Forensic Techniques: Chemical, Biological, and Analytical

The exploration of forensic chemistry illuminates its vital role in rewriting the narratives of crime.

Forensic chemistry is the backbone of the criminal investigation process. It utilizes various chemical, toxicological, and analytical techniques to identify evidence from a crime scene. Most of the chemical analysis uses spectroscopy/spectrometry and chromatography techniques, spanning High-Performance Liquid Chromatography (HPLC), Gas Chromatography-Mass Spectrometry (GCMS), Liquid Chromatography-Mass Spectrometry (LCMS), Thin Layer Chromatography (TLC), Atomic Absorption Spectroscopy, and Fourier Transform Infrared Spectroscopy (FTIS). Spectroscopy is concerned with the investigation, measurement, and quantification of the absorption or emission of electromagnetic radiation. Chromatography is the process by which components of a mixture can be chemically separated.

Mass spectrometry is a tool used to measure the mass-to-charge ratio of one or more molecules, which helps to determine a compound’s identity and molecular composition. The mass spectrometer uses a 4-step process to establish the ratio: acceleration of the particles to first give them all the same kinetic energy, ionization of the particles (which gives them a charge), deflection of the particles, and then their subsequent detection. Lighter, more charged particles are deflected more and detected earlier. A mass spectrum graph is produced for the particles detected. Occasionally, forensic scientists also use “quadrupole time-of-flight (Q-TOF) mass spectrometry, which separates sample components based on how long it takes for them to cross a certain distance within a vacuum,” using time instead of a mass-to-charge ratio to separate fragments (News-Medical). Mass spectrometry has helped forensic scientists determine the identity of an unknown illicit substance and can establish whether two samples of trace evidence came from the same source. However, mass spectrometry alters and destroys the evidence through analysis, proving it difficult to use in cases where limited evidence is retrieved.

Raman spectroscopy, a light-scattering process, can also elucidate chemical structure though it is non-destructive which preserves the evidence in its original state. It can also provide key information on trace evidence and residue, such as the “color of a paint chip, the origin of a textile, and how it was made” (AZO Life Sciences). Another kind of spectroscopy is UV/Vis (referring to UV and Visible light), which is used to examine inks and fibers and is complemented by chromatography. Spectroscopic techniques have further developed fingerprinting, as they can identify the chemical components of a fingerprint collected. Imaging with infrared micro-spectroscopy can capture the “most comprehensive view” of fingerprint chemistry. Meanwhile, Fourier transform infrared (FT-IR) spectroscopy, which characterizes unidentified samples based on “the absorption of [infrared] light by characteristic functional groups of organic molecules,” has been used to estimate the time since a fingerprint was deposited. FTIR can identify different glandular secretions that are further analyzed to determine how they change over time and under different temperatures. This helps model how changes in the chemical composition of the fingerprint may reflect the time since it has been deposited.

Chromatography is also heavily used by forensic scientists and is often coupled with spectroscopic techniques. Chromatography involves a mobile phase that moves the compounds or sample components and separates them and a stationary phase where the compounds move through. There are two main kinds – column chromatography (like GCMS and LCMS) and planar chromatography (like TLC). In column chromatography, samples are put vertically through a high-pressure column, whereas in thin-layer chromatography, samples are spotted flat on a paper-like material.

Chromatography methods separate compounds while spectrometry characterizes them, and the combination of these two methods can narrow down the substances that need to be identified. In GCMS and LCMS, the GC and LC principle is that molecules in a sample will separate based on differences in their size and chemical properties, which affects how attracted they are to the stationary phase. The more they are attracted to one material, the less they want to move with the other, the mobile phase – these differences in attraction by the different components of a compound affect how much they separate from each other. The MS principle turns the components from the column into ionized particles and then separates them by a mass-to-charge ratio.

Liquid chromatography differs from gas chromatography in that liquid is used to first dissolve the sample and acts as a mobile phase to move the compound and separate its components, rather than gas and heat being used as it is in GCMS. And because the sample is moving through a liquid and not heated gas, the sample material is less likely to be worn down during the chromatography analysis. LCMS helps analyze volatile substances like gunpowder residue, fibers, toxins, and dried blood and is used to “confirm the findings of presumptive drug tests” (News-Medical). LC/MS-based screening techniques have “enabled hundreds of drugs and their metabolites” to be detected at unprecedented speed, in a matter of minutes.

In forensic investigations, gas chromatography has been used in toxicology screening to determine which fluids and compounds are present inside a human body after death. Upon analysis of bodily fluids and fluid composition, a forensic scientist can establish whether a person was intoxicated from alcohol, drug abuse, poison, or other harmful substances at the time of death – “imperative knowledge to determining cause of death and possible motive and culprit in the case of foul play” (Chromatography Today). Another notable use of gas chromatography is in suspected cases of arson when flammable liquids and accelerants may need to be analyzed. GCMS analysis of trace evidence of explosives may also point investigators not just to the composition but also to the manufacturer.

Planar chromatography like TLC is used heavily because it is reproducible, cost-effective, and fast. Samples are added to paper-like material, usually, aluminum sheets coated with silica, and placed in a solvent, where capillary action draws up the analyte, whose components begin to separate accordingly. TLC has been used to identify ink on banknotes and link it to the same ink on the hands of thieves, in addition to forgeries, dyes, and different drugs.

As the cornerstone of the criminal investigation process, forensic chemistry employs an array of tools — chemical, toxicological, and analytical — to unveil hidden truths within crime scenes.

Sources:

https://collegedunia.com/exams/forensic-chemistry-definition-methods-job-profile-chemistry-articleid-2167