flat glass plate (stationary phase). Polar compounds tend to interact more with the stationary phase, therefore they “run less” on the TLC plate; whereas less polar compounds interact more with the mobile phase, therefore they “run more” on the plate and in this way they can be separated from compounds that have a different polarity.
All the above techniques are applied to recognize and identify specific substances. However, the work of a forensic chemist also involves other types of analysis. The colour tests, for example, are utilized to probe drug samples for their chemical properties.
These reactions are carried out on ceramic or plastic dishes called spot plates, which contain several wells. Each well is filled with a small amount of the questioned drug and a chemical known to produce a coloured product in the presence of the drug. This allows the presumptively identification of a drug. Some drugs can be also recognized through microcrystalline tests, which analyse representative crystalline structures under the microscope with transmitted illumination. These tests, though, are more complicated than the colour tests as not many forensic chemists are trained to recognize the characteristic crystalline structures.
Different analyses can be performed to verify the properties of soil that can be related to a specific location where a crime was committed.
This can help to confirm, for example, if the soil found under the shoes of a suspect or on his car is the same of the crime scene. Soil is a mixture of organic and inorganic materials whose properties can be analysed (pH, density, composition, colour, texture) through microscopic techniques, sieves to determine the size distribution of a sample, density gradient to evaluate the distribution of densities comprising in the soil mixture.
Other common items of evidence collected on a crime scene are fingerprints. All fingerprints are unique for each individual (even twins have different fingerprints!) and they can be very useful to identify a suspect, especially if their fingerprints had been previously recorded in databases of prints collected by law enforcement agencies. Latent fingerprints cannot be visualized by eye but development techniques can be used to observe them.
One of these is called powder dusting and involves the use of powders (dark, light o fluorescent) to see the contrast and highlight the fingerprints, which can then be lifted and preserved using a fingerprint tape. Another substance that can be used to colour fingerprints is called ninhydrin, which is used in biochemistry for qualitative and quantitative determination of a-amino acids. This substance gives a purple-coloured product when reacts with the amino acids contained in fingerprints residues. Further techniques that can be used for fingerprint development include the silver nitrate reaction (reaction between silver nitrate and soluble sodium and potassium salts to give a white solid product called silver chloride, which produces silver and chlorine purple-black gas when exposed to ultraviolet light) and the iodine sublimation (based on the absorption of iodine vapours by the fingerprints products to give a amber-coloured product).
Finally, if the fingerprints contain traces of blood, these can be observed by employing luminol, a derivative of phtalic acid, which allows the detection of blood traces through the reaction with metal cations (e.g. the iron-shaped cation present in the heme group of haemoglobin) to give a blue luminescencent product called 3-aminophtalate.
Many different techniques are used to analyse evidence, which are essential to solve a case. Therefore, although it is not well known, forensic chemistry gives a vital contribution to all investigations. Think about it next time you watch a police TV series…