Forensic science is an interdisciplinary field defined broadly by the application of scientific methodology to analyze crime scene evidence. The field was largely popularized with the release of crime dramas, true-crime documentaries, and podcasts. While television and movies shed some (highly dramatized) light on forensic science, they rarely discuss forensic methods or their safety implications within a lab setting. Although forensics encompasses areas such as cybercrimes, analysis of accounting records, anthropology and dentistry, fingerprint analysis, bloodstain patterns and ballistics, the emphasis of this article will be on chemical and instrumental techniques. These include toxicology and identification of bodily fluids, trace evidence analysis, serial number restoration, and DNA profiling.
Forensic toxicology and identification of bodily fluids – Forensic toxicology involves the identification and quantification of illicit drugs, medications, alcohol, poisons, and/or toxins in samples of blood, urine, or other bodily fluids. Analysis of these samples may be conducted using instrumental techniques such as mass spectrometry, UV-visible spectroscopy, infrared spectrophotometry, gas chromatography, or high-performance liquid chromatography (HPLC). Bodily fluids can be identified using similar techniques, but are more commonly identified using less instrument-intensive methods such as luminol, a chemical mixture that reacts with bodily fluids to produce light, antibody staining or enzymatic methods.
Trace evidence analysis – Trace evidence analysis includes the analysis of any trace evidence such as hairs, fibers, gunshot residue, explosives, paint, ink, and fire debris left at a crime scene. Hairs and fibers can be examined under a high-powered microscope. However, other evidence must be subjected to chemical analysis. Similar to toxicological methods, trace evidence analysis of explosives, ink, and fire debris may use gas and/or paper chromatography to analyze sample composition and compare it to known reference standards.
Serial number restoration – Serial numbers that have been scratched or filed off of a metal surface can sometimes be restored using chemical (most common), electrolytic, ultrasonic, magnetic, and thermal methods.
DNA profiling – DNA collected from crime scenes in the form of bodily fluids, skin cells, hair, etc. can be analyzed and compared with samples from a known origin or person. The DNA from the samples is extracted, quantified, and then amplified using polymerase chain reaction (PCR). The amplified DNA can then be analyzed using short tandem repeat (STR), Y-chromosome, mitochondrial, or the less common restriction fragment length polymorphism (RFLP) analysis techniques.
Ductless hoods can be an invaluable tool for evidence handling and forensic science. The type of ductless hood required depends heavily upon the forensic application and the type of protection required (operator v. process). Serial number restoration and chromatography methods rely on the use of acids and organic solvents, respectively. Both chemical classes are generally considered hazardous, and inhalation of the vapors from these chemicals should be avoided. Luminol and other commonly used stains or dyes can also cause respiratory irritation. Therefore, when possible, serial number restoration, blood detection on small articles, staining of fixed samples, chromatography methods, and other applications utilizing hazardous chemicals should be conducted in a fume hood to protect the operator from hazardous fumes. A ductless fume hood with a horizontal laminar flow, which pulls air from near the operator, across the work space, and up the back of the hood where it is filtered prior to exhaust, should be used for these types of applications. The hood should be fitted with the appropriate carbon filter to capture the chemical vapors, and operators should always don appropriate personal protective equipment to protect against splashes. In instances where the area or article to be analyzed cannot be handled within a ductless hood, other respiratory protection should be used.
For other applications, such as DNA profiling, process protection, rather than operator protection may be needed to preserve evidence integrity and prevent environmental contamination. Most DNA profiling methods utilize a PCR amplification step. PCR is incredibly sensitive; in theory, a single copy of DNA can be amplified to 1 billion copies in a single reaction. Therefore, the contribution of even small amounts contaminating DNA in the form of skin cells or hair from workspace users or other samples is quickly amplified during the reaction. DNA contamination can lead to false, inconsistent, or uninterpretable results for an often irreplaceable sample. PCR setup in a ductless hood with a vertical laminar flow can help protect against environmental contamination during DNA analysis. A vertical laminar flow hood pulls air from the top of the hood, through a high-efficiency particulate air (HEPA) filter and onto the work surface. This filtered air is free from dust, debris, and contaminating environmental debris to preserve sample integrity. The air then escapes out the front sash. Therefore, vertical laminar flow hoods should only be used for applications where operator protection is not a concern.