DNA and Evidence Collection Principles

The dawn of a new age has arrived in law enforcement in the form of DNA research and testing. We in law enforcement, especially those of us working the crime scenes need to be aware of what we can do “in the field” to assure that proper evidence collection techniques are followed. Only then will the groundwork for successful evidence examinations be in place when we submit the case to a forensic laboratory for analysis.

Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) is the DNA evidence analysis technique which is being practiced at the state laboratory at the Division of Criminal Investigation in Des Moines. PCR is a sensitive, fast, and highly discriminatory method of analysis. One of the most essential aspects of DNA evidence analysis at the lab is that a basis knowledge of evidence collection principles is necessary at the initial stage. PCR allows the criminalist to examine evidence which has been properly collected and preserved with expectations that accurate results will be found as result of the analysis.

Impact and Exchange

It is widely embraced within the law enforcement forensic field that, to at least some degree, the process of impact and exchange occurs at every crime scene. For example, a “run” vehicle impacts the accident scene and exchange occurs with the transfer of paint to the victim vehicle; a burglar impacts a scene with the approach of the area, and exchange occurs when footwear impressions are left behind. As law enforcement officers and crime scene specialists, it is our job to collect and preserve evidence at the scene – evidence which may not only connect the suspect to the scene – but connect the suspect to the incident itself.

Swab Method

The collection and preservation of evidence which will be subjected to DNA analysis is best accomplished by the seizure and submission of the original item. For example, it would be desirable to collect and submit undergarments worn after an incident involving suspected sexual assault rather than cutting or swabbing the specimen. Sometimes, however, the submission of the original item is impossible or impractical. Imagine a shooting or stabbing scene where there is evidence of considerable blood loss on a tile or linoleum floor. The practice of swabbing for the evidence is then practical for collection of possible DNA evidence.

It is preferred that swabs to be submitted to the D.C.I. Lab in Des Moines be made with cotton tipped swabs (ie: Q-tiptm). The process is simple, and the following outlines the procedure:

Slightly moisten a cotton tip swab with clean water.

concenrate the stain as much as possible.

avoid potential sample-to-sample contamination during the process.

avoid contamination by the collector (wear protective clothing).


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