Making Invisible Traces Visible
Advanced techniques that transform microscopic evidence into objective, court-proof data for criminal investigations
In forensic chemistry today, the scope extends far beyond simple drug tests. Modern analytical laboratories are highly technical environments where highly sensitive instruments and innovative methods decipher even the smallest chemical traces that often represent crucial evidence in criminal investigations5 . These advanced procedures transform invisible evidence—whether in the form of a tiny gunshot residue particle, an inconspicuous fiber, or a minimal blood trace—into objective, court-proof data that can reconstruct the events of a crime.
Chromatographic methods form the backbone of forensic sample preparation. They separate complex mixtures, as commonly found at crime scenes, into their individual components.
This is an especially powerful tool. Samples are first separated in the gas chromatography column before the isolated substances are ionized in the mass spectrometer and identified based on their unique fragmentation pattern. This technique is indispensable in fire investigations, where it detects accelerants in fire debris, as well as in drug analysis for identifying narcotics like heroin or cocaine5 .
This method is particularly suitable for substances that are not vaporizable or are temperature-sensitive. In forensic toxicology, it is used to quantify non-volatile drugs such as certain opioids or antidepressants in biological samples5 . A further development, Liquid Chromatography-Electrospray Ionization-Tandem Mass Spectrometry (LC-ESI-MS/MS), even allows the characterization of 38 different nitazene analogs—a class of synthetic opioids—and helps distinguish them based on their product-specific ions1 .
While chromatography separates, spectroscopy identifies. It utilizes the interaction between matter and light to create a unique chemical fingerprint of a substance.
FTIR measures how a sample absorbs infrared light. Specific chemical bonds and functional groups in a molecule vibrate at characteristic frequencies, producing a spectrum typical for each substance. Forensic scientists use FTIR to identify polymers in fibers, the chemical composition of paint particles in traffic accidents, or various types of plastics in drug packaging5 .
This special sampling variant has proven revolutionary for solving violent crimes. Researchers at the University of Murcia showed that ATR FT-IR combined with chemometrics can accurately determine the age of bloodstains at a crime scene3 . This technique helps narrow down the time of the crime, which is crucial for reconstructing the sequence of events.
| Analytical Technique | Forensic Application | Analyzed Material |
|---|---|---|
| ATR FT-IR | Age determination of bloodstains3 | Blood |
| Raman Spectroscopy | Forensics and cultural heritage preservation3 | Various organic and inorganic materials |
| Handheld X-ray Fluorescence (XRF) | Distinguishing tobacco brands3 | Cigarette ash |
| Scanning Electron Microscopy/EDX (SEM/EDX) | Analysis of suspected cigarette burns3 | Injured tissue |
Fingerprints have always been golden evidence in forensics. But one crucial question often remained unanswered: How long has the print been at the crime scene? Conventional analysis of ridge patterns provides no answer to this.
Petr Vozka and his team used two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOF-MS) to get to the bottom of this question1 .
Fingerprints are deposited on standardized surfaces.
Samples are stored under controlled conditions for defined periods.
Organic substances are extracted from the fingerprints.
Extracted samples are analyzed using GC×GC-TOF-MS. This procedure offers exceptional separation power and captures even minute amounts of hundreds of thousands of chemical compounds.
The massive amounts of data obtained are evaluated using chemometric models to identify time-dependent changes in the chemical signature.
The research showed that the chemical components of a fingerprint—a complex mixture of sweat, skin cells, and sebum—change predictably over time. Volatile compounds evaporate, oxidation processes begin, and new substances are formed. The GC×GC-TOF-MS can detect these time-dependent chemical changes1 .
By applying machine learning to these chemical profiles, the team was able to develop a predictive model that can estimate the age of a fingerprint with high accuracy. This represents a monumental advancement, as it provides investigators with a "clock" for this widely used type of evidence, significantly improving the temporal classification of events at the crime scene1 .
Interactive chart showing chemical concentration changes over time
| Chemical Component | Concentration (Fresh) | Concentration (7 Days) | Concentration (30 Days) |
|---|---|---|---|
| Squalene | High | Medium | Low |
| Certain Fatty Acids | Medium | High | Medium |
| Oxidation Products | Not detectable | Low | High |
Today's experiments would not be possible without a range of highly specialized equipment and reagents. The forensic scientist's toolkit is full of high-tech instruments.
| Device/Reagent | Function in the Analysis Process |
|---|---|
| Ion Mobility Spectrometer (IMS) | Enables mobile drug detection on surfaces on-site, e.g., at crime scenes or in public facilities6 . |
| Salting-Out Assisted Liquid-Liquid Extraction (SALLE) | A sample preparation method validated for the reliable detection of amphetamine-type stimulants and cocaine metabolites in forensic toxicology1 . |
| Solid Phase Microextraction (SPME) | A contact-free extraction technique for volatile substances, used, for example, to distinguish crude oil sources in environmental forensics1 . |
| Ultra-High Performance Liquid Chromatography (UHPLC) | Enables extremely fast and high-resolution separations, often coupled with mass spectrometry, e.g., for profiling tissue metabolites to determine the postmortem interval1 . |
The boundaries of forensic chemistry are continuously being expanded by new technologies. Artificial intelligence and machine learning are revolutionizing data analysis. For example, portable electronic noses, equipped with 32 metal oxide semiconductor sensors, combine advanced machine learning algorithms to distinguish between human and animal samples, determine the postmortem condition, and even estimate the postmortem interval1 .
Also, Next-Generation Sequencing (NGS) of DNA allows genetic material to be analyzed with unprecedented precision, even if it is severely damaged or present in only minimal amounts7 . At the same time, DNA phenotyping opens up the possibility of inferring external characteristics such as hair, eye, and skin color of a potential perpetrator from a DNA trace—information that can be crucial when searching for unknown individuals7 .
Advanced data analysis and pattern recognition
Predicting physical characteristics from DNA
On-site analysis with laboratory precision
Modern analytical methods in forensic chemistry have evolved from purely identificatory tools into highly complex systems that can extract temporal, spatial, and biographical information from the tiniest traces. They give silent witnesses—the molecules—a voice and thus contribute decisively to ensuring that justice no longer depends on the size of a piece of evidence but on the sharpness of the analytical perspective. As technology continues to evolve, the future promises even more precise, faster, and more informative insights into the hidden chemical truth of every crime.