Forensic techniques are used to investigate and help solve many different crimes, including theft, drug trafficking and use, murder, and terrorism. Forensic scientists, therefore, need to use the most advanced techniques available to them to help examine evidence and secure convictions for criminals involved in crimes. This article will provide an overview of how chromatography is used in forensic science.
Image Credit: borzywoj / Shutterstock.com
Chromatography – An Overview
Chromatography is a laboratory technique for separating compounds within a mixture, first devised in Russia in 1900 by Mikhail Tsvet. There are two phases of chromatography – the mobile and stationary phase. The mobile phase is where the mixture is dissolved in a fluid (either a gas, solvent, or water) which is then carried through a system (for example, a column or a capillary tube) on which is affixed a material. This is the stationary phase.
Within the mixture, different compounds have different affinities for the stationary phase. Depending on their interaction with the material in question, these compounds can stay affixed to it for longer or shorter periods. Thus, the compounds in the mixture separate due to traveling at different velocities in the mobile fluid. Properties such as size and how strongly they bind with the stationary phase allow for proper separation and analysis. A compound that binds more strongly with the stationary phase moves more slowly within the mobile phase, for example.
There are many different types of chromatography that can be used to analyze many different materials. These include Thin Layer Chromatography (TLC) or Paper Chromatography (which are planar methods) and column-based methods such as High-Performance Liquid Chromatography (HPLC) and Gas Chromatography (GC.) Other techniques include Ion exchange chromatography and highly specialized techniques such as reversed-phase chromatography and chiral chromatography.
Use in Forensics
Forensic scientists can be called upon by law enforcement agencies and other organizations worldwide to perform analysis on a vast range of compounds, from ink and lipstick to explosives used within bombs. As each compound has a unique set of chemical properties, there is no one chromatographic technique that is suitable for all purposes.
Planar techniques can be used to identify substances such as ink on banknotes (and to detect the same ink on the hands of thieves) or forgeries, dyes, and different drugs. TLC is one commonly used technique for this purpose wherein analytes are drawn through a thin layer of stationary phase via capillary action at different rates, allowing for easy identification of compounds.
Planar techniques are advantageous due to their quick and inexpensive nature. However, more complex chromatographic techniques are far better suited for forensic applications.
The column-based technique HPLC is widely used in forensic science. In this method, the mobile phase is forced through the column at high pressure rather than dripped through it, as it is in other liquid chromatography methods. The sample which needs to be tested is injected as a solute into the mobile phase. The mobile phase, therefore, acts as a solvent.
HPLC is used mainly for analyzing the contents of explosives as different substances that are used in them have different retention times due to their differing chemical and physical properties. HPLC can also be used to detect certain drugs and has been used in investigations into terrorism, drug cartels, murders, and organized crime syndicates.
GC is another widely employed technique, otherwise known as gas-liquid chromatography, which differs from other chromatography methods by using a liquid stationary phase and a gaseous mobile phase. In forensic investigations, gas chromatography is used in toxicology screening to determine if a deceased person has ingested drugs or alcohol prior to death.
It can also be used to tell if a victim of crime has been poisoned. When trying to determine the cause of death, this information can be crucial for investigators. Samples such as blood and fibers can also be investigated with GC.
Gas chromatography is also employed in arson investigations. Most cases of arson are started with accelerants such as gasoline and kerosene, and gas chromatography can separate the hydrocarbon components of these accelerants. A unique patterned graphic can then be produced for each compound present, allowing easy visual analysis of them.
Coupling Chromatography with Mass Spectrometry
Chromatography is only one part of an effective forensic investigation. Whilst it separates them, it still needs a way to effectively detect and categorize the compounds. Some compounds can be easily mistaken for other similar compounds as well.
An effective method that can be coupled with chromatography techniques is mass spectrometry. Samples that are separated by chromatography can then be put through the spectrometer, which separates them by size. This narrows down the range of possible substances and allows for accurate identification of them. Both GC-MS and LC-MS techniques are established in forensics, which can identify a wide range of compounds present in explosives, drugs, and many others that must be identified as part of an effective investigation.
Chromatography methods are a well-established, powerful suite of methodologies in forensic science. They can be employed for easy identification of a plethora of chemical compounds that may be present in samples from terrorist incidents, drug busts, murders, and robberies, to name but a few.
Wood, M. et al (2006) Recent applications of liquid chromatography – mass spectrometry in forensic science J. Chromatorgr A. 13:1130(1):3-15 [Accessed Online 29th December 2020] https://pubmed.ncbi.nlm.nih.gov/16716330/
Maurer, H.M (1998) Liquid chromatography-mass spectrometry in forensic and clinical toxicology Journal of Chromatography B: Biomedical Sciences and Applications 713:1 pp. 3-25 [Accessed Online 29th December 2020] www.sciencedirect.com/science/article/abs/pii/S0378434797005148
Sampat, A et al. (2016) Forensic potential of comprehensive two-dimensional gas chromatography TrAC Trends in Analytical Chemistry 80 pp, 345-363 [Accessed Online 29th December 2020] https://www.sciencedirect.com/science/article/abs/pii/S016599361530050