The Science Behind Forensic Evidence Collection

Julia Koblitz |

Forensic science evolved from societal concerns rather than natural or physical sciences. It is a significant difference. For example, a crime scene investigator should carefully collect an object containing DNA traces without collecting interfering materials. It prevents contamination of the evidence. The same applies to other physical evidence, such as fibers.

Physical Evidence

There are two categories of evidence: physical and testimonial. Physical evidence is measurable, such as blood or bullet fragments. This kind of evidence needs to be appropriately documented and can be gathered by tapping, swabbing, or scraping.

Those who collect physical evidence need to be aware of the importance of the chain of custody. They should also understand that a forensic technique that works on one occasion may not work another time and must be empirically validated before being used at a crime scene.

To reduce contamination, evidence should be collected from various sources to create the most complete picture possible for laboratory analysis. The investigator should also ensure that the sample is not contaminated by touching it with a dirty hand or clothing, as this could render the evidence useless for testing.

Physical evidence collection can include blood, body fluids, hair and fibers, stains, paint flakes, and glass fragments. In most cases, these samples must be preserved and stored after being collected at the crime scene. It includes keeping them dry and ensuring the collection bag or envelope is not sealed prematurely, as moisture can hasten its deterioration. The most straightforward tool for collecting trace evidence is tweezers, but vacuums and tape lifts can also be employed.

Chemical Evidence

Whether an object at a crime scene has evidentiary value, all objects must be collected and adequately preserved to avoid contamination or degradation. It is essential for evidence that can be analyzed using chemical processes. For example, chemical evidence includes drugs (both illegal and prescription), explosives, poisons, and adulteration of petroleum products. All of these types of chemicals have specific characteristics that are unique to each. It makes identifying the substance by a qualified forensic chemist possible through scientific methods.

In some cases, chemical analysis can identify suspects or determine punishment. However, it is challenging to study the impact of forensic evidence on criminal justice outcomes without being able to evaluate a range of different factors.

For example, forensic laboratory analysis depends on the ability of a technician to do forensic evidence collection handle, and store samples under controlled conditions. Additionally, a forensic chemistry technician must be able to recognize the presence of various chemical compounds in a sample and then determine which testing reagents are required for analysis. This process requires a great deal of training and experience. In addition, law-enforcement communication and priorities can influence evidence collection and preservation decisions. It can cause forensic examiners to sacrifice appropriate methodology for expediency.

Digital Evidence

Forensics experts must preserve all digital evidence, from computer hard disks to video files. They must also ensure that all tools used in this process are valid and can extract data without causing damage or altering it. It is a complex task, and the results can be devastating for any investigation that relies on it.

Historically, the decision to collect and preserve crime-scene evidence has been based on an officer or evidence technician’s perceptions of the seriousness of the case. While some of this is inevitable, agencies can ensure greater accuracy by establishing policies based on scientific standards and research.

In addition, forensic methods should be subject to peer-reviewed and published studies, establishing their scientific basis and validity. The 2009 NRC report notes a “notable dearth of such studies.”

Finally, the collection and preservation of digital evidence must be done to allow for further analysis in the future. It could include the need for a new type of analysis or reopening a case in light of additional information. Agencies should establish policies and prioritize cases to ensure that forensic evidence is properly preserved for the long term. It may require other resources and the establishment of a new crime laboratory dedicated to forensic evidence work. Ideally, this work would be conducted by professionals independent of law enforcement and subject to a high degree of oversight by scientists familiar with the collected and analyzed evidence.

Biometric Evidence

Biometric evidence refers to methods that use physical or behavioral attributes of an individual for identification purposes. These include facial features, voice patterns, fingerprints, palm prints, iris scans, eye structures (i.e., the retina and iris), and gait. These techniques are becoming increasingly used in law enforcement and private sector applications, with massive databases of such data already being stored by companies and government agencies.

Agencies should be able to insist through policies that only scientifically valid evidence-collection methods are used. In addition, they should ensure that any limitations of the method are known to those who use it. National scientific bodies and academics must research the validity and reliability of forensic evidence methods, as crime laboratories need more resources to do so independently.

Scientists and crime lab professionals must be part of developing policies and procedures concerning how forensic evidence is collected, analyzed, presented, and preserved. Mistakes can be made without that involvement due to haste, inexperience, and lack of a scientific background. Bias, slanted interpretations, and even falsification can occur.

Ideally, the individuals involved in collecting, analyzing, and presenting forensic evidence to court should be independent of law enforcement and subject to scientific oversight. However, communication and cooperation between law-enforcement personnel and forensic evidence professionals is appropriate and essential. Still, steps should be taken to create policies that prevent bias through directive or task-irrelevant communications.

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